The Blower only runs thirty seconds, there's nothing wrong

[sjc1989]

Remember me! I had the furnace inspected by a pro and he diagnosed the problem or lack there of! The temperature of the house is only set to about 52 degrees right now since it's unoccupied. When you set the thermostat for 68 and walk away it only runs thirty seconds and the blower shuts off while the burners continue to run after a while the blower kicks on for another 30 seconds or maybe a little longer and then shuts off while the whole process repeats itself.

The reason being, there's a temperature probe in the heat exchanger which is on a limit switch. The "house" thermostat signals the burners to start, the burner starts and raises the temperature of the heat exchanger to a preset level (in my case 150 degrees), then the blower kicks on. Under normal conditions, when the house gets up to the preset temperature, "house" thermostat signals the burners to stop, and when the heat exchanger cools to a preset temperature (105 degrees in my case) the blower stops.

However, in my case, since my furnace is sucking in rather cold 52 degree air the heat exchanger cools below the preset low limit of 105 degrees and shuts off the blower 'til the heat exchanger gets back up to the high limit of 150 degrees. The pro says he thinks it'll work fine once the house gets back up to a "normal" operating temperature. He also said we could tinker with those limits, but they're probably set correctly for the system already. The blower cycle did get longer the warmer the house got, although, we didn't even wait for the house to get to 60 degrees.

I didn't ask, but I imagine this is one of the many reasons that manufacturers went from the "pot metal" heat exchanger to the cast iron heat exhanger with its much higher heat coefficient.

Thanks for all the replies!

Steve Cox

[daveferg]

Steve---I'm by no means an expert---I've done a little work on heaters and thermostats. What the pro says makes some sense, but yet, something doesn't seem right. While the blower may have limit switches, from what you said, it sounds like the burners are also shutting down prematurely.

I seem to remember that on older style, analog thermostats, under the cover is an adjustment that affects the cycling time of the heater. Could this be off? Again, I'm no expert, but it sounds like a candidate.

[sjc1989]

I'm sorry, I might not have been clear. The burners NEVER shutdown while it's trying to reach the temp set on the house thermostat, just the fan shuts off.

Thanks,

Steve Cox

[Grandad]

Heat anticipator. Electronic thermostats seem to have gotten away from this, prefering to measure how well they hit the target temperature. For example, if you set your heat down 10 degrees when you are not home, the thermostat starts heating up the house 2.5 hours earlier than when you get home. It does so to try to get the house 10 degrees hotter in time. If it takes longer, it starts earlier the next day, if shorter it starts later. Honeywell uses 1 degree per hour as the target, thus the 2.5 hour estimate.

[TDHofstetter]

I kinda' wondered if maybe the heat exchanger was cooling off too quickly & shutting down the fan. Too bad nobody's installed a manual override for things like your situation. Seems like it'd be a problem every time you have an extended power outage - the blower will short-cycle EVERY TIME the house gets chilly, no?

Ya might look into sticking a toggle switch in parallel with the low-limit (blower) switch in the heat exchanger. It'd be totally safe; the worst that could happen is the the blower would run continuously if you forgot & left it turned on.

-- Tim --


E Lignus Unum

[sjc1989]

I could override it by setting the low limit to something like 90 or 95 degrees. However it may just cool to 90 and then shut off, but it would run in longer cycles.

Steve Cox

[TDHofstetter]

If your low limit is adjustable, it seems like that'd be worth doing. The blower motor takes LOTS more power to start up than it does to run - therefore, the more often it starts up the more power it burns. Besides, it seems like that'd heat the house quicker - even if it's blowing cooler air.

Like in your car on a cold day. Start it, then turn the heater blower on low. You'll get warmer faster than if you flip on the blower, wait 30 seconds, flip it off, wait a while, flip it on, et cetera.

If the blower runs longer, it won't drive you nuts, either. :)

-- Tim --


E Lignus Unum

[dicklaxt]

I doubt seriously that the problem is because it hasn't run in a while .I have never heard of a system like that but I don't have any answers either just an opinion,

dick

[CnA]

Guess my snap disk assumption was wrong.

Oh well, it wouldn't be the first wrong idea. Won't be the last!

[TDHofstetter]

I'd say you were mighty close, CnA! I thought maybe the low limit WAS a snap disk - surprised that it was adjustable. Cool, huh?

-- Tim --


E Lignus Unum

[sjc1989]

I believe CnA was correct about the fan control, wrong about the type. The high and low limit operate off the same probe and it is adjustable. I don't think snap disks are adjustable. Also, I believe there's nothing wrong with the control, just the over all design of the system allows it a much narrower operating range than I'm currently using it for.

Steve Cox

[douglas stein]

One more opinion... I would first examine the blower motor, see if it has an internal thermal overload switch with a built in reset. If the motor is heating up all by itself, then the overload may be shutting it down in order to protect the windings (like it's supposed to), then as the cold air cools down the motor, it cycles all over again.

As for the "how should it work" ... The furnace is told by the house themostat when to fire up and when to stop trying to heat the house to the house setting. The house thermostat does not tell the blower to start or stop. When the themostat senses the house temp has met the thermo setting it tells the furnace to go through a fire up and heat up cycle. The ingnitor lights the burner, the burner in turn heats up a chamber in the furnace below the plenum, when the plenum reaches a designated "high" temp range, it signals the blower to now start and take the heated air and blow it into the ducts to heat the house. Once the heat has been removed from this heat chamber and the continual burning of the fire ring can no longer sustain the heat range in the fire box then the blower will be told to shut down about 1 to 2 minutes after the gas has been turned off, in order to exhaust all remaing heat into the ducts. There are really two temp sensors in the firebox / plenum area. A high and a low sensor (setting), when the heat hits the high setting, the blower is told to blow the accumulate heat out of the box, when the heat lowers to the low sensor setting, the blower is told to stop blowing and waits for the cycle to start over and repeat the heat up process.

My guess would be that if the blower motor is OK, bearings are not bad, dirt not causing the motor to overheat, then I would take a look at the high and low sensors in the heat box chamber. The sensors themselves are up inside the chamber, but the control signal wires should come down to a terminal board or directly to the starter for the blower.

If the wiring or the system is scary, I would ask the furnace repairman to specifically look at this portion of the furnace's design.

Let me know what you found, I keep a running memory in my history book of what was broke and how did that other guy fix it.

doug