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!

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 far end of the room from the vent is warm, look at where the thermostat is.

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 certain 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.

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!

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.

Turn the thermostat up to 95 degrees and see if the burner and blower will operate continuously for more than 10 to 15 minutes. If this is successful, there is absolutely no problem with the polarity, gravitational pull of the moon, surface ignitors, or anything within the furnace. Your problem lies solely within the THERMOSTAT. Plug hole in wall behind thermostat with caulk or tissues to block cold air inflow. Check vents in ceiling to make sure they blow air toward windows, and not across ceiling and toward the thermostat. If you do not understand how to adjust heat anticipator, then try to construct some sort of tent over the thermostat. You are trying to decrease the sensitivity of the thermostat. Cut a styrofoam cereal bowl like a half moon and hang it aver the thermostat like an awning. See if that "shelters" the thermostat a bit. Good luck. Paul.

Holy cow... I hope this isn't still a problem almost 4 years later... :confused:

Xx

This is an obscure solution but I hope it helps someone who has tried everything else. I had checked all the usual suspects that could be causing this problem, clogs in the blower motor or venting, tested all my sensors by jumpering them, overrode my thermostat, checked the heat anticipator setting, all was good but the furnace was still short cycling. I was down to replacing either the igniter board or the motherboard but these are both fairly expensive. However, these will often go bad due to hairline cracks in the soldering, the most likely place for this to occur is on the back side of the furnace control terminals where you hook up the wires running to the thermostat. These hairline cracks will cause voltage variations that make the furnace think it is receiving on/off commands from the thermostat. It will often take the furnace a minute or so to cycle down at which point it immediately starts back up senses another voltage drop and goes back into another one minute cycle down, thus the short cycling. The fix took about 10 minutes after spending hours troubleshooting but isn't that how it always goes. I turned the power and gas off, removed the motherboard leaving all the wires intact, flipped it around and resoldered the 5 points where the terminals were joined to the motherboard. Keep in mind that you may have weak solder points in other places or in the igniter module. But these terminal connection points receive the most stress from wires being screwed onto them. Wah lah it now runs consistently until reaching the set point. Hope this helps someone, stay warm.

Xx

I just had a technician in for my furnace that would work for 15 minutes then cycle. It would completely shut down after 15 minutes and start up again -- run for 15 minutes and the cycle would start all over again. The answer was simple. The furnace filter I had, though not dirty, was too restrictive for my furnace. It was causing the airflow to be below what was needed. As soon as it shut down the first time, he check some sensor attached to the area the flame burned and it was overheated. The technician said lack of proper airflow was causing it to overheat. We took out the filter and the furnace worked without any problems. I put in a new, less restrictive air filter and have not had problems since.

Check the inside of your thermostat for the cycle settings. If you have an high efficiency furnace, setting 2 should be set to on. Then your funace will cycle on 3 times per hour. I just had a new furnace installed and it cycled on 6 times per hour. Installer said my old thermostat was not working properly and installed a new thermostat. It worked a little better but It was cycling on too frequently . Later when I read the booklet & checked the cycle settings I noticed that settings 1 & 3 should be set to "no" and setting 2 should be set to "on" because my new furnace unlike my old one was high efficient. Works great now.

Bingo... B I N G O!!

Hello Scooby,

I have the same problem since some years.
My furnace goes on, the room feels ware quickly, it goes off, the room feels cold after 10-15 minutes and it goes on again and so on.

Can you please explain one more time how you fixed the issue?

Thank you in advance

Hello Scooby,

I have the same problem since some years.
My furnace goes on, the room feels ware quickly, it goes off, the room feels cold after 10-15 minutes and it goes on again and so on.

Can you please explain one more time how you fixed the issue?

Thank you in advance

I have a three minute cycling story, cycling that I put up with for 15 years and was finally resolved yesterday.
After trying all of the usual filters, thermostat, etc. mine turned out to be a Plenum up in the attic that was half the size it should have been for this 4 ton furnace that was supposed to service the big addition to the house.

I had service people tell me that this furnace was too big for the house, or the flu was to small and last year a guy was trying to sell me a new furnace unit for $14,000.

Well, after six hours of repair work, and $400.00, I have a Furnace that, well, after a short while, I have to turn it down.

I am elated...

1. Never listen to internet people when it comes to your gas heater. Find a friend who knows a heater guy and pay him to deal with it. None of them know what they're doing so any is as good as the next.. but a friend of a friend might not also rip you off on purpose (just by accident by not knowing how to find and fix the REAL issue)
1a wait for it to start short cycling
1b when it stops, is the mercury telling it to stop? If not, it ain't in the stat.
2. jump the thermostat or turn up to max heat. Does it stop cycling and run continuously? No- proceed to 2a. Yes- its in the stat.. Set anticipator 1/4 higher and try again.
2a watch the flame that hits the flame detector to see if it flickers or goes out just before the whole heater shuts off. The sensor works not by heat, but by electrical current running through the flame. Slight fluttering will shut her off. Too much gas velocity will push the flame front past the sensor. Not anough gas in the air/gas mix will mean no ignition. Clean the carbon out of the little holes in the end of the gas manifold... then worry "why was there carbon in there in the first place"
3. if flame looks good, it still shortcycles with the stat pegged full heat- so it's in the heater not the stat. There's a rotory high temp limit setting with a dial face inside the cabinet. Watch it and see if it rises past the lever setting. Feel the duct above the heating section- burn yourself? The high limit stat has been saving your life continuously- the filter is clogged or the fan is damages and running too slow, or your vents are all closed. Cut a 1 foot hole in both the supply and return duct and repeat step3. If it still locks out, it's a slow fan not clogs in the ductwork.
4. If the high limit isn't tripping it may be one of the remaining air pressure differential lockouts meaning your vent fan process is to blame, or a pressure variance between the inside of the exchanger and your house is not what it should be.

Consider an electric heat bank replacment instead of gas.. much easier to tell why it's busted and it won't let an incompetent service man use it to kill you in the night.

Oh- next time one of you thieves go to rip off an old lady with the ol "your heat exchanger is cracked maam you need a new unit $8000 please" make sure her son isn't a LEED PE. Dumbasses. What? My fan is dirty and it's 17% less efficient so I need a new $800 fan motor? Um yeah OK.