HVAC question

[yarcraft91]

It's home heating season again. Yippee!

My faithful old (1988) Amana 92% NG furnace does something odd, but only occasionally. The furnace cycle will start, run and end normally, but then it immediately starts another cycle that also starts, runs and ends normally. This might happen on the first start after the overnight temperature setback ends or it might happen later during the day. This morning I got a look at the thermostat (Ace ATX9000 series, made by LUX) immediately after the second cycle started and saw that the reported room temperature was below the setpoint. So, it seems the question isn't "why did the second cycle start?", it's "why did the first cycle end prematurely?". I first noticed this odd behavior near the end of the last heating season and it seems to be happening more often now, at least once each day.

I suspect the thermostat, which was installed at least 20 years ago. I might suspect the heat exchanger high temperature limit switch, but I have a dial thermometer with long probe installed in the ductwork a few feet away from the furnace where it cannot "see" the heat exchanger. The thermometer reads a maximum of 137*F during a normal furnace cycle, just as it has for years.

Any thoughts?

[elite203]

I believe your issue is "short cycling"

http://www.grayfurnaceman.com/gas-fu...ving%20problem.

probably need a professional to check it out.

hope this helps

[Pokey Fisherman]

It sounds like the first cycle is ending by a over temp and is being shutdown by a senor and not the thermostat.

[REW]

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[black white face]

If the heating call is interrupted by a limit switch opening, the main blower on the furnace will continue to run until the furnace retries it's ignition sequence, that is easy to discover if that is the issue.

I would look at the thermostat as being the cause- the heat anticipator cycler may be defective in the control or the control itself just bad.

[yarcraft91]

Originally Posted by Pokey Fisherman

It sounds like the first cycle is ending by a over temp and is being shutdown by a senor and not the thermostat.

I considered that, but I would expect that problem would be accompanied by an increase in air temperature in the ductwork, which I am not seeing.

[AllenW]

Originally Posted by black white face

If the heating call is interrupted by a limit switch opening, the main blower on the furnace will continue to run until the furnace retries it's ignition sequence, that is easy to discover if that is the issue.

I would look at the thermostat as being the cause- the heat anticipator cycler may be defective in the control or the control itself just bad.

Yes, while diagnosing with out being there is risky, I'd check thermostat also.

Al

[Paule]

Originally Posted by yarcraft91

I considered that, but I would expect that problem would be accompanied by an increase in air temperature in the ductwork, which I am not seeing.

Not necessarily. If the blower motor is loosing RPM's the duct temperature may not raise much if any. As mentioned before, not having the unit within eyeshot makes it difficult and risky. Easy diagnostics become difficult. The video that was sent is quite good. I have seen flame sensors unplugged and plugged back in solving some problems. That being said, one plug and one screw for removing a flame sensor would make cleaning it after all the years it was in service a good idea in my opinion.

[JiMinneye]

It is definitely short cycling, but why? It could be as it is running the first cycle it is building up condensation and as it does it will eventually cause the pressure switch to open. That will turn of the burner and the vent motor will continue to run as the thermostat isn't satisfied. As the condensate drains away the pressure switch then closes and allows the burner to come back on. And the whole thing starts all over again. Make sure all of your condensate lines are clean. This is just one scenario that could be happening. I have seen condensate lines that look like they are clean and in reality they are partially plugged. It could also be a crack in a heat exchanger that as the furnace is starting out one of the limit switches is closed but as it heats up it makes(opens)the limit. Then as the main blower runs it cools down the switch, it cools off, resets itself and the whole vicious circle starts all over again until the thermostat is finally satisfied. It could be a dirty air filter or main blower motor, or dirty squirrel cage that isn't moving enough air fast enough and it makes the main limit switch. There are so many variables that trouble shooting without being there is next to impossible. If you understand the wiring diagram you can jump out all switches (one at a time) and your thermostat to see what is happening but if you don't, I would call a HVAC company that you trust.

[yarcraft91]

An update:
I perform each of the following items every fall, as these have proved to be important maintentance items with this furnace.
- Lubricate induction and main blower electric motors via their lubrication ports.
- Remove the flame sensor and shine it up with steel wool.
- Remove the condensate drain plumbing and clean it.
- Replace AA batteries in thermostat.
This has already been done this year. Condensate drain plumbing will be removed and cleaned again in February. The filter is an AprilAire with the large pleated paper air filter. It does not need changing annually, more like every 3 years and, even then, it is not visibly dirty. It has a very large filter area.

Today, I removed the thermostat from the wall plate and checked the wiring. I found the screw connecting the yellow wiring a bit loose and corrected that. Hmmmm. Remaining connections were properly tight.

I hooked up my HOBO data logger to record supply duct air temperature and induction blower on/off vs time. This will reveal whether the furnace continues this "double-cycle" behavior and how often.