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what is causing a sudden increase in the amount of condensate dripping into the drain line of my high efficiency furnace?

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I have a forced air gas furnace that’s 12 years old. The rated efficiency is 92%. Just at the end of this year’s cooling season and more noticeably at the start of this year’s heating season, a larger than usual amount of condensing has occured from the 2" exhaust PVC vent that runs from the outside of the house through the foundation. It travels approximately 50 ft horizontally underneath the floor joists in my basement from the blower motor on the furnace and where the trap leads out taking the condensate through a drain line to my sump pump well. I measured the amount of condensate dripping into the sump pump well to be approximately 1 ½ gallons per day. This is an huge increase in the amount of condensate I had during both heating and cooling seasons combined over the previous 11 years. In fact, I never even had a sump pump installed in the well because it wasn’t perceived as a necessity due to the small amount that used to drain into the well. Now I’m having to pump out the 12 to 14 gallon sump pump well once a week!

I don’t understand what could have caused this sudden change in the amount of condensate. ? I already had a service technician take a look at the situation and all they can tell me is that I need a sump pump installed now in the well. I also have the furnace humidifier turned off as well as the water line shut off to the humidifier so no water can enter into the humidifier.

I appreciate any information you might be able to share on why there is a dramatic increase in condesate occuring. I also look forward to any advise you may be able to offer to reduce the amount condensate dripping. Thank you in advance.
asked in Heating System by jess (150 points)
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1 Answer

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Hi Jess,
It is difficult to predict exact volume of condensate produced by a high efficiency appliance because it depends on its cycle rate (how often it’s operating), design (2 or 3 heat exchangers), size (BTU’s), etc. For example, a 60 000 BTU rated high efficiency furnace with two heat exchangers operating for 30 minutes can create approximately a quart of condensate. 
I was searching for more information about condensate amount from different size high efficiency furnaces but can’t find any details.
If you are positive that water flooding the sump pump well has nothing to do with ground waters / elevated water table, and it is only a result of the condensate produced by your furnace, the thermostat settings hasn’t been changed, ambient temperature is about the same as previous years, I don’t really know what could be responsible for the increase in condensate amount.
The condensate amount sometimes gets down because of a clogged heat exchanger but more condensate is usually related to longer operating time of the furnace.
Try to compare your November / December gas bills from the last 2-3 years; look for gas usage and an average temperature in the particular month. Maybe this will shine some light on this unusual situation.
Please let me know, just click “comment” below, and I'm sorry that I can't give an "absolute" answer.
answered by darekrudy (21,730 points)
In regard to water flooding the sump pump well, I can’t confirm it has something to do with ground waters / elevated water table, or just instead of only a result of the condensate produced by the furnace.It’s possible it’s been a contributing factor because this area received record breaking rainfall this year in August. I can say for sure though the thermostat settings haven’t been changed and the ambient temperature is about the same as previous years. I have a 80 000 BTU rated high efficiency furnace with two heat exchangers that when operating now for 1 hour creates approx  1- ¾ gallons a day.


A possible way I thought of solving this sump pump well flooding is to install a condensate pump into the drain line. I also have a humidifier that the condensate pump would capture water from in the drain line, but that amount of water from the humidifier can be slowed down or even turned off so it wouldn’t contribute to the main source of the problem which is the furnace condensation.  


Could you tell me if a condensate pump would solve the problem (bypassing the sump pump well) by running drain tubing from the condensate pump outside through a basement foundation opening such as where the Freon line leads to the outside? I don’t know whether or not the end of the tubing left sticking outside would freeze over in the winter and therefore prevent the pump from discharging water. The other concern I have that I hope you can help me with is the length of the tubing I will need to run. The condensate pump will need to be able to pump the amount water a distance of approximately 30 ft from the pump to the outside. it's rated at 100 gallons per hour at a discharge height above pumping level of 10 ft. and only 30 gallons per hour at a discharge height above pumping level of 15 ft. (At least I think what the spec is referring to is that the pump can discharge vertically up to 15 ft at 30 gallons per hour ). The manual doesn't describe the total distance the water will travel to be expelled out. I need to run tubing from the pump vertically for approx 6 ft and then horizontally for an additional  24 ft  to expel the water outside. Also, the kit only provided a 20 ft piece of tubing which may be the maximum distance it can meet within it's rating.


You can simply put a bucket / container / bottle under the condensate drainage tubing for one day to see how much of the liquid it will collect. If it’s almost 2 gallons per 1 hour of operation… that seems to be a lot and I would contact the manufacturer for explanation.
Condensate discharge to exterior in cold climates is not recommended due to possibility of freezing. If it freezes you’ll flood the furnace and the surrounding area. 
You could use condensate pump to discharge it somewhere else inside the house, possible condensate discharge locations are explained in my  attic drip pan post. Some may not be permitted in your jurisdiction and remember that condensate is slightly acidic (somewhere around 3-5 pH) and might cause permanent damage to various materials (neutralizing filters are available).
For your particular pump I'd need to see the specs. Could you copy the brand and the model # for me so I could look it up?
The condensate pump brand is Flotec and the model # is FPCP-20ULST.  I did hear that the discharge to the outside in cold climates wasn't recommended because of freezing, however, my next door neighbor has been using a condensate pump for 12 years now with drain tubing running to the outside and without ever flooding the furnace (this was why I thought this operation might still work after being told that the way my neighbor has it set up shouldn't be attempted because of the cold climate).    As for discharging it somewhere inside, there really isn't anywhere else except the sump pump well. I'll certainly look over the "attic drip pan" post to see if that provides another alternative.


It’s hard to find any positive reviews for this condensate pump; I checked Home Depot and Amazon, very few positive ones. I couldn’t find any explanation for the distance you can run the discharge tubing but since it doesn’t produce much pressure the maximum recommended distance might be that 20’ of tubing provided with the pump. Maybe longer if you provide some down-slope along its horizontal run and let the gravity do the job.
In Flotec FAQ’s section there’s one suggestion but I'm not sure if it applies to condensate pumps or to general utility pumps:
Q. How long of a hose can I use with this pump?
A. In general, try to keep discharge hoses shorter than about 25’. Most utility pumps don’t build a lot of pressure. If using a hose that is too long, the pump’s performance will be severely reduced and the pump will wear faster than normal. Also, garden hoses come in different diameters (3/8”, ½”, 5/8”, ¾”). Use as fat a hose as you can find. Also, the higher you have the discharge hose going up, the larger diameter hose it should be (within reason).
Yes, I completely agree on the quality (or lack thereof) of this particular pump. After inspecting it when I brought it home, my first thought was that I do not  need a plastic pump that might not function very well. I found the Q&A statement that said MOST utility pumps don't build alot of pressure convincing enough,and after having looked at enough pumps sold in the stores, I would be hard pressed to find a very good one where I wouldn't be worried about the pressure failing or even with freezing outside that could allow condensate to drip back to the furnace. The way the bulder of the house intended the condensate to discharge (through a dripline into the sump pump well) may be the most logical way to go.  I believe that the solution lays in a quality submersible sump pump installed in the well to have the water discharged outside.