Tee fitting at water heater/furnace vent

Would you comment on this & if so, why? House is 14 years old and furnace & water heater operated/vented properly. I understand with natural draft that a wye would be better here, but does anyone have any documentation supporting this install and what affects it can have with venting? BTW, the water heater vent is properly pitched up to the tee fitting.

Thanks in advance.

I don’t have a problem with it. The pitch looks OK.

I have a problem. The water heater appears to be the lower vent. It’s hard to tell from the picture. If it is… then

EDIT: On a second look, it looks like the furnace is the lower appliance in the picture. Ignore below

Is the lower BTU appliance connected above the higher BTU appliance?

The water heater should be connected above the furnace.

I’m thinking the water heater is the upper vent.

If I were to call out every Tee connection at the exhaust area, it would be in 1/3 of the homes I inspect.

I simply note the connection.

As for the sizes of the vents and their placement…the smaller vent connector should vent into the chimney above the larger vent connector…




Thanks for the replies, fellas.

It should be okay as long as there is not a draft motor on one of the appliances.

I am actually really glad I found this thread. I run into this on nearly every home I inspect.
Two things: first, I write it up every single time as a performance and safety issue IF the natural draft water heater is first in line and there’s a power vented (fan assisted) after. Why? Because a natural draft water heater has trouble venting it’s gasses (in the sense of it doesn’t vent properly because it’s blocked by assisted air pushing against the natural draft.) Second, I read every manual for every water I inspect and nearly every one of them say they CANNOT be commoned with a power vented (fan assisted) appliance. Back drafting and exhaust spillage back into the living space.
I always recommend a WYE fitting and explain my reasoning with references and direct quotes from the user manuals with pictures. Here is a picture of a house I just inspected it two tee fittings AFTER the water heater.

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I see draft inducer gas furnaces hooked to the flue below natural draft WH all the time. No backdrafting, perfectly acceptable.

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That’s the key. If the WH was second in line than I don’t have an issue. It’s when the WH is the main flue and there’s a powered unit TEE’d into it. Otherwise, don’t use a tee use a wye fitting or follow the mfg specs.

Both a fan assisted furnace and a natural draft water heater are both category one appliances. These are not positive pressure. They can both be tied into the same natural draft flue pipe utilizing tee or Y fittings. It doesn’t make a hill of beans difference to me which one ties in above or below the other.

On the contrary, most of the time when I read the manuals it is not permitted per the mfg. I’ve read enough to where I just always recommend a wye. Not saying you’re wrong, or HVAC guys are wrong when installing them (but they kinda are cause they could easily read the manual) so I just write it up.

You said “You run into this on almost every inspection”. The reason for this is the AHJ has passed the installation. You said a fan assisted vent is a power vent. This leads me to believe you are not a licensed HVAC or plumber in fact your knowledge in this field may be improved. One may assume the AHJ holds a license in this field.

You mention back drafting and spillage. This is a poor assumption as a fan assisted cat 1 appliance may join a cat 1 appliance in the same flue pipe that is natural draft.

When you mention the installation manual are you talking about the furnace and water heater manual or the flue pipe manufacturers installation instructions?

Duravent says you can uses a tee or wye when connecting multiple appliances. Page 6 step 3. They are the industry leader in residential flue piping material.

State water heaters makes no mention of a tee or wye in the flue pipe configuration. They say consult with the local gas company as flue pipe arrangements can greatly vary. I guess they know a thing or two just like Duravent.

Please post a PDF installation manual of cat 1 appliances requiring wye B vent fittings.

Unless I am completely misunderstanding the MFG specifications, the State WH you posted says “common venting that WH will adversely affect the operation.”

This is what the MFG says about the WH in the picture I posted.

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Or, the HVAC installers never read the manuals. Devils advocate.

I honestly don’t disagree with this statement. I could always use more knowledge, hence this conversation.

I agree, but in my opinion it depends on where the connection is. In my photo, I do not believe that natural draft WH will properly vent it’s exhaust with two fan assisted appliances running in congruent with the WH when the WH is the first in line. Maybe if the WH terminated AFTER the two T’s I would hold a different opinion but that is not the case.

I read all MFG manuals to determine whether or not the flues can be commoned or not. The flue itself might have a different recommendation on the installation but I follow what the MFG manuals say for furnaces, boilers and WH’s.

This may be true, but the appliances may not allow this. This is why I read what they recommend.

This in someway is along the lines of what I am getting at, which again, is why I read all the manuals to determine how it should be installed.

I can sense the sarcasm. No reason to be mad.

That is for a power vent water heater. A category 1 fan assisted appliance is not a power vented appliance.

If you get a chance can you please post the furnace manual that requires wye B vent fittings.

When you mentioned that State water heaters said common venting the water heater may adversely affect its performance. Read further this is due to deleting or adding systems. Every vent system must be properly sized. When you delete an appliance from a vent system, you have changed the required size of the main vent. This is common knowledge for those of us in the profession. This is why this is a trade that requires a license. When regular people read through these manuals they misunderstand important information.

Don’t worry sweetheart I’m not mad. I’ve been in the trades for 35 years, you’ll know when I’m mad.



Absolutely correct but easily misunderstood.

Correct again, changes MAY cause problems.

Smallest above larger, or side by side - T or Wye acceptable depending on headroom or configuration.


Sorry for the sloped photo :crazy_face:

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Brian you are spot on!

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@mwilles My question is how to identify a cat 1 vs cat 2, 3 or 4? Will the appliance have that information on the data plate?

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Brian that’s a good question. The appliance data plate should tell you their category and vent pipe requirements. As a home inspector the following photograph will help you understand which category the appliance is by observing their vent pipe.