Minimum furnace run time? - Page 3

Ormston · 11-18-14, 04:44 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

jeff5may · 11-19-14, 06:42 PM

Quote:

Originally Posted by Ormston

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

This type of control would definitely put an end to a system that was short cycling for some reason. It would definitely work better for a hydronic setup, though. Just think what would happen if it were rigged to a short-cycling air handler or blast furnace! What would the homeowner say if they turned up the thermostat and the furnace didn't immediately start blasting hot air???

Fordguy64 · 11-18-14, 07:32 PM

been away for a while but 2 weeks ago i replaced my 30 year old 40-60% eff (per the only info i could find on it) with a new 2 stage 95% eff furnace with ecm blower. i also replaced the lux thermastate that allowed me to adjust the swing with a nest learning thermostat.

well its been very cold the last few days down into the single digits at night and upper 20s low 30s during the day. its hard to tell how long its actually run but i know it has only kicked into the high stage once (on its own) but i did force it before just to make sure it worked

Daox · 11-20-14, 12:34 PM

Looks like I'll have to measure vent outlet temperatures.

The cyberstat has a bunch of settings for temperature variance. I've never heard of any other thermostat with a +/- 3F swing on it. There is no setting for minimum run time, but it will control a heat pump, so you can probably control a minimum off time.

MN Renovator · 11-21-14, 07:52 AM

I just ran a little test.

There is 1 minute 15 seconds of burner run time prior to the blower starting.
Blower Time, Temp rise, percentage of max temp rise, %*57000BTUhr
1 minute, 25 degrees, 33%, 18810
2 minutes, 40 degrees, 53%, 30210
3 minutes, 50 degrees, 67%, 38190
4 minutes, 60 degrees, 80%, 45600
5 minutes, 65 degrees, 87%, 49590
6 minutes, 70 degrees, 93%, 53010
9 minutes, 75 degrees, 100%, 57000 (or about 10 minutes of the gas burner running which is actually 12 minutes of call for heat since my furnace takes 2 minutes from the call for heat until the burner kicks on)

After shutting down the furnace
1 minute, 50 degrees, 67%, 38190
2 minutes, 35 degrees, 47%, 26790

I'm not sure how to calculate all of this but if I add all of the figures together and average them I suppose I'll get somewhat close. It seems that I'm getting 67.75% of the steady state heat output in the first 10 minutes of operation including the cooldown period or about 38618 of the 57000. Which is 51.49% of the 75000 input rate. Seems a little hideous to me. What's worse is the previous owners had a thermostat that only had the gas valve running for 5 minutes which looks like an even worse proposition considering the temperature through the registers is still rising. If I take the 10 minutes and change it to 20 minutes, things look better at 91% of the heat output versus 67.75% of the 10 minute figure and 94% if we make a 30 minute run.

Granted, this math is rough but I think it illustrates the point.

Daox · 11-21-14, 08:33 AM

Interesting numbers. Ill have to do some data collecting of my own and share it.

Servicetech · 11-24-14, 09:11 AM

Installing a SMALLER FURNACE solves a lot of this. Cut the furnace size in 1/2 and run time each cycle triples. The time in between cycles doubles. Instead of running (4) 3minute cycles per hour, you run (2) 9 minute cylces. 12 minutes per hour @ 100K vs. 18 minutes @ 50k.

I found this when I changed my own furnace from 88k to 44k (yeah it was THAT oversized). The furnace actually runs long enough to get to to the full 45f rise instead of cycling off before reaching steady state. My funace runs 45 seconds before the blower kicks on, blower runs 3 minutes after burners switch off. There is about a 10f rise once blower kicks off. Even though the furnace cycles HALF as much thereis LESS overshoot/droop compared to the old furnace. Old furnace would overshoot 2-3 degrees, new only 1 degree.

gtojohn · 11-28-14, 11:42 PM

Try moving to a lower fan speed for heat. This should allow you a faster warm up and higher output tep. I improvised a " variable speed profile" for my blower interrupting the board output with a spdt fan relay and an adjustable delay on make timer. NC on relay was low speed and NO was high speed.

Servicetech · 11-29-14, 08:13 AM

Quote:

Originally Posted by gtojohn

Try moving to a lower fan speed for heat. This should allow you a faster warm up and higher output tep. I improvised a " variable speed profile" for my blower interrupting the board output with a spdt fan relay and an adjustable delay on make timer. NC on relay was low speed and NO was high speed.

Blower is single speed, one of my previous mods was replacing the OEM 3 speed 1075PRM motor with a single speed 825RPM. PSC motors have awful efficiency on low speed, old motor used 355W, new uses 150W. Temp rise did go up about 7 degrees when I changed motors. Furnace is rated 25-55f, 45f is on the high side, but still 10f from max.

I did do the mod you are talkind about on my old furnace. It had to be on HIGH speed to stay off limit, and the giant blower produced a cold draft when it 1st kicked on. A sequencer was my "time delay" of choice, had a fixed 30 second delay. Long enough to let the blower clear the cold air out of the ductwork while still on low speed.

Dumping the oversized beast and installign a furance 1/2 the size fixed a lot of comfort issues. No need for a low speed anymore since the blower isn't overpowering to begin with.

MN Renovator · 11-30-14, 10:04 PM

Quote:

Originally Posted by Servicetech

Blower is single speed, one of my previous mods was replacing the OEM 3 speed 1075PRM motor with a single speed 825RPM. PSC motors have awful efficiency on low speed, old motor used 355W, new uses 150W. Temp rise did go up about 7 degrees when I changed motors. Furnace is rated 25-55f, 45f is on the high side, but still 10f from max.

I did do the mod you are talkind about on my old furnace. It had to be on HIGH speed to stay off limit, and the giant blower produced a cold draft when it 1st kicked on. A sequencer was my "time delay" of choice, had a fixed 30 second delay. Long enough to let the blower clear the cold air out of the ductwork while still on low speed.

Dumping the oversized beast and installign a furance 1/2 the size fixed a lot of comfort issues. No need for a low speed anymore since the blower isn't overpowering to begin with.

Is the 44k an 80% unit?
44000*.8=35200
35200/(45*1.08)= 724 CFM
You are getting 724CFM from 150 watts? Is this 825RPM motor a PSC motor?

The reason I ask is I replaced my old shaded pole furnace motor with a PSC motor and I thought I was impressed by going from 804 watts down to 320 watts on the highest speed tap. I'm a little under 800CFM based on my own output capacity and temp rise.

11-20-14, 12:34 PM	#4
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Looks like I'll have to measure vent outlet temperatures. The cyberstat has a bunch of settings for temperature variance. I've never heard of any other thermostat with a +/- 3F swing on it. There is no setting for minimum run time, but it will control a heat pump, so you can probably control a minimum off time. Attached Thumbnails __________________ 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.

11-21-14, 08:33 AM	#6
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Interesting numbers. Ill have to do some data collecting of my own and share it. __________________ 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.

11-24-14, 09:11 AM	#7
Servicetech Apprentice EcoRenovator Join Date: Nov 2013 Location: Moore Oklahoma Posts: 267 Thanks: 108 Thanked 23 Times in 21 Posts	Installing a SMALLER FURNACE solves a lot of this. Cut the furnace size in 1/2 and run time each cycle triples. The time in between cycles doubles. Instead of running (4) 3minute cycles per hour, you run (2) 9 minute cylces. 12 minutes per hour @ 100K vs. 18 minutes @ 50k. I found this when I changed my own furnace from 88k to 44k (yeah it was THAT oversized). The furnace actually runs long enough to get to to the full 45f rise instead of cycling off before reaching steady state. My funace runs 45 seconds before the blower kicks on, blower runs 3 minutes after burners switch off. There is about a 10f rise once blower kicks off. Even though the furnace cycles HALF as much thereis LESS overshoot/droop compared to the old furnace. Old furnace would overshoot 2-3 degrees, new only 1 degree. Last edited by Servicetech; 11-24-14 at 09:13 AM..

11-18-14, 04:44 PM	#1
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

11-18-14, 07:32 PM	#3
Fordguy64 Journeyman EcoRenovator Join Date: Oct 2011 Location: Cincinnati ohio Posts: 338 Thanks: 40 Thanked 35 Times in 31 Posts	been away for a while but 2 weeks ago i replaced my 30 year old 40-60% eff (per the only info i could find on it) with a new 2 stage 95% eff furnace with ecm blower. i also replaced the lux thermastate that allowed me to adjust the swing with a nest learning thermostat. well its been very cold the last few days down into the single digits at night and upper 20s low 30s during the day. its hard to tell how long its actually run but i know it has only kicked into the high stage once (on its own) but i did force it before just to make sure it worked

11-21-14, 07:52 AM	#5
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	I just ran a little test. There is 1 minute 15 seconds of burner run time prior to the blower starting. Blower Time, Temp rise, percentage of max temp rise, %*57000BTUhr 1 minute, 25 degrees, 33%, 18810 2 minutes, 40 degrees, 53%, 30210 3 minutes, 50 degrees, 67%, 38190 4 minutes, 60 degrees, 80%, 45600 5 minutes, 65 degrees, 87%, 49590 6 minutes, 70 degrees, 93%, 53010 9 minutes, 75 degrees, 100%, 57000 (or about 10 minutes of the gas burner running which is actually 12 minutes of call for heat since my furnace takes 2 minutes from the call for heat until the burner kicks on) After shutting down the furnace 1 minute, 50 degrees, 67%, 38190 2 minutes, 35 degrees, 47%, 26790 I'm not sure how to calculate all of this but if I add all of the figures together and average them I suppose I'll get somewhat close. It seems that I'm getting 67.75% of the steady state heat output in the first 10 minutes of operation including the cooldown period or about 38618 of the 57000. Which is 51.49% of the 75000 input rate. Seems a little hideous to me. What's worse is the previous owners had a thermostat that only had the gas valve running for 5 minutes which looks like an even worse proposition considering the temperature through the registers is still rising. If I take the 10 minutes and change it to 20 minutes, things look better at 91% of the heat output versus 67.75% of the 10 minute figure and 94% if we make a 30 minute run. Granted, this math is rough but I think it illustrates the point.

11-28-14, 11:42 PM	#8
gtojohn Journeyman EcoRenovator Join Date: Jul 2014 Location: Austin, TX Posts: 314 Thanks: 39 Thanked 50 Times in 45 Posts	Try moving to a lower fan speed for heat. This should allow you a faster warm up and higher output tep. I improvised a " variable speed profile" for my blower interrupting the board output with a spdt fan relay and an adjustable delay on make timer. NC on relay was low speed and NO was high speed.