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Community and Q&A

Dehumidify only?

swtjr | Posted in Mechanicals on

In Boston area, 5A region. I own a condo in an old building with average insulation and air sealing and a gas forced-air furnace.

I currently don’t have central AC. My furnace needs replacing, and AC is cheaper as a package install. My city has about 400 cooling degree days per year, and my 1000 sqft condo is on the first floor and shaded by other buildings, so the heat is not too bad. Humidity is, however. A whole-house dehumidifier with a DIY install would be cheaper than AC, and we could conceivably take it with us if we moved. It would also handle humid days in the spring and fall.

If I brought in cooler night air via window fan and ran the dehumidifier and ceiling fans during the day, would that be more or less efficient than AC? Would it be cool enough compared to running the AC at a 78 degree set point? The cooling load calculation I ran online had a sensible load of 18000 BTUH and a latent load of 1500 BTUH, with a reference temperature at 75 deg.

My thought is that the dehumidifier handling the latent load would use less energy than the AC handling both loads, and the dehumidifier would raise the temperature a little but allow for evaporative cooling.

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  1. Expert Member
    Dana Dorsett | | #1

    If you're allowed to hang stuff on the outside of the building (say in an alley or facing the next building rather than on the main street) a ductless mini-split heat pump could handle most or even all of it, including heating.

    If the gas furnace is currently in the basement of the building (as opposed to inside your condo) it could even be cheaper to heat with ductless heat pumps than with gas, but it depends on a number of factors, not the least of which is your power rates (NStar is pretty pricy compared to most MA municipal power utilities, National Grid is a bit behind NStar but still not cheap in most places I've directly compared.) And your gas rates of course...

    Uninsulated ducts in basements give up a sizable fraction of their heat to the basement, and LEAKING ducts in the basement will give up even more, and even a 92% condensing gas furnace will only be putting 70% of the source-fuel heat into the conditioned space in a "typical" system that hasn't been upgraded with very tight and insulated ducts.

    Nighttime ventilation schemes are TERRIBLE in Boston, due to the high summertime dew points. It takes roughly 2x the energy to lower the dew point by 5F than it takes to lower the air temp by 5F. A room at 78F with 50% RH is pretty comfortable, but that air has a dew point of ~57F. The average summertime dew points in Boston are in the mid-60s, often bumping into the low 70s.

    A dehumidifier merely converts latent load into a sensible load (with a bit more heat added from the motor running the pump). So to lower the dew point of 67F air to 57F to make it comfortable raises the air temp by quite a bit! The thermal mass of the building & it's contents keep the air-temps from going off the charts, but it's fair to think of a 500watt dehumidifier as a 2000W space heater in terms of air temp, unless the condenser coil is outside of conditioned space. With AC the condenser is always outdoors, and that latent heat is given up as higher temperature to the outdoor air, but not so with dehumidifiers.

    Ducted mechanical systems are rarely legally-separable from the building at the time of sale since it disrupts the other functions of the ducts. Basically, if it's mechanically fastened to the building or is hard-wired rather than plugged in, it stays.

    Unless you have a lot of unshaded west facing windows (doesn't sound like it) I'd be surprised if your sensible load was anything as high as 18KBTU/hr, but your heating load might be around that depending on how much exterior wall & windows you have. A high-efficiency 1.5-2 ton ductless mini-split heat pump typically runs about $4-5K, installed. If the condo is a long skinny unit that would need two interior heads to keep temperatures relatively balanced you might be looking at $5.5-6K, but not much more. Most units have a "dehumidify" mode that dehumidify efficiently, but will also be doing at least some sensible cooling as a by-product in that mode, and most don't have dehumidistat controls- it just runs in that mode until you put it in heating or cooling mode. But for the low-sensible load muggy days it's not a bad feature.

    SFAIK only the Daikin Quaternity series allows independent RH (dehumidistat only) & temperature setpoints, and can dehumidify in either heating or cooling mode. They're not cheap, but they're not outrageously more than other high-end ductless units.

    The outside heating design temp for Boston is about +12F and most better mini-splits have decent performance down to +5F. Design temps are a bit cooler outside the city, but inside of 128 you can safely use +8F as a design temp. (Worcester's design temp is still +5F, after all.) With a mid-winter gas bill with exact usage & dates (or an average temperature over the billing period and a therms/day number) it's possible to work backward to a fairly hard upper bound on the true heating load for the unit at any arbitrary design temp. (Cooling loads are harder due to solar gain factors which can exceed other gains.)

  2. GBA Editor
    Martin Holladay | | #2

    The answer to your question depends on your own comfort range. If you install a stand-alone dehumidifier in your condo, it will raise the indoor temperature as it lowers the indoor humidity. Some people might conclude that, on balance, the hot dry air felt good. Others might say, "This doesn't help. Our condo is hotter than ever."

    If you are worried about comfort, an air conditioner is a safer choice. An air conditioner will lower the indoor temperature as well as lower the indoor humidity.

  3. Expert Member
    Dana Dorsett | | #3

    But opening up the windows at night for sensible cooling just adds a big latent load for the dehumidifier to work on- it's not really a very effective or efficient approach most of the time.

    A DIY-installed name-brand cooling-only 3/4 ton mini-split costs about the same as a bare-bones whole house dehumidifier, and will almost certainly handle both the humidity and sensible load more efficiently by not taking on a big slug of moisture every night, and dumping the (now smaller) latent-heat outside rather than heating up the condo with it. Installing a mini-split isn't rocket-science, but you'll probably want to read up on in first, and you may want to pay a tech to adjust the refrigerant charge if it doesn't seem to be working well.

  4. swtjr | | #4

    Many thanks to you both for your answers.

    It sounds like the dehumidifier is not a good fit, and neither is night-time ventilation if we have any sort of conditioning system.

    I dug through our utility rates and determined that the operating cost of a heat pump and a furnace/AC would be about the same. The ductless system sounds good, but wouldn't I need supplemental ventilation to condition closed-off bedrooms? (edit: I see that there's another Q&A topic on this.)

    Our ducts are in the basement with relatively short runs, and I have mastic and duct insulation already purchased; would a ducted system like this one make sense?

    I ran an aggressive load calculation and got 15.5K/27.5K BTUH cooling/heating. (The HVAC contractor said 20.4K/50K BTUH and wants to install 2 tons and 60K.) I also did the average therm/HDD times design day calculation with my mid-winter gas bill and got 30K BTUH. Since then, we have had cellulose insulation blown in, and I'm replacing a grossly oversized (meaning more inefficient?) 120k BTUH furnace.

    The reason I'm balking at the AC is a different environmental issue: the compressor runs at 78 dB, which would annoy the neighbors, who are only 30 feet away. Daikin outside units are about 25 dB quieter. Having a heat pump would also allow for the possibility of using solar (2 kW) for heating in the future. The furnace/AC install would be $8k. I contacted some Daikin dealers for estimates and to see whether they install this particular system. I'll also look into a DIY two-zone ductless install. Thanks again for your help.

  5. Expert Member
    Dana Dorsett | | #5

    The 30K number on the "before" picture of heating is probably accurate, and includes duct losses and a lower efficiency than the actual name-plate efficiency. (If you didn't adjust for burner efficiency you're probably at least 20% too high.

    The efficiency losses of oversizing on low-mass hot-air furnaces are nothing like what they are for oversized high-mass boilers, so don't de-rate it a whole bunch on your fuel use calc.

    I'd definitely look at Mitsubishi & Fujitsu too, both of which have decent 1.5 -2 ton dual-head or single head ductless systems, and have wider support networks. They all have dehumidfy modes and DO remove moisture in normal cooling modes as well. You can set a pretty high cooling setpoint and it'll still do a decent amount of dehumidifying, but you may need to switch to "dehumidify" mode when outdoor dew points are in the 70s if your cooling setpoint is 80F or above.

    A standard method of getting SOME amount of temperature balancing with ductless systems is to use an active ventilation system, preferably an ERV, configured with the ventilation air supply is in the same space as the ductless head(s), and all other rooms have only ventilation exhaust, with door cuts/grilles or jumper-ducts etc to supply the return path. An ERV system for a condo might run $2500 or so, and would likely use 3" ducts.

    All inverter-drive mini-split compressor units are whisper quiet, barely audible even standing next to them except when running at highest fan speed. The interior heads are usually much quieter than metal-ducted air-handler driven solutions- quieter than a typical refrigerator at low-speed (where they will be running MOST of the time.) Comfort factor is also quite high- for highest efficiency you "set and forget" and let it modulate, since they run with phenomenal efficiency at part load, but not-so-phenomenal efficiency at high speed, which is where they would be running on a recovery ramp if you're setting back overnight in heating mode.

    A 2- head 2- ton ductless solution would probably handle the whole thing for under the $8K quoted for the gas-fired ducted + AC solution. If the floor plan is open enough to be able to get by with a single-head it'll be under $6K.

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