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Can I use a common vent pipe for gas-fired HWH and boiler?

0 votes

Hi Dariusz,


In my 100-year-old house, the hot water heater and boiler, both gas-fired, currently vent through an old masonry chimney (see photo below). I want to use the chimney for a new wood-burning fireplace and reroute the other two vent pipes (4" HWH; 5" boiler) through a side wall. Space is tight so I'm wondering if I can use a single vent pipe and a common connector -- or is there some prohibition against mixing the two kinds of flue gases? Also, can I use PVC instead of metal for any portion of the run?


Thanks -- John



asked in Heating System by boatster (120 points)
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1 Answer

0 votes


Hi John,
  1. Yes, you can use a single vent pipe and a common connector to vent your boiler and water heater because they are both using the same fuel… or are they? At the beginning you are saying “both gas-fired” and towards the end “mixing the two kinds of flue gases”, and you’ve also used “oil and gas appliance venting” tag... 

    For information about oil and gas exhaust mixing follow this link - Gas and oil burning appliance venting into a common chimney 

    Unless there are two separate flues inside the chimney, they are vented pretty much the same way right now, into the same flue. 
  2. An appliance must be listed / approved by its manufacturer for side-wall venting. If you have a regular, natural draft water heater it must be vented through the chimney. Some boilers are listed for side wall venting (sometimes using special conversion kit) but you’d have to contact its manufacturer to obtain information that applies to your particular boiler model.
  3. You cannot use PVC for venting any of your current appliances since they are designed for chimney venting and they would simply melt PVC. 
I hope that helps. Just click “comment” below if you have more questions.
answered by darekrudy (21,730 points)