Dehumidifying fresh air ventilation/ make up air
Oliverancher
| Posted in GBA Pro Help on
We are currently constructing a home with insulated concrete panels. Once completed and sealed, we anticipate it being a very tight house. We have a fresh air intake duct (hole) in the wall opposite our very large vent hood.
Question: What is the best way to dehumidify outside air being pulled through the damper?
Additional information:
• We do not have a central/ducted A/C system.
• We live in SE Texas- high humidity.
• Energy efficiency is desirable as we are powered by off-grid solar.
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Replies
A dehumidifier.
You can get specially made dehumidifers for this purpose that do almost all latent heat removal (which is basically dehumidification). They aren't cheap though. You need a good air filter ahead of these units to keep crud from outdoors from clogging up the coils. You will use lots of energy running this type of unit.
You could use an ERV instead of an HRV, which isn't going cut down on humidity intake as much as a dehumidifier will, but it's better than just straight air exchange. There is an article about the difference between ERVs and HRVs on Energy Vanguard here: https://www.energyvanguard.com/blog/why-you-probably-need-an-erv-not-an-hrv/
An ERV is going to use less energy than a dehunidifier.
Bill
The Building Equinox CERV2 has several dehumidification / makeup air use cases on their web site.
Will you have any fuel burning appliances? If so, will they be sealed combustion? Certainly, a huge range hood in a tight home with an atmospherically vented water heater would be a very bad idea without makeup air.
Very few people actually bother to turn on their large vent hoods.
What I am saying don’t spend a ton of money before you have tried living with out the vent. If you come to the decision that life would somehow be better with the vent, at least try it without dehumidification.
Understand a standard dehumidifier is a super-efficient heater with a COP above 1. When it removes water from the air it releases 100% of the BTUs that were used boiled the water into a vapor plus 3412 BTUs for every kWh the equipment draws.
I think you want a small air conditioner that could dump the BTUs outdoors. I don’t think it is an off the shelf item. Getting it sized right and the controls to work as a DIY project sounds hard.
I have to ask what is the off-grid AC plan in your climate. Seems like this dehumidifier is a rounding error compared with your AC requirements.
Walta
I would agree that there's no way to know before-hand if extra dehumidification is even needed, or if the AC will handle it. And if you don't have combustion appliances I wouldn't assume makeup air is needed.
Yes, I have a large blue star propane range that requires that size vent hood. I am positive that I will use it often - I cook and bake a LOT. Our water heaters are tankless and mounted on the outside of the structure.
We also have a gas/propane clothes dryer.
We will have a wood burning fireplace with a propane line and it has its own dedicated fresh air intake (required for installation).
We plan to install mini- splits (110v) as needed but ductless don't dry the are ase much as a ducted system ( in my experience). So we feel we will probably need some dehumidification to make the air more comfortable. We find in our current house in Houston that 78° @ 50-55% humidity is comfortable for us.
As of today, we have all the wall and roof panels set, but not grouted (sealed) and no windows and doors installed. The temperature was 90° 62% RH and it was 10-12° cooler under roof. So we have high hopes for the insulating qualities of the 7" concrete walls.
As for the solar, we have a robust system with 40kW battery storage. We have had the system for 4 years and have run in- efficient air conditioners all summer. We are realistic - installing an on demand back up generator to charge batteries during low sun or high demand. As you alluded, our climate gets a lot of sun!
We are fairly confident in our power generation capabilities, but can add additional solar if needed.
I am a bit confused about ERV's. I have read many articles - they all reference units that are integrated into central A/C systems.
The appliances on your list do not seem likely to back draft and would not scare me.
Yes, you will have the oversized range and you will use it a lot by a lot you mean 4 hours of burner run time a day? My gut says you are not using it like a commercial kitchen with 6 burners going 12 hours a day.
My in-laws liked to bake in AZ without AC they had an oven outdoors for use in the summer.
“We plan to install mini- splits (110v)” are you talking about portable AC units with hoses to the outdoors? Most of the 120-volt equipment I have seen is built to the lowest price point at the cost of being less energy efficient.
Yes, thermal mass can work in climates where there is a big swing between the high and low temp each day and you are present the open and close the windows to cool the house each night. In general, that is a dry desert climate because you don’t see the bid changes with that much moisture around.
How large is this house have you done a manual J for it at your projected set points?
Ideally all ERV would have dedicated ductwork but that costs more so they get connected the AC ducts.
Again, dehumidifiers are heaters that mostly lower the relative humidity by warming the air. Even without removing any moisture warming air lowers its relative humidity.
Walta