Help - new AC causes low humidity?

[SQLit]

You have not spent much time in the SW deserts. Check Phoenix, Tucson, Las Cruces NM or El Paso TX

[Gary R. Lloyd]

Assuming a room temp of 75F, your supply air temp leaving the evap coil would have to be 26F or less in order to give you 20% RH. I don't think so. Much more likely you need better instruments to measure your RH.

Gary

[Gary R. Lloyd]

Let me clarify this, so there is no misunderstanding:

It didn't happen. No way in hell. The humidity indicator is WRONG.

Gary

[nicksanspam]

Agreed, but why 26 F? The dew point of 75 F (535 R) air at 20% RH is about 535/(1-535ln(0.2)/9621) = 491 R or 31 F.

Maybe the coil is under freezestat control.

Nick

[Gary R. Lloyd]

Yep, I stand corrected. I misread my psych chart. Although the SA temp would need to be somewhat below dewpoint to balance against latent load and/or bypass, so it probably isn't that far off.

Is it technically possible? Yep. With an A/C unit? Very highly unlikely. The airflow would need to be very low, in which case it wouldn't handle any appreciable heat load.

Realistically, it just didn't happen.

Gary

[gerry]

Indirect RH measurement may well be the culprit. As you (Gary) said, it didn't happen. What was not mentioned is what was used to measure the RH!

Calibration of indirect measurement hygrometers is not easy to maintain and some are just junk near extremes or with age. Was a good old wet bulb used? They are not too vulnerable to aging, though salt build up on the wick can induce some error.

Even lab quality dew point hygrometers need maintenance to be accurate. One of the classic laboratory methods is optical detection of fog on a chilled mirror. Fail to keep the mirror very clean mucks them up.

I suspect a piece of bad equipment was used to measure it.

gerry

[Tekkie]

Gary R. Lloyd posted for all of us....

Hey Gary, welcome back!! Missed you.

[nicksanspam]

Water freezes at 32. An interesting coincidence.

After some sort of control failure...

Maybe an iced up coil on a mild day...

Nick

[Gary R. Lloyd]

Oops. Make that 31F.

Thanks, Tekkie.

Every once in a while I drop by and skim through a few thousand new posts as time permits. And of course, the one I choose to respond to I misread my psych chart and get the dewpoint wrong... LOL

Gary