Minimum furnace run time? - Page 2

stevehull · 01-15-13, 09:14 PM

Is this an air type heating system? I find that leaving the fan on the "on" position reduce hot/cold spots (and yes does use kWhr).

See if the cycling is reduced by putting the fan to "on" instead of auto.

Steve

Daox · 01-15-13, 09:27 PM

I'm not really worried about cycling time. Its on for a half hour, that has to be plenty.

Mikesolar · 01-16-13, 06:46 AM

It takes a minute or 2 for any gas combustion to get to proper combustion temp (not air or liquid temp) which is when it will start to burn efficiently. Oil is way longer. The real issue is start/stop cycles on the equipment as the contracts are rated for a limited number of cycles. Hence....modulation....

Daox · 11-17-14, 10:28 AM

I'm bumping this.

What are some of you guy's typical furnace run times?

I'll have to remeasure mine. I now have the thermostat set to +/- 2°F. So, the temperature swing is fairly large. Sadly, my newer cyberstat wifi thermostat only does +/- .5, 1, 2 or 3. I liked the +/- 1.5°F my old thermostat had.

Daox · 11-17-14, 08:48 PM

I timed the furnace on time tonight. It was 12F ambient. My thermostat is set to 70 +/- 2F. The run time was 29 minutes on. I didn't time the off time.

I'm contemplating going to +/- 1F. That would keep it more comfortable, and I don't think that a 15 minute on time is horribly short.

What do you guys think?

jeff5may · 11-17-14, 10:28 PM

As long as the system spends most of its cycle time burning gas and blowing hot air, it isn't short cycling. Many apartment and condo units have furnaces that have very little "dead" time waiting for heating calls in between run cycles. I lived in one a number of years ago that I swear cycled fifteen or more times an hour. Vent 1 minute, light burner, blower kicks on, burner shuts off 2 minutes later, blower shuts off 1 minute later. 3 Minutes later, vent kicks on and here we go again. Not a well designed heating system.

theoldwizard1 · 11-17-14, 11:27 PM

I can only tell you approximate numbers and something to watch for.

I have a 1400 sq. ft. trilevel. It has above average insulation and double pane thermo-windows (far from "super" insulation). Tonight, the outside temp is in the 20s. The wind gusting up to about 30 mph. It takes a 2 degree drop on my digital thermostat to trigger the "call" for heat from my 85% efficient horizontal furnace. Run time is less than 10 minutes. Off time is 10-20 minutes.

When it is EXTREMELY cold (below zero) and the wind is blowing hard my furnace DOES short cycle from hitting the over-heat limit switch. According to one service person (who I agree with), this is because the furnace has to run for a longer than typical AND the horizontally mounted furnace (mounted in the crawl space) does not allow "free flow" of air out and into the house. Warm air actually has to make 2 - 90° turns before it gets into the main supply duct.

With this combination of weather occurs, the inside temp can get down to below 64 even when the thermostat is set to above 72 ! Electric heat is the only way to get the temp up to 70. OR ... put on a sweat shirt and an extra blanket on the bed.

Blower speed is at maximum. Burner pressure is at minimum.

NiHaoMike · 11-17-14, 11:45 PM

Quote:

Originally Posted by theoldwizard1

When it is EXTREMELY cold (below zero) and the wind is blowing hard my furnace DOES short cycle from hitting the over-heat limit switch. According to one service person (who I agree with), this is because the furnace has to run for a longer than typical AND the horizontally mounted furnace (mounted in the crawl space) does not allow "free flow" of air out and into the house. Warm air actually has to make 2 - 90° turns before it gets into the main supply duct.
...
Blower speed is at maximum. Burner pressure is at minimum.

The ductwork is way too restrictive.

theoldwizard1 · 11-18-14, 01:20 AM

Quote:

Originally Posted by NiHaoMike

The ductwork is way too restrictive.

Yes ! 2 - 90s before the main trunk !!!

Ormston · 11-18-14, 05:44 PM

I was once asked to look at a device that had been retrofitted to an oil boiler heating a swimming pool.
The device had been sold as a fuel saver for what seemed like a lot of money at the time, I think it was something like £200 (around 23 years ago). The owner reconed it had paid for itself in the first year and was very keen for me to fix it as it had stopped working.

This £200 box contained a small hand soldered board with a 555 timer switching a relay. The fault turned out to be a failed relay so was an easy fix.

The device worked by delaying the startup of the burner when the thermostat called for heat, there was a pot on the fromt of the box labeled low to high saving. The higher the saving selected the longer the delay before starting the burner. I think it was something like 20 to 30 mins delay.
This delay has the effect of increasing boiler run times by forcing longer off periods.

I have no idea how well this works in a domestic application, it's on my to do list to try with my heat pump.

Steve

01-15-13, 09:14 PM	#11
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Is this an air type heating system? I find that leaving the fan on the "on" position reduce hot/cold spots (and yes does use kWhr). See if the cycling is reduced by putting the fan to "on" instead of auto. Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990

01-15-13, 09:27 PM	#12
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	I'm not really worried about cycling time. Its on for a half hour, that has to be plenty. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

11-17-14, 10:28 AM	#14
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	I'm bumping this. What are some of you guy's typical furnace run times? I'll have to remeasure mine. I now have the thermostat set to +/- 2°F. So, the temperature swing is fairly large. Sadly, my newer cyberstat wifi thermostat only does +/- .5, 1, 2 or 3. I liked the +/- 1.5°F my old thermostat had. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

11-17-14, 08:48 PM	#15
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	I timed the furnace on time tonight. It was 12F ambient. My thermostat is set to 70 +/- 2F. The run time was 29 minutes on. I didn't time the off time. I'm contemplating going to +/- 1F. That would keep it more comfortable, and I don't think that a 15 minute on time is horribly short. What do you guys think? __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. Last edited by Daox; 11-17-14 at 08:51 PM..

01-16-13, 06:46 AM	#13
Mikesolar Master EcoRenovator Join Date: Aug 2012 Location: Toronto Posts: 958 Thanks: 40 Thanked 158 Times in 150 Posts	It takes a minute or 2 for any gas combustion to get to proper combustion temp (not air or liquid temp) which is when it will start to burn efficiently. Oil is way longer. The real issue is start/stop cycles on the equipment as the contracts are rated for a limited number of cycles. Hence....modulation....

11-17-14, 10:28 PM	#16
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,431 Thanks: 431 Thanked 619 Times in 517 Posts	As long as the system spends most of its cycle time burning gas and blowing hot air, it isn't short cycling. Many apartment and condo units have furnaces that have very little "dead" time waiting for heating calls in between run cycles. I lived in one a number of years ago that I swear cycled fifteen or more times an hour. Vent 1 minute, light burner, blower kicks on, burner shuts off 2 minutes later, blower shuts off 1 minute later. 3 Minutes later, vent kicks on and here we go again. Not a well designed heating system.

11-17-14, 11:27 PM	#17
theoldwizard1 Apprentice EcoRenovator Join Date: Oct 2014 Location: SE MI Posts: 105 Thanks: 3 Thanked 12 Times in 9 Posts	I can only tell you approximate numbers and something to watch for. I have a 1400 sq. ft. trilevel. It has above average insulation and double pane thermo-windows (far from "super" insulation). Tonight, the outside temp is in the 20s. The wind gusting up to about 30 mph. It takes a 2 degree drop on my digital thermostat to trigger the "call" for heat from my 85% efficient horizontal furnace. Run time is less than 10 minutes. Off time is 10-20 minutes. When it is EXTREMELY cold (below zero) and the wind is blowing hard my furnace DOES short cycle from hitting the over-heat limit switch. According to one service person (who I agree with), this is because the furnace has to run for a longer than typical AND the horizontally mounted furnace (mounted in the crawl space) does not allow "free flow" of air out and into the house. Warm air actually has to make 2 - 90° turns before it gets into the main supply duct. With this combination of weather occurs, the inside temp can get down to below 64 even when the thermostat is set to above 72 ! Electric heat is the only way to get the temp up to 70. OR ... put on a sweat shirt and an extra blanket on the bed. Blower speed is at maximum. Burner pressure is at minimum.

11-18-14, 05:44 PM	#20
Ormston Apprentice EcoRenovator Join Date: Mar 2013 Location: UK Posts: 131 Thanks: 13 Thanked 35 Times in 32 Posts	I was once asked to look at a device that had been retrofitted to an oil boiler heating a swimming pool. The device had been sold as a fuel saver for what seemed like a lot of money at the time, I think it was something like £200 (around 23 years ago). The owner reconed it had paid for itself in the first year and was very keen for me to fix it as it had stopped working. This £200 box contained a small hand soldered board with a 555 timer switching a relay. The fault turned out to be a failed relay so was an easy fix. The device worked by delaying the startup of the burner when the thermostat called for heat, there was a pot on the fromt of the box labeled low to high saving. The higher the saving selected the longer the delay before starting the burner. I think it was something like 20 to 30 mins delay. This delay has the effect of increasing boiler run times by forcing longer off periods. I have no idea how well this works in a domestic application, it's on my to do list to try with my heat pump. Steve