Old Carrier Furnace with weird controls problem

[djgiesbrecht]

I have an older Carrier furnace, (58GS100) that is acting a little weird. The furnace will run through its heating cycle absolutely normally, until the blower turns off a minute or two after the gas valve. Then the blower will come back on for a second or two, turn off for a second or two, then come back on, then off. This little on/off thing may happen up to 4 times.

It doesn't seem like this could be a limit switch problem, or the furnace would be turning off prematurely, and it isn't. Could this be the time delay on the main circuit board acting up?

[ACtechGUY]

Your problem is your fan limit switch. That is the device that (after you take the cover off, if it has one) has a dial indicating temperature of the heat exchanger.

Replace the fan limit. When Fan/Limit controls start to wear out they will do exactly what you have posted. One important thing to keep in mind when purchasing a new device is length of the bimetal sensing element. The available sizes range from 4 inches to 14 inches. It is important that you match the size of the device.

You may be able to adjust it for the short term. There is usually an adjustment on the device. You need to change the fan off setting to a higher setting. that is the tab in the midddle of the half moon on the device.

[djgiesbrecht]

My furnace doesn't have an adjustable fan limit switch. It looks like the attached photo. It is permanently set to open at 250 degrees and close again at 210. It appears to be in the circuit for emergency purposes only. The service instructions refer to a "time-delay circuit". Quote: "After approximately 105 seconds, heat relay HFR is energized and blower motor MTR stops."

Do you still advise me to replace the Limit Switch? It is fairly cheap.

[RegUS_PatOff]

this may be your 58GS Service & Installation c.1979

[Budget Appliance Repair]

djgiesbrecht, That is not the fan switch, it is the hi-limit safety switch.

The 250 degrees is the temp that it will open or close at depending on how the circuit is setup.

If the heating chamber gets to the 250 degree trip point something is wrong and when this t-stat cycles it

would shut the furnace down and should turn the blower fan on until the heating chamber get back down below the 210 degree set point of the t-stat.

The fan switch is usually set-able in a range of 90 to 150 degrees,(some also have a set-able fan off point and if it doesn't the fan off will be a set deferential from the on temp), it should be set at the temp you want the blower fan to cycle on at and start blowing hot air out the vents, you don't want it set to low because if the on temp is too low when the blower comes on it will cool the heating chamber down and could shut the blower off prematurely.

Some of the fan switches are just like the the hi-limit safety that you have pictured and have a fixed fan on degrees but it would be in the range of the 90-150 degrees somewhere not over 200 degrees.

From your info, "The service instructions refer to a "time-delay circuit". Quote: "After approximately 105 seconds, heat relay HFR is energized and blower motor MTR stops.", it sounds like maybe your furnace has an electronic control board that turn the the blower fan on/off and powers the gas valve. If this is the case then it is programmed into the circuit board to turn the blower on after 105 seconds and generally you would find another place on the board with a jumper to set the fan off delay from 90 seconds to 180 seconds after wall t-stat has been satisfied.

I went back and read the PDF file that RegUS linked for you, the fan on/off temps or times are not changeable on your unit. It's a time delay built into the circuit board which turns the blower fan on 75 seconds after the the pilot flame has been proven and turns the blower fan off 105 seconds after the t-stat call for heat has been satisfied.

It's possible, (but not real likely), that the 250 degree hi-limit safety t-stat is failing and tripping at a too low of temp and may be what is turning the blower fan back on after call for heat has been satisfied or the circuit board, (time delay circuit or relay contact points), is bad.

[dilkey]

your 58gs has a fixed limit switch not a mechanical one, i would first go back to the basics, your pilot has three wires right? also your gas valve has a hold and pick coil, and does the circuit board have the old ice cube relays? and dont forget the draft circuit board, its an old unit that had a lot of issues, take it from me, its time to replace it,

[djgiesbrecht]

Thanks for your help, guys.

I replaced the circuit board and all is well. There was nothing to see on the old board,(no burned points on the relay, for instance). I'm guessing that the problem was related to the time-delay circuitry.

Thanks again.