I had an Aprilaire AA700M humidifier installed on my gas furnace last year. Everything worked fine from what I could tell. Fast forward to this winter, and the humidifier isn't keeping up with the humidity set point on the thermostat. I have an Ecobee 3 smart thermostat, and the desired humidity level is set to 38%.

I have a 3-story town house, approx 2000 sqft. In the past week or so, the humidity levels were over 40%, although it had been raining for a day or two at that time. It's been dry for roughly a week. Lows overnight have been into the mid-20s F and daytime highs in the mid-40s to low-50s F. Right now, the thermostat is reading 29% humidity in the house and 68F temperature, with an outside temperature of 55F and 18% humidity. Earlier in the winter when temperatures were down to 10F, humidity levels in the house were below 20%.

I would expect the Aprilaire 700 to be able to properly humidify a 2000 sqft house (its specs indicate it can humidify up to 5300 sqft), but I am not seeing that. The water panel was just changed at the beginning of the season. Is this normal, or is there something wrong with the humidifier or the install? The humidifier turns on and off when commanded (I can hear the solenoid click open and closed, and I can see and hear water draining through the drain line).

I previously had the Ecobee set to Frost Control for the humidifier, but the HVAC technician indicated that was a problem when the outside temperatures got too low (and the humidifier wouldn't turn on at all? Hard to tell exactly how Ecobee's Frost Control works from what I read online...).

The house was built around 2000, Northern Virginia in the US. It does not seem to be very "tight" - there's no separate vapor barrier that I can see (it may be part of the sheathing?), electrical outlets have foam gaskets but no sealant, windows are original and losing vacuum seals, etc.

I know the RH of air drops as you warm it (especially with a gas furnace) - my question is to what extent the humidifier should be able to counteract that effect.

  • turn the hvac fan from auto to on so that the humidifier can run 24/7.
    – dandavis
    Jan 18, 2023 at 5:52

3 Answers 3


Depends how badly the house leaks air, more than the square footage of the house, really.

If the house is leaky, cold dry air comes in and warm moist air goes out and your humidifier has limits on how much water it can put into the air.

Grab the caulking gun and start sealing cracks.

You might also want an Energy Recovery Ventilator (ERV) which will exhaust air as well as bring in fresh air, and transfer heat and humidity to the air coming in (which a random leaky crack won't do...)

Do you understand that relative humidity outside is "relative to 100% at that temperature" and that the same air pulled inside and heated can hold much more water, so its relative humidity drops when it comes inside?

  • It'll take a lot more than a caulking gun. Leakage is generally at places you actually want to open, such as doors and windows. So seals have to be repaired. Jan 16, 2023 at 20:59
  • 2
    That depends on the age of the home. Before the 90s most homes didn't have continuous vapor barrier, sealed device boxes, or taped windows. Most leakage would be other than at operational window and door seals in that case. I could feel the air coming in around my electrical boxes in my 1950s home.
    – isherwood
    Jan 16, 2023 at 21:41

Yes one would expect that humidifier to do a pretty good job. "Time for a new pad" was the first thing that came to mind but you did mention having already replaced that.

The 700 is a fan-driven model -- have you confirmed that the fan is operational? After an operating cycle, have you confirmed that the evaporator pad is actually wet (ie water is distributing well and wetting the whole pad)?

Reading through the question again it occurs to me that you may face two separate issues.

The Ecobee frost control feature automatically reduces your humidity setpoint during periods when outdoor temperatures are very cold. This is an effort to prevent condensation forming on the inside of window panes or inside of walls. It's based on their idea of "typical" construction; it's possible that your house could be insulated above-average and tolerate a higher humidity level than what they picked. Anyway, since you had frost control enabled earlier when temperatures were down to 10F, this could be why the humidity remained low then.

The second problem you may be facing is related to the mild weather you're experiencing now. During mild weather the thermostat calls for heat less frequently and the heat cycle time is shorter. That translates to fewer opportunities for the humidifier to operate. Further, if the humidifier water solenoid opens at the same time as the furnace begins heating, then the humidifier pad may not even be fully wetted by the time the furnace has satisfied the call for heat. That translates to very little productive humidification.

You've already addressed the "frost control" problem by disabling the feature. The problem of short/infrequent heat cycles is more difficult. There are a few things you could try, some of which might require different controls.

  • Give the humidifier a head start. When it's time for heat, activate the humidifier first, delay a bit, then activate the furnace.
  • Run the humidifier even when there's no need for heat. Some humidifier controls, such as the Aprilaire automatic humidistat, should be able to activate the furnace blower and run the humidifier as needed independent of the need for heating.

Verify the humidifier is hooked up to hot water and that the pipe to the humidifier is not insulated. The hotter the water, the more moisture it can put into the air when it is running.

If your ecobee is on WiFi, you can get a visual time trend of various operating parameters including run time temperature humidity etc.

If you want to see if the humidifier is working, wait for a few days of consistent weather, and turn the humidifier off for have of the days. You should see in the trend a much different pattern for the days that it is running.

I have a 2000 ft2 house and our Apriaire humidifier keeps the house in the mid 30% RH. Getting up to 38% might be a stretch. We do have a heat pump, which does run for longer period.

  • I am not sure whether this answers the question Jan 20 at 1:36

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