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Old 02-12-2012, 09:13 AM   #1
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Default Furnace turns on every 10 minutes

I'm a new(ish) homeowner so I never noticed such things before, but I installed a thermostat, a Honeywell from HD that ran about $60, and the thing turns the furnace on about every 10 minutes and runs it for 7-8 minutes. With my old thermostat, I don't remember it ever operating this frequently. I looked in the manual, there's no option to adjust how sensitive it is (i.e. how much the temperature has to drop before it will turn on). I changed the thermostat in the first place to get the weekday and weekend scheduling options, not sure if that actually saves any money. Seems to me it'd be more efficient to try and keep at a constant temperature rather than losing all that heat during the day and trying to make it up in the evening.
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Old 02-12-2012, 09:16 AM   #2
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The span time is probably too low, so it's short cycling. Is it doing it more when it's really cold? I'm not familiar with that thermostat but see if there's an option called "span", though it may be called something else.

I've been working on a fully custom thermostat, and I'm actually going to set the temp in ranges. It cost less money to run the furnace for a long time, then to run it in a bunch of short cycles so the idea will be to avoid short cycling as much as possible.

Though if you have high heat loss it's harder to avoid.
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Old 02-12-2012, 09:17 AM   #3
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what kind of 'furnace' is it?
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Old 02-12-2012, 09:22 AM   #4
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We need to know the type of furnace. Oil or Gas fired, hot water baseboard heat or hot air heat. All that makes a difference in how the thermostat options are set up. We also need the exact model of the thermostat that you bought from Home Depot. There are usually a few screw terminals on them and depending on the furnace, you may need to jumper a couple of them or there may be a screw that sets the amount of "on time" before it will go off or come back on again. For example, in a hot air system the fan will stay on a couple of minutes after the furnace goes off.
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Old 02-12-2012, 09:23 AM   #5
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what kind of 'furnace' is it?
Brand I don't remember, but I am in a ~1500 sq ft townhouse in Virginia and it is a gas furnace with electric blower. Today is about 30 degrees outside.
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Old 02-12-2012, 09:28 AM   #6
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Sounds like the hysteresis is set to tight. Your older T-stat must have had a wider hysteresis setting. I'm not sure if you are able to change that on the new one.
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Old 02-12-2012, 09:28 AM   #7
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30 degrees F? Yeah the furnace should not be running that often. I could see that when it's like -20 or so. When it stops, is it at the temp you set it at though? If yes, then you could also have major heat loss going on, maybe not enough insulation in the attic.
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Old 02-12-2012, 09:33 AM   #8
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Quote:
Originally Posted by Analog View Post
Sounds like the hysteresis is set to tight. Your older T-stat must have had a wider hysteresis setting. I'm not sure if you are able to change that on the new one.
I don't think I can. It is a Honeywell RTH6450. The only thing that comes close in the manual is a "heating cycle rate," which only has the following options:
Set to 5 if you have gas or oil furnace
Set to 9 if you have electric furnace
Set to 3 if you have hot water or high efficiency furnace
Set to 1 if you have gas/oil steam or gravity system

So mine is obviously set to 5. There is nothing else in the manual that I can see that would change this.
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Old 02-12-2012, 09:38 AM   #9
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Quote:
Originally Posted by Triumph View Post
I don't think I can. It is a Honeywell RTH6450. The only thing that comes close in the manual is a "heating cycle rate," which only has the following options:
Set to 5 if you have gas or oil furnace
Set to 9 if you have electric furnace
Set to 3 if you have hot water or high efficiency furnace
Set to 1 if you have gas/oil steam or gravity system

So mine is obviously set to 5. There is nothing else in the manual that I can see that would change this.
I don't know about yours, but some have set-screw adjustments on the back side for such things.
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Old 02-12-2012, 10:12 AM   #10
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Quote:
Originally Posted by Triumph View Post
I'm a new(ish) homeowner so I never noticed such things before, but I installed a thermostat, a Honeywell from HD that ran about $60, and the thing turns the furnace on about every 10 minutes and runs it for 7-8 minutes. With my old thermostat, I don't remember it ever operating this frequently. I looked in the manual, there's no option to adjust how sensitive it is (i.e. how much the temperature has to drop before it will turn on). I changed the thermostat in the first place to get the weekday and weekend scheduling options, not sure if that actually saves any money. Seems to me it'd be more efficient to try and keep at a constant temperature rather than losing all that heat during the day and trying to make it up in the evening.
my lux programmable thermostat says my system should cycle 3 to 6 times per hour.

so every 10min sounds like its within range.
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Old 02-12-2012, 10:29 AM   #11
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I turned the heat off after the last cycle, and it took about 25 minutes to drop 1 degree. Granted it is getting warmer outside but I figure this is a good enough test of my insulation (which I know I have problems in the entry hall). I would prefer to adjust this thermostat so that it turned on after dropping 1 degree instead of 1/10th of a degree or whatever it's set to. There's nothing in the manual nor a screw adjustment on the module anywhere that speaks to adjusting this sensitivity.

Anyone have a suggestion for a good thermostat?

Quote:
Originally Posted by JEDI View Post
my lux programmable thermostat says my system should cycle 3 to 6 times per hour.

so every 10min sounds like its within range.
Thanks for that, that's good feedback.
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Old 02-12-2012, 10:31 AM   #12
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I believe the heating cycling rate is exactly what you should be looking at. Change it to 3 and see if there's any improvement.
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Old 02-12-2012, 10:32 AM   #13
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That's a perfectly good thermostat-I have two of them in central CT and both run perfectly.

Check to see if all the bottom switches are set correctly, then I'd follow Jedi's suggestions and if that doesn't work, call the Honeywell hotline-they are very helpful.
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Old 02-12-2012, 10:34 AM   #14
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Quote:
Originally Posted by Analog View Post
Sounds like the hysteresis is set to tight. Your older T-stat must have had a wider hysteresis setting. I'm not sure if you are able to change that on the new one.
This. Most of the programmable thermostats that I've seen have a 1,2 or 3 degree hysteresis.

OT: My new system simply keeps the temperature where it's set by varying the blower speed up/down like a servo system. Most consistent (and cheap - 95.5% efficient) furnace I've ever had.
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Old 02-12-2012, 10:44 AM   #15
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Quote:
Originally Posted by arcenite View Post
I believe the heating cycling rate is exactly what you should be looking at. Change it to 3 and see if there's any improvement.
why 3?

changing from 5 to 3 will increase sensitivity, thus more cycles?
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Old 02-12-2012, 11:00 AM   #16
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How many btu/hr is the furnace?
Where is the thermostat located? Usually it's in a hallway away from drafts from doors and located near a return. Digital stats are far more sensitive than aneroid bi-metal old school stats like T87F etc.
Does it have an anticipator?
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Old 02-12-2012, 11:02 AM   #17
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ohh im going to piggyback on this thread (sorry op). but I think my thermostat is going (only after 7 years!) it keeps letting the temp drop. the other day it got to 61.

i had it set to 74 during the day and 69 at night. I had to set ot override and to 74 only room that is cool is the office/playroom (haven't got new windows in this room yet)
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Old 02-12-2012, 11:08 AM   #18
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Recheck your settings. Also the type of heating system (hot water baseboard or hot air) and wether it is a newer high efficiency furnace will have to use the correct settings.

Set to 5 if you have gas or oil furnace ... this would be for normal, old hot air furnace
Set to 9 if you have electric furnace .. not many homes have this type
Set to 3 if you have hot water or high efficiency furnace .. hot water baseboard heat or new high efficiency furnace
Set to 1 if you have gas/oil steam or gravity system ... only used if you have steam radiators found in many very older homes

Note you can check the nameplate on the furnace. If it is within say 4 years or so old and has a rating of 80% or higher, it is a high efficiency type of unit.
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Old 02-12-2012, 11:17 AM   #19
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Quote:
Originally Posted by JEDI View Post
why 3?

changing from 5 to 3 will increase sensitivity, thus more cycles?
It has nothing to do with sensitivity to temperature.

The lower the number the less times per hour the system will kick on and off.

5 = 60/5 = Tstat cycles a maximum of once every 12 mins
3 = 60/3 = Tstat cycles a maximum of once every 20 mins

That's my understanding anyway.

Last edited by arcenite; 02-12-2012 at 11:20 AM.
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Old 02-12-2012, 11:57 AM   #20
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Quote:
Originally Posted by Engineer View Post
This. Most of the programmable thermostats that I've seen have a 1,2 or 3 degree hysteresis.

OT: My new system simply keeps the temperature where it's set by varying the blower speed up/down like a servo system. Most consistent (and cheap - 95.5% efficient) furnace I've ever had.
Second the hysteresis theory.
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Old 02-12-2012, 12:04 PM   #21
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Quote:
Originally Posted by bruceb View Post
Recheck your settings. Also the type of heating system (hot water baseboard or hot air) and wether it is a newer high efficiency furnace will have to use the correct settings.

Set to 5 if you have gas or oil furnace ... this would be for normal, old hot air furnace
Set to 9 if you have electric furnace .. not many homes have this type
Set to 3 if you have hot water or high efficiency furnace .. hot water baseboard heat or new high efficiency furnace
Set to 1 if you have gas/oil steam or gravity system ... only used if you have steam radiators found in many very older homes

Note you can check the nameplate on the furnace. If it is within say 4 years or so old and has a rating of 80% or higher, it is a high efficiency type of unit.
It is a 20 year old Rheem unit. I had the AC serviced last summer and the heat checked out in the fall. It works good, the HVAC guy said it was one of his favorite units and to run it until it dies. Which I plan on doing. So the #5 setting is correct for it.

Quote:
Originally Posted by arcenite View Post
It has nothing to do with sensitivity to temperature.

The lower the number the less times per hour the system will kick on and off.

5 = 60/5 = Tstat cycles a maximum of once every 12 mins
3 = 60/3 = Tstat cycles a maximum of once every 20 mins

That's my understanding anyway.
Even though the #5 setting is technically correct for me, I will try setting it to 3 and see what happens. I don't like this unit, the manual should make some explanation of what this setting is, without you and I having to guess at it.
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Old 02-12-2012, 12:12 PM   #22
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Quote:
Seems to me it'd be more efficient to try and keep at a constant temperature rather than losing all that heat during the day and trying to make it up in the evening.
it's not.
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Old 02-12-2012, 12:45 PM   #23
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Quote:
Originally Posted by ElFenix View Post
it's not.
Yeah, in fact that's pretty much the sole reason programmable timer t-stats were invented.
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Old 02-12-2012, 01:17 PM   #24
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To answer your other question, yes, it saves energy. Your house loses energy at a rate proportional to the difference in temperature between indoors and outside.
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Old 02-12-2012, 01:24 PM   #25
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I can honestly say that everyone I have sex with always smokes after!

Then again I am the furnace operator at the crematorium.
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