UK Central heating plumbing and control question. - Page 1
Forum section: electronics / general

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 08, 2025, 01:19:57 pm
content: UK bungalow built about1975. Central heating installed professionally when the previous owner had it built. Query is re hot water cylinder heated by oil fired central heating boiler and with immersion heater and separate thermostat.I have never really got to grips with how the system is supposed to work. It has two pumps, but the second *MAY* have been added when an upstairs room was added after the bungalow had been built and in use for some years. Both pumps run when the central heating is on under control of the programmer. In the piping to the water heating coil in the hot water cylinder is a two port zone valve. Currently it's seized up, either the motor or the valve itself, so I guess that's why neither the boiler and neither pump runs when the hot water is turned on at the programmer.Why is there just a single 2 port zone valve? How might it work, if it's open is it designed for the main pumps to flow water through the heating coil as well as the radiators based on pressure differentials?There are no room stats. Every radiator has a thermostatic valve on it. The boiler has a built in thermostat control. The bathroom towel rail is always flowing so would act as a bypass, I don't see any pipe based bypass for water to flow if all rads are turned off.Finally, when it was working, if the hot water was turned off at the programmer, but the central heating on, the water in the cylinder rose to very high temperatures over a couple of days. My thoughts are the zone valve is leaking or it's micro-switch and / or thermostat are wired up wrong and the hot water zone valve is opening and water at central heating circuit temperature is flowing slowly through the coil without proper flow regulation.I'm puzzled, fitting a three port valve or a second two port one would be a real challenge , there's very little space and it already looks like a snake charmers nightmare.Thanks for any ideas. Have you come across a system like this with just a single two port zone valve?

author: NE666
author_rank: Frequent Contributor
timestamp: October 08, 2025, 02:42:12 pm
content: Qualification: I'm not a heating engineer but I've DIY renovated and upgraded a couple of systems from a similar era.I've not seen the use of a single two-port valve before but I think that on the basis of your description, you've correctly fathomed both its principle of operation and the failure. Zone valves are notorious for short life spans, both due to calcification of the valve internals and failure of their synchronous motors. I've replaced one such unit at least twice in the past decade.As yours currently is, it's not a great design. It doesn't allow for prioritisation of water heating over room heating. If I were going to be living in that house for any length of time, I'd seriously consider making the change to the more typical three-port system. Unless it's a 'big' bungalow, two pumps seems excessive too, even for an upstairs room.Again, assuming that it made financial sense to invest in the property, I'd also be upgrading the thermostat situation. Currently, there's no active call signal to the boiler when a room is below set temperature, it just relies on the boiler running continuously (when its return temperature drops back down). This must mean that the pump is working excessively too.Two years ago I upgraded my CH using the Heatmiser UK system - wired, heated wax valves on every radiator, driven from a central control unit which accepts either wired or wireless thermostats for each room. When any room is below set point, the control unit calls for heat from the boiler as would a single, central thermostat. That level of control has greatly improved the comfort level of every room.control unit:https://www.amazon.co.uk/dp/B01N9H4YHVactuators:https://www.amazon.co.uk/gp/product/B00BI0QL8QOther systems with the same operating principle are available too, including some with wireless radiator actuators so that you don't have to provide 12v cabling to each rad.

author: nali
author_rank: Frequent Contributor
timestamp: October 08, 2025, 02:59:09 pm
content: Sounds like the HW has been plumbed in just like another radiator. It may act as a sort of priority if it's closest to the pump flow/return by shunting the water flow, unless there's some sort of flow control going on too.Note my Dad's old bungalow had 2 pumps, in that instance the oil boiler's water jacket was partitioned into a split system with 2 fully independent water loops. One pump did the water heating the other the house radiators... not the best of ideas but that's what it was.I used to use Heatmiser thermostats years ago with a DIY manifold & 2-port zone valves in my old house. Now I use the Wiser system which simply replaces the radiator valve heads and Honeywell timeswitch. Can be a bit laggy as it's cloud based but it is possible to run them locally on Home Assistant if need be - just I haven't bothered.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 08, 2025, 03:10:33 pm
content: Thanks for the replies so far. Initially I wondered if the two pumps were for separate systems, one for hot water and one for the CH. But a quick in my head count up says the place originally had fifteen radiators, all on the ground floor, then another bedroom was built in the (big....) roof space, with a big double rad in it.. I guess the flow was insufficient and a second pump added? Both run with just the CH activated. I must make a better effort at pipe tracing, but as I said, it's a bit of a nightmare...I suspect converting to an "S Type" system with two 2 way zone valves would be easier than a "Y Type" with a three way valve, I need to re-address this. And yes, it's a fairly big bungalow, well, for the UK at least.

author: Electrodynamic
author_rank: Regular Contributor
timestamp: October 08, 2025, 03:46:17 pm
content: Chris WilsonNo problem, I used to work with building and HVAC automation systems .So I think you have one boiler for both central heating and a heat exchanger (your cylinder) for domestic hot water heating?. It sounds like the two port valve ie. regular valve is seized partially open which explains the domestic water temperature creeping up over a few days.The first rule is don't fix it if it ain't broken. Don't start changing and adjusting stuff which may have no relation to the actual problem. This only complicates things.I suspect the boiler is tripping because of the temp high limit switch on the domestic water heat exchanger. So the boiler has series circuit with a bunch of limit switches on it. If any one switch opens the circuit loop is opened and the boiler shuts down. My favorite trick is to get a short loop of wire or alligator clip and short the limit switch for a few seconds . If the boiler fires that's your problem. The limit switches could be 120v but more likely in the 24v control loop. Take care with electricity, always assume it's live.As well, being a farmboy I like to fix things rather than replace them. If you isolate (close the valves on either side) then open and clean up the seized valve there is a 90% chance it will work just fine. Does your system have a side stream filter?. It's usually a small stainless steel cylinder with a small circle/view port to indicate the flow rate. It filters sludge and debris in the system, highly recommended.Good luck, winters coming.

author: Cyclotron
author_rank: Unknown
timestamp: October 08, 2025, 03:54:31 pm
content: I'm not sure if you'll find it helpful, but systems like this are often best understood when I sketch them out like a schematic.Basically, I have a tendency to create a schematic and block diagram for more complex systems, so I can make logical guesses about where problems are or where I might want to change/augment a system.Good luck.

author: Electrodynamic
author_rank: Regular Contributor
timestamp: October 08, 2025, 03:57:04 pm
content: ChrisQuoteSo most larger systems always have two circulation pumps. One is the primary circulation pump and the other a secondary or back up. In most cases if the low circulation flow switch trips the programmer will fire up the secondary pump. If the pumps are in parallel and have a check valve after them it usually indicates one is primary the other a secondary. If in series then the last pump is acting as a booster pump.I find it absurd that many smaller systems have no back up circulation pump which should be mandatory. If the circulation pump goes and they do were basically hooped. Always have a back up plan.

author: NE666
author_rank: Frequent Contributor
timestamp: October 08, 2025, 03:59:28 pm
content: Quote from: Chris Wilson on October 08, 2025, 03:10:33 pmYes, that's a fairly large install for the UK. Probably does need the extra flow assistance, especially if it's all/mostly in 15mm.Quote from: Chris Wilson on October 08, 2025, 03:10:33 pmPossibly so, as you've said, it rather depends on the space available and ease of access to it all (which is why my new home includes a 'plant room' for all this stuff).I don't know what your CH wiring looks like but, if it's original, it's quite possibly a complete rat's nest too. Full of now sticky cables, peeling insulation tape and 'chocolate blocs'. I found it far easier when making additions and modifications to just junk it all and rewire completely using one of these:https://www.cityplumbing.co.uk/p/honeywell-home-wiring-centre-230-v-42005748-001/p/834330

author: nali
author_rank: Frequent Contributor
timestamp: October 08, 2025, 08:33:34 pm
content: Forgot to mention, if your valve is a Honeywell it may not be actually physically seized. They use apolycarbonatePolypropylene dropper capacitor (~2.2uF from memory) followed by a bridge rectifier & zener to power a 24V motor. Mine failed a couple of times from degraded caps and IIRC one had a zener go short. You can normally disconnect the motor head to see if the valve physically turns (make sure you remember and put it back to the same position before replacing the head).

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 10, 2025, 12:53:29 pm
content: Turns out the thermostat for the hot water cylinder ws stuck open circuit. I gave it a hefty tap and it's working now, at least until I can pop out for a new one.The system is a bit Heath Robinson. The two pumps seem to be split, not paralleled. One, probably the original, does the downstairs central heating rads. The second one, probably fitted when an upstairs bedroom was added, does that room and the coil in the hot water cylinder. When the CH is turned on but not when the just the hot water is turned on) both pumps run. There's just the one two port zone valve that's just used to open or close this heating coil in the hot water cylinder.Her indoors is in charge of the heating, as the monumental bills will testify, it seems it's only possible to have the boiler heat the hot water cylinder if the central heating is also turned on.I will investigate the logistics of fitting a second two port valve to enable independent control of oil fired hot water and central heating.Thanks for all the advice, have a good weekend folks