Whole House Dehumidifier – Is it worth while?
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I’m building a new home in Climate zone 6 (central PA). It will be well insulated and air sealed. While I’m still wrestling with the ERV vs dedicated bath exhaust fan decision, my HVAC contractor has just further muddied the water. We are going with a closed loop geothermal heat pump. He has recommended we install a whole house dehumidification system (with outside air capability). Specifically he recommends installing a Honeywell DR90. In theory this makes sense for those days that are not hot, but still humid. Any thoughts or opinions? Is it worth the net installed cost, $1400 net.
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Replies
Jim,
Assuming that your ground-source heat pump will provide cooling during the summer, you won't need a supplemental dehumidifier in your climate. Save yourself the $1,400.
Since your HVAC contractor is giving you bad advice, you might want to double-check the contractor's other suggestions.
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HRV or ERV?
I ran into this problem in our house, in climate zone 6 (NH). The heat pump for a well insulated and tight house will be sized for heating load and ends up grossly oversized for cooling. Last summer, our first in the house, bore this out. At times, the heat inflow just wasn't high enough to run the heat pump long enough to dehumidify without overcooling, and we had to be very careful not to do cooling unnecessarily, so as to have that need when we needed dehumidification.
I ordered the heat pump (two ton; A/C need less than 1 ton in worst conditions) from Climatemaster with their "Climadry" feature, which provides reheat after cooling to dehumidify, to solve the problem. Unfortunately, their documentation for water valve wiring was not correct for an open loop (standing column well) design. I have addressed that issue, so that this summer we should have a good test of the feature.
So my advice isn't necessarily the same as Martin's in this case. You may be comfortable enough with the A/C provided by the heat pump much of the time, but I suspect that with the house you describe you may find a means of dehumidification without overcooling quite desirable. It's a matter of what you are willing to pay for drier air all the time vs only when the outside temp is hot enough to have the A/C running often enough.
Dick - your response is exactly what the HVAC rep was telling us. I ran into the same situation when I replaced the HVAC in our last house. In the middle of the Iowa summer it was either too cool in the house or too damp. Thanks
Jim,
Interesting information, Dick. I'm willing to learn -- my advice might have been hasty. If indeed a GSHP system has trouble dehumidifying in northern climates, as you and Jim imply, it certainly sounds like one more argument in favor of a cheaper ductless minisplit air-source heat pump system.
Spend your money in the best building envelope possible, reduce your Heating and Cooling loads to a minimum and you won't need to spend money in a GSHP. Only then you can hire a good HVAC contractor to give you the right advice.
It isn't so much a matter of the HVAC being GSHP. It's that any heat pump (GSHP or ASHP) will be reversible and provide both heating and cooling. Especially in CZ6, where the unit size almost certainly will be determined by the heating load, the thing winds up grossly oversized for the cooling load of a very well insulated and very tight house. If the heating system were separate, then the A/C unit could be sized for its own duty and have long run cycles.
If Jim's location were in the rest of PA, CZ4 or CZ5, the heating and cooling loads likely would be closer and the heat pump wouldn't create the chilled-but-clammy issue as often. As it is, I do think there will be times when some means of dehumidifying without overcooling will be desirable.
Proper management of inside temperature and humidity with just the heat pump can handle his situation much of the time. This I found out by trial and error last summer. Without our Climadry reheat feature operable, I just left the thermostat setting a bit on the warm side much of the time, with an eye to the weather report. Now and then I'd lower the set point for a while to cool and dry the air to a tolerable level. I avoided too low a setting, so that when the humidity got too high I still had some wiggle room on temperature. This summer I'll have the water valve control issue resolved and ought to be able to make good use of the Climadry feature. Based on last summer's dilemma, I think I['ll be glad I spent the extra money for it.
Jim - I believe you will be well served by the DR90 unit. The ERV and exhaust fan will not control your indoor humidity like the DR90 during the warm months.
As we insulate, we reduce the sensible heat load on the house. This reduces the capacity/run time of the cooling equipment. Unfortunately insulating does not reduce the latent heat load on the house (air sealing helps to reduce latent and sensible heat loads). The occupants still create latent heat in their daily activities and fresh ventilation air carries a latent load during the warmer months. Insulating does not alter these latent loads. The cooling equipment removes less latent heat (since it responds to sensible heat load) and (at times) additional dehumidification is required.
Adding a separate dehumidifier will decouple your cooling system from the latent heat load and allow it to maintain the temperature (sensible heat load) while the DR90 handles the relative humidity (latent heat load) in an efficient manner. The DR90 will dehumidify your fresh make-up air before it is introduced into your house (if necessary) allowing your GSHP to control the indoor temperature.
The latent and sensible loads on a house are rarely balanced and this makes it difficult to use a single device to control both latent and sensible heat loads together, yet maintain a comfortable indoor condition. A cooling system with an adjustable SHR (sensible heat ratio) is needed to do a good job controlling both sensible and latent heat loads simultaneously. The cooling equipment with reheat is intended to overcome this issue, but there are drawbacks to this method compared to a separate piece of equipment for dehumidification.
I upgraded my GSHP about 3 years ago and ran into the same problem.The new unit is more efficent in the cool mode and the run times were too short to really get the humidity down.I inquired about a dehumidifier in the system but was put off by the cost. Since then I have conditioned my crawl space an added attic insulation.This has improved things greatly,but I still am inclined to run a small dehumitfier this summer.It is cheaper than running the GSHP.
I live in the NE corner of NC about 50 miles south of Va. Beach
The GSHP has been great ,since my first installation in 1985.
It seems to me that the larger coils of the newer high eff units reduce the latent cooling. I thought the larger coils are not as cold and dont pull as much moisture from the air.