High Humidity with A/C

[kasterko]

High humidity in new 2600 sq. ft. 2-story home on a crawlspace (6 months old) with A/C running normally (65-70% Rh as indicated on t-stat) also feels very muggy and supposedly T-stat humidity reading was compared to 2 other humidity meters and compared. So we’ll assume the humidity reading is correct. The Heat Pump condenser unit is a Carrier model 25HPA324300, serial 2206E22576. From what I can find out this is a 13 SEER, 2006 model and I think the 24 may indicate 2 ton (puron). The air handler is also a carrier model 58MXB080-F-10116, serial 3405A09316, with propane backup located in the garage (ceiling mounted). These units control 4 zones from one t-stat. The clients had been complaining to the builder about the humidity and the builders HVAC person, who has been to the home a dozen times, downsized the condenser. Makes sense ? Anyway the home is still humid and the HVAC person installs a squirrel cage fan in the crawlspace access door to circulate air in the crawl; so the fan wasn’t really pulling air through the crawl. The home is still humid. While I inspected the home the A/C was running and not much condensate came out of the line. I checked the attic, ran a lot of water, crawled the entire crawlspace and nothing was wrong. The attic was as dry as a bone, nothing leaked, and the crawlspace was clean as a pin, covered completely with plastic. It was damp under the plastic but the insulation wasn’t hanging down and it was all in place. There were no signs of water infiltration around the foundation.

So what things could cause the home to be so humid? Is the sizing different for the newer puron systems? Are the newer systems complicated and not well understood by your basic HVAC folks. I told these clients that I was at a loss to explain this and recommended that they request a Carrier regional technical specialist to come on site to analyze and resolve this issue. But I’m curious and I wanted to get your input. HVAC isn’t my strong area but I thought A/C removed moisture from the air and if the unit was too large and it cooled too quickly it didn’t dehumidify properly. That’s probably what the HVAC guy thought. If the 24 in the model number is for 24,000 BTU’s, is that enough to cool a 2600 sq. ft. home? Any thoughts?

Thanks in advance for any help,

Jim, in Calvert County, MD

[Terence McCann]

Rough rule of thumb is 600 square feet for a ton of cooling (400 per ton in commercial bldgs). A Carrier model 25HPA324300 (*might be a 2.5 ton unit). A flat 2 ton unit seems small for this home. Having said that heat load calcs should be done to be 100% sure.

If the humidity was high then temperature should have been as well if the unit was way undersized. If the unit is oversized you get a problem with humidity being to high but temperatures are OK (cold and clammy).

Downsizing the condenser is OK to have the unit run longer thereby removing more humidity but again, it has to be big enough to keep the space cool as well.

Lack of refrigerant could cause this problem but then you would see ice and the humidity and temperature would be high. Was all the supply ductwork blowing 5x degree supply air and did it have enough return air?

Punt and have another HVAC company check all calculations.

[Bill Kibbel]

In the mid-Atlantic, and the house is reasonably well built, it should probably be a 3.5 ton and the fan speed is probably too high.

Disclaimer: The above is not based on any intelligent science or calculations, just my own experiences. (But I'd bet I'm on the right track.)

[JEuriech]

Jim

I agree with you on size and SEER rating. It is a 13 SEER, two ton unit. See Product Data/Model Number Nomenclature on page 2

Click here: http://www.xpedio.carrier.com/idc/group ... a3-1pd.pdf

My first guess would be that the unit is too small for the application. We tend to use 1 ton of air conditioning for every 500 square feet of space out here. The only way to know for sure is to do a heat load calculation.

A heat pump that is too small will be unable to remove all of the heat and humidity. A heat pump which is too large will "short cycle." It will not run long enough to adequately remove excess humidity. Heat pumps should be sized for the exact heat gain or slightly larger. Properly sized equipment runs at peak efficiency.

My second guess would be high air flow or fan speed set too high. The slower the air speed the more condensation you will get. It could also be that the blower runs too long after the unit shuts down.

Third guess would be low refrigerant charge or refrigerant flow-related problem. (restriction/bad metering device)

I am assuming that all other obvious moisture generating items have been checked and are not an issue. Example: Dryer exhaust vents to the outside, exhaust fan over the range, exhaust fans in the bathrooms, etc.

Jeff Euriech

Peoria, Arizona

[Jim Katen]

Originally posted by Terence McCann

Rough rule of thumb is 600 square feet for a ton of cooling (400 per ton in commercial bldgs). A Carrier model 25HPA324300 (*might be a 2.5 ton unit). A flat 2 ton unit seems small for this home. Having said that heat load calcs should be done to be 100% sure.

If the humidity was high then temperature should have been as well if the unit was way undersized. If the unit is oversized you get a problem with humidity being to high but temperatures are OK (cold and clammy).

Downsizing the condenser is OK to have the unit run longer thereby removing more humidity but again, it has to be big enough to keep the space cool as well.

Lack of refrigerant could cause this problem but then you would see ice and the humidity and temperature would be high. Was all the supply ductwork blowing 5x degree supply air and did it have enough return air?

Punt and have another HVAC company check all calculations.

Terry, the first thing to strike me as odd about this problem is that it's a heat pump with 4 zones. In my area this sort of system is unusual and would have a bypass duct. If the bypass duct were screwed up - for instance if the damper were always in the open position - couldn't that explain what's happening? A small amount of air would be short circuiting through the indoor coil becoming very dry, thus yielding very little condensate. Enough of this very cold, dry air would make it to the registers to satisfy the stat but not to dehumidify the house.

What do you think?

- Jim Katen, Oregon

[Terence McCann]

That's a possibility however, a true zoned system usually has t-stats to control the dampers to their zone (and are allowed to initiate a call for clg & htg). As these zones cut back the bypass damper opens up due to a signal from static pressure control. Then, as the zones cut back, the Delta T across the evaporator really increases so they have a low-limit t-stat to shut off the unit to prevent liquid slugging. Perhaps Jim could chime back in with a little better explanation of the zoned system and if there is a bypass damper or not. I've seen some zoned systems that do not have a bypass damper as it follows the fan curve to achieve the same thing however, this was in commercial office bldgs. It would also help to know the return air vs the supply air temperatures. A cheapo sling psychrometer would be beneficial as well to measure RH in numerous areas of the home as well as the crawl space. Other question would be, is the unit bringing in any non-conditioned outside air, what was the outdoor ambient temperature and humidity at the time?

Obviously these are outside the scope but it helps with understanding the systems better.

[kasterko]

Now I'll really have to show my ignorance. I don't think it had a bypass duct. It had a humidifier(it wasn't on) in the only return I saw and I don't recall seeing anything that looked like what might be a bypass duct. I saw all 4 zone dampers on the supplies, nothing on returns (that I saw). I will get a humidity tester for future problems like this. The zones didn't operate properly. For example when only zone 1 should have been cooling, zone 4 was also cooling. I'm not sure why this house was zoned anyway, especially 4 zones, 2 on each floor of a 2 story 2600 sq. ft. house. I did recommend the owners get the Carrier regional technical guru out. I really appreciate all the help you guys provided. In addition to helping the clients, I sure learned alot.

Thanks,

Jim

[Terence McCann]

Hi Jim:

Google Carrier VVT or Trane VariTrac for more info.

Locate your local Trane or Carrier dealer and request some brochures on the various systems.