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.