which vent on top

[chris wright]

Hello,I have a 6 inch vent for a oil furnace going into a brick chimney below and on the other side of a 3 inch gas Hot Water heater vent will this cause back drafting and possible carbon monoxide leaking into the basement?

[Ashley Eldridge]

As long as the smaller vent enters the chimney above the larger connector you should be ok. Also, keep in mind that the flue must be properly sized for the two appliances to operate properly.

Ashley Eldridge
Chimney Safety Institute of America
Director of Education

[Tom Rees]

First of all, make sure the chimney is lined. In general gas and oil flues should not be shared, but if they must then make sure the oil flue is below the gas flue.

[John Arnold]

...Also, keep in mind that the flue must be properly sized for the two appliances to operate properly.

Here's something that's always bugged me: how can the flue be properly sized for the two appliances together and each appliance separately? In many areas the water heater is going to be the only appliance venting into the flue for most of the year, so if the flue is big enough for both, it must be too big just for the water heater, no?

[Ashley Eldridge]

Here's something that's always bugged me: how can the flue be properly sized for the two appliances together and each appliance separately? In many areas the water heater is going to be the only appliance venting into the flue for most of the year, so if the flue is big enough for both, it must be too big just for the water heater, no?

The simple answer is that venting for gas allows up to 7 times the area of the outlet. Naturally, the flue will not function as well when it shares two appliances for the very reason you have described. Concessions have to be made that will allow both to vent into the same flue. That doesn't mean it cannot work.
The worst case scenario comes when condensation occurs from a vastly oversized flue. I am more concerned with the mixing of the acids from two appliances that are not venting well. I agree that the chimney should have a liner, BTW.

Ashley Eldridge
Chimney Safety Institute of America
Director of Education

[chris wright]

Thank you for all your help,you all do good for others thanks again!

[Bob Harper]

... In general gas and oil flues should not be shared, but if they must then make sure the oil flue is below the gas flue.

Tom, can you provide a reference for this?
TIA
Bob

[Tom Rees]

Bob, I remembered reading it in one of my old NHIT study books so I dug them out and found it in there. It didn't say why the oil pipe should be lower, I would defer to your expertise if you could clarify for us all.

[David Bell]

The smallest vent should be the highest to assist the the vent to prime the chimney for proper draft. The higher pipe run before entering the chimney the better.

[Bob Harper]

David is correct as is his explanation but keep in mind, it is a 'recommendation'-the code still allows common vent manifolding 'IF'. The IF means the common vent must be capable of handling both appliances including sufficient draft, the connectors are sized according to the mfr. and codes and the appliances have primary safety controls. Hmmm. Does that mean it has to have bilateral spill switches? Good question because the code is unclear. Therefore, it would fall back on the AHJ.

In add cases, the whole thing should start with a professional level II inspection. You can bet the level II will reveal the need for a liner regardless. In addition, most mfrs. now either require or recommend a properly sized liner.

Back to David's point, the code's intent is to increase the vent rise on the smaller appliances to assist with venting. What it does not take into account are things such as how to penetrate a terra cotta liner with a 3" pipe and have sufficient access to seal the junction to the flue wall with a handful of furnace cement. It's required by code but I've NEVER seen it done before my arrival. I can barely do it with a 5" pipe, much less a 3".

Bob

[DavidR]

Back to David's point, the code's intent is to increase the vent rise on the smaller appliances to assist with venting.


And it still never works like it's supposed to in the field. :D

[chris wright]

Hi out there just a word of thanks to Ashley,Tom,John,Bob,David and David for your input,you have been very helpful. I feel confident that this issue has been addressed correctly and i can move forward thanks again.