Venting Considerations for Condensing Boilers & Water Heaters

How do I vent my condensing boiler or water heater? What materials should I use? What size vent should I use? The quick answer to all of these questions is “check the manufacturer’s installation manual.” Condensing boilers and water heaters are Category IV appliances so the manufacturer is required to tell you how they want their boiler vented.

In most cases, combustion air can be ducted with common B-vent and/or PVC piping.  Combustion air can also be room air in many cases. Again, the individual boiler manufacturers will note how they want the boilers’ combustion air handled and what material should be used.

There are two main materials in venting condensing boilers and water heaters. Larger boilers (>1000MBH inlet) generally require AL29C stainless steel venting such as manufactured by Heat Fab. Smaller condensing boilers may allow the use of PVC or CPVC venting. Again, check the boiler installation manual for details and restrictions.

Venting can generally be run in vertical or horizontal directions. Keep in mind that any horizontal runs should be pitched BACK to the boiler, not away from the boiler as we do with non-condensing boilers. Also, if you are doing a sidewall vent with a condensing boiler, be aware of where the vent is discharging. There will be some condensate in the vented gas which may cause damage and/or ice build-up.

Here are some quick links to some various boiler installation manuals:
Laars Neotherm
Laars Rheos+
Aerco Modulex
Aerco Benchmark
Aerco KC1000
Aerco Vent Guideline (GF4050)

In next week’s issue, we will start the discussion of matching boilers and water heaters with systems that maximize their benefits.