Patent Publication Number: US-2023138744-A1

Title: Heating Apparatus, Recreational Vehicle With Heating Apparatus and Method for Heating Fluids in a Recreational Vehicle

Description:
This 35 U.S.C. § 371 National Stage Patent application claims priority to PCT Patent Application No. PCT/EP2021/056839, filed Mar. 17, 2021, which claims priority to and benefit of DE Patent Application Serial Number 102020203422.2, filed Mar. 17, 2020, all of which is incorporated by reference herein. 
    
    
     Present embodiments relate to a heating apparatus, a recreational vehicle with such a heating apparatus and to a method for heating fluids in a recreational vehicle, in particular two distinct fluids in a recreational vehicle. 
     A heating apparatus for a recreational vehicle usually comprises a burner and a heat exchanging unit. While the burner is provided for burning a mixture of fuel gas or liquid and combustion air, the heat exchanging unit is provided for transferring heat from the exhaust gases from the combustion within the burner to one distinct fluid that has to be heated. The fluid to be heated is typically air from the interior of the recreational vehicle and/or water for sanitary or cooking purposes. 
     Meanwhile, also several configurations of heating apparatus, in particular for recreational vehicles, are known which are configured to heat distinct fluids more or less independently of each other. 
     However, the known configurations suffer from several drawbacks. In particular, known configurations often are expensive, require intensive maintenance and are in particular very bulky and heavy. Moreover, it is the permanent aim to increase the efficiency of such systems while reducing the consumption of energy at the same time. 
     Accordingly, there is much space for further developments of such heating apparatus. 
     The present embodiments overcome at least some of the aforementioned drawbacks of prior art devices. 
     This object is achieved by the heating apparatus as well as by the method for heating fluids according to the appended claims. The present embodiments also cover a recreational vehicle comprising such a heating apparatus. 
     According to a first aspect, a heating apparatus, in particular for recreational vehicles like campers or caravans, comprises a heating unit and two separate heat exchanging units. The heat exchanging units are coupled to the heating unit in parallel with each other. The heating unit comprises one burner for each heat exchanging unit and one common single combustion air fan unit. The single combustion air fan unit is configured to supply the burners with combustion air. The burners are configured to burn fuel gas or liquid further supplied to each of the burners together with the combustion air received from the single combustion air fan unit to get hot exhaust gasses. The heat exchanging units are configured to receive the exhaust gasses from the burners and to transfer heat from the exhaust gasses to the fluids to be heated, provided within the heat exchanging units. 
     In other words, according to the present embodiments, a heating apparatus has two independent heating circuits. One heating circuit is provided for heating a first fluid and another one is provided for heating a second fluid. Thus, it is possible to heat both fluids independently of each other. However, both heating circuits are supplied with combustion air by only one common single combustion air fan unit. Providing only one common single combustion air fan unit allows to save space and energy and results in reduced maintenance efforts as well. In other words, instead of being supplied by two separate combustion air fan units only one single combustion air fan unit is provided and supplies combustion air to both heating circuits so that only one has combustion air fan unit to be maintained. 
     The first heat exchanging unit is configured to transfer heat from the exhaust gasses to a liquid, in particular water for sanitary or cooking purposes, to be heated. Furthermore or alternatively, the second heat exchanging unit can be configured to transfer heat from the exhaust gasses to a gas, in particular ventilation air from an indoor room of the recreational vehicle, to be heated. With such a configuration, it is possible to heat as first fluid a liquid and as second liquid a gas. Thus, it is possible to heat different fluids resulting in a highly flexible implementation. 
     Further, the heating unit comprises a printed circuit board assembly. The printed circuit board assembly is coupled to the burners and configured to operate the burners independently of each other to heat the fluids provided in the heat exchanging units independently of each other. This configuration allows to heat both fluids independent of each other depicting a highly functional and flexible implementation. 
     Further the burners are coupled to the single combustion air fan unit in parallel with each other, such that a flow of combustion air generated by the single combustion air fan unit is split between the burners. By splitting the flow of combustion air from the single combustion air fan unit both burners are supplied with fresh combustion air. This allows to operate the burners more independently of each other and with efficiency. 
     Further the single combustion air fan unit has only one single combustion air fan for generating the flow of combustion air. In particular the single combustion air fan comprises only one single fan wheel, for example in the form of an impeller. Such a configuration is space saving. Moreover, such a configuration is reliable due to the small number of various elements. Furthermore, impellers allow extra small configurations. 
     In such a configuration, it may be that the single combustion air fan unit further comprises two housing elements. The housing elements are coupled to each other to form a combustion air flow path from one combustion air inlet opening via one single combustion air fan chamber to two combustion air outlet openings. The combustion air inlet opening is provided in one of the two housing elements. The one single combustion air fan chamber is enclosed by the two housing elements and contains the single combustion air fan. The combustion air outlet openings are provided in the housing elements. In particular, both combustion air outlet openings are provided in the same housing element, and in the same housing element than the combustion air inlet opening. Each of the combustion air outlet openings is coupled to one of the burners. Such a configuration is space saving and robust against external influences like dust or shocks as the single combustion air fan is enclosed by the housing elements. 
     In such a configuration, it may be that the single combustion air fan unit further comprises a combustion air fan driving unit coupled to the single combustion air fan and configured to drive the combustion air fan. The combustion air fan driving unit is provided on an outer surface of one of the housing elements. Further, the combustion air fan driving unit is coupled to the combustion air fan via a driving rod lead through a driving rod through hole provided in the housing element on which the combustion air fan driving unit is attached. In particular, the driving rod through hole is provided in one of the housing elements having no combustion air inlet or outlet openings. Providing the combustion air fan driving unit not within the combustion air fan housing unit allows easy access thereto and, thus, a facilitated maintenance thereof. 
     In such configurations, it may be preferred that the single combustion air fan unit further comprises two combustion air valves. Each of the combustion air valves is configured to close a section of the combustion air flow path from the single combustion air fan chamber to one of the combustion air outlet openings. This configuration results in the possibility to control the amount or rate of combustion air supplied to the burners independently of each other such that the heating apparatus can be operated in a flexible and efficient way. 
     At least one of the burners comprises two nozzles configured to supply fuel gas or liquid to a combustion area in which the fuel gas or liquid is to be burned with the combustion air. Each of the nozzles is coupled to its own fuel gas or liquid valve to control the fuel gas or liquid supply for each of the nozzles independently of each other. Providing two parallel nozzles with independently controllable valves for one burner allows to supply the combustion area with distinct amounts or rates of fuel gas or liquid and, thus, to control the combustion reaction at the corresponding combustion area in a flexible and efficient way. 
     In such a configuration, it may be that the fuel gas or liquid valves are provided as monostopable valves being switchable between an opened and a closed operation state. Such monostopable valves are inexpensive and reliable. 
     In such configurations, it may be that the nozzles of one burner differ from each other, in particular one nozzle may differ from the other in cross section of its supply opening and, thus, in its through put rate. This configuration allows to increase the number of various supply rates which can be set by opening and closing the various fuel gas or liquid valves. 
     The heating unit further comprises a secondary air supply arrangement. The secondary air supply arrangement is configured to provide a flow of secondary air to at least one, in particular to both of the burners. 
     Such a secondary air supply arrangement in particular is configured to be operated at distinct operation states with different supply rates for the secondary air. This implementation results in an increased flexibility for operating the heating apparatus in an optimized manner. 
     According to a further aspect of the present embodiments, a recreational vehicle, in particular a camper or caravan, comprises at least one of the above described heating apparatus. Thus, it is possible to take advantage of the above described technical effects in the recreational vehicle. 
     According to another aspect of the present embodiments, a method for heating two distinct fluids with one of the above described heating apparatus comprises the following steps:
         operating the single combustion air fan unit to generate a flow of combustion air from an external environment of the heating apparatus to each of the burners;   supplying fuel gas or liquid to each of the burners; operating the burners to burn a mixture of the combustion air with the fuel gas or liquid;   supplying a first fluid to be heated to the first heat exchanging unit and supplying a second fluid to be heated, different from the first fluid to be heated, to the second heat exchanging unit; guiding the exhaust gasses of the combustion within the first burner from the first burner to and through the first heat exchanging unit to transfer heat from the exhaust gasses to the first fluid; and   guiding the exhaust gasses of the combustion within the second burner form the second burner to and through the second heat exchanging unit to transfer heat from the exhaust gasses to the second fluid.       

     With this method according to the present embodiments it is possible to heat two distinct fluids independently from each other in a highly efficient manner as only one common single combustion air fan unit is to be operated. 
     For example, the combustion air fan unit is operated in such a manner that it provides the flow of combustion air to the burners simultaneously and in parallel with each other. Thus, the fluids can be heated flexible and/or independently of each other. 
     Further, the burners are operated simultaneously such that the fluids are heated simultaneously. In particular, the burners are not operated in a periodical manner, and in particular not in an alternating manner. This implementation allows to heat both fluids with the maximum heat outputs of the burners simultaneously. 
     Further, after being guided through the heat exchanging units the exhaust gasses are used to preheat the combustion air. Such an implementation is highly energy efficient as the remaining heat of the exhaust gasses which is not transferred to the fluids is used to preheat the combustion air and, thus, is at least in part transferred to the fluids in a second step of heat transfer. 
    
    
     
       These and other features of the embodiments will become more apparent from the following detailed description of a non-limiting embodiment, with reference to the accompanying drawings, in which: 
         FIG.  1    is a spatial view of a heating apparatus according to one exemplary embodiment; 
         FIG.  2    is a further spatial view of the heating apparatus of  FIG.  1   ; 
         FIG.  3    is a partially exploded illustration of the heating apparatus of  FIGS.  1  and  2   ; 
         FIG.  4 A  is a spatial view of an exemplary the coupling member for the heating apparatus; 
         FIG.  4 B  is an enlarged spatial view of the coupling member of  FIG.  4 A ; 
         FIG.  5    is a spatial view of an exemplary burner for the heating apparatus; 
         FIG.  6 A  is an exploded illustration of an exemplary combustion air fan unit for the heating apparatus; 
         FIG.  6 B  is a spatial view of another exemplary combustion air fan unit for the heating apparatus; 
         FIG.  7    is another partially exploded illustration of the heating apparatus of  FIGS.  1  and  2   ; 
         FIGS.  8 A to  8 C  are various spatial views of the heating apparatus with distinct elements omitted to show the inner structural configuration of the heating apparatus; 
         FIG.  9    is a spatial view of the heating apparatus illustrated in the above referenced FIGS. with lines indicating various cross-sectional planes; 
         FIG.  10 A  is a cross-sectional view of the heating apparatus along line A-A of  FIG.  9   ; 
         FIG.  10 B  is a cross-sectional view of the heating apparatus along line B-B of  FIG.  9   ; 
         FIG.  10 C  is a cross-sectional view of the heating apparatus along line C-C of  FIG.  9   ; 
         FIG.  11    is a schematic illustration of the structural configuration of the heating apparatus illustrated in the above referenced figures. 
     
    
    
     With reference to the accompanying drawings, a heating apparatus  1  according to the present embodiment comprises a heating unit  10 , a first heat exchanging unit  30  and a second heat exchanging unit  40 . The first heat exchanging unit  30  and the second heat exchanging unit  40  are both coupled to the heating unit  10  in parallel with each other. 
     The heating unit  10  comprises a primary housing  12 , a secondary housing  14  and a coupling member  16 , coupled to each other and housing further components of the heating unit  10 . The secondary housing  14  is coupled fixedly, for example via screw members, to the coupling member  16 . Alternatively, the secondary housing  14  can be formed integrally with the coupling member  16  as one-piece unitary member. The primary housing  12  of the heating unit  1  is coupled to the secondary housing  14  and to the coupling member  12  in a releasable manner, for example via clamping members or via a form fitting. This results in a configuration in which the primary housing  14  can be disengaged from the secondary housing  14  and the coupling member  16  in an easy manner. This enables access to the interior components of the heating unit  10  for maintenance or the like. However, alternatively the primary housing  12  can be coupled to the secondary housing  14  and/or the coupling member  16  via a configuration requiring the use of appropriate tools. 
     The primary housing  12  is provided with an opening  12   a . The opening  12   a  is covered with a removable lid (not illustrated). The lid allows access to the interior of the heating unit  10  in an easy but limited manner. This opening  12   a  is for example provided to connect electrical connections for power, control and/or 230 Vac electrical power to a circuit panel of a printed circuit board assembly (described later) of the heating unit  10 . In addition thereto or alternatively, other desired operations on the interior components of the heating unit  10  are possible via the opening  12   a  without the need of removing the primary housing  12  or parts thereof. 
     At least the primary housing  12 , in particular also the secondary housing  14 , is provided with some ventilation slots  12   c  and  14   c . The ventilation slots  12   c ,  14   c  allow air from the exterior of the heating apparatus  1  to enter the interior of the heating unit  10 . In the illustrated configuration, in the assembled state of the heating unit  10  the ventilation slots  14   c  provided in the secondary housing  14  are configured to extend corresponding ventilation slots  12   c  provided within the primary housing  12 . In the present embodiment, on each of three different sides of the primary housing  12  a set of eight horizontal ventilation slots  12   c  is provided. However, also configurations with more or less than eight ventilation slots  12   c  per side, and/or other structural configurations for the ventilation slots  12   c  and  14   c , like for example ventilation slots having circular or elliptical shapes or ventilation slots extending in vertical direction, are suitable within the scope of the present embodiments. 
     As illustrated in  FIGS.  4 A and  4 B  the coupling member  16  comprises basically a base portion  16   a , a preheating portion  16   b , a printed circuit board assembly coupling portion  16   c , and two burner coupling portions  16   d  and  16   e  coupled to each other. Here, the coupling member  16  is a one-piece unitary member. The coupling member  16  can thus be formed in a single cast process. However, some or all of the various portions  16   a  to  16   e  of the coupling member  16  can be provided as independent parts coupled to each other in an appropriate manner, for example with screws or bolts. 
     The base portion  16   a  of the coupling member  16  comprises a bearing surface  16   a   1 . The bearing surface  16   a   1  has a plurality of through holes  16   a   2 . In the present configuration bearing surface  16   a   1  has two through holes  16   a   2 . The number of through holes can vary depending on the specific need. The through holes  16   a   2  allow appropriate bolts or screws to pass therethrough such that the base portion  16   a  of the coupling member  16  can be fixed to a surface of the recreational vehicle like, for example, a wall, floor or ceiling area of the recreational vehicle or to any other suitable surface. The base portion  16   a  can be integrated into the secondary housing  14 . 
     The preheating portion  16   b  comprises a combustion air flow duct  16   b   1  and an exhaust gasses flow duct  16   b   2 . The exhaust gasses flow duct  16   b   2  is completely enveloped by the combustion air flow duct  16   b   1 . The two air flow ducts  16   b   1  and  16   b   2  are at least partly separated from each other by only one heat transmitting separation wall  16   b   3 . Thus, heat is transferred between air within the two air flow ducts  16   b   1  and  16   b   2 . The combustion air flow duct  16   b   1  comprises an inlet opening  16   b   1 A and an outlet opening  16   b   1 B. The exhaust gasses flow duct  16   b   2  comprises two inlet openings  16   b   2 A and  16   b   2 B as well an outlet opening  16   b   2 C coupled to each other, respectively. 
     The printed circuit board assembly coupling portion  16   c  is configured to attach a printed circuit board assembly (PCBA)  18  thereto in a releasable manner. In the illustrated configuration this can be done via appropriate screws engaging threaded bores  16   c   1  provided within the PCBA coupling portion  16   c . Furthermore, the printed circuit board assembly coupling portion  16   c  comprises a port that allows the sensing of the intake air pressure. In other embodiments, the structural element labeled with reference sign  16   c  is not provided to attach the printed circuit board assembly  18  thereto. Instead, it is provided for the port for sensing the intake air pressure only. 
     Each of the two burner coupling portions  16   d  and  16   e  is provided as circular frame member  16   d   1  or  16   e   1  having an interior opening  16   d   2  or  16   e   2  and several engaging recesses  16   d   3  and  16   e   3 . A single burner  20  and  22  can be inserted into each of the burner coupling portions  16   b  and  16   a  to be coupled to the corresponding heat exchanging unit  30  or  40 . Each burner  20 ,  22  can be locked in this position via appropriate engaging members like screws or bolts. 
     The heating unit  10  is enclosed by the primary housing  12 . In the inner of the heating unit  10  the secondary housing  14  and the coupling member  16 , the above references PCBA  18 , two burners  20  and  22 , a fuel gas or liquid piping  24  coupled to the two burners  20  and  22  and having an inlet port configured to be coupled to a storage for fuel gas or liquid (not illustrated), a combustion air fan unit  26  and a ventilation air driving unit  28  are provided. Here, the term burners  20  and  22  refer to fuel manifolds as illustrated. 
     The two burners  20  and  22  are inserted into the burner coupling portions  16   d  and  16   e  of the coupling member  16 . The two burners  20  and  22  protrude from the inner of the heating unit  10  with their flame generating side.  FIG.  5    illustrates an example for the structural configuration of a dual nozzle fuel manifold of the burners  20  and  22 , here in particular an exemplary embodiment of the second burner  22 . According to this configuration the second burner  22  comprises a combustion air flow duct  22   a , several (here in particular two) nozzles  22   b   1  and  22   b   2 , an ignition arrangement  22   c  and a controlling arrangement  22   d.    
     The combustion air flow duct  22   a  is configured to lead a flow of combustion air from a combustion air fan unit  26  coupled to an inlet opening  22   a   1  of the combustion air flow duct  22   a  (here the lower end thereof) to a combustion area  22   e  of the burner  22 . 
     The two nozzles  22   b   1  and  22   b   2  are provided with passive flow disturbance devices (not illustrated). The two nozzles  22   b   1  and  22   b   2  receive gas or liquid via corresponding fuel gas or liquid piping  24  coupled thereto. The two nozzles  22   b   1  and  22   b   2  are thus configured to supply fuel gas or liquid to the combustion area  22   e  of the burner  22 . The two nozzles  22   b   1  and  22   b   2  consist of a first nozzle  22   b   1  and a second nozzle  22   b   2 . The second nozzle  22   b   2  is differing from the first nozzle  22   b   1  in a cross section of its supplying opening. Accordingly, the first nozzle  22   b   1  and the second nozzle  22   b   2  have different throughput rates. 
     The ignition arrangement  22   c  is configured to ignite the mixture of combustion air from the combustion air flow duct  22   a  with the fuel gas or liquid from the two nozzles  22   b . In the illustrated embodiment, the ignition arrangement  22   c  is provided as electric arc or spark generating arrangement having two elongated electrodes  22   c   1 . Here, the elongated electrodes  22   c   1  serve also as flame detector and feedback unit which will be referred to later. 
     The controlling arrangement  22   d  is configured to be coupled to the PCBA  18  of the heating unit. The controlling arrangement  22   d  is coupled to the ignition arrangement  22   c  and two fuel gas valves or liquid valves (not illustrated here but further referred to below). The PCBA  18  is configured to supply electrical power to the ignition arrangement  22   c  to operate the ignition arrangement  22   c  appropriately, for example, by generating an electric arc or spark to ignite therewith within the combustion area. Each of the two fuel gas valves or liquid valves is coupled to one of the two nozzles  22   b   1  and  22   b   2 . By the fuel gas or liquid valves the fuel supply for each of the two nozzles  22   b   1  and  22   b   2  can be controlled independently of each other. Both fuel gas or liquid valves are monostable valves having an opened operation state and a closed state. Such monostable valves are well known in the art, which is why a detailed description thereof is omitted here for the sake of brevity. 
     With such a configuration, in principle, four states for the fuel supply at the combustion area  22   e  and, thus, four different heat outputs of the second burner  22  can be selected by the controlling arrangement: 
     In a first operation state, both valves, the fuel gas valve and the liquid, valve are closed such that no fuel gas or liquid is provided to the combustion area  22   e . Accordingly, there is no combustion process in the combustion are  22   e  and the heat output is zero. 
     In a second operation state, one of the fuel gas valve and the liquid valve coupled to the first nozzle  22   b   1  is opened while the other one of the fuel gas valve and the liquid valve coupled to the second nozzle  22   b   2  is closed. Accordingly, fuel gas or liquid is provided to the combustion area  22   e  as defined by the throughput rate of the first nozzle  22   b   1 . This results in a first heat output. 
     In a third operation state, one of the fuel gas valve and the liquid valve coupled to the first nozzle  22   b   1  is closed while the other one of the fuel gas valve and the liquid valve coupled to the second nozzle  22   b   2  is opened. Accordingly, fuel gas or liquid is provided to the combustion are  22   e  as defined by the throughput rate of the second nozzle  22   b   2 . This results in a second heat output differing from the first heat output. 
     In a fourth operation state, both valves, the fuel gas valve and the liquid valve, are opened such that fuel gas or liquid is provided to the combustion area  22   e  with a third fuel supply rate defined by the combined throughput rate of the first nozzle  22   b   1  and of the second nozzle  22   b   2 . This results in a third heat output substantially corresponding to the sum of the first heat output and of the second heat output. 
     In the illustrated embodiment, the first burner  20  in principle has the same structure as the second burner  22 . But in the present configuration the first burner  20  comprises one single nozzle  20   b . Moreover, only one fuel gas valve or liquid valve is coupled thereto instead of two, as is illustrated in  FIG.  8 C . In the illustrated embodiment, it is not necessary to provide both burners  20  and  22  with the possibility to switch between four distinct operation states. Thus, providing the first burner  20  with only one nozzle  20   b  can save costs. Nevertheless, the first burner  20  can have the same configuration as the second burner  22 , if desired. 
     As illustrated in  FIG.  6 A , the combustion air fan unit  26  comprises two housing elements  26   a  and  26   b , one single combustion air fan  26   c , and a combustion air fan driving unit  26   d  (seen in  FIG.  3   ) for the combustion air fan  26   c.    
     The first housing element  26   a  comprises a combustion air inlet opening  26   a   1  and two combustion air outlet openings  26   a   2  and  26   a   3 . The combustion air inlet opening  26   a   1  is coupled via a common O-ring (not illustrated) to the outlet opening  16   b   1 A of the combustion air flow duct  16   b   1 . The first outlet opening  26   a   2  of the first housing element  26   a  is coupled via a common O-ring (not illustrated) to the inlet end opening  20   a   1  of the combustion air flow duct  20   a  of the first burner  20 . The second outlet opening  26   a   3  of the housing element  26   a  is coupled via a common O-ring (not illustrated) to the inlet opening  22   a   1  of the combustion air flow duct  22   a  of the second burner  22 . 
     The second housing element  26   b  is coupled to the first housing element  26   a  with several coupling members, like for example bolts or other suitable coupling means (not illustrated). The first housing element  26   a  and the second housing element  26   b  are configured to form a combustion air flow path. The combustion air flow path leads form the combustion air inlet opening  26   a   1  to a combustion air fan chamber  26   e . Further, the combustion air flow path leads from the combustion air fan chamber  26   e  via two separate flow path sections to each of the two combustion air outlet openings  26   a   2  and  26   a   3 . The two sections of the combustion air flow path leading from the combustion air fan camber  26   e  to the combustion air outlet openings  26   a   2  and  26   a   3  can be provided with combustion air valves. The corresponding section of the combustion air flow path can be closed by the combustion air valves. Thus, it is possible to control a supply of combustion air to the two burners  20  and  22 , for example, to enable an emergency shut down of the corresponding burner  20  or  22  and/or to increase the amount of combustion air flow provided to the other burner  22  or  20  coupled to the section of the combustion air flow path which is still open. 
     In some embodiments, the one single combustion air fan  26   c  is provided with one single fan wheel. In particular, the single fan wheel is implemented as an impeller. Such a configuration allows to save space. The combustion air fan  26   c  is positioned within the combustion air fan chamber  26   e  which is formed by the two housing elements  26   a  and  26   b . The combustion air fan  26   c  is positioned in a plane perpendicular with respect to the central axis of the combustion air inlet opening  26   a   1 . The combustion air fan  26   c  is configured to generate a flow of combustion air from the combustion air inlet opening  26   a   1  towards both of the combustion air outlet openings  26   a   2  and  26   a   3  and, thus, in the assembled state of the heating unit  10 , to the two burners  20  and  22 . As such fans are commonly known, a detailed description thereof is omitted for the sake of brevity. 
     The combustion air fan driving unit  26   d  is provided on an outer surface of the second housing element  26   b . A driving rod (not illustrated) extends through a driving rod through hole  26   b   1  provided within the second housing element  26   b . The combustion air fan driving unit  26   d  is coupled via the driving rod (not illustrated) to the combustion air fan  26   c . The combustion air fan driving unit  26   b  is configured to drive the combustion air fan  26   c  for generating the above described flow of combustion air. 
     In  FIG.  6 B , another example for a combustion air fan unit  26  is illustrated. This combustion air fan unit  26  has basically the same structural configuration as the one illustrated in  FIG.  6 A , but comprises two separate combustion air fans  26   c , each coupled to only one of the two combustion air outlet openings  26   a   2  and  26   a   3  (seen along the combustion air flow path). In other words, each distinct combustion air flow path for one of the burners  20  or  22  has its own combustion air fan  26   c . Thus, it is possible to control the supply of combustion air to the two burners  20  and  22  independently of each other. 
     The ventilation air driving unit  28  is a ventilation unit well-known from the state of the art. The ventilation air driving unit  28  is configured to generate, in particular with a ventilation air fan provided therein, a flow of ventilation air from an inlet opening  28   a  of the ventilation air driving unit  28  towards an outlet opening  28   b  of the ventilation air driving unit  28 . As such ventilation air driving units are well-known from the state of the art the detailed description thereof is omitted for the sake of brevity. 
     The ventilation air driving unit  28  is coupled, for example with bolts or screws (not illustrated) or by other suitable means, to the secondary housing  14 . However, the ventilation air driving unit  28  can also be coupled to one of the elements of the heating apparatus  1 , for example it can be coupled to the coupling member  16 . Thus, the inlet opening  28   a  of the ventilation air driving unit  28  is positioned in the vicinity of the ventilation slots  12   c  and  14   c  of the housings  12  and  14 . A ventilation air outlet section  14   b   1  is provided within the second heat exchanging unit opening  14   b  of the secondary housing  14 . The outlet opening  28   b  of the ventilation air driving unit  28  is positioned on the ventilation air outlet section  14   b   1 . Thus, the ventilation air driving unit  28  is configured to generate a ventilation air flow from the environment of the heating apparatus  1  through the ventilation slots  12   c  and  14   c  to the ventilation air outlet section  14   b   1 . 
     As indicated above, the PCBA  18  is attached with the PCBA coupling portion  16   c  to the coupling member  16 . The PCBA comprises a control signal receiving unit, a processing unit coupled to the signal receiving unit and several control signal wires coupling the processing unit with the controlling arrangements  20   d  and  22   d  of the two burners  20  and  22 , the combustion air fan driving unit  26   d , a controlling arrangement of the ventilation air driving unit  28  and a controlling arrangement for a bypass gas valve which can be provided. 
     The control signal receiving unit is configured to receive control signals via wired or wireless communication from a control signal input unit coupled to the control signal receiving unit. The control signal receiving unit is further configured to forward control signals to the processing units. For example, the control signal input unit can be a specific remote-control device or a common smart phone with an appropriate app sending control signals to the control signal receiving unit. This can be achieved by blue tooth or by other suitable wireless communication. Alternatively, the control signal input unit can be provided as a control panel coupled to the control signal receiving unit via cable. It is to be noted that control signals do not have to contain only direct control instructions. Control signals can also comprise, for example, various sensor signals. Sensor signals may, for example, comprise temperature sensors provided in the heating apparatus or in the recreational vehicle, etc. Moreover, the control input signal unit can be provided as one single device. It can, however, also comprise or consist of several independent devices like serval sensors and/or input devices transmitting control signals to the control signal receiving unit. 
     The processing unit is configured to receive and to process the control signals received from the control signal receiving unit and to generate appropriate instruction signals for the various components coupled to the processing unit. In particular, the processing unit comprises a memory. The memory can be one of a volatile or non-volatile memory. The memory can contain programs or the like allowing the processing unit to generate appropriate instruction signals from the received control signals. The generated instruction signals do not necessarily have to contain just digital signals, which then have to be processed by the respective components receiving such signals. The generated instruction signals can also include analogue signals for directly operating the respective components. For this purpose, the PCBA further can contain a separate electrical power supply, like for example a battery. Alternatively or in addition thereto, the PCBA can be configured to be coupled to an external energy source like the power grid of the recreational vehicle etc. 
     In the illustrated embodiment, the PCBA  18  is configured to control and/or operate the controlling arrangements  20   d  and  22   d  of the two burners  20  and  22 , the combustion air fan driving unit  26   d , the ventilation air driving unit  28  and a bypass gas valve which can be provided. Further components controlled and/or operated by the PCBA  18  will be discussed later. Within the scope of the present embodiments other configurations for the PCBA  18  are possible. In particular, the PCBA  18  can be configured to use information about various temperatures, for example of an air temperature within the recreational vehicle or of the environment of the recreational vehicle, of a liquid temperature, for example of a fuel liquid of the heating apparatus  1  or a liquid to be heated with the heating apparatus  1 , etc., various pressure values and/or a flame ionization of the burners  20  and  22  to control the various components of the heating apparatus  1  in an appropriate manner. 
     As indicated above, the heating apparatus  1  further comprises a first heat exchanging unit  30  and a second heat exchanging unit  40  coupled to the heating unit  10 . The first heat exchanging unit  30  and the second heat exchanging unit  40  will be described in the following with reference to  FIGS.  1  to  3   . 
     The first heat exchanging unit  30  is configured to allow heat exchange between exhaust gases from the first burner  20  and a liquid to be heated. The first heat exchange unit  30  comprises a liquid tank  32 , an exhaust gasses piping  34 , a cold liquid piping  36  and a hot liquid piping  38 . Both of the cold liquid piping  36  and the hot liquid piping  38  are provided with a coupling section to be coupled to an external cold liquid storage (not illustrated) or an external hot liquid storage (not illustrated), respectively. In the present configuration the liquid to be heated is water for sanitary or cooking purposes. However also other liquids could be used and heated with the first heat exchanging unit  30 . Furthermore, also all other sensors/switches, and electrical fuses/breakers work with the electrical heating devices as well. 
     The liquid tank  32  comprises a tubular main body  32   a . The tubular main body  32   a  is enclosed on a bottom side thereof by a bottom plate  32   a   1  and opened on a top side of the main body  32   a . The top side of the main body  32   a  is sealed with a lid member  32   b . The lid member  32   b  has a central exhaust gasses inlet opening  32   b   1 , an exhaust gasses outlet opening  32   b   2 , a cold liquid inlet opening  32   b   3 , a hot liquid outlet opening  32   b   4 , two further equipment insertion openings  32   b   5  and  32   b   6  and a specific mounting structure configured to mount the lid member  32   b  and thus the liquid tank  32  to the heating unit  10 . Alternatively, the further equipment insertion openings  32   b   5  and/or  32   b   6  can be moved to the bottom plate  32   a   1 . 
     The exhaust gasses piping  34  is provided to one end thereof with a combustion chamber section  34   a . The combustion chamber section  34   a  is coupled to the lid member  32   b  of the liquid tank  32  in such a manner that in the assembled state of the heating unit  1 , the combustion air flow duct  20   a  of the first burner  20  is coupled to the combustion chamber section  34   a  of the exhaust gasses piping  34  in a sealed manner. Therefore, a common O-ring is provided between the first burner  20  and the exhaust gasses piping  34 . Moreover, in the assembled state of the heating unit  1 , the combustion area  20   e  of the first burner  20  is located within the combustion chamber section  34   a  of the exhaust gasses piping  34  such that the combustion reaction of the first burner  20  is taking place in the combustion chamber section  34   a  of the exhaust gasses piping  34 . 
     The exhaust gasses piping  34  further has a tubular exhaust gasses leading section  34   b . The tubular exhaust gasses leading section  34   b  is coupled at one of its ends to the combustion chamber section  34   a  in a sealed manner. At its other end the tubular exhaust gasses leading section  34   b  is coupled to the exhaust gasses outlet opening  32   b   2  of the lid member  32   b . In particular, the exhaust gasses piping  34  is a one-piece unitary member having the combustion chamber section  324   a  and the exhaust gasses leading section  34   b . However, other configurations are possible. As is illustrated in the figures, the exhaust gasses leading section  34   b  is provided in several loops within the main body  32   a  of the liquid tank  32 . This is to increase the contact surface between the exhaust gasses piping  34  and a liquid provided within the liquid tank  32 . By doing so a heat transfer from the exhaust gasses within the exhaust gasses piping  34  to the liquid within the liquid tank  32  is increased. For the same purpose, the tubular exhaust gasses leading section  34   b  may be provided with surface extensions (not shown), for example in the form of a radially extending fin or, for example, in form of a plurality of fins, to further increase the contact surface between the exhaust gasses piping  34  and the liquid provided within the liquid tank  32 . The tubular exhaust gasses leading section  34   b  and the surface extensions may be a one-piece member, for example the surface extensions may be extruded directly with the tubular exhaust gasses leading section  34   b  as one part. Alternatively. The surface extensions, for example in form of fins, can be separate members being coupled to the tubular exhaust gasses leading section  34   b  by suitable means, like for example bolts, or methods, like for example welding. Of course, other suitable means to increase the contact surface between the exhaust gasses piping  34  and the liquid provided within the liquid tank  32  can be realized as well. The exhaust gasses outlet opening  32   b   2  of the lid member  32  is configured such that in the assembled state of the heating apparats  1 , it is positioned on the first inlet opening  16   b   2 A of the exhaust gasses flow duct  16   b   2 . Thus, the exhaust gases can flow from the exhaust gasses piping  34  into the exhaust gasses flow duct  16   b   2  of the coupling member  16 . The contact between the exhaust gasses piping  34  and the exhaust gasses flow duct  16   b   2  is sealed with a heat resilient O-ring, for example in form of a silicon O-ring. The O-ring is highly heat resistant to seal this connection reliably. 
     The cold liquid piping  36  goes through the could water inlet opening  32   b   3  of the lid member  32   b  into the liquid tank  32 . Thus, the liquid to be heated can be supplied to the inner of the liquid tank  32 . The hot liquid piping  38  goes through the hot water outlet opening  32   b   4  of the lid member  32   b  out of the liquid tank  32 . Thus, the hot liquid can be discharged from the inner of the liquid tank  32 . In the assembled state and the finally set configuration of the heating apparatus  1 , the hot liquid outlet opening  32   b   4  and the hot liquid piping  36  have to be positioned at an upper position as compared to the cold water inlet opening  32   b   3  and the cold water piping  36  to achieve an expedient overall configuration. 
     In the illustrated embodiment, two electrically driven heating members  39  are inserted through the two equipment insertion openings  32   b  and  32   b   6  into the liquid tank  32 . These, heating members  39  are coupled to the PCBA  18  to be controlled and/or operated thereby. They are provided to provide a further possibility to heat up the liquid within the liquid tank  32  either instead of or together with exhaust gases form the first burner  20 . Thus, an increased heating rate is achieved. Moreover, if desired, it is possible to heat the liquid within the liquid tank  32  by electrical power only. Other equipment components like temperature sensors or the like can be inserted through one of the equipment insertions openings  32   b   5  and  32   b   6  and/or at least one of the equipment insertion openings  32   b   5  and  32   b   6  which can be closed by a removable lid member or may be sealed permanently. 
     The second heat exchanging unit  40  is configured to enable heat exchange between exhaust gases from the second burner  22  and ventilation air. Therefore, the second heat exchanging unit  40  comprises a ventilation air enclosure  42  and an exhaust gasses piping  44 . 
     The ventilation air enclosure  42  comprises a tubular body section  42   a  and a lid section  42   b . The tubular body portion  42   a  is sealed at one side thereof with an end plate  42   a   1 . The end plate  42   a   1  is provided with a ventilation air inlet opening  42   a   1 A and an exhaust gasses inlet opening  42   a   1 B. The end plate  42   a   1  is configured such that it can be coupled to the secondary housing  14  and/or the coupling member  16  in such a manner that, in the assembled state of the heating apparatus  1  in which the second heat exchanging unit  40  is coupled to the heating unit  10 , the ventilation air inlet opening  42   a   1 A of the ventilation air enclosure  42  is positioned on the ventilation air outlet opening  14   b   1  of the secondary housing  14 . Thus, the combustion area  22   e  of the second burner  22  protrudes through the exhaust gasses inlet opening  42   a   1 B into the ventilation air enclosure  42 . Furthermore, the end plate  42   a   1  further comprises an exhaust gasses pipe outlet opening  42   a   1 C. The exhaust gasses pipe outlet opening  42   a   1 C is configured to be coupled via a silicone O-ring (not illustrated) to the second inlet opening  16   b   2 B of the exhaust gasses flow duct  16   b   2  of the coupling member  16 . The O-ring is highly heat resistant. 
     The lid section  42   b  is coupled at the other side of the body portion  42   a  to confine the inner space of the ventilation air enclosure  42 . In the present configuration the lid section  42   b  is provided as separate element. The lid section  42   c  comprises four ventilation air outlet openings  42   b   1  to  42   b   4  arranged in pairs on two opposing side surfaces of the lid section  42   b . Of course, also other configurations for the ventilation air outlet openings  42   b   1  to  42   b   4  can be realized, like configurations with less or even more ventilation outlet openings and/or configurations having further elements like pipes or safety meshes. 
     The exhaust gasses piping  44  is provided to one end thereof with a combustion chamber section  44   a . The combustion chamber section  44   a  is coupled to end plate  42   a   1  of the ventilation air enclosure  42  in such a manner that in the assembled state of the heating apparatus  1 , the combustion air flow duct  22   a  of the second burner  22  is coupled to the combustion chamber section  44   a  of the exhaust gasses piping  44  in a sealed manner. Therefore, a common O-ring is provided between the second burner  22  and the exhaust gasses piping  44 . Moreover, in the assembled state of the heating apparatus  1 , the combustion area  22   e  of the second burner  20  is located within the combustion chamber section  44   a  of the exhaust gasses piping  44 . Thus, the combustion reaction of the second burner  22  is taking place in the combustion chamber section  44   a  of the exhaust gasses piping  44 . 
     The exhaust gasses piping  44  further has a tubular exhaust gasses leading section  44   b . One end of the tubular exhaust gasses leading section  44   b  is coupled to the combustion chamber section  44   a  in a sealed manner. The other end of the tubular exhaust gasses leading section  44   b  is coupled to the exhaust gasses outlet opening  42   a   1 C of the ventilation air enclosure  42 . In particular, the exhaust gasses piping  44  is a one-piece unitary member having the combustion chamber section  44   a  and the exhaust gasses leading section  44   b . However, also other configurations are possible. As may be taken from the  FIGS.  8 A to  8 C , the exhaust gasses leading section  44   b  is provided in several loops within the body portion  42   a  of the ventilation air enclosure  42 . By providing the several loops the contact area between the exhaust gasses piping  44  and a ventilation air provided within the ventilation air enclosure  42  is increased. This enables an increase of the heat transfer from the exhaust gasses within the exhaust gasses piping  44  to the ventilation air. For the same purpose, the tubular exhaust gasses leading section  44   b  may be provided with surface extensions (not shown), for example in the form of a radially extending fin or, for example, in form of a plurality of fins, to further increase the contact surface between the exhaust gasses piping  44  and the ventilation air provided within the ventilation air enclosure  42 . The tubular exhaust gasses leading section  44   b  and the surface extensions may be a one-piece member, for example the surface extensions may be extruded directly with the tubular exhaust gasses leading section  44   b  as one part. Alternatively. The surface extensions, for example in form of fins, can be separate members being coupled to the tubular exhaust gasses leading section  44   b  by suitable means, like for example bolts, or methods, like for example welding. Of course, other suitable means to increase the contact surface between the exhaust gasses piping  44  and ventilation air provided within the ventilation air enclosure  42  can be realized as well. The exhaust gasses outlet opening  42   a   1 C of the main body  42   a  is configured such that, in the assembled state of the heating apparatus  1 , it is positioned on the second inlet opening  16   b   2 B of the exhaust gasses flow duct  16   b   2 . Thus, exhaust gases can flow from the exhaust gasses piping  44  into the exhaust gasses flow duct  16   b   2  of the coupling member  16 . The contact between the exhaust gasses piping  44  and the exhaust gasses flow duct  16   b   2  is sealed with a heat resistant silicon O-ring. Thus, this connection is sealed reliably. 
     As for example can be seen in  FIG.  7   , the heating apparatus  1  further comprises a heat exchanging units housing shell  50  and a supplementary mounting member  52 . 
     The heat exchanging units housing shell  50  is a tubular member configured to be pushed onto the two heat exchanging units  30  and  40  and to be fixed to the heating unit  10 . The heat exchanging units housing shell  50  comprises a first heat exchanging unit section  50   a  and a second heat exchanging unit section  50   b  corresponding a respective one of the two heat exchanging units  30  and  40  in cross sectional shape as seen along their longitudinal axes. With the heat exchanging housing shell, the overall configuration of the heating apparatus  1  gains more structural stability and protection against external influences. 
     The supplementary mounting member  52  serves as lid member for the heat exchanging units housing shell  50 . The supplementary mounting member  52  is coupled to the heat exchanging units housing shell  50  on the side opposing the side to which the heating unit  10  is coupled. The supplementary mounting member  52  is coupled to the heat exchanging units housing shell  50  in an appropriate manner like, for example, via form-fitting or separate coupling means. The supplementary mounting member  52  comprises a first heat exchanging unit section  52   a , a second heat exchanging unit section  52   b  and at least one mounting section  52   c  coupled to each other. 
     The first heat exchanging unit section  52   a  of the supplementary mounting member  52  is configured to seal the first heat exchanging unit section  50   a  of the heat exchanging units housing shell  50 . 
     The second heat exchanging unit section  52   b  of the supplementary mounting member  52  consist of a frame defining a central opening. Through the central opening the lid section  42   b  of the second heat exchanging unit  40  can be coupled to the body portion  42   a  of the second heat exchanging unit  40  in the longitudinal direction thereof. 
     In particular, the heating unit  10 , the first heat exchanging unit  30  and the second heat exchanging unit  40  are provided as independent self contained arrangements coupled to each other in a releasable manner to form the heating apparatus  1 . The heat exchanging units housing shell  50  is configured to enclose the two heat exchanging units  30  and  40  at least partly, such that the two heat exchanging units  30  and  40  cannot be separated from each other without removing the heat exchanging units housing shell  50 . The heat exchanging units housing shell  50  is configured to be coupled to the heating unit  10  in a releasable manner. The heat exchanging units housing shell  50  is configured to couple the heating unit  10  and the two heat exchanging units  30  and  40  to each other in such a manner that for removal of the heat exchanging units housing shell  50  and, thus, for disassembling of the heating apparatus  1 , the heat exchanging units housing shell  50  has to be decoupled form the heating unit  10  first. 
     As, for example, is illustrated in  FIG.  7   , the supplementary mounting member  52  comprises two mounting sections  52   c . A mounting section  52   c  is coupled to the first heat exchanging unit section  52   a  and the other mounting section  52   c  is coupled to the second heat exchanging unit section  52   b  of the supplementary mounting member  52 . Each of the mounting sections  52   c  is provided with a bearing surface  52   c   1  having at least one through hole  52   c   2 . The at least one through hole  52   c   2  allows appropriate bolts or screws to pass there through. Thus, the supplementary mounting member  52  and thus the overall heating apparatus  1  can be fixed to a desired or suitable surface of the recreational vehicle like, for example, a wall, floor or ceiling area of the recreational vehicle. 
     It is to be noted that the above described configuration is merely exemplary embodiment of a heating apparatus. However, the present embodiments and in particular the final scope of protection is defined by the appended set of claims. In particular, many of the above described structural features of the embodiments can be replaced by others or adapted if desired or necessary. Such modifications lie in the abilities and freedom of a skilled artisan without leaving the overall disclosure of the present embodiments. 
       FIGS.  10 A to  10 C , illustrate several cross sections of the above described heating apparatus  1  to facilitate the understanding of the specific configuration of this exemplary embodiment (in particular with regard to the various flow paths), wherein in  FIG.  9    the various cross-sectional planes are depicted. 
     Although not illustrated herein explicitly, another aspect refers to a recreational vehicle provided with a heating apparatus according to the present invention, like for example with the above described heating apparatus  1 . Such recreational vehicles are well known in the art and it lies within the abilities of a skilled artisan to provide any heating apparatus within such a recreational vehicle. Accordingly, for the sake of brevity, a detailed description thereof is omitted. 
     In the following, referring to  FIG.  11   , a method for heating fluids of the above described heating apparatus  1  is described as merely exemplary embodiment according to another aspect thereof. 
     Within the method according to the present embodiments the combustion air fan unit  26  is operated to suck combustion air from an external environment of the heating apparatus  1 . The combustion air is sucked or forced through the combustion air flow duct  16   b   1  of the preheating portion  16   b  of the coupling member  16 . The sucked combustion air is forced towards each of the two burners  20  and  22  generating an overpressure within the respective combustion areas  20   e  and  22   e . Accordingly, no second combustion air fan unit has to be provided. This greatly reduces costs and the susceptibility to errors of the overall heating apparatus  1 . 
     Each of the two burners  20  and  22  is further supplied with fuel gas or liquid from a fuel gas or liquid storage coupled thereto. Due to the fact that the second burner  22  is provided with two nozzles  22   b   1  and  22   b   2  to supply the fuel gas or liquid to the combustion area  22   e , the rate with which fuel gas or liquid is supplied to the combustion area  22   e  can be switched between four various operation states by operating the respective valves. Accordingly, it is not necessary to provide an expensive and error-prone burner with a complex structure to achieve various operation sates. The combustion air and the fuel gas or liquid are mixed with each other within the combustion areas  20   e  or  22   e  and the obtained mixtures are ignited by the respective ignition arrangements  20   c  or  22   c  to burn within the respective combustion areas  20   e  or  22   e.    
     Hot exhaust gasses from the first burner  20  are guided through the exhaust gasses piping  34  of the first heat exchanging unit  30  and transfer some of their heat to a corresponding liquid provided within the liquid tank  32  of the first heat exchanging unit  30 . Permanently, fresh cold liquid from a cold liquid storage is supplied to the liquid tank through the cold liquid opening  32   b   3 , while hot or at least heated liquid is discharged through the hot liquid opening  32   b   4  to a hot liquid storage. Thus, cold liquid is supplied to the heating apparatus  1  and hot/heated liquid is discharged by the heating apparatus  1   
     Hot exhaust gasses from the second burner  22  are guided through the exhaust gasses piping  44  of the second heat exchanging unit  40  and transfer some of their heat to ventilation air. The ventilation air is forced by the ventilation air driving unit  26   b  to move from an indoor room of the recreational vehicle, respectively a space in which the air is to be heated, through the body portion  42   a  of the second heat exchanging unit  40  back to the indoor room. Thus, cold ventilation air from the indoor room or interior of the recreational vehicle is sucked into the heating apparatus  1  and heated ventilation air is discharged into the indoor room or interior form the heating apparatus  1 . 
     The exhaust gasses leaving both of the heat exchanging units  30  and  40  and which still have high temperature are guided into the exhaust gasses flow duct  16   b   2  of the preheating portion  16   b  and, thus, preheat the combustion air suck through the combustion air flow duct  16   b   1 . This preheating results in a very efficient heating operation. Afterwards, the exhaust gasses are discharged into the external environment. 
     Although, in the above described process, both ventilation air and liquid are heated simultaneously the heating apparatus  1  also can be used for heating only one of the two liquids. As a skilled artisan will be able to consider various possibilities to operate the above described heating apparatus  1  resulting from its specific structural configuration, no detailed listing of all possible modes of operation is given here for the sake of brevity. 
     Now, an exemplary method for starting the above described heating apparatus  1  is described in detail. 
     When it is desired to start the heating apparatus  1 , firstly the fans of the combustion air fan unit  26  and of the ventilation air driving unit  28  are switched on with a predetermined rotation speed. Then, the respective ignition arrangements  20   c  and/or  22   c  are/is activated to provide an electric ignition, in particular an electric arc or at least sparks and the gas valves for the nozzles  20   b ,  22   b   1  and  22   b   2  desired to be operated are opened such that the mixture of combustion air and fuel gas or liquid is ignited at the respective combustion areas  20   e  and/or  22   e . A flame detector is provided in the burners  20  and  22  and coupled to the PCBA. The flame detector verifies if a flame has been ignited in the corresponding burner  20  or  22 . If not, the respective gas valve(s) is/are closed, while the combustion air fan  26   c  is still operated for a predetermined time interval, in order to dispose of the gas previously injected into the respective combustion area  20   e  or  22   e . Then, a new attempt for ignition is made by opening the gas valve and providing an electric arc or sparks at the respective combustion area  20   e  or  22   e . After the successful ignition of a flame, the respective ignition arrangement  20   c  or  22   c  is turned off. 
     In the following, some exemplary modifications are described that even improve the characteristics and/or functionality of the herein disclosed heating apparatus  1 . 
     According to a first modification, the heating apparatus can be provided with a secondary air supply arrangement. By this secondary air supply arrangement, a supply of secondary air from the external environment to at least one of the burners can be realized. In such a configuration the combustion air depicts the primary air. While the primary air is supplied to start a combustion reaction with the fuel gas or liquid, the secondary air is provided to complete the reaction. This results in an improved flame stability and emission characteristics. A burner with such a functionality is also called flat surface burner. A flat surface burner can be further provided with a wall having a plurality of apertures and being arranged to further separate the flow of primary air or combustion air from the flow of secondary air. In such a configuration, the dimension of the apertures is optimized as a function of the rate of secondary air, which in particular also depends on the fans. 
     Although, referring to the above embodiment a configuration with only two burners is described, also implementations having more than two burners can be provided if desired. Even if more than two burners are provided, all of the burners can be supplied with combustion air by the one single combustion air fan, to transfer the therewith obtained simple and reliable setting to such a configuration. 
     Although within the scope of the above embodiment a configuration with only two nozzles  22   b   1  and  22   b   2  in the second burner  22  is described the second burner  22  (and also the first burner  20 ) can be provided with more than two nozzles to implement more than four distinct operation states. 
     Although within the scope of the above embodiment a configuration in which both nozzles  22   b   1  and  22   b   2  of the second burner  22  have different structures (cross sections of their supply openings) are described the nozzles  22   b   1  and  22   b   2  can have the same structure. Effectively, only three various operation states are, thus, realized. 
     A preferred embodiment of a monostable valve, as used herein, includes a valve body having an inlet for connection to a fuel supply and an outlet for connection to the respective nozzle. A valve seat is positioned between the inlet and the outlet of the gas body. A valve member is linearly movable between a closed position or state and an open position or state. In the closed position or state the valve member is seated on the valve seat, and in the open position or state valve member is spaced from the valve seat. Usually, a spring acting on the valve member keeps the valve member in the closed position. A solenoid (electro-magnet) is fixed to the valve body and, when energized, acts on the valve member to move it to or keep it in the open position, i.e. when a gas injection is required. Conversely, the spring, when the solenoid is not energized, moves the valve member to or keeps the valve member in the closed position, i.e. when a gas injection is not required. Thus, the closure device moves linearly with respect to the electro-magnet, which is axially fixed. The solenoid is driven directly by the PCBA. No movable elements are provided for actuating the gas valve. The solenoid may be excited with two different current values: a constant first value and a constant second value, lower than the first one. Thus, when the gas valve is actuated for opening, the solenoid is firstly excited with the first value until a predetermined time, for example 100 ms, has passed, then it is excited with the second value. The transition from the first value to the second value is actuated by the PCBA depending on the time only, i.e. independently on the position of the valve itself. However, since the valve could not open instantaneously, for example in less than 100 ms, it is likely that the valve reaches the open position when the coil is excited with the second (lower) value. Then the valve is hold in the opening position as long as the coil is kept excited with the second value. 
     The liquid tank is further coupled to a frost valve. The frost valve comprises a pressure valve and is configured to perform at least the following basic functions: manual drain, frost draining (automatic) and pressure relief. However, also other configurations are possible. 
     The heating apparatus  1  further comprises an external container. In the external container all of the elements of the heating apparatus  1  are arranged. The external container shields the various components of the heating apparatus against harmful external influences like, for example, sun radiation, water or dirt. Further, the external container is made of molded plastic, as such a material is highly resistive to external influences and forces. 
     According to a further modification, the heating apparatus further includes a supplementary electric heater. The supplementary heater provides a heating coil to achieve a supplementary heating of the ventilation air. The electric heater can be positioned inside the external container, next to the second heat exchanger and is driven by the PCBA. Thus, it is possible to increase the heating power of the heating apparatus  1 . 
     The heating apparatus may be provided with a master valve for cutting the supply with fuel gas or liquid centrally. 
     It should be noted that the above described configurations are preferred but merely exemplary embodiments but do not limit the achieved scope of protection as defined by the appended set of claims. A skilled artisan will be able to imagine various modifications of the above described configurations without contravening the basic idea of the teaching and/or leaving the scope of protection as defined by the appended set of claims. 
     REFERENCE NUMERALS 
     
         
           1  heating apparatus 
           10  heating unit 
           12  primary housing 
           12   a  opening 
           12   c  ventilation slots 
           14  secondary housing 
           14   a  first heat exchanging unit opening 
           14   b  second heat exchanging unit opening 
           14   b   1  ventilation air outlet section 
           14   c  ventilation slots 
           16  coupling member 
           16   a  base portion 
           16   a   1  bearing surface 
           16   a   2  through holes 
           16   b  preheating portion 
           16   b   1  combustion air flow duct 
           16   b   1 A inlet opening 
           16   b   1 B outlet opening 
           16   b   2  exhaust gasses flow duct 
           16   b   2 A first inlet opening 
           16   b   2 B second inlet opening 
           16   b   2 C outlet opening 
           16   b   3  separation wall 
           16   c  printed circuit board assembly (PCBA) coupling portion 
           16   c   1  threaded bore 
           16   d  first burner coupling portion 
           16   e  second burner coupling portion 
           18  printed circuit board assembly (PCBA) 
           20  first burner 
           20   a  combustion air flow duct 
           20   a   1  inlet opening 
           20   b  nozzle 
           20   c  ignition arrangement 
           20   d  controlling arrangement 
           20   e  combustion area 
           22  second burner 
           22   a  combustion air flow duct 
           22   a   1  inlet opening 
           22   b   1  first nozzle 
           22   b   2  second nozzle 
           22   c  ignition arrangement 
           22   d  controlling arrangement 
           22   e  combustion area 
           24  fuel gas or liquid piping 
           26  combustion air fan unit 
           26   a  first housing element 
           26   a   1  combustion air inlet opening 
           26   a   2  first combustion air outlet opening 
           26   a   3  second combustion air outlet opening 
           26   b  second housing element 
           26   b   1  driving rod through hole 
           26   c  combustion air fan 
           26   d  combustion air fan driving unit 
           26   e  combustion air fan chamber 
           28  ventilation air driving unit 
           28   a  inlet opening 
           28   b  outlet opening 
           30  first heat exchanging unit 
           32  liquid tank 
           32   a  main body 
           32   a   1  bottom plate 
           32   b  lid member 
           32   b   1  exhaust gasses inlet opening 
           32   b   2  exhaust gasses outlet opening 
           32   b   3  cold liquid inlet opening 
           32   b   4  hot liquid outlet opening 
           32   b   5  first equipment insertion opening 
           32   b   6  second equipment insertion opening 
           34  exhaust gasses piping 
           34   a  combustion chamber section 
           34   b  exhaust gasses leading section 
           36  cold liquid piping 
           38  hot liquid piping 
           39  heating members 
           40  second heat exchanging unit 
           42  ventilation air enclosure 
           42   a  body portion 
           42   a   1  end plate 
           42   a   1 A ventilation air inlet opening 
           42   a   1 B exhaust gasses inlet opening 
           42   a   1 C exhaust gasses outlet opening 
           42   b  lid section 
           42   b   1  first ventilation air outlet opening 
           42   b   2  second ventilation air outlet opening 
           42   b   3  third ventilation air outlet opening 
           42   b   4  fourth ventilation air outlet opening 
           44  exhaust gasses piping 
           44   a  combustion chamber section 
           44   b  exhaust gasses leading section 
           50  heat exchanging units housing shell 
           50   a  first heat exchanging unit section 
           50   b  second heat exchanging unit section 
           52  supplementary mounting member 
           52   a  first heat exchanging unit section 
           52   b  second heat exchanging unit section 
           52   c  mounting section