Furnace Condensate & New D.O.E. Rules – Part 2


In my last post, I addressed the question of the pH of the condensate that comes out of 90+% furnaces.  Since that post has come out, I have been asked the question, How much condensate will be produced by a 90+% efficient furnace?   Good question!

We need to understand that during the combustion process, water vapor is produced. This water comes from both the water moisture contained in the gas fuel and in the combustion air.

Now, the maximum theoretical quantity of condensate that could be produced is 1.1 gallons per hour for a 100,000 BTU input furnace operating continuously. Most applications will not produce that much.

The actual quantity produced will be determined by many factors. These include: (1) fuel supply, (2) combustion air, (3) furnace efficiency, (4) furnace temperature rise, and (5) return air temperature. Typical high-efficiency furnaces will actually produce about 0.8 gallons per hour of condensate  for a 100,000 input BTU furnace running constantly

Since furnaces do not run constantly, this amount of condensate is reduced even more.  For example, an average 80,000 BTU furnace operating on a 50% duty cycle would likely produce less that 8 gallons in a 24 hour period.

That may seem like a lot, but you need to keep in mind how much water goes down the drain every day in the average household.  When you compare it to that, it is really quite small. This also ties into the pH of the condensate since it is “diluted” by the rest of the home’s waste water.

In an up-coming post, I will be addressing some venting application, as chimneys used with 90+% efficient furnaces will pretty much be gone in the northern tier of states so we need to look at that issue next.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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3 Responses to Furnace Condensate & New D.O.E. Rules – Part 2

  1. Don Desilets says:

    good post , but this is the total condensate produced and delivered out the flue and not just what is directed to the floor drain . Do you have a rule of thumb for that percentage

    • Actually, the 8 gallons per day is the condensate that goes down the drain. A 95% efficient furnace puts 95% 0f the heat into the house and 5% up the chimney in the form of products of combustion and water vapor. That water vapor is minimal compared to what goes down the drain.

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