OK, Labman and others proved very helpful in the past with my ignitor problem so now I have another question: How long should a proper furnace run between cycles?

I have read about the flame sensors and "limit switches" but I'm not sure if this is related to either. My heater seems to go off and on very frequently. For example, it has gone off and on about 6 times in the past hour and a half. Each time it runs for anywhere from 2 to 3 minutes but blows plenty of hot air. I have my thermostat set to 68 degrees at all times and the house seems to be maintaining the heat. At one point I set the desired temp to 72 degrees and it ran for 8 minutes before shutting itself off. The thermostat never went up in temperature so I assume it was keeping itself from overheating? So I'm not really sure if I should be complaining or even considering calling the same HVAC tech out (another service call? sigh).

So now I'm wondering if this is how an efficient gas furnace should operate or if this is a sign that yet something else is out of sync. Another tidbit of info is every morning when I get up around 6 am the usage log tells me that it has been running for approximately 2 hours throughout the night.

My instincts tell me this is not normal (any maybe why my ignitor burned out so quickly last time) but maybe someone can tell me otherwise?

Thanks!

labman

That sounds too often to cycle. Your house could have some sort of major heat loss cooling it down too quickly. The thermostat could be too close to a heat vent. The furnace comes on, heats up the area near the vents, and then the thermostat kick it off. Then the local heat spreads into he cold room, and kicks it back on. If the whole room is warm when the thermostat shuts off, I would look for heat loses. Is any one room colde thann the rest? If the furnace quits before the the far end of the room from the vent is warm, look at where the thermostat is.

James W. Acker

There are two things come to mind about a furnace cycling as much as you say your's is:
First: It could be that your house is losing too much heat, therefore the furnace is trying to keep the house warn. The thermostat only knows that the air around it is a certian tempature and acts accordingly.

Second: On your thermostat there is a devse called a "heat anticipator" and it has amp markings settings on it. It has to be set to the apm ratings of the gas valve of the furnace. If you take the front cover off the thermostat it is possible that you may see this heat anticipator. The scale goes from probably .2 .3 .4 .5 so on through the scale till it gets to the whole number 1 the proper setting is going to be some where around .45 or so. There is an amp reading on the gass valve but sometimes it is hard to find. so if you do find the scale move the setting up a little if this is not enough move it up more. How this heat anticipator works is that it has a small coil of wire and when the thermostat calls for heat electricity flows through this coil and creates heat. if the setting is too low it heats fast and turns off the thermostat because it thinks the house is warm enough. But the air is still cool and so it cools off the coil rapidly and the thremostat turns on and starts the cycle over again.
I hope you can understand what I am trying to tell you and I hope you can find the right setting. I feel like I am giving more info that you really want.
Like the old saying you ask for the time of day and someone tells you how to build a watch.

Jeorgiagirl

Thanks, Labman and James, I had a feeling I would be hearing from you two again. Your thorough answers are actually very much well appreciated. I will investigate the thermostat tomorrow morning when I will not disturb the rest of the sleeping family.

I actually dug up the original contractor installation instructions for the Rheem furnace and have been browsing through the troubleshooting algorithms. My problem seems to follow the "lockout mode":

1) call for heat runs the induced for 30 seconds to prepurge
2) surface igniter heats for 30 seconds, inducer continues to run
3) gas valve opens for a 9 seoncd trial for ignition
4) IF FLAME IS NOT SENSED during the 9th second after the gas valve opens , the gas valve closes and the igniter de energizes.
5) after a 30 second interpurge the igniter heats for 30 seconds. After 30 seconds, the gas valve opens for 9 seconds. If no flame sensed, gas valve closes and igniter de energizes. BOTH THE MAIN BLOWER AND THE INDUCER OPERATE FOR 180 SECONDS BEFORE THE NEXT IGNITION TRIAL.
6) it repeats the precess up to 4 times before going into a soft lock for a 1 hour delay

The timing of this correlates to the 6 times in one and a half hours as well as the approximately 120 minutes throughout the night. What throws me off is the fact that I could get it to run for 8 minutes straight the other day. Now I need to narrow down the possible troubleshooting explanations:

-check polarity of 115 VAC supply
-check continuity of ground wire
-check insulation of igniter leads
-check flame sense current; clean sensor with steel wool if yellow LED flashes
-if check are OK , replace IFC

All of these electrical links are suspicious since we had our electrical service upgraded from a 100 amp box to a 200 amp box this past summer. Maybe our electrician mixed the polarity or did not properly ground? I will not touch the electricity and leave that to the experts if it comes to it. I do plan on shutting everything off and trying to clean the flame sensor. The "insulation of igniter leads" is curious too. I noticed our old burned out igniter had a black sleeve around the leads but the new replacement one (put on by a HVAC tech) did not. Is it possible that something this minor is causing the problem? I could slide the old sleeve onto the new igniter leads although I would rather not deal with the fragile igniter unless I have to.

Thanks again!

labman

This is a new problem, you didn't have the short cycle problem other years? That would leave out the poorly located thermostat. It is also unlikely the setting in the thermostat changed. Did you have enough other work done over the summer, that could have made a major disturbance to your insulation or something?

I do not understand polarity problems with the AC feed. You should have a black, white and bare wire connected from the house. Normally current flows through the black and white wires with the white being grounded at the box. The bare wire only serves to ground the cabinet in case of a short. If either the black or white were poorly connected at the new box, it could shut the furnace off completely, no blower continuing after the gas goes off.

If the gas stays on long enough for the blower to start, and warm up the house, I doubt it is the temperature sensors in the furnace. Do the ignitor leads have places where a bare wire could touch any metal?

Does the furnace indicate any malfunction codes? Some furnaces flash an LED to give a code.

One thing you could try is to shut the furnace off, disconnect the little white wire from the thermostat, and connect a little wire from the red wire to where the white was connected. This bypasses the thermostat. Then turn the furnace on. It should come on and run as long as you leave it that way. If it does, the problem is in the thermostat or its wiring. If it goes off, the problem is in the furnace or the wiring from the house.