WELCOME
Condensing vs. Non-Condensing

Gas Appliances
Gas Non-Condensing Furnace

• Burners are ignited 

•Blower motor is turned on

•Cool or return air enters, is 

filtered and blown through 

furnace heat exchanger

•Heat from the combustion 

chamber goes through the 

heat exchanger

•Air absorbs heat and is passed 

out through supply duct

•Exhaust gasses are vented out 

through type B vent to roof 
Non-Condensing 

Furnace Installation 

• Requires type B metal vent up to and through 

the roof

• Requires indoor combustion air

• Should have minimal professional maintenance

• Minimum 80% efficient
Gas Condensing Furnace

•Second heat exchanger used to heat 

the air from condensed exhaust gasses

•Less expensive venting (Usually PVC) on 

new construction
Condensing Furnace Installation

• Requires PVC venting

• Will not connect to existing venting when 

replacing existing furnace

• Most models are direct vent (does not require 

indoor combustion air)

• Can require annual professional maintenance

• Minimum 90% efficient
Orphaned 

Water Heater

• Many homes utilize a combination venting system through a chimney where the 

WH & Furnace share a vent.

• In existing homes where an older non-condensing Furnace is replaced by a 90+ 

efficiency appliance, an “Orphaned Water Heater” can occur.

• When the new condensing furnace is vented directly outside, the remaining vent 

for the WH is drastically oversized and serious CO concerns may arise.

• Methods to address this problem can be a major financial constraint.
Orphaned Water Heater

Connected to oversized flue Liner installed through flue
Gas Non-Condensing Water Heaters

•Cold water enters through the 

inlet and is released at the 

bottom

•The gas burner heats the 

water 

•Warm water rises and exits 

through the outlet pipe

•Exhaust from burner exits 

through a baffled flue and is 

vented outside (type B vent)
Standard Storage-Type 

Water Heater Installation

• Requires 6” of clearance for type B ventilation

• 4 - 6 sq ft ground space

• Requires indoor combustion air

• Seamless installation when replacing existing 

water heater (reconnect to existing structures)

• Should be flushed by homeowner annually

• Requires very little other maintenance 
Gas Condensing Water Heater

• Hot exhaust gases are captured and 

utilized to heat the water in addition 

to main burner

•Less expensive venting in new 

construction

•Cannot connect to existing venting 

when replacing old WH

•More Efficient

•Low NOx (Nitrogen Oxides) Emission
Condensing Water Heater Installation

• Requires PVC venting, no clearance to combustibles 

(some models may be direct vent)

• 4 - 6 sq ft ground space

• Requires new venting when replacing existing water 

heater

• May not require indoor combustion air

• Annual professional maintenance recommended
MID-EFFICIENCY VS. HIGH-EFFICIENCY

AFUE

(Annual Fuel

Utilization 

Efficiency)

VENTING BASIC OPERATION

ENHANCEMENTS PROS CONS

80-83

Mid 

Efficiency

• Traditional metallic piping

and/or chimney liner

• Utilize existing venting on

remodel/upgrade

• Fan-assisted draft

• Should be through roof

• Electronic Ignition

• Fan induced Draft

• Small Diameter Flue

• Several fan/burner 

controls

• Retrofits are simple

• Less expensive

• Less maintenance

• Proven/familiar

designs 

• Cannot be direct vent

• Consume more fuel

• Produce more 

byproducts

90+

High 

Efficiency

• Must be vented according

to manufacturer’s

instructions.

• Most likely require PVC

• Can be direct-vent or fanassisted draft

• Can be through wall

• Sealed Combustion

chamber

• Utilize second heat

exchanger to extract

heat from flue gas

• Lower temp flue gas

• Condensing

• Many fan/burner 

controls

• Consume less fuel

• Produce less

unwanted byproducts

• Easier install on new

construction

• More expensive

• More maintenance

• Newer designs not

as well proven

• More parts to break

• Condensate disposal

• Retrofits are complex

• Orphaned WH
THANK

YOU