Furnace cutting power to thermostat and cycling

[CarrierDude]

Hello - hope you can help. Carrier has no customer support - lame! I have a Carrier 58RAV induced-combustion downflow furnace. It is controlled by a Honeywell Chronotherm III thermostat (T8600C). This is apparently a "pwer-stealing" thermostat. Here are my issues:

1. During a heating cycle, after the burners have shut off but the fan is still running, I get a "-AC" code on the thermostat, which, I am told, indicates a loss of AC power to the thermostat. Once the heating cycle is over (fan shuts off) the normal display of the current temperature is shown.

2. In the morning, I have noticed that the furnace is cycling A LOT. It will come on, run for maybe 5-6 minutes, then shut off. It then kicks on again after about a minute or so and the whole thing repeats over and over. I know this thermostat has "adaptive" technology where it tries to ramp up the temperature prior to the set time. The "wake" time we have set is 4:30AM. This is still occurring at 6:30AM. The thermostat reads 70, which is the set point. It is pretty cool outside the past few mornings (under 40 degrees F), but I just don't know if this is normal behavior or not.

Thanks for your help!

CarrierDude

[wprowe]

I have this issue now on my system. Any help would be appreciated ... my house seems to stay at a constant 68 degrees regardless of what I set the heat at.

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[davegmoore]

quote:Originally posted by CarrierDude

Hello - hope you can help. Carrier has no customer support - lame! I have a Carrier 58RAV induced-combustion downflow furnace. It is controlled by a Honeywell Chronotherm III thermostat (T8600C). This is apparently a "pwer-stealing" thermostat. Here are my issues:

1. During a heating cycle, after the burners have shut off but the fan is still running, I get a "-AC" code on the thermostat, which, I am told, indicates a loss of AC power to the thermostat. Once the heating cycle is over (fan shuts off) the normal display of the current temperature is shown.

2. In the morning, I have noticed that the furnace is cycling A LOT. It will come on, run for maybe 5-6 minutes, then shut off. It then kicks on again after about a minute or so and the whole thing repeats over and over. I know this thermostat has "adaptive" technology where it tries to ramp up the temperature prior to the set time. The "wake" time we have set is 4:30AM. This is still occurring at 6:30AM. The thermostat reads 70, which is the set point. It is pretty cool outside the past few mornings (under 40 degrees F), but I just don't know if this is normal behavior or not.

Thanks for your help!

CarrierDude

I have a similar problem with my Honeywell T8600C, except at heat up it cycles on and off every 5-10 seconds for 1-2 hours. It's very loud. Any ideas?
DGM
p.s. This just started last year. No problems during the prior 8 years.

[LazyPup]

Whoever told you the thermostat is stealing AC power simply has absolutely no understanding of HVAC controls systems. There is no AC power to steal.

In your circuit breaker panel there are two circuit breakers for your HVAC system. One for the furnace or heat section and the other for the condenser or AC section. Both the furnace and condenser are then supplied by separate breakers and cabling so there is no interconnection of power between them whatsoever.

For convenience the low voltage step down transformer is normally inside the furnace control section simply because it is supplied by the furnace manufacturer and installed in the unit at the factory, but there is no specific requirement for it to be there, and it could be mounted at any convenient location nearby.

The control transformer is commonly a 120v primary x 28v secondary step down transformer, althogh there are some that have a 220v primary. The purpose of the transformer is to convert line voltage to low voltage for the control system.

There are two poles on the secondary side of the transformer. The red wire which is attached to one of the poles on the transformer supplies power to the thermostat. The white wire which is attached to the other pole is then connected to the common side of all relays in the control circuit, both in the furnace and the AC condenser Primary voltage relay.

The cable to the Thermostat has four or five wires. ( heat pumps require an additional wire to control the reversing valve.)

In the thermostat cable the red wire is supplying control voltage to the thermostat, and the remaining wires are then returning control voltage to thier respective components.

White to the burner for heat
Yellow to the AC condenser primary relay.
Green to the inside fan motor relay (manual fan mode).
and on a heat pump,,Blue to the Reversing Valve in the AC condensing unit.

Some thermostates have a separate power source to the control board for heat (RH) and AC (RC). They require a jumper be installed between RH and RC.

Some digital thermostates have small LED indicators to show when the different functions are running. Some have three lights, Fan, AC and Heat, while others only have two lights Heat and AC.

The FAN light indicates when the inside fan motor (motor inside the furnace) is running in the Manual fan mode.

The AC light indicates the inside fan motor is running in an automatic mode, signaled by other internal controls (Automatic mode).

The HEAT light may be configured to show whenever the HEAT section is powered, or it may be indicating the inside fan is running in conjuntion with the burner controls.

The burner is then controlled by a temperature sensor in the plenum. When the burner is on the thermotate would show the HEAT light, indicating both the burner and the inside fan motor are running. When the temperature in the plenum reaches a preset high limit point, the burner is switched off, but the fan continues to run until the heated air has been moved out of the plenum, thus reducing the temperature and signaling for the burner to start again.During this fan only mode the theromstat lights may show AC light, indicating the FAN is running but the burner is off.

If the fan and ductwork are properly sized to the unit, the air in the plenum should never go high enough to trip the high limit, but instead it should be moved out by the fan as it is heated, thus keeping the average temp in the plenum below the set point of the high limit switch.

If there is a substantial restriction to the air flow, which is often caused by:
1.Dirty Air Filters
2. Too many discharge vents closed causing a back pressure on the duct system.
3. Furnishings or personal possessions blocking the return air grilles.
4. Improper RPM or a loose squirrel cage wheel on the inside fan.

You then have an inadequate air flow, resulting in the plenum temperature rapidly rising above the high limit set point and short cycling the burner.