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Old 07-07-15, 07:11 AM   #1
stevehull
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Default dehumidifier vs heat pump?

I have a well insulated 1600 sf (~ 60 sq meters) guest house on the property that I don't use too often. Problem is non-use humidity build up.

Background: My Mom lived there until she died a couple years ago. For the years she was here, to keep her happy in winter, I put in a two stage 24K BTU "pump and dump" (open loop) GT heat pump (EER 35 or so, COP 5). This enabled me to keep it at the winter elderly "comfort zone" of 76 F without breaking the bank! Almost never ran on stage two, winter or summer. Summer relative humidity with cooking, showers, etc of about 35-38%. Very dry and healthy.

Anyway, while it is empty, I set the thermostat at 85F, as my Yankee heritage doesn't allow me to set the temp lower. But the humidity crawls up to 48-52% or so after a week or so of non-use. The house is tight and well insulated; R25 in the walls, R40 in the ceiling, R20 below the floor, low e double pane windows and is tight as a drum. I put a plastic liner on dirt crawl space and it is well ventilated. No interior water leaks as I can turn water off to house (except GT heat pump) with same results.

The average summer daytime high temp here in central Oklahoma is about 95F, night low of 75F (100% humidity at night) with a daily average of about 85 F, so the heat pump barely works in the daytime (less than 30 minutes) to offset the small ten degree temp difference (outside to inside).

Of course, this nighttime humidity eventually creeps in and permeates all the interior volume and mass (sheet rock, furnishings, furniture, wood trim, etc). When guests are there (like this past weekend), it takes a day or two of running the GT heat pump (74F) to wring this moisture out.

Is it worth getting a small dehumidifier to pull this water out or should I turn down the thermostat to the GT heat pump? I have an extra 50 pint dehumidifier, but it runs at about 250 watts - and of course gives off heat.

I sure wish some smart company would invent a small capacity (6 K BTU) GT heatpump unit with 40 EER. Then I could run that and be happy that I am dehumidifying, cooling and heating the hot water tank.

Ideas?

Thanks in advance.

Steve

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Old 07-07-15, 07:57 AM   #2
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I would say running the main unit is of course the best outcome because you are circulating the entire volume of the house, BUT you will be using the most power. The amount of heat given off by the dehumidifier might cause your GT unit to run a minute more during cycles. But you are right they run a lot of watts over a long period of time.
Options I think might help would be a heat pump hot water heater. Sadly this is only going to help as much as the hot water you use, and the house being empty means its not going to run that much.
You could build a small mini split, 5,000 btu window unit with the condenser outside. This would solve the added heat problem with the dehumidifier. Also running R290 would save you some watts.
Or you could drill you a bore hole and do a DX Geothermal unit. I would guess you could get a fairly high EER value from that.
If building a system is not a option for you, how about one of those small portable AC units. I know the higher end models have a dehumidifier setting and they vent the hot air outside. Amazon.com - Honeywell MN12CES 12, 000 BTU Portable Air Conditioner with Remote Control - Black/Silver - Bucketless Standing Ac Units

Just some ideas. Good luck and death to humidity!
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Old 07-07-15, 08:01 AM   #3
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Rig a humidistat to the heat pump, set it to your target humidity, and connect it to your stage 1 cooling controls. The thing will only run a little while at night.
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Old 07-07-15, 01:06 PM   #4
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Jeff,

I have done something similar (manually turned on stage 1 only) but it cools off the house too much without latent water removal. Stage 1 cooling is about 18K BTU/hr.

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Old 07-07-15, 05:36 PM   #5
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The control scheme you conjure will determine how effective your unit will operate. From your comments, fan speed is an issue. To wring high humidity out of the air takes lots of energy and low airflow. The newer units that have a dehumidification function basically run the air handler at the slowest possible speed while cycling the compressor on and off.

The idea is to force the evaporator to partially frost up. Once a relatively small amount of frost forms, it begins to choke the airflow through the hx. What air makes it through the evaporator is cold and dry.The longer the compressor runs, the more frost forms.

After a short time, the compressor shuts off and the low flow of air melts the frost as it is wrung of moisture. The cold evaporator continues to draw moisture from the circulated air as it warms its way to the dew point. After some time has elapsed, the compressor starts back up, and the cycle repeats. The evaporator is charged and discharged like a thermal capacitor or battery to dehumidify the air without removing too much heat too fast.

If it were mine, I would be tempted to find a portable dehumidification unit with a leak or burnt out compressor and rob its guts. The defrost thermometer and relay outputs could be easily rigged to what you have and assume limited control pretty darn simple. If you don't want to do anything custom, a duty cycle timer could toggle between "fan only" mode and cooling stage 1 if the fan speed could be slowed.
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Old 07-07-15, 08:04 PM   #6
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I completely agree and have the system on the dehumidification setting already. Still too large capacity to dehumidify with only a ten degree F outside to inside temp difference.

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Old 07-08-15, 07:21 PM   #7
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So... What you're saying is what? The dehumidifier you own is using too much power, and your gshp is too good at doing its job? What you have can be made more effective, but what you are after seems to require a different approach.

What I think you should do is build a solar desiccant waterfall. Make a feng shue fountain or something. You would have your dry indoor air, and the sun would pump your brine as well as evaporate your humidity. Modern day magic.
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Old 07-08-15, 09:31 PM   #8
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Have any links to the water fall magic you speak of Jeff?
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Old 07-09-15, 02:14 PM   #9
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http://ecorenovator.org/forum/solar-...umidifier.html

Search Keyword ldac or devap

And here it is on youtube:


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