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  • Nov 2nd, 2007 2:03 am
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Deal Addict
May 11, 2003
2220 posts
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Furnace cycling?

I've noticed that my furnance runs about every 5 minutes or so. Is this considered cycling? I've got a Honeywell programmable thermostat.

I checked the Honeywell site, and they said this:

"For instance, gas or oil forced air systems have a recommended cycle rate of 6. With a cycle rate of 6, the heating system, at a 50% load, will cycle 6 times per hour. This breaks down to about 5 minutes on and 5 minutes off. Again, the actual on and off time of the heating system will vary as the load on the heating system varies."

So according to this, the 5-on, 5-off would make sense. I can't change the cycle rate, but I'm assuming that it is a cycle rate of 6.

My friend said that he thinks the furnance may be too big, which is why it is cycling (100,000 BTU for about 1900-2000 sq ft or so - I don't know the exact size).

Also, the duct work just above the furnace itself gets REALLY hot when it is running. The same friend suggested that they might not have cleaned that area when they replaced the furnace (2 year old furnace).

Thoughts?
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Nov 13, 2005
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GTA
e-man wrote:
Oct 29th, 2007 9:55 am
I've noticed that my furnance runs about every 5 minutes or so. Is this considered cycling? I've got a Honeywell programmable thermostat.

I checked the Honeywell site, and they said this:

"For instance, gas or oil forced air systems have a recommended cycle rate of 6. With a cycle rate of 6, the heating system, at a 50% load, will cycle 6 times per hour. This breaks down to about 5 minutes on and 5 minutes off. Again, the actual on and off time of the heating system will vary as the load on the heating system varies."

So according to this, the 5-on, 5-off would make sense. I can't change the cycle rate, but I'm assuming that it is a cycle rate of 6.

My friend said that he thinks the furnance may be too big, which is why it is cycling (100,000 BTU for about 1900-2000 sq ft or so - I don't know the exact size).

Also, the duct work just above the furnace itself gets REALLY hot when it is running. The same friend suggested that they might not have cleaned that area when they replaced the furnace (2 year old furnace).

Thoughts?
Wholly cow. I have a 100,000 btu 2 stage Bryant furnace installed in a 3800 sq ft home and that's good enough for us. Wow I would call the manufacturer of the furnace and ask them for advice.

sk
[OP]
Deal Addict
May 11, 2003
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sunnybono wrote:
Oct 29th, 2007 10:33 am
Wholly cow. I have a 100,000 btu 2 stage Bryant furnace installed in a 3800 sq ft home and that's good enough for us. Wow I would call the manufacturer of the furnace and ask them for advice.

sk
I need to look into it then. My friend was suggesting that a 70-80,000 BTU was sufficient, so it sounds weird that your house is twice the size without twice the BTUs. But then again, my furnace is a mid-efficiency. I need to look into it some more.
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Jul 18, 2003
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Definitely take into account the efficiency.

A 100,000BTU furnace with 80% efficiency is only 80,000BTU. The other 20,000BTU went up the smoke stack.

But something else may be worth considering..... how does your thermostat turn on and shut off the furnace? Does it have a preset cycle time or totally dependent on temperature? If based on temperature, then check the air flow.

I've seen houses where the thermostat is on a wall opposite the register. But the furniture is placed in such a way that hot air is directed towards the thermostat. So, within 60 seconds of the furnace turning on, the area around the thermostat is warm enough to shut off the furnace while the rest of the house remains colder. This results in frequent furnace cycling.
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May 11, 2003
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eelfliw wrote:
Oct 29th, 2007 1:19 pm
Definitely take into account the efficiency.

A 100,000BTU furnace with 80% efficiency is only 80,000BTU. The other 20,000BTU went up the smoke stack.

But something else may be worth considering..... how does your thermostat turn on and shut off the furnace? Does it have a preset cycle time or totally dependent on temperature? If based on temperature, then check the air flow.

I've seen houses where the thermostat is on a wall opposite the register. But the furniture is placed in such a way that hot air is directed towards the thermostat. So, within 60 seconds of the furnace turning on, the area around the thermostat is warm enough to shut off the furnace while the rest of the house remains colder. This results in frequent furnace cycling.
My thermostat is just off to the side of a big cold air return, so I'm guessin the location isn't an issue. I suspect that the cycling is time-based. Like the Honeywell manual said (see my original post), it looks like it turns on and off every 5 minutes by design.

Regarding the BTUs, I read somewhere that you should have about 40 BTUs per square foot. But is that ACTUAL BTUs, or does that take into account the efficiency rating?
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Newbie
Jul 26, 2007
72 posts
Toronto
Definitely take into account the efficiency.

A 100,000BTU furnace with 80% efficiency is only 80,000BTU. The other 20,000BTU went up the smoke stack.
.... true - but when they calculate what's needed for the house, these losses should be taken into account already. i got a 1400 sq foot home, and because it's old, there's no insulation the exterior walls. despite that, i still have a (if i recall correctly) 60k btu mid efficiency furnace.

My thermostat is just off to the side of a big cold air return, so I'm guessin the location isn't an issue. I suspect that the cycling is time-based. Like the Honeywell manual said (see my original post), it looks like it turns on and off every 5 minutes by design.
is it your actual furnace that's firing up every 5 minutes, or is it just your fan that's turning off and on? for single stage fans (if you have one) the frequent firing might also be a common phenomenon....
Jr. Member
Dec 26, 2006
117 posts
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phatmanmd wrote:
Oct 29th, 2007 5:46 pm
is it your actual furnace that's firing up every 5 minutes, or is it just your fan that's turning off and on? for single stage fans (if you have one) the frequent firing might also be a common phenomenon....
I used to have a digital programmable honeywell thermostat, after the furnace had generated enough heat to the setting temperature, the fan kept cycling on and off with a period of about 1 minute each. I had no idea how to fix it.

I then replaced the thermostat with an old type (non digital) with the slider, the problem hasn't occurred.

Anyone has ideas to fix this problem?

I really would like to switch back to the digital programmable thermostat to save energy during the night. I called the handyman, he suggested to replace the whole circuit board inside the furnace (hmm!??). That would cost like 50% of the whole furnace. I wouldn't want to do this because the furnace is still working well except just this weird problem.
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May 11, 2003
2220 posts
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homeless06 wrote:
Oct 30th, 2007 3:44 am

I really would like to switch back to the digital programmable thermostat to save energy during the night. I called the handyman, he suggested to replace the whole circuit board inside the furnace (hmm!??). That would cost like 50% of the whole furnace. I wouldn't want to do this because the furnace is still working well except just this weird problem.
I'm obviously not an HVAC expert, but the fact that your furnace cycles with the new theromstat only suggests that it is a thermostat issue, not a furnace
one. I don't think a new board would help (but what do I know).

My friend had the same problem with his Honeywell (which is why he gave it to me). I read somewhere that you should put some insulation BEHIND the thermostat if you are having problems. The frequent cycling may be related to the cycling that seems to be built-into the Honeywells, which is 5-mins on, 5-mins off...basically what I'm experiencing now.


phatmanmd wrote:
Oct 29th, 2007 5:46 pm
is it your actual furnace that's firing up every 5 minutes, or is it just your fan that's turning off and on? for single stage fans (if you have one) the frequent firing might also be a common phenomenon....
Yeah, I gotta look into that to see if the heat is actually going on or not. I'll try and check that tonight. Also, isn't 60,000 BTU a little small for 1400 sq ft?
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Deal Addict
Sep 19, 2005
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Get a prog thermostat you can adjust the cycle rate and preheat cycles,,

mine used to come on quite often,,,, adjust the rate and now it works great!
Deal Expert
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Aug 9, 2004
21622 posts
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Mississauga
Ours used to do a weird thing when after the furnace brought the housee up to the set temp, it would cycle on and off every 5 secs or so for about 10-15 mins. It ended up being a circuit board that I bought online for about $80. Works fine since.
I was advised that it might be a good idea to sign up for the gas companies furnace protection plan, and then have them take a look at it and fix if necessary (then cancel the plan later on if you like).
Thanks for the memories, RFD.
Good-bye.
Jr. Member
Dec 26, 2006
117 posts
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stealth wrote:
Oct 30th, 2007 2:04 pm
Ours used to do a weird thing when after the furnace brought the housee up to the set temp, it would cycle on and off every 5 secs or so for about 10-15 mins. It ended up being a circuit board that I bought online for about $80. Works fine since.
hi stealth, yes, that's it. That's the exact problem I have. I switched with 2 different honeywell thermostats, 1 old and 1 brand-new, but experienced the same problem on both. The brand-new one went back to Home Depot.

The handyman said something about the voltage or current that caused the cycling. He said that I should either have the whole board replaced or he could install a relay for about $200, parts and labor included.

Can you give me more info on the circuit board that you bought online for $80? ... thanks.
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Jul 9, 2007
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The way I understand things, the cycling is a feature to keep your furnace from burning out. Just like the op said.
The five minutes cycling sounds a bit frequent but ok. I would call honeywell and find out.
I have a Noma programmable one, it works fine. It can also control the humidifier.
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Aug 9, 2004
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homeless06 wrote:
Oct 31st, 2007 3:46 am
hi stealth, yes, that's it. That's the exact problem I have. I switched with 2 different honeywell thermostats, 1 old and 1 brand-new, but experienced the same problem on both. The brand-new one went back to Home Depot.

The handyman said something about the voltage or current that caused the cycling. He said that I should either have the whole board replaced or he could install a relay for about $200, parts and labor included.

Can you give me more info on the circuit board that you bought online for $80? ... thanks.
Homeless, google "furnace fan control board" to learn more about the problem. Then, you'll want to get the part number from your existing circuit board and see if you can find a match / replacement online. Unfortunately, I cant find the package from the one I ordered in order to refer you. My furnace is a Keeprite, but the board I believe was made by Honeywell. It should look something like this
http://www.amazon.com/Honeywell-ST9103A ... B00095OL1U
Image
Thanks for the memories, RFD.
Good-bye.
Jr. Member
Dec 26, 2006
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Hi stealth, thanks so much. From your info, I am certain that the fan control board is exactly what I need to upgrade. My furnace is Heil 90+% efficient.
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homeless06 wrote:
Nov 1st, 2007 7:19 pm
Hi stealth, thanks so much. From your info, I am certain that the fan control board is exactly what I need to upgrade. My furnace is Heil 90+% efficient.
No problem. I found my old invoice. I had ordered from www.furnaceparts.com. You could prob email them for advice, or at least to confirm your suspicion.
The part I ordered was:HQ1170063HW, Honeywell ST9120C5013
Product Code: 1170063
But of course yours is likely to be different.

Good luck.
Thanks for the memories, RFD.
Good-bye.
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