Multiple gas appliance venting to common chimney?

Question

I would appreciate if i could get an answer from an actual HVAC installer or plumber who knows this stuff.

I have installed water heaters before and I am considering installing a new gas water heater in place of my current electric one. Both of these would be located in the basement.

My home currently has a forced air gas furnace that is vented directly to one side of a chimney. It is a fairly typical model furnace (10-20 years old) that pulls in fresh air from ducts in my kitchen and hallway upstairs.

I am wondering if it would break any building code rules for me to cut a second hole on another side of this chimney to vent my other heater.

This would mean 2 gas appliances are venting to a common chimney.

I live in Wisconsin and the building codes here (SPS 323.155) (https://docs.legis.wisconsin.gov/code/admin_code/sps/safety_and_buildings_and_environment/320_325/323/V/11) seem to indicate that this would be OK as long as I buy the correct kind of water heater and vent on a different side of the chimney.

I know this can potentially be very dangerous if done incorrectly. I intend to do it correctly. Part of that is asking questions such as this. Please spare me from comments about how dangerous this can be if done wrong. I fully understand the risk of carbon monoxide. Thank You.

David B · Accepted Answer

That answer does not sound right what so ever.  I see two appliances exhausting in one chimney all the time.  Sometimes two pipes with two holes other times two pipes joined and than to chimney.  The only time the appliances would fight each other is if one or more has a fan to push exhaust out.  If they both draft exhaust it should be fine.

Rusty Nail · Answer

I have one chimney going out of the roof of my house.  Both the furnace and the gas water heater use this same pipe.  This was installed 45 years ago.  Furnace has ben changed, and so has the water heater.  Still use the same roof vent stack.

Ec · Answer

Yes you can, You can use the same chimney for multiple appliances as long as it is using the same fuel. Class A for Class A, B for B. You just can't intermix. So long as they are both venting gas into the same chimney thats fine.

ranger_diy · Answer

Why not just get a power vent water heater that vents using PVC? They are super easy to vent. They can cost a few dollars more but you can often get a rebate from your power utility.

Gerrit · Answer

Only with an 80% furnace all others vent separately.

XTX · Answer

one vent will cause another to blow itself OUT === get this done by a professional  ... it is your home and SAFETY considerations are 1st ....

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Multiple gas appliance venting to common chimney?

6 Answers