Gas furnace cycling on and off

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Old 02-13-05, 05:43 PM
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Gas furnace cycling on and off

I have a Rheem gas forced air furnace, Model RGDA-15EE-JR (approx 15 years old) that keeps cycling on and off. When the thermostat calls for heat, the ignitor lights (glows), pilot "rail" lights, burners ignite, blower comes on, but after about 4-5 minutes the burners will shut off. The blower will continue for a couple of minutes then it shuts off. Wait for about 7 minutes and the whole procedure starts all over.

I had replaced the old thermostat last summer with a Honeywell CT3200, and noticed the cycling recently. I thought it might have been the thermostat so I tried another (same model), same thing. The fan limit control is a Honeywell (can't see the model), but it's set at 90 / 120-130 / 190. The ignition control is a Robertshaw, model HS780 34NL-306A.

It doesn't seem to matter if the house temp is close to the desired temp or not for the cycling to occur.

Now, as I've read previous posts...

The filter (pleated) is brand new, all vents are open (no obstructions), and the A coil appears to be clean (no significant clogging can be seen from the one side). I've pulled the motor (direct drive - to replace a start cap) and the squirrel cage is not clogged. <I'm wondering if the constant cycling is what caused the starting cap. to go... Hmmmmmm.> No obstructions of the flue pipe can be seen.

Any suggestions? If my hunch about the starting cap is correct, I'm concerned the motor is going to be next, that is after my heating bills!!!

Thanks in advance for any help.
 
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Old 02-13-05, 07:53 PM
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On the older Rheem, they used the HSI (Hot surfance ignitor) as a flame sensor.. I would say it's edn of it's life and needs to be replaced. Have you had it changed out before?
 
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Old 02-14-05, 09:17 AM
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Been there, done that...

Yes, that has already been replaced - remember it dearly... Coldest day of December late last year... Hard to forget that one!

Anyway, I replaced that myself. Don't have the part number right here with me, but I believe I still have the old one and the box from the new one @ home (if necessary). If I remember correctly, there was no distinguishable part number available on the old one tho...

While I'm at it, and I don't mean to confuse/muddy the issue, this whole problem originally started with the blower not coming on. I did not include it before because I didn't think the blower problem was related. But just for history purposes, here goes:

Last week the motor did not come on. I checked the furnace and heard a loud humming coming from the motor compartment. Thought it was the starting cap, but wasnít quite sure if it wasnít the motor. So, I did a bit more research and came to the conclusion (maybe incorrect) it was the starting cap because I was able to turn the squirrel cage by hand and the motor started up. I bought the cap. this morning, but have not yet installed it. The motor is starting without the new cap, but I don't trust it because it would not start twice in the past few days, so Iím replacing the cap.

While doing all of this, I noticed that the furnace was cycling a lot more frequently than I had noticed earlier. In the original post I stated that I replaced the thermostat because of the cycling, but the problem didnít go away with the new thermostat. When the motor wasnít starting, I began to watch the furnace operation a bit closer and thatís when I realized how often it was cycling.

So thatís where we are at the moment. The furnace is cycling quite frequently, the cap will be replaced when I get home, and the HSI is about 2 months old. Nothing else has been replaced or repaired.
 
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Old 02-14-05, 09:22 AM
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Go ahead and put the cap in and try it but i bet your motor is shot to. You should never leave a furnace in service that is short cycling because you are taking a chance of cracking the heat xchanger.

When is the last time you've had this thing cleaned and serviced by a pro?
 
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Old 02-14-05, 01:22 PM
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I also have the CT-3200, and the way it maintains a tight temperature differential is by cycling the system six times per hour. Refer to the instructions that came with the stat and adjust for a longer cycle time. Mine was so bad in milder temperatures that it would satisfy the call for heat before the blower even started (i.e., a call for heat would last less than one minute).
 
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Old 02-14-05, 08:22 PM
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In reply to Mattison:

The furnace was serviced in the fall of 2003 - no problems - other than the guy trying to sell us a new one just because it was "older". AC was serviced this past summer due to a freon leak.

In reply to Eclipse:

How are you adjusting the cycle time? Does your stat have screws for the adjustment? Mine only has 3 DIP switches - 1) select degrees in C or F, 2&3) select warm air (mine set here), hot water, or electric. I've looked on Honeywells website and apparently there once was a model (earlier I'm assuming) that has screws for these selections (see: http://hbctechlit.honeywell.com/tech...0s/69-0653.pdf - page 6). The really frustrating part is in the back of my manual in the FAQ it states "IF THE FURNACE CYCLES TOO OFTEN OR SYSTEM CYCLE LENGTH IS TOO SHORT OR TOO LONG, THEN ADJUST THE SCREWS ON THE BACK OF THE THERMOSTAT."

Slight problem... THERE ARE NO SCREWS TO ADJUST ON MY STAT!!!!!!!!!

I think this explains it tho... I had the furnace turned off during the day, so the temp was 63F when I got home. Turned it on and it kept going until approx 68F, then shut off. Turned on again and cycled for approx 15 min periods until it reached 71F. Then it cycled on/off for shorter periods @ 71!!!

The Honeywell website also has a FAQ for the stat, and it states:

#12 Why is my thermostat cycling so often?

Honeywell digital thermostats are designed to maintain temperature within 1įF of the setpoint. In heating, the thermostat is factory set for forced air systems. This means lighter heating or cooling loads will change the cycling of your thermostat to shorter on and off times, which is normal for forced air systems. To ensure your thermostat cycle rate is matched to your system type, see "How do I set the cycle rate?"

Also.

#19 Why does my thermostat turn off before it reaches my set temperature?

In order to prevent overshooting your programmed temperature, the CT3200 is designed to shut the system off (in both heating and cooling) before reaching the set point. Shortly thereafter, it will bring the system back on, bringing the room temperature to your programmed temperature. This is considered normal operation. On rare occasions (e.g. very hot afternoons), the thermostat may cycle the system on and off more than once prior to reaching your programmed temperature.

My questions on this are: What type(s) of havoc is this type of cycling wreaking on my heating system? Wouldn't the 6 times an hour cycling eventually wear down a motor? Is keeping the constant temp within 1 degree of setting done really at the expense of fuel (constantly heating the chamber, then cooling it down)?

OK, I'm done ranting now... This has been a frustrating event, but at least I know it's not the furnace.
 
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Old 02-14-05, 10:06 PM
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I think I would clean the lighting rail. Spiders build nests inside and that slows down the gas flow. Flame does not hit the HSI properly and causes shot cycling.
 
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Old 02-15-05, 05:34 AM
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Yes, my CT-3200 is an earlier model (circa 1996). Apparently Honeywell has replaced the screws with switches but negelected to update the manual to reflect this. Try setting it to the hot water setting. That should reduce the cycling. And yes, too-frequent cycling will increase wear on and reduce the efficiency of your furnace.
 
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Old 02-16-05, 09:32 AM
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Thanks for everyones help one this.

I picked up a new stat last nite and replaced the CT3200 - no more cycling.

I also wrote a note to Honeywell cust. srvc. about having information re: furnace cycling on the web but nothing in their product manual that comes with the unit. Had it been there a bit clearer, I wouldn't have been chasing this around.

And in closing, Mattison was right... Replaced the motor starting cap, but the motor still occassionally (very infrequent) fails to start. Looks like a new motor is in my future...

Thanks again.
 
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