Furnace Cycling

reddog

I have an American Standard Freedom 80 2 stage furnace. Over the past year I have noticed a change in the way its been operating. For instance if
my indoor temp is 58 and I set the thermostat to 68 the furnace comes on and runs BUT NOT continuously until it reaches 68. It will run for awhile, the indoor temp climbs to about 61, it shuts off for a few minutes, comes back on and runs for awhile, the indoor temp reaches about 63, then shuts off again for a few more minutes, it continues this routine once or twice more until the indoor temp 68 as set on the thermostat. I thought it might have been the thermostat but that was changed with a new one and the problem still exists. A new flame sensor was also installed. Any appreciated

Reddog

747

I bet doc knows the answer to that. Hopefully he will stop bye.

reddog

Thanks 747. Yes Doc is DA MAN!

tinner73

could it be shutting down on limit? do you get any codes once the burners shut down? sounds like an airflow issue.

reddog

When it shuts down its Flashing SLOW, (normal, no call for heat). The furnace operates normally except for this strange problem. All heating vents to the rooms are open, the filter and squirrel cage are clean. Beats me what the problem is. I checked all electrical and tube connections. I'll probably be calling a service person if I can't figure out what the problem might be.

HVAC Doc

Red,

Ok, gonna start with a bunch of questions for you so we can see if we can narrow it down.

1. Type of thermostat- Is it a 2 stage heat or Single with the control board doing the switching from high to low fire? Is it digital or mercury bulb?

2. Anything new that produces heat close to or under the stat such as a t.v., computer, lamp, etc. ?

3. Have you watched the L.E.D. lights to see if it is changing from low to high or from high to low fire when it does this?

4. If the new stat is digital, is it doing this on an early morning program setting when it might be using an energy saver program to gradually bring the heat up?

reddog

Hi HVAC!


1) Thermostat is a White Rodgers 1f83-261 digital non-programmable multi-stage thermostat. The old one was a mercury bulb type whiterodgers 1f56-301 mechanical. The furnace started this behavior with the old mechanical thermostat. It carried through with the new thermostat. Most likely there is nothing wrong with the old thermostat.

2) No, the thermostat is mounted in the hallway at the proper height. It has always been in this area since this house was built in 1943. There is nothing indirectly influencing/heat producing the thermostat.

3) The thermostat LCD has an indicater icon showing which stage its in(a 1 for single stage and a 1+2 icon for second stage). I see the indicators changing from 1 to 1+2 when second stage kicks in. I also hear the relay within the thermostat click as it toggles between 1st and 2nd stage. It seems to be operating properly.

4) No. Its not a programmable thermostat. You can only Raise/Lower the temperature by pressing the UP or DOWN arrow button on the thermostat. Of course it does this behavior in the morning because it gets cold in the house at night but it does not just do this in the morning. It does this when there is a variation of Indoor temp to what I set the thermostat to. Like if the indoor temp is 59 and I set the thermostat to 68 then it will produce this behavior..no matter what time of day. After the furnace cycles on and off several times and it finally meets the thermostat setting of 68, then the furnace will operate normally in that it will come on and cycle once(as it should be) to maintain the 68F setting on the thermostat. If I was to raise the theromstat to 75F then the furnace will start cycling(come on and off several times) before it reachs the 75F thermostat setting. This is because of the larger temperature variation between 68 to 75F.

Thank you HVACDOC! I appreciate your taking the time to respond.

reddog

I just noticed after the furnace shut down sooner than it should have(as mentioned in this post) the diagnostic indicator(red LED that flashes on the furnace) is FLASHING 3 FLASHES which indicates PRESSURE SWITCH PROBLEM. I also notice the flue exhaust pipe gets really hot. You spit on it and it sizzles! Not sure if thats how hot its supposed to get. I removed the tubing to the pressure switched and they appeared clean and free of obstruction. I also cleaned the tubing connector on the Flue exhaust motor with a piece of wire and vaccumed it out. Reinstalled the tubing and the problem still exists. Perhaps the pressure switches need replacing. I can see the flue exhaust motor fans spinning so its functioning.

Some thoughts...maybe the pressure switches are bad. Maybe the flue exhaust motor(2 speed) is only running at one speed..maybe it runs on low speed for 1st stage then jumps to high speed at 2nd speed and perhaps the motor is not changing to high speed. Not sure if this is how a 2 speed flue exhaust works(that is, its speed follow the 1st and 2nd stages respectively)

HVAC Doc

It is possible that the inducer isn't changing speeds (you would have to stand there and watch/listen to it to verify) but it is also possible you have something like a bird nest in the flue or cap. It could be only enough that on high fire, it makes the high speed pressure switch but on low fire, it is too much for it. Normally if a pressure switch fails, it keeps the ignition cycle from ever happening. You could also get a tech to check the pull on the inducer motor and make sure the motor itself isn't getting hot and losing RPM's and slowing down as it runs till it finally breaks the pressure switch and kills the cycle. He would use a magnahelic gauge to measure the vacuum on it. You could however check to see if the motor is going on high and low speed yourself if you felt comfortable, just drop me an e-mail and I can explain how.

reddog

Thanks again HVAC DOC! I sent you an email. I appreciate your help sir.