Timer controlled thermostat

[Jim]

But I don't mean your run of the mill programmable type.

Presently we are having very warm and humid days. The central air conditioning handles this properly. Then late at night as the outside temperature drops the system doesn't call for cooling as often. Once again this is normal operation.

But during this time the humidity indoors rises. Very good chance the interior temperature will be stable enough not to call again for air conditioning for the remainder of the night. Net result being that sticky feeling.

Wondering if it is possible to find a thermostat, or other computer controlled device perhaps, that will instruct the air conditioner to cycle say once every 15 minutes for a 1 minute duration. User adjustable, to assist in controlling the relative humidity without running for excessive amounts of time.

Anything like this exist?

[RayV]

While Y'all are pondering Jim's question add this:

A programmable thermostat that allows a timer or cycle setting for the circulation fan. I have a two story center hall colonial and of course heat rises so upstairs is always warmer.

[JoeSpareBedroom]

Running for such a short time wouldn't give the system a chance to remove much humidity. It would be like having an AC unit too big for the house.

Is your house "leaky"? My system behaves like yours, but I don't notice humidity building up. The lack of air movement is bothersome, though, after the AC stops running at night. So, I have a quiet floor fan in the bedroom, running on its lowest setting, pointed straight at the ceiling. The brand is Vornado. Keeps the room very comfortable.

[Art]

Short cycling a ac wears it out. Bad idea.

[Jim]

For the sake of clarification define short cycling? Only very hot days it is not uncommon for my scroll compressor to run for say 90 minutes, shut down, then come back on 10-15 minutes later.

[Knute]

you mention (not quoted above) every 15 min for 1 min which others have already pointed out is a bad idea. however let me add that as an idea it is not bad at all with proper timelimits.

I've seen several installations in s.florida where the thermostat maintained a temp setpoint (as you mentioned) but humidity behaved erratically and it was solved with a thermostat calling separately for low speed (variable speed blowers) and stage1 (multi-stage system) for a given duration in programmable timelimits

one such example was a dual compressor (2 ton & 4 ton) system where the VS blower at very low speeds initiated the 2 ton compressor when humidity setpoint (54%) was reached and it ran the system for 30 minutes and then shut down. this was accomplished with a 24vac macromatic relay (timer On/Off) tied into the stage1 (Y1) circuit from the thermostat RH% sensor (DH). every time the DH 24vac relay opened, it sent a signal to the macromatic On and this in turn ran to Y1 which activated the 2 ton compressor, after 30 min (adjustable dial on the macromatic) the relay shut down.

there are other types of macromatic relays, my personal favorites are the interval On, interval Off where you can program the time duration on and time duration off and the cycle repeats. thus you could have indoor temp at

77F while maintaining a precise 50% humidity and if the time interval results in too little or too much cooling (likely not a problem with variable speed blower and small stage1 compressor), it's simple and easy to adjust

hope that helps

[mm]

Please put what you mean in the text of the post. It's annoying to me to have to guess what you are talking about, and then to have to hunt for clues.

My words are only theory, not based on actual AC experience, or even AC theory, but.... Why would the humidity rise? Did you take a steamy shower, leave wet towels hanging on a towel bar, make spaghetti, boil eggs, run the dishwasher, run the dryer with an indoor vent?

Or maybe there is an air leak with the outside. What I don't know is if it is possible that the humidity would leak in faster than the temperature. Maybe, but I don't know.

People perspire no matter what, but it's a lot slower if it is cool in the house. How many people live in your house?

I don't think that would remove any more humitidy than it would cool in so little time.

[Art]

That sounds normal. A system that is too large for a house will turn on, cool the house quickly and turn off quickly. Every turn on and turn off shortens the life time of an ac. In a perfect world it would be sized to run forever keeping the house at the right temperature and never turn off. Of course the outside temperature is always changing so that a perfectly sized ac system is impossible to design. That is why they have multistage systems. In any case your thermostat idea is forcing it on and off often and it would fail prematurely.

[M Q]

Actually, while you are right that the reason for multistage systems is so that the AC does not turn on and off as often, the major reason that this cycling is undesirable is not AC lifetime, but energy efficiency and humidity control.

When the AC turns on, you are spending energy for a while cooling down you evaporator, ducts, etc. before you get to full efficiency. When you turn on and turn off, you are blowing previously condensed moisture back into your home.

[mm]

Your reason below certainly makes sense, but I don't think this one actually does so much.

When the evaportator, ducts etc. warm up between AC cycles, they are sucking in heat from their surroundings. This cools the house, and keeps the surroundings from warming the air when the AC is on.

And at he same time, the condenser is cooling down, which should make the condenser more efficient until it warms up again.

Yes, but I'm sure you mean some of it. Not the stuff that has drained from the pan and gone into the sump or the condensate pump.