Passive house – dehumidification
I bought a ~10 year old passive house in Maine this spring.
It’s become clear that dehumidification is needed.
I’ve tried a portable unit, but draining it is inconvenient (no basement). More importantly, the portable dehu is adding significant heat. The house has no mechanical cooling and is already uncomfortably hot (~77 degrees) without the heat the dehu is adding. Small bathroom I was initially draining the dehu in got up to 91 degrees very quickly.
I’m trying to figure out if adding a couple mini-splits will be enough to solve the humidity problem while also addressing the overheating problem.
Or if it’s possible I’ll add mini-splits and find that I still need a whole house dehumidifier.
I’ve gotten a quote for a Santa Fe, but would like to spend money wisely to tackle the humidity — if the mini splits will do it and I’ll want them anyway, that seems the most straightforward route.
For context, the ERV is a Zehnder ComfoAir200.
I’m new to passive houses and by no means a building scientist, so plan to speak with a passive house consultant but thought I’d check here as well.
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Replies
I do not think you want /need a dehumidifier you want air conditioning. As you found out dehumidifiers are super-efficient heating units.
Consider staring with a window unit or 2 that will let you figure out how much cooling you really need likely not much and if cooling one room will make other rooms comfortable enough.
When it is hotter and more humid outdoors than indoors consider turning off the ERV.
Please tell us about your heating system.
Does the house have forced air heating? Probably not but I have to ask.
What fuel do you use for heating? Propane boiler heating the floors?
When you go shopping for an central air, consider a heat pump ducts in every room would be best but maybe almost impossible. Maybe you could give up a closet and drop the ceiling in the hallway for the ductwork. Avoid putting the system in the attic.
If you go mini split shopping avoid multi split that will have more than one indoor unit connected to one outdoor unit. Avoid oversizing the system with heads in every room. Note oversized systems will not lower the humidity because they will have very short run times forcing you to lower the set point and the house will be cold and clammy.
Walta
I've written quite a bit here about dehumidification lately, for example this piece:
https://www.greenbuildingadvisor.com/question/why-sizing-dehumidifiers-is-hard
The short answer is there is no short answer. Trial and error seems to be the only method. It sounds like you're going to need cooling, the question is whether that's sufficient by itself.
You can look at the ASHRAE site I link to in that posting to get an idea of what extremes in humidity are like where you are.
Appreciate both of your replies.
It sounds like experimenting with a portable AC is the first step to understanding my cooling needs and whether a bit of cooling brings down the humidity. Makes sense, and I'll try that. (It'll need to be a portable AC and not window unit because the windows are all european tilt/turn)
Walta, the heating is via electric baseboards powered by solar. So ducting for the ERV is the only ducting in the house.
Also, there is no attic to speak of, the upper floor is a "hot roof."
I'm in hot humid Dallas and we're currently experiencing an extremely humid summer.
Then I have two ervs bringing in gallons of moisture daily.
I have two portable dehus but rarely use them. Setting the mini split to cool mode and the indoor humidity can get to 66%
But when it's set to dry mode it is able to keep indoor humidity around 55%
With the heat in your passive house a small mini split set to dry mode will do wonders for your humidity and comfort.
When you go shopping for portable ACs only consider the units that require two hoses one brings in air from outdoors and the other expels the now heated outdoor air. The single hose machines are likely to actually warm your home as they are expelling conditioned air from the home that gets replace with unconditioned air.
Consider that 12kBTU portable will provide considerably less cooling than the same 12K window or mini split because the compressor is located inside the conditioned spaces and its heat is escaping into the conditioned space.
If you install a heat pump or mini splits that would cut your heating costs by about 75% and might actually pay for itself while increasing your comfort level.
Walta