Higher Humidity upstairs after Air Sealing Attic ducts and floor
I live in Zone 4A – 2 story home built circa 2005. Air handler and HVAC ducts are in the attic. The upstairs HVAC equipment was replaced in 2015 with a single stage heat pump + VS air handler. (TEM6 A/H)
Recently had some insulation work performed in the attic:
– Removed all the old R-30 fiberglass
– Air sealed the attic floor with foam (fixtures, etc)
– Air sealed the HVAC ductwork
– Used some mastic paint near the air handler with new tape to seal it up. I also noticed some spots near the air handler had mold on the insulation blanket, so replaced them as well.
– I also took the liberty of sealing all the areas on the air handler (according to what was posted on https://www.nachi.org/inspecting-hvac-cabinet-seams-air-leakage-sealing.htm).
-I took off the cover for the air handler and cleaned the coils and checked for mold, etc. I did notice the dipswitch settings for the air handler (which I’ll get into later)
The GOOD news – According to my TStat, my runtimes for the HVAC are down about 10 runtime hours, whereas the last 4 years of data during the hot months, it would be over 15 and nearly to 20. Actually, I’m impressed that it’s run times are shorter and it’s hotter outside than it has been for the last 2 years.
I was told by the insulation contractor that I might have higher indoor humidity levels upstairs now and I should discuss this with my HVAC company (as well as having a static pressure test performed as I changed the mechanics of the air ducts by sealing them to make sure I don’t ruin equipment or have higher electricity bills)
Sure enough, the upstairs humidity is near 60% all the time now. I verified this with 2 different probes. I checked some data loggers for humidity and the last 3 years it was around 49% in July/August. It remains consistently high (the downstairs is about 41% as of this writing)
After doing some equipment checks, I realized that – of course – the builders of the home oversized the outdoor unit, and oversized the air handler. The outdoor, upstairs unit is 2.5 ton heat pump and the air handler is a 3 ton. I checked the dip switch settings against the published documentation: It’s configured for 3 ton (367cfm/ton 1100cfm/total and 343W of power). But it can be configured for lesser tonnage via dipswitch settings – ie: SW1 sets it to 2.5 ton, normal, 340cfm/ton 863/cfm/total 211W of power) which would give better dehumidification behavior due to the lower air speed but I’m not sure this is the way to go, because I had a manual J/S performed after the air sealing service, and it recommends a 1.5 ton Heat pump + Air Handler with the new insulation work and others for the upstairs, and a 2.5 ton matching for the downstairs.
Would lowering the air handler speed be better than dropping thousands on a whole house dehumidification product?
FWIW – the installer of the upstairs HVAC equipment told me yesterday that an oversized air handler is normal (huh) to get the SEER rating.
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Replies
Sorry I don’t have anything to add about dehumidification, but we are building in a home in Zone 4, 2 stories, 2,800 sq ft. Our Manual J says 1.5 ton per floor, so your calculation of 1.5 is more in line. Unless it’s cost prohibitive, I would downsize the equipment before getting dehumidifier. Can you utilize the same ducts?
Not sure, working with an engineer on the manual D part right now
Right now the cost of HVAC equipment around here is 40% more. I was quoted a 1.5 ton unit (Single Stage), Seer15/HSPF 9.5 with a VSAH and it was just shy of $9k.
I would add a separate dehumidifier. Either get fancy and have your HVAC company line in a santa fe dehumidifier that will empty itself, filter the air, and last for years, or start out with a portable dehumidifier. Now that your AC runs less frequently it is no longer running often enough to manage the humidity.
I live in what was zone 4a though recent changes mark it as zone 3a now. With home humidity in the 60s in the fall and spring with little AC use I picked up a midea 50 pint cube dehumidifier that can either hold 4 gallons of water or pump the water down a drain and have been happy with the results with it placed near one of the two air returns in my two story 2100 square foot house. For now I empty it every couple days and use it to water plants outside. Recently I set up a santa fe advance 90 in my crawl space as part of an encapsulation project and brought humidity beneath the house from the mid 70s/80s down to the 50s. At some point I may have it connected to my air handler but for now I appreciate the simplicity of two separate dehumidifiers and no additional ductwork. Adding the sante fe to the crawl space did reduce how often I need to empty the midea.
10h runtime per day probably puts you at even less than 1.5 ton, 1.5 ton is not far off though.
There is nothing wrong with reducing the blower speed in cooling to increase the SHR of the unit. You do have to watch the temperature after the coil to avoid freezing it up. The CFM/ton is calculated based on the outdoor unit not the indoor unit, so use 2.5. Getting down to 340 cfm/ton is a bit low but doable if you watch the discharge temperature.
For heating you want to run higher cfm/ton, so only adjust the cooling settings only.
Adjusted the CFM to high speed, 2.5 ton (995cfm) and enabled Comfort-R which ramps up at 80% for a few minutes, then 100% as needed. Will see if that makes a difference.
Something about the question sounds wrong to my ear.
If seems unlikely that the humidity upstairs could be much different than down if the temp is about the same.
Of Corse the classic symptom for an oversized system is a cold and clammy house.
Installing a larger indoor coil is a very old school attempt to increase system efficacy and it does would in a way. More sensible cooling and less laten cooling.
There are limits to how much you can reduce the air flow when the coil cools the air more than 25° parts of the coil begin falling below freezing and ice begins forming. The more ice the less air flow and death spiral begins. When too little air is flowing the liquid refringent fails to boil into a gas, should the liquid reach the compressor it could damage the compressor.
Some thermostat have programs for dehumidification. They over cool the house by a few degrees the longer run times and lower temps remove more moisture the unit does not start again until the temp is at the set point. They are somewhat affective.
If you want to play the dehumidifier game start with one of the disposable units from a box store. Don’t expect the box store units to last more than a few years then you can decide if you want to spend the money for a quality unit. Just understand the dehumidifier removes moisture IE the latent heat and converts it to sensible heat the oversized unit can easily deal with.
Walta