Issues with Dehumidifier + Fresh Air
codyhazelwood
| Posted in Mechanicals on
Hello, We recently moved in to our new (soon to be certified) ZERH home in Zone 4A and we’re having trouble with our HVACD.
Some background: it’s a mostly DIY build, 1600 sq ft, R26 SIP walls, R40 polyiso over heavy timber great room, R40ish cellulose over bedrooms, with a conditioned crawlspace. If I did all of the air sealing right, then the goal is to be about 1 ACH 50.
The heating and cooling is a 1.5 ton dual-stage split system. The dehumidifier is an Aprilaire 1870 with fresh air intake.
The heating and cooling seems to work fine. It keeps up well and doesn’t run very often.
The dehumidifer is not working well. The HVAC crew attached a fresh air intake with a motorized damper, joined to a crawlspace air intake feeding the return of the dehumidifer. The damper is opened on a timer to meet ASHRAE requirements for fresh air.
They have tried several configurations for the output of the dehumidifier:
1. 100% of the output feeding into the heating and air return (unit controls were separated).
This option did not work well. The hot fresh air would blow out of the return in the living room and quickly heat up the loft.
2. 50% of the output feeding into the heating and air return and 50% of the output feeding into the crawl space (unit controls were separated).
This is the option we have now and is the least terrible, but the hot fresh air still heats up the loft, and it doesn’t dehumidify the living area very well anymore (running about 56% humidity right now – I’ve seen it get as high as 70%).
3. 100% of the output feeding into the heating and air return, but with the controls interconnected (so the dehumidifer would call for the heating and cooling blower to run and would open zone dampers).
This option was by far the worst. The air ran all day long, usually ran about 5 degrees above where we set the temp, and more than doubled the electric bill. They said it probably didn’t work well because of the programming of the dual stage unit.
The heating and air company seems to be out of ideas. I suggested returning the output of the dehumidifer to the regular living area vents to spread the fresh air around the house, but they objected because they were concerned about the pressure build up with both systems running at once.
Is there something I can try (or have them try) that would work better? Or should I save up a bit and remove the fresh air and replace it with an ERV? I originally asked for an ERV, but they strongly recommended the dehumidifier + fresh air over the ERV.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I'd need to know more about your climate, but I'm thinking a dehumidifier isn't the right tool for the job. A dehumidifier is only appropriate when you don't need cooling (or even need heating). If you need cooling a dehumidifier puts more of a load on the AC than just bringing in the outside air untreated, and letting the AC dehumidify it. The dehumidifier dumps the heat removed from the air back into the building whereas the AC dumps it outside.
Is there any particular reason why you decided to go with a dehumidifier in the first place if all you wanted was a way to get fresh air intake? During the summer your AC should dehumidify to an extent and during the winter the furnace will remove humidity as well. Perhaps it’s very humid where you are though.
Also, why did they take all their return air from a crawl space? The crawls pace is probably going to be pretty damp and full of dirty air so the dehumidifier is going to be trying to dehumidify and dilute that air. So your starting off your process with air that you don’t even really want in your house.
I have only worked in HVAC for a year but I have never seen a dehumidifier that takes in fresh air as well. It’s kinda odd because usually during the summer the outdoor air is very humid, as a matter of fact usually (unless it’s within the first year or so of new construction) you turn an Air Exchanger off during the summer so it’s not sucking in hot, humid air which will make your dehumidifiers and AC units work harder.
Now I’m not saying to give up on a whole-house dehumidifier, they’re great! If you have a lot of trouble with humidity they can work wonders. But something does seem odd. Have you worked with this company before?
We're in Middle TN, so it's fairly humid in the summer - but not to FL or TX levels.
The crawl space isn't damp and dirty in this case. We have a concrete floor down there and there are vents off the main duct conditioning it. I think their goal was to dehumidify both the crawl space and the living area. My only concern with the air quality of the crawl space would be radon, since it can be much higher in the crawl space (or basement) and this would be sucking that air up and pushing it into the living space. I'm not too concerned with it though since we installed a passive radon vent from the start.
I haven't worked with this company before, but they were highly recommended by my HERS rater (as well as using the dehumidifier + fresh air approach instead of an ERV). The AprilAire and UltraAire dehums that we looked at both had controls & timers for a fresh air damper.
I agree - something is odd, but I don't have enough knowledge about HVACD to know what it is. I think this being such a tight (and somewhat small) house makes any problems with it more visible.
An ERV will be more energy efficient than using outside air directly - you should have one.
In most climates, a dehumidifier IS needed for periods when cooling load is low. Distribute the dehumidified air throughout the house, but to the extent this isn't possible, to central areas. Preferably with dedicated ducts/registers that will have the right balance and throw. If this isn't enough, then cycle the furnace fan just enough to create mixing throughout the house.
> they were concerned about the pressure build up
Where? A diagram of the entire system would help.
I've attached the duct drawing for the HVAC and then attempted to sketch the dehumidifier + fresh air setup as it stands now.
Thanks for the advice. I'll start researching ERVs some more.
Ideal would be to supply fresh air with an ERV, and then have the dehumidifier available for moderate-temperature high-humidity days when the A/C isn't doing enough dehumidification. Then the supply air from the ERV is cool, and you can distribute it without overheating anywhere. Since the dehumidication becomes more of a supplemental thing, and isn't on super-hot days, you don't need to worry as much about how it's distributed.
I'm looking into ERVs and reading through the articles on here about proper install. It seems like dedicated ducts will be easy enough to do in our situation since we have easy crawlspace access. Are there any ERVs that are more DIY friendly than the others? I think we need about 40 CFM based on ASHRAE 62.2.
Any suggestions on what we could do better for now until we can get an ERV?
I haven't used it myself but the Zehnder duct system seems highly regarded for ease of installation and good performance.
Independent of ASHRAE 62.2 (a body odor standard), I'd deliver 15 CFM/person to bedrooms and closed door offices (for healthy sleep and clear thought).
Most ERV boxes themselves are pretty straightforward, its the ductwork install that will take a significant effort in a finished house.
Some of the newer ones offer fans that will try and achieve the target CFM which simplifies balancing (Panasonic intellibalance, Vanee AI Series). Although I think a measurement is always a good idea. Don't confuse flow balancing and pressure balancing though, you sometimes have to run unbalanced flows to achieve the balanced space pressure you're trying to achieve.