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Discussion Starter #1
i know there are a lot of variables. but is there some kind of rule of thumb that states about how much a furnace should run ? example, say the house is properly insulated and the hvac is properly installed and operating. the furnace should run about 15mins every hour.
 

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Run time is more dependant on temperature, insulation, outdoor climate, windows....etc. Every home I've ever been in has a different situation. The only way to even get close to structured run times is to oversize the equipment. Unfortunately that creates more problems than it solves.:yes:
 

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Plus your thermostat may have a CPH setting, and the stat will vary its run time by that setting also.

At 65 outdoor temp, the furnace shouldn't need to run. At design outdoor temp the furnace should basically be running 24/7. Convert your outdoor temps to % of load, and that is the % of time a properly sized furnace should have to run.

EG: -10 outdoor design, 65 no heat needed. Thats 75 degrees difference. 100/75=1.33, so if the outdoor temp is 30. 30*1.33=40. At 30 degrees outdoor temp the furnace should be running 40 percent of the time. Or on a thermostat with a CPH setting of 3, 8 minutes on 12 minutes off, repeated.

Thats an approximation, as 30 degrees during the night ill take more heat then 30 degrees during the day when your home is receiving solar gain.
 

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i know there are a lot of variables. but is there some kind of rule of thumb that states about how much a furnace should run
Your first sentence says it....a lot of variables. You could hook up an hour meter to the gas valve which is what I did. Anytime the gas valve is open...receiving voltage...the hour meter advances.

I use the hour meter to gauge run time and when to change the filter which for me is about every 125 hours.

A few photos below.

The bottom photo shows the gas valve with the hour meter wired in. I just used single to double disconnects.
As a note...the middle photo in the left column appears to show the greenfield passing thru the 4" square box. It does not...it passes behind it. The only think in the 4" box is the hour meter and its wires.
 

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I've been interested in this only because I'm datalogging our thermostat in mysql and have the numbers to mess around with. I'm trying to come up with a baseline number so I can compare before/after changes like attic insulation, new windows, etc that we plan on doing but in the end, there are just too many variables.

Sunlight hours, wind, whether we're home or out or in/out a lot, etc. We've had a few days lately where we've had over 6 hours runtime in a day, so that's 15 minutes out of each hour, but one hour solid has been getting the house back up to temperature in the morning from setback.

In the end, I just accept it for what it is, and make longterm decisions on how to improve things.
 

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Discussion Starter #7
thanx for the replies.

the reason i am asking, is that i think my furnace is running too much. its on 20mins and off about 35. this is at 30deg outside temp on a non windy day.
my gas bill was $110 for nov, and the furnace is not old. i asked a coworker, and his was $65, and he has 3 more people in the house. but i do know that his newer house is much better built and insulated than mine currently is. and it is not even really cold out yet. btw, we like it at 72 in here and do not touch the stat.
 

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Fix'n it...does the local utility offer energy audits free of charge? May be worth your while to look into that.
 

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Discussion Starter #14
i put a known good/accurate thermometer on a stick hanging out in open air. i am dealing with a 4 deg temp swing. 70 on, 74 off. i think i need more attic insulation and more air sealing.
 

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Fix'n it:

I'm with you on this thread, I just had a high efficiency unit installed (95%) and it seems as if runs more than my old unit (78%). My home was built in the early 90s.

I've been watching the gas meter every few days and comparing my bills with last year, and I am using about the same amount of gas. Any one here able to tell me if this is to be expected?
 
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