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Ventilating dehumidifier to complement mini splits?

gridlach | Posted in Mechanicals on

After getting a trio of single-zone Mitsubishi mini splits last summer we’ve had a tough time controlling humidity in our 1200 sqft home. Total cooling BTU is 24k – a 6k and two 9k; We sized above the roughly 18k BTU cooling the Manual J called for to account for additional winter heating load.

With a summer and a half of testing, it seems like there are a specific set of climate conditions that overwhelm the Mitsubishis: cool-to-moderate (75-85ºF outdoors), cloudy, and damp. Unfortunately these conditions make up a fair chunk of our summer weather in southern Ohio, with our county on the boundary between Zones 4 and 5.

In these conditions neither “cool” nor “dry” modes make much of a dent in indoor relative humidity, which can stray as high as 70% and routinely sits in the 60-65% range. I’d like it to stay pegged around 50% if possible. I’ve reduced fan speed and tried turning off a unit to test for it being an oversizing issue, but these tweaks haven’t made any difference.

The house is a relatively recent build – late 90s on the original structure, mid-2010s on an addition. In spite of this the older part of the house is quite leaky: 2×4 stud walls with fiberglass, no crack sealing, no house wrap. The addition is much tighter, but connected to the old structure on an open floor plan. We’re under a lot of tree cover, and three stories with a shed roof probably leads to strong stack effect. I realize that better windows, insulation, and sealing in the old house are probably the long-term solution, but these improvements aren’t in the cards for at least a couple more years.

At the moment, my idea is to connect a ventilating dehumidifier with a fresh air inlet to an existing duct system that used to do distribution for a gas furnace – the house could also use more positive fresh air ventilation than it currently gets. I’ve looked at the Santa Fe Ultra70 and the AprilAire E070 or E080.

My questions are: is the above a smart strategy, or are there other, better approaches? And if a ventilating dehumidifier is the ticket, would one of the models I note or a different one be a good choice?

Thanks in advance for your help and advice.

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  1. walta100 | | #1

    Have you tried just running a box store dehumidifier in the house?

    Seems to me the outdoor air you bring in to the house thru the dehumidifier is likely to be more humid than the air it will displacing from your house by leaking out the cracks and gaps.

    Seems to me you need blower door directed air sealing done. They use a fan to depressurize the house measure how much it leaks. While the house is depressurize, they use smoke to find the leaks later they fix the leaks and measure again. In a house as old as yours it is very likely to be extremely leaky. 10 air changes per hour would not be uncommon at 50 Pascals.

    If you don’t care about knowing the numbers, taping a box fan into a window and some incense sticks will locate the leaks.


  2. gridlach | | #2

    I have a wheeled dehumidifier that I use for a finished basement space. I've brought it up to test and it quickly knocks humidity down to the desired level, but it runs continuously to keep it there and is awfully loud. Kind of negates one of the main benefits of the mini-splits, their whisper quiet operation. The type of dehumidifier I was looking at would sit in a soundproofed basement room rather than chugging away up in the living space.

    As far as the blower door test goes, I'm looking at getting one done. The original part alone has nearly two dozen windows, and I figure from what I've seen doing repair work on them that the builders didn't bother to insulate around the frames. Just sealing all of those might go a ways towards tightening the house up.

  3. jonathanb | | #3

    I'm in a similar situation as you, @gridlach. I have ducted Fujitsu mini splits, and have humidity issues.

    I recently bought an Airthings View to measure IAQ. What I've learned: humidity is only part of the problem. In the bedroom at night, CO2 levels quickly rise to 1300+ after closing the door. It's got an en suite bathroom, and humidity rises to 70%.

    We can use the bathroom exhaust fan for exhaust-only ventilation, and it quickly brings the CO2 level down to a healthy range, <800. The bathroom exhaust does nothing much for humidity, interestingly. Also, it pulls in outside air, and that can mean VOC and PM 2.5. And it's a bit noisy and more exhaust air than we really need for that room.

    We're considering a quiet indoor dehumidifier and putting the bathroom exhaust fan on a timer, having it run just enough to keep CO2 levels down. That's the inexpensive solution to humidity and CO2. The better solution would be what you're considering: a ventilating dehumidifier.

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