Ventilating dehumidifier vs ERV + dehumidifier for hot humid climate
3700 sf house in Houston, getting completly gutted and renovated. Goal is to be as air tight as possible when done. The entire house will be conditioned space, unvented roof design.
Using Mitsubishi mini splits for cooling and heating.
Since I plan on making the envelope very tight, I was looking into ERVs for necessary ventilation. Being in Houston, I expect to also need a whole house dehumidifier, at least during spring and fall when the AC isn’t running 24/7. So then my research brought me to the Santa Fe Ultra Series vented whole house dehumidifiers. Now I’m stuck with two options and can’t decide.
Operational wise, would it be cheaper to run such a combined unit vs running an ERV + separate dehumidifier in my climate zone? I expect the combined unit would probably be somewhat cheaper for install, but I’m more concerned about my long term energy cost.
Does the ventilating dehumidifier still ventilate in times when dehumidification is not needed?
I don’t have existing ductwork, since planing on non ducted mini splits (might do ducted mini splits upstairs, haven’t decided yet)
If doing ERV and separate dehumidifier, would/could the two share the same supply and return ducts? Or better to run two lines parallel and keep everything as a separate system? I plan on putting the units in my tiny attic space and I’m somewhat limited in space for ducts going from upstairs to downstairs since I have no crawl spaces. So the less ducts the better, but not if it would affect performance.
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My opinion is ERV/HRVs is a silly idea in your warm moist climate.
Simply put most of the year the difference between the indoor temp and the outdoor temp is so small the there is almost no energy to transfer. The other thing that works against them in your climate is that the outdoor humidity is almost always higher than the desired indoor humidity so operating the ventilator makes the home more uncomfortable. If your home feels stuffy crack open a window.
Yes, some will try to convince everyone that every building must have a ERV or terrible things will happen, I say BS! Some people do need them when they live in a cold climate in a small tight home with lots of occupants. Without one the high indoor winter humidity would make mold grow.
I do think a dehumidifier would be a good idea in your climate. Because many days the outdoor temp if about the same as the indoor temp but the high humidity makes the home uncomfortable.
Walta
Yes I will get a whole house dehumidifier either way.
I guess what im trying to figure out is, when pulling outside air through the dehumidifier, the unit obviously needs to work harder and use more energy then if it were to only dehumidify return air from inside the building.
At the same time, with option B, I would still need to run a dehumidifier, plus the ERV in addition to that. But the dehumidifier would run less as some of the outside air already gets dehumidified by the ERV as it’s coming in. Is that assumption correct?
There may not even be a way to measure what option would be cheaper to operate. I’d just hate to make the wrong choice and then regretting it every month when my electric bill comes in.
I’m from Katy. A ventilating dehumidifier will lower your ability to dehumidify the house air because all of the fresh humid air you just brought in from the outside will need dehumidification as well. A non-ventilating unit would be better.
An ERV will bring in fresh air, but temper the humidity by mixing with the indoor exhaust air. So you won’t have as big of a latent heat load penalty as a ventilating dehumidifier. If you really plan on getting a good (<1 ACH50) blower door test I would say an ERV is a good idea and I would also make it double as my bath fan exhaust so I wouldn’t have to worry about make up air for those. Your kitchen vent hood should have a dedicated make up air system still though since the ERV won’t be anywhere near as powerful as it.
I would urge you to consider a fully ducted HVAC system for both floors. What is your plan for filtration if only using mini split heads? If your dehumidifier unit is strong enough you can use the same ductwork as the indoor air handler. Just be sure to pull from the return duct before the air handler and push out to the supply ductwork after the air handler.
Corbett Lunsford just came out with a dehumidifier video this week that was very informative on his YouTube channel. He is based in Atlanta so he understands our humidity problems.
You bring up some interesting points.
As far as central AC goes, it’s just not gonna happen. I dormered out the entire upstairs, so I really have no crawl spaces left to work with. I can probably put a small ducted mini split, ERV and dehumidifier in the attic and that’s pushing it. We’re talking a tiny space you need to crawl in. There’s just no space for plenum, air handler and ducts off this size. I can be creative and make something work for small 6” ducts for dehumidifier/ERV but no big AC ducts. Plus, for years we were uncomfortable with hot upstairs and cold downstairs so, I’d have to do two units. Not gonna happen.
I know it’s somewhat unconventional for this area, but I had 3 HVAC companies take a look and after initially pushing for central air, they all agreed that mini splits were the only option, after actually looking at the house.
As far as using the ERV for bathroom venting, that’s something I originally considered, but then read here on GBA that that’s not a good idea as the cfm you’re getting from the ERV are not sufficient for bathroom ventilation. I guess I’ll have to do more research on that. We have 3 kids who take a lot of showers and baths and I hate walking into a steamy bathroom, so proper ventilation here is important.
As far as filtration goes, the Santa Fe Ultra dehumidifier I was looking at, actually has a Merv1 13 filter, which seems to be adequate for filtration?
So really, there seems to be no way around of sucking in humid air. Unless I just don’t ventilate at all. Kids go in and out so often throughout the day, sometimes leaving the back door wide open. Might be enough …..ha
I would find a spot with a drain for a 70-pint dehumidifier from the box store knowing it will fail in a year or 3. Set it to 50% and let it run whenever the windows are closed. I think you will spend less money than the expensive ducted ones.
Understand a dehumidifier is a super-efficient heater that removes moisture IE latent heat and converts it too sensible heat that your AC system can easily remove.
Bringing in the moist air from outdoors is counterproductive. My guess is even after the outdoor air goes thru a dehumidifier it will still have more moisture than the air in the house currently does.
Seems to me it is silly to condition the empty attic. Conditioning the attic all but forces you to use expensive spray foam insulation. Spray foam is very ungreen putting a ton of nasty chemicals into your home that do not always cure as intended. For 1/3 the cost you could get 2 maybe 3 times the R value of 100% post-consumer recycled cellulose. If you must condition the attic full it with leaky duct work to keep the attic cool and dry. If the attic is not well connected to the rest of the home it could turn into a moldy mess.
Note You said you were getting mini splits be sure that you get mini splits and avoid the multi splits that have more than one head connected to the same outdoor unit. Also avoid over sizing your systems as the oversized units will not run long enough to remove any moisture. This forces you to set the thermostat to a lower setting to feel somewhat comfortable. The classic complaint is the house is cold and clammy. Try to make room for some duct work with a concealed miny ducted head for the bedrooms.
Walta