Humidity issues in Studio w/ small minisplit + ERV
Hi,
I’m in the first summer of what I had hoped would be an excellent indoor air quality solution for my attached studio. It’s master bedroom of a single family home that shares a crawl space with the rest of the home – but is sealed off and has a private entry to the back patio. Combined square footage of room and bathroom is 350 sqft. Climate zone is 3C, San Diego
Issue:
-Relative humidity rises to 80% at night, but drops to 50% by Noon, then begins rising again
HVAC:
– 9k Mitsubishi (MSZ-FS09NA /MUZ-FS09NA) set to 69F
Ventilation:
-Panasonic FV-04VE1 WhisperComfort Balanced Air Solution ERV, Set to boost (40cfm exhaust/30 cfm supply) because it helps keep the room smelling fresh since its a short term rental
-Bathroom fan, manually actuated timer
Building envelope:
Exterior walls: Stucco on lathe, no insulation
Attic: R38 Mineral Wool + Air Sealing
Crawl Space: partially encapsulated, dehumidified and steady at RH 50%
Air sealing: well sealed, crawl space band joist spray foamed + all outlets and attic penetrations
Possible solutions:
-Replace ERV with better model
-Add standalone dehumidifier or ducted dehumidifier
-Reduce ventilation rate?
-Add humidistat to bathroom fan
-Add fresh air intake to exterior wall to prevent negative pressure from bathroom fan
Thanks in advance, really eager for some suggestions here!
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Replies
I am a little surprised that you have a Mitsubishi hyper heat in that climate. I believe other models remove more moisture, and in your climate that unit will never see the kinds of low outdoor temperatures that it was designed to operate in for heating.
The FS does have a dry mode that may be helpful if your cooling load isn’t high enough to keep the unit running. Put the fan on its lowest setting—it’ll remove more moisture with a greater delta over the coil.
If that still isn’t satisfactory I’d get a dehumidifier.
Thanks for taking the time to think about this with me!
My understanding was that the hyper heat models were slightly more efficient (28.4 vs 29.8 SEER 2) with a few more bells and whistles on the remote and wall unit. The outdoor hyper heat unit is supposedly slightly larger and a tad louder. Both units specifications list the same moisture removal rate of 0.6 Pints/h.
Thanks for the note on the dry mode, I have been using that between guests, but I think it gets a little too chilly for some when that mode is on. Even with dry mode on, it still climbed to 70%RH last night with a temp of 67F, dew point: 58°F so not bad. That's compared to the outdoors which was 65.8 °F, 83 % RH, Dew point: 60.6 °F
You have two issues. You are over-ventilating and your cooling load is very low thus your hvac is way oversized.
350sqft is about 3000cuft of volume.
Your ~30CFM ventilation is 1800CFH, so you are exchanging the air in there every 1.6h. That is way over ventilating such a small space with no people in it. No matter the efficiency of the ERV, the interior dewpoint will always approach outside air dewpoint. If you are cooling the place, this means higher RH than outside.
First step would be to reduce the speed of the ERV. I would change the jumper on it to the lower speed or at least don't run at on boost all the time. You can crank up the speed once people are there.
The oversized hvac ship has sailed, so not much that can be done on that front. Your best bet is to operate it at the lowest fan speed.
If reducing the ventilation doesn't get your humidity under control, consider keeping the place warmer when not occupied.
If all that doesn't work, a dehumidifier will definitely fix your humidity issues.
P.S. Try not to start two threads for the same topic.
Thanks for your time, all your advice has been applied accordingly! I also edited that other post to link back to here since I couldnt delete it.
I should add, my understanding was that MSZ-FS09NA /MUZ-FS09NA modulated down to 1600 btu in cooling mode, which originally alleviated my oversizing concerns. However now I am concerned there's reduced moisture removal capacity in the hyper heat models. I also noticed a significantly lower moisture removal rate for the MSZ-FS06NA
Separately, since San Diego has quite temperate weather, my shoulder seasons seem to necessitate supplementary dehumidification since I cannot depend on a cooling load.