Central air problem

[Claude Lawrenson]

I am looking for possible causes of the following. It is a central air unit with exterior condensor unit and indoor A-coil, above the furnace. It is a 18 year old unit, and a 10 year forced air gas furnace.

The central air temperature thermostat is set to 74 degrees F. The compressor operates adequately, at that temp, however, the concern is that after running for several hours during the day - a small amount of heat discharges from the "supply" floor registers after the cycle has finished when the fan is on. When the air cycles back on the air cools again.

Comments please.

[Roy D. Cooke, Sr]

Originally Posted by clawrenson

I am looking for possible causes of the following. It is a central air unit with exterior condensor unit and indoor A-coil, above the furnace. It is a 18 year old unit, and a 10 year forced air gas furnace.

The central air temperature thermostat is set to 74 degrees F. The compressor operates adequately, at that temp, however, the concern is that after running for several hours during the day - a small amount of heat discharges from the "supply" floor registers after the cycle has finished when the fan is on. When the air cycles back on the air cools again.

Comments please.

Not an AC expert but has this happened other years or do you know .
Could it be going into a defrost cycle to make sure the coils are not covered in Ice.

[Scott Gilligan, CMI]

It's a bit out of the SOP, but check the thermostat wiring to make sure that it is wired properly and there are no wires contacting where they shouldn't be. Just had to fix someone's (friend from work - not a client) system not to long ago because the previous home owner installed a digital thermostat and crushed the wires in the housing causing the heat control wires to contact some of the AC wire contacts. Everytime the AC would turn on, it would also turn on the heater. Both of her systems pretty much ran constantly.

[relliott]

How could heat come through unless the burners are fired up?
Anyone?

[Charley L. Bottger]

Originally Posted by clawrenson

I am looking for possible causes of the following. It is a central air unit with exterior condensor unit and indoor A-coil, above the furnace. It is a 18 year old unit, and a 10 year forced air gas furnace.

The central air temperature thermostat is set to 74 degrees F. The compressor operates adequately, at that temp, however, the concern is that after running for several hours during the day - a small amount of heat discharges from the "supply" floor registers after the cycle has finished when the fan is on. When the air cycles back on the air cools again.

Comments please.

You are describing a downflow furnace with the registers in the floor but say the A-coil is above the furnace telling me that the return air is being drawn thru the A-coil. This does not compute with me as the standard installation would be on a down flow system the A-coil would be below the furnace with the fan discharging thru the A-coil.

On a system of that age I would think the fan would cycle off when ever the outside unit cycled off not continuing to run. It sounds to me you have an improperly wired control system and a improper location of the A-coil.

I would also check all of the return air duct for loss of insulation; sounds also as if you are picking up excessive heat from the return side of the system.

Very hard to trouble shoot a system from this monitor.

Roy does not do home inspections from pictures but I can trouble shoot from pics do you have any

[David A. Andersen]

I have had a lot of units run in heat and cooling at the same time.

In this case Robert is correct. Did you check the furnace to see if it was running?

If it is, it is the T-Stat or control wiring.

By the way; furnaces do not have a defrost cycle unless they are a duel-fuel (at this age it is unlikely).

Newer furnaces use an electronic control board which is often the cause of a run-a-way furnace.

[lfoster]

Would it be the end of the cycle and the unit is clearing the conditioned air from the ducts? Like the heating does?