Home>Discussions>INSULATION & HVAC>heat system cycling on and off
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garywood
heat system cycling on and off

i installed a 3 ton package air conditioner with heat strips in a 680 square foot home. the cooling works great, but the heat cycles on and off at about 30 second to one minute cycles. ive noticed that the heat strips work correctly if the fan is set to on, but if the fan is set to automatic, the fan runs for 30 seconds to about one minute, which does not allow the heat strips to go through a full heat cycle. is my problem in my thermastat or the fan relay? i have had 2 professionals look at the unit, and they could not solve my problem. i need help!

canuk
Re: heat system cycling on and off

Dosen't seem like the fan relay since it's the same relay for both auto and on.

3 ton for 685 sq ft sounds quite large to me.

JLMCDANIEL
11q1

I agree with Canuk, 3 ton unit sounds awfully large for such small house.

The fan speed may be so high that it cools the heat exchanger below the low temp much to quickly.
Jack

Sten
Re: heat system cycling on and off

You mentioned Heat Strips, usually these are install in an air handler and used with a heat pump. If you do have a heat pump are you using a heat pump T-Stat?

garywood
Re: heat system cycling on and off

the unit i am using is a intertherm 3 ton package unit. it is not a heat pump, so i am using a heat-air thermastat for a furnace and ac. i have tried several other thermastats, including a set back. none of them have fixed the problem. what baffles me is why do the heat strips work ok while the fan switch on the thermastat is in the vent position, but not when it is on the automatic position? and if it the unit is too large for the home, why does the air conditioner work perfectly? do i need to just sell the unit and get a smaller one, or is there something else i can do if the unit is too large?

Sten
Re: heat system cycling on and off

I don't know why you have a three ton unit in your 680 sq. foot home but if it's working OK and not short cycling (turning off and on continuously) don't worry about it. If you take the face plate off of your Stat there should be a dip switch on the inside that is for diff. heat types, yours should be on electric. The on switch at your stat is to circulate air, I don't think your heat strips should be coming on when the stat is in the on pos. It sounds like you stat wiring could be wrong at one end or the other.

Bob Gabrilson
Re: heat system cycling on and off
canuk wrote:

Dosen't seem like the fan relay since it's the same relay for both auto and on.

3 ton for 685 sq ft sounds quite large to me.

Do you think? I read that and thought something has to be wrong.
Either the house is bigger or the A/C (size) smaller. Unless, of course, he lives on the sun. LOL

JLMCDANIEL
Re: heat system cycling on and off

Do you have a register blowing directly on the thermostat? Are you positive it is not a heat pump/electric furnace package?
Jack

Sten
Re: heat system cycling on and off
JLMCDANIEL
Re: heat system cycling on and off

A basic description of how a heating and AC units works.

There are generally 3 components, AC compressor unit, heating unit (gas flame or electric elements) and the blower controlled by a control unit.

When the thermostat is set on AC, the thermostat sends a signal to the control unit which turns on the compressor unit and the blower. Both units are on as long as the thermostat calls for cooling. There are no temperature limit switches involved. When the temperature is reached the thermostat turns off the signal and the controller shuts down the compressor. In some units the blower will continue to run for a short time.

When the thermostat is set for heat it sends a signal to the control unit. The control unit turns on the heat source (other items like an inducer motor and draft vacuum switch may be involved in gas units) once the temp in the heat exchanger reaches the low limit a low limit switch closes and the blower is turned on. The heat and blower will continue until the temperature is reached and the thermostat cuts the signal (normal operation) However if the temperature in the heat exchanger gets to high a high limit switch opens and turns off the heat source but the blower continues.If heat is still being called for by the thermostat, once the heat exchanger cools below the high limit the heat unit will be turned back on. Once the anticipated temperature is reached the heat source is turned off and the blower continues to run until the temperature in the heat exchanger goes below the amount set by the lower temp limit switch.

Because the unit seems to work with the blower in the on position, it indicates that enough air is being moved to prevent the temp from opening the high limit switch.The problem described by the OP could be caused by a low limit switch and a high limit switch temperature being set to close together, the low limit switch set to high, control unit being defective, or something improperly wired.

The proper way to trouble shoot this problem is to go through a heat cycle, monitor the limit switches and find out which unit is causing the problem.
Jack

Sten
Re: heat system cycling on and off

The Heat strips should not be coming on while the stat is in the vent or on position. As I said the vent or on pos. is for air circulation. It very well could be going off on high limit, but any Tech that looks at this system and sees that the Heat Strip is coming on when that stat is at the on or vent pos. will have to fix this first. IMHO it's not High Limit. Please don't take this as an argument I'm just going by what the OP posted.

Then again all I know is duct sealing, according to some.

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