Oversized unit and high humidity problems?

helloimchristieJune 12, 2012

When I moved into my house, my dad told me that he had a bigger heat pump installed than what was required - thinking bigger is better. I had no idea at the time that this was actually bad instead of good. The house I'm living in is very old, and this is the first time central heat and air has ever been installed. Ever since we moved in, we've been having very high humidity problems. Lots of mold (I think it's pretty harmless though, but I am constantly cleaning it) and sweating windows, sometimes even condensation on the walls, etc. It tends to be worse in the winter than in the summer. I never thought to connect this problem with the heat pump, but now I have. What can I do? Installing a new unit is not an option, due to money. How can I live with my current heat pump and control these humidity levels? I do have a dehumidifier - I keep it in my bedroom where the humidity tends to be worse. I don't run it all the time though. I have Damp-rid buckets in several rooms. I read online somewhere that setting the thermostat fan setting to "ON" rather than "AUTO" would help, but then upon doing more research, some sites suggest this could make the humidity worse. Should I have my fan setting at "ON" all the time? Any advice would be greatly appreciated. Thank you!

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High humidity in the cooling mode can result from an oversized air conditioner/heat pump. High humidity in the winter will not result from an oversized heat pump.

These suggestions are not a solution, only suggestions to put a band-aid on it. Do not run the blower continuously. Do you have more than one blower speed? If so, use the low speed until you find the cause of the problem. In addition to that, run the cooling manually by letting the temp drift high and then turning it down to a lower temp. When the it reached the set point and turns off, turn the temp up a few degrees, repeat. Forget the Damp-rid and run that dehumidifier all the time until you can find the root of your problem. Get an inexpensive hygrometer or two, digital. Put them in different areas of your house. Do this today! If one dehumidifier will not keep the humidity below 50%, get another one, quick.

Where do you live? (What is the climate like?) I do not think this is an HVAC problem, but someone here might be able to help if you tell us more about your house. (Do you have a basement for starters). You need to consult with a building expert about where the moisture is coming from. If you want to start something yourself, see if your library has "My House is Killing Me", by Jeffry C. May.

You need to fix this before your house and you turn to moldy mush. You might already have done serious damage. How long has this been going on? If you have water condensing on your walls, you have a very serious problem.

    Bookmark   June 12, 2012 at 6:36PM
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we get this question all the time here
in hot humid Louisiana...

don't set fan to on position.
this will allow the moisture collected
on the coil to re-circulate throughout the house.

instead locate your stand alone dehumidifier
in a central part of your house.
leave it on all the time.
expect to empty it often as it removes
leave interior doors open.

shoot for below 50%RH as it will keep
mold from growing.

best of luck

    Bookmark   June 12, 2012 at 6:38PM
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We would need some more information about your system. When your air conditioning system comes on, how long does it typically run before the thermostat is satisfied and the unit shuts off. Yes, record the time and let us know. Unit's that are over-sized will generally have a short run time to satisfy the set-point of the thermostat. This short run time is why the humidity cannot be efficiently removed from the air. There are a couple of things that can be done.

I am jumping the gun a bit, as i don't know enough about your system yet but.... One step that can help is to install an anti short cycle device on the condensing unit. This device is a timer which gets wired into the low voltage wiring preventing the compressor contactor from energizing until the timer has completed its count down cycle. Some of these devices can be set all the way up to 15 minutes which would not let your unit to run all too often since it cools your house down quickly. In most cases this would also cause your unit to run longer each time it came on. But once again, we would really need to know more about your system and it's run times before giving solid advice.

Secondly, if your condensing unit was over-sized, then likely your indoor fan motor was also over-sized. If your evaporator has a TX valve, then the airflow can be put on a lower setting during the AC run cycle. This is a more complicated step, and i would not recommend a home owner to try this themselves. A qualified technician would need to do this and monitor the operating pressures of the system.

    Bookmark   June 12, 2012 at 10:28PM
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"It tends to be worse in the winter than in the summer."

I don't think this is an AC cycling problem. There is a source of moisture in the house which needs to be understood.

    Bookmark   June 13, 2012 at 7:32AM
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There is not enough information in the original post to allow a detailed explanation of the problem, but here are some thoughts:

Heat pumps are rarely right sized for both the heating and cooling seasons. They will be oversized or undersized for one season depending on the house and climate.

Since your issues are occurring primarily in the winter, I suspect that the ventilation in your house changed when the heat pump was installed. What type of heating/cooling system was in place before the heat pump? Fireplaces? Wood or Coal stoves? An open combustion furnace? All of these devices will exhaust a large amount of indoor air when heating causing outdoor air to enter the house. If you replace these heating devices with a heat pump and do not mechanically ventilate, the amount of moisture in the house during the winter will increase.

Did you have any air sealing or insulation done when the heat pump was installed? This can reduce the air changes in your house and cause moisture to accumulate.

Often the best solution for high indoor humidity in the winter(outdoor dew point less than 40F)is to increase the ventilation in the house. The lower dew point (outdoor) air being brought in will reduce your indoor humidity. In some cases operating a dehumidifier is best as it reduces the indoor humidity and adds heat to the house. Changing the cycle on your heat pump in heating mode will not reduce the humidity inside your house when the heat pump is heating.

Many of the suggestions in prior posts will help reduce your indoor humidity during the cooling season. Reducing the amount of outdoor air that enters your house when the outdoor dew point is above 55F will lower the humidity inside your house. A whole house dehumidifier such as the Ultra Air unit can ventilate and dehumidify your house all year round.

Your best solution may be to address any ventilation and insulation issues in your house before replacing the heat pump.

    Bookmark   June 13, 2012 at 1:48PM
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