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Suspect home has wrong size vent for furnace & hot water heater

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Re: My hot water heater  not venting correctly.  It is melting the plastic on the unions on top of it.  very hot air about 1 1/2" from vent is not going up vent pipe.  I suspect the 4" insulated vent pipe is not sufficent and should have been 5" (currently there is a reducer from 5" to 4" just before it goes up through the celing of the basement)   (The furnace guy did remove a wasps nest out of the chimney near the cap, but the hwh still seemed to run hot.)The  The city uses the International Fuel Gas Code, but when I ask him to calculate it with the numbers I give him, he is reluctant and is giving me the run around.

The furnace is 80%, 8 years old, 66,000 BTU. The hot water heater is one year old 3" vent 50 gallon, 38,000 BTU.  The vent from the ceiling of the basement to the top of the chimney (vent pipe) is 16'-18'.  There is an offset of the vent pipe in the attic to move it about 6' toward the rear of the house.

With these numbers, can you calculate it for me?  Or tell me who could?


Lady Senior Citizen Homeowner  (that means I am helpless & uninformed)  lol
asked in Heating System by sassycat44 (120 points)
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1 Answer

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Hi Sassycat44,
Correct me if I’m wrong; your furnace and water heater are discharging exhaust gasses into a common 5” diameter vent pipe, which has been downsized to 4” diameter vent pipe before penetrating the ceiling. This is wrong and dangerous; you not supposed to downsize the vent pipe!
If both of your appliances are natural draft type / with draft hoods (no induced draft motors to force exhaust gases into the vent pipe) you don’t need to use Gama Venting Tables for calculations.
The simple formula to determine the diameter of the vent pipe is 100% of the larger pipe diameter (measured at the draft hood’s collar) plus 50% of the smaller vent pipe. If your furnace uses 4” and WH 3” diameter vent pipe, the minimum required common vent diameter would be 4”+1 ½” = 5 ½” - the closets available size would 6”. 
If both of those natural draft appliances require 3” diameter vent pipes, the result of this calculation would be 4 ½” and you can use 5” diameter vent pipe (they don’t make half sizes).
Assuming that your furnace utilizes induced draft motor for venting you’re supposed to use Gama Venting Tables, but the information you provided is not complete to make those calculations. Also, the connector between the water heater, furnace, and that 5” common vent pipe is important, it must be “Y” shaped, look at the picture beneath the video in my article: furnace and water heater venting
For calculations go to this link - Gama tables and look at the examples starting on page 20, pick the one that applies to your installation and just change the numbers / use appropriate table listed in that particular example. First pages of Gama tables contain information on how to use them and more info about appliance venting. 
If there’s something you don’t understand or if you have any trouble with getting the final number, please let me know.  Without those few missing values it looks like you are looking at a 5” diameter vent pipe.
Just click "comment" below if you need more help.
answered by darekrudy (21,730 points)