DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   HVAC (
-   -   furnace run time ? (

Fix'n it 12-12-2012 10:06 PM

furnace run time ?
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.

Missouri Bound 12-12-2012 10:18 PM

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:

beenthere 12-13-2012 05:14 AM

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.

hammerlane 12-13-2012 07:22 AM

5 Attachment(s)

Originally Posted by Fix'n it (Post 1072103)
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 passes behind it. The only think in the 4" box is the hour meter and its wires.

hammerlane 12-13-2012 07:26 AM

Also here is a thread I did last year about The Natural Gas Cost For Furnace 1 Hour Run Time

SuperJETT 12-13-2012 09:22 AM

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.

Fix'n it 12-13-2012 09:33 PM

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.

Missouri Bound 12-13-2012 09:39 PM

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

Fix'n it 12-13-2012 09:46 PM

idk. i will ask them.

Fix'n it 12-13-2012 09:54 PM

i think the only way this would be worth it, was if they used a thermal imager. AND gave ME the results.

Missouri Bound 12-13-2012 09:57 PM

Did you take the survey on that link?:eek:

Fix'n it 12-13-2012 10:38 PM

what survey ?

did i sign up, no way.

Missouri Bound 12-14-2012 09:36 AM

I was referring to the "pre-assessment evaluation"

Fix'n it 12-15-2012 10:11 AM

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.

hammerlane 12-15-2012 01:43 PM


Originally Posted by Fix'n it (Post 1073462)
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.

Hanging out where in open air?

What does 70 on, 74 off mean?

All times are GMT -5. The time now is 04:10 AM.

User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.