Water Heater Draft Problem During Summer

Home Maintenance Questions and AnswersCategory: Water Heater VentingWater Heater Draft Problem During Summer
Question by Joe

Here is the problem:

furnace and water heater vents chimney entranceIn the basement “boiler room” 10ft  x 12ft I have a low efficiency old furnace with galvanized exhaust 6″ pipe connected to a masonry chimney, one story – ranch house. The water heater 3″ single wall pipe (properly pitched) is also connected to the same chimney 10″ above the furnace pipe. The square chimney is lined with 8″ hard clay pipe from the basement bottom to the top of the chimney. ( just for the record, please note the HVAC duct work above the WH exhaust pipe).

While ago I noticed that on the top of the water heater where the in/out water pipes enters the WH are plastic collars and the are melted.

Immediately I was suspicious that hot fumes from WH are entering the room instead of going up the chimney.

Ok, and here is the interesting thing.

The WH fumes are not spilling in to the “boiler room”  I would say for 8 months (colder months) but when the AC is on (on the hot days-summer), then  WH  exhaust fumes spills into the basement. But ones I ajar the walkout basement door just an inch all is good again and the fumes are going up the chimney.

Obviously the negative atmospheric pressure in the basement on hot days is playing the role here and I do not know what else I can do to balance it.

This what I have done to correct the problem (one item at the time):

  1. Changed the 3″ WH pipe for single wall  4″ including oversize draft diverter (did not help at all)
  2. made a whole in the basement wall, (above ground) and install 5″ fresh air pipe all the way to the basement floor  (did not help at all)
  3. install chimney top  8″ x 2ft extension with 8″ wind turbine chimney cap on the top of the 2 ft extension (did not help at all)

Water heater draft hood and single wall connectorOnly one thing is is helping and that is ajar the basement walk out door. 20ft away.

I was thinking of drilling the bottom of the door and install few little round vents – not really fan of that idea because the basement is finished and I do not want the outdoor stairway smell in my basement.

Not sure what may help any more. My options are limited.

We have not enough AMPs in the basement for electric WH, it would require major complicated rewiring.

Power vent gas WH is out of question because we have many windows above in that basement area

Maybe to install separate double wall 3″ pipe for WH only and run it inside the clay pipe, just like on the picture you have on your website (attached)

Is it allowed by the code?

Any idea would be deeply appreciated.


Low efficiency furnace vent conector at chimney stackChimney above the roof with extension

1 Answers
Dariusz Rudnicki Staff answered 1 year ago

Hi Joe,

  1. It appears from your picture that you have a newer water heater; although it’s a natural draft but it is most likely slightly more efficient than the one it replaced or one that the chimney was built for. More efficient means it should burn less gas, produce less heat as a bi-product, and in effect keep the vent pipe connector and the chimney cooler.

During the cold season this may not be a problem because you have a low efficiency working furnace that throws a lot of heat onto the chimney helping to create sufficient draft. Only small portion of your chimney is exposed above the roof so the chimney may not completely cool down between the cycles of the furnace and even when the furnace is off, the smaller amount of WH’s exhaust is enough to be propelled outside and spilled back into the basement.
When you stop using the furnace during warm weather, WH itself doesn’t produce enough hot exhaust gasses to propel themselves all the way up the chimney. One reason could be its liner but I don’t have details about the WH (BTU’s), galvanized pipe / connector length, and masonry chimney height. The very basic rule for a 3” WH draft hood outlet or flue collar is that the diameter of a chimney liner must be 7” max or less, according to your email it is 8” (or is this exterior diameter of a clay pipe). As I said, there are a few other variables that need to be incorporated. You can find tables to calculate proper chimney dimensions here – https://www.rhs1.com/support/references/GAMA_Vent_Tables.pdf / you can read everything but the shortest way will be to start from page 7, figure 12, and scroll to Table 8 on page 17.

  1. To improve draft of your WH you should have at least 12” high vertical connector sitting on top of a draft hood before any elbows are attached. There is zero inches now and you could probably put at least a few inches long connector there and still keep ¼” per foot rise on the horizontal, that should help as well. I would probably start from this option since it’s  the fastest solution if it works. I can’t figure out from you picture if the WH’s connector entrance to the chimney can be raised one more block / you could replace it with B-Vent / double wall if too close to floor framing.
  1. Combustion air (your 5” fresh air pipe) – take a look at this article I wrote some time ago – http://www.checkthishouse.com/6236/combustion-air-requirements-for-fossil-fuel-burning-appliances.html .

Again, I don’t know details about your appliances and basement dimensions but you should find step by step directions on how to make sure your appliances have enough oxygen for combustion.

  1. Power vent WH would need to discharge above the roof if there is no room on sidewalls / because of the windows.
  2. Putting another liner inside the existing clay liner would only work if you connect both appliances to it. You cannot utilize old liner for the furnace while there’s another pipe serving water heater running inside that same liner. You can calculate the smallest required chimney liner diameter for both appliances (Using Gama Tables link above) and connect both of them to that single liner.

That would be it, let me know,

Dariusz Rudnicki Staff replied 1 year ago

Hello Darek

Thanks for your fast reply.

In my case it is impossible to extend the 3″ vertical exhaust pipe above the draft hood to min 12″ above the draft hood, as you recommended, just because the AC duct work above (please see the picture) will not allow that and subsequently 1/4 pitch per foot for the rest of the exhaust pipe connected to the chimney would not be met.
Unfortunately, cannot relocate the WH due to lack of space.
Only solution to this issue is to replace the “TALL” WH (57″ tall) for a short WH (47″ tall)
I am going buy shorter 40 GAL Kenmore WH, for $ 440.00, replace and extend the draft hood vertical pipe as high as I can and will use double wall “B”- 3″ exhaust pipes-duct.
I have high hope for this solution.
I hope that this interesting problem would help many people with same issue I have.

Will keep you posted,


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