Two stage furnace

petehouk · 01-21-13, 12:05 PM

Hi everybody. I am considering a new thermostat, and I have a question about single- vs. two-stage furnaces.

I have a two-stage, forced air, gas fired furnace. I was looking for a new thermostat recently, and one that I was considering said that it was only compatible with single-stage furnaces. Until then, I didn't know that there was a difference between single- and two-stage thermostats.

So I looked at the installation instructions from my furnace, and it has a few different ways to wire up the thermostat; the one that my installer used was "two-stage heating with single-stage thermostat." There was also an option for "two-stage heating with two-stage thermostat."

Is there an advantage to using a two-stage thermostat in conjunction with a two-stage furnace? I paid extra for the two-stage feature. Am I getting the full advantage from it? Or will I get better performance with two-stage thermostat?

I am working on my basement now, and, among other improvements, I am tidying up the wiring before I paint the joists. If I need to replace the 4 conductor thermostat wire with a 5 conductor (which the two stage thermostat needs), then this would be the time to do it.

stevehull · 01-21-13, 12:26 PM

Pete,

Exactly what is the second stage on your gas burner? Does the fan turn faster (more CFM)? or does the furnace turn up the burners and the fan speed.

petehouk · 01-21-13, 01:20 PM

Quote:

Originally Posted by stevehull

Pete,

Exactly what is the second stage on your gas burner? Does the fan turn faster (more CFM)? or does the furnace turn up the burners and the fan speed.

I'm not totally sure.

But I looked through the installer's guide. There are Input BTUH ratings for 1st stage (52,000) and second stage (80,000), so I am assuming that the burner has two different stages.

The fan definitely has at least two speeds. It starts on a slower speed and then after a few minutes I can hear and feel it blowing faster.

BTW, model number is TUD080R936K.

Daox · 01-21-13, 01:33 PM

My guess is he just has it wired up to blast out the 80k BTUs. The advantage of the lower BTU setting might be slighly higher efficiency and less cycling of the furnace. If you have a thermostat with that option I'd definitely rewire it for 2 stage.

petehouk · 01-21-13, 02:28 PM

I just found a note on the wiring diagram that says that if this furnace is wired into a single stage thermostat, then second stage heating will begin 10 minutes after first stage.

I don't exactly know how a two stage thermostat would be better...would it sometimes detect that first-stage heating is "good enough" since the temperature was rising quickly? And in that case it would save gas by not kicking on the second stage?

Lately I have been cutting down on natural gas usage by using the wood stove instead. I don't think the furnace runs enough for me to realize any appreciable energy savings from a two stage thermostat. But, I would like a way to run my furnace fan a little more often. I'll start a new thread on that.

Edit: here's the link to my thread about getting the fan to run more often:
http://ecorenovator.org/forum/conser...html#post27607

Daox · 01-21-13, 02:34 PM

I'm not sure of the logic of it all. But I would think that the first stage (lower BTU) would be used to hold a set temperature in the house. The second stage would kick in when you have a large temperature gap and need to bring the house up to temperature. For instance I set my temperature to 60F at night and 68F when I'm home. To get the house to heat up more quickly it would kick on the 2nd stage, but once it is there it would use the first stage. Using the first stage will reduce cycling and probably keep the house's average temperature more constant which will make it feel more comfortable. I would also think the first stage is probably a bit more efficient.

stevehull · 01-21-13, 03:50 PM

Pete,

On the coldest of days, how much does the gas furnace run? Today is probably a good day to test (actually tomorrow AM).

For example, if you are just running on stage 1, and on the coldest of days it is running almost all the time (90%+) then do not connect up the second stage.

The highest efficiency is observed when the furnace is running 100%.

And the lower stage is more efficient.

Hope this helps,

MN Renovator · 01-21-13, 05:52 PM

The lower stage is actually less efficient based on everything I've ever read from the HVAC-talk folks. It's still over 90% efficient and condensing but I think more heat is proportionately lost through the exhaust in lower stage. According to them lower stage is more of a function of comfort and that you really need to make sure your upper stage is sized properly for the load and not just used as a 'just in case' or 'bigger is better but let's make lower stage correct but have a higher stage anyway'. I'm running a heat load test right now. This morning the heat load was under 25k the entire morning from midnight until sun-up at -10f(average -7f). Tomorrow morning will be a deeper load but it seems that the only way to get the furnace to run constantly will likely be in the -15f to -20f range for a 25k stage. I'm not actually looking to get a 2 stage though but will get a variable speed blower with a condensing furnace to remove the exhaust and subsequent backfill infiltration needed to cover the flue losses. This should reduce the heat load even more. Longer cycles are a positive thing though, very nice for comfort and might be better on efficiency depending on how long the cycles are in each stage. For me, a 40k would draw the 12 minute cycles with 10 minutes of gas burn to about 14-15 minute cycles in moderate weather and 20 minute cycles on days like these to about 26 minutes or possibly longer versus my current 57k natural draft furnace.

stevehull · 01-22-13, 09:05 AM

MN is exactly correct on gas burner efficiency. I am including the energy necessary to run the higher fan speed in the stage 2 setting (less efficient) together with the efficiency of the burner (higher) in stage 2 together with the more rapid cycling in stage 2 (lower efficiency).

In general, the longer a gas burner runs, the more efficient it is. But MN is correct that other issues are factors as well.

Some of the variable gas inlet values do allow for adjustments so that you can tweak the settings for maximal efficiency. Don't know if this gas fired unit has that.

Gas fired units that have an external temperature sensor do use the delta T (inside to outside temp) to maximize efficiency, use long run times and allow the unit to provide enough heat for the outside conditions thereby minimizing cycling.

The variable scroll Copeland compressors on heat pumps do exactly the same thing. Their advantage for dehumidification is superb as run time is long thereby maximally reducing the latent humidity (dewpoint) while minimally lowering the inside temperature.

Hope this helps . . .

Servicetech · 11-09-13, 09:31 AM

Quote:

Originally Posted by MN Renovator

The lower stage is actually less efficient based on everything I've ever read from the HVAC-talk folks. It's still over 90% efficient and condensing but I think more heat is proportionately lost through the exhaust in lower stage. According to them lower stage is more of a function of comfort and that you really need to make sure your upper stage is sized properly for the load and not just used as a 'just in case' or 'bigger is better but let's make lower stage correct but have a higher stage anyway'. I'm running a heat load test right now. This morning the heat load was under 25k the entire morning from midnight until sun-up at -10f(average -7f). Tomorrow morning will be a deeper load but it seems that the only way to get the furnace to run constantly will likely be in the -15f to -20f range for a 25k stage. I'm not actually looking to get a 2 stage though but will get a variable speed blower with a condensing furnace to remove the exhaust and subsequent backfill infiltration needed to cover the flue losses. This should reduce the heat load even more. Longer cycles are a positive thing though, very nice for comfort and might be better on efficiency depending on how long the cycles are in each stage. For me, a 40k would draw the 12 minute cycles with 10 minutes of gas burn to about 14-15 minute cycles in moderate weather and 20 minute cycles on days like these to about 26 minutes or possibly longer versus my current 57k natural draft furnace.

The fact you have a 25k loss with a 57k furnace installed is just more evidence of how much furnaces are routinely oversized. Most 2stage units you can simply unhook the 2nd stage and never miss it.

01-21-13, 12:05 PM	#1
petehouk Lurking Renovator Join Date: Dec 2012 Location: Ann Arbor, MI Posts: 13 Thanks: 0 Thanked 0 Times in 0 Posts	Two stage furnace Hi everybody. I am considering a new thermostat, and I have a question about single- vs. two-stage furnaces. I have a two-stage, forced air, gas fired furnace. I was looking for a new thermostat recently, and one that I was considering said that it was only compatible with single-stage furnaces. Until then, I didn't know that there was a difference between single- and two-stage thermostats. So I looked at the installation instructions from my furnace, and it has a few different ways to wire up the thermostat; the one that my installer used was "two-stage heating with single-stage thermostat." There was also an option for "two-stage heating with two-stage thermostat." Is there an advantage to using a two-stage thermostat in conjunction with a two-stage furnace? I paid extra for the two-stage feature. Am I getting the full advantage from it? Or will I get better performance with two-stage thermostat? I am working on my basement now, and, among other improvements, I am tidying up the wiring before I paint the joists. If I need to replace the 4 conductor thermostat wire with a 5 conductor (which the two stage thermostat needs), then this would be the time to do it.

01-21-13, 12:26 PM	#2
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Pete, Exactly what is the second stage on your gas burner? Does the fan turn faster (more CFM)? or does the furnace turn up the burners and the fan speed. __________________ consulting on geothermal heating/cooling & rational energy use since 1990

01-21-13, 01:33 PM	#4
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	My guess is he just has it wired up to blast out the 80k BTUs. The advantage of the lower BTU setting might be slighly higher efficiency and less cycling of the furnace. If you have a thermostat with that option I'd definitely rewire it for 2 stage. __________________ 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.

01-21-13, 02:28 PM	#5
petehouk Lurking Renovator Join Date: Dec 2012 Location: Ann Arbor, MI Posts: 13 Thanks: 0 Thanked 0 Times in 0 Posts	I just found a note on the wiring diagram that says that if this furnace is wired into a single stage thermostat, then second stage heating will begin 10 minutes after first stage. I don't exactly know how a two stage thermostat would be better...would it sometimes detect that first-stage heating is "good enough" since the temperature was rising quickly? And in that case it would save gas by not kicking on the second stage? Lately I have been cutting down on natural gas usage by using the wood stove instead. I don't think the furnace runs enough for me to realize any appreciable energy savings from a two stage thermostat. But, I would like a way to run my furnace fan a little more often. I'll start a new thread on that. Edit: here's the link to my thread about getting the fan to run more often: http://ecorenovator.org/forum/conser...html#post27607 Last edited by petehouk; 01-21-13 at 04:53 PM.. Reason: adding link to another relevant thread

01-21-13, 02:34 PM	#6
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	I'm not sure of the logic of it all. But I would think that the first stage (lower BTU) would be used to hold a set temperature in the house. The second stage would kick in when you have a large temperature gap and need to bring the house up to temperature. For instance I set my temperature to 60F at night and 68F when I'm home. To get the house to heat up more quickly it would kick on the 2nd stage, but once it is there it would use the first stage. Using the first stage will reduce cycling and probably keep the house's average temperature more constant which will make it feel more comfortable. I would also think the first stage is probably a bit more efficient. __________________ 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.

01-21-13, 03:50 PM	#7
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Pete, On the coldest of days, how much does the gas furnace run? Today is probably a good day to test (actually tomorrow AM). For example, if you are just running on stage 1, and on the coldest of days it is running almost all the time (90%+) then do not connect up the second stage. The highest efficiency is observed when the furnace is running 100%. And the lower stage is more efficient. Hope this helps, __________________ consulting on geothermal heating/cooling & rational energy use since 1990

01-21-13, 05:52 PM	#8
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	The lower stage is actually less efficient based on everything I've ever read from the HVAC-talk folks. It's still over 90% efficient and condensing but I think more heat is proportionately lost through the exhaust in lower stage. According to them lower stage is more of a function of comfort and that you really need to make sure your upper stage is sized properly for the load and not just used as a 'just in case' or 'bigger is better but let's make lower stage correct but have a higher stage anyway'. I'm running a heat load test right now. This morning the heat load was under 25k the entire morning from midnight until sun-up at -10f(average -7f). Tomorrow morning will be a deeper load but it seems that the only way to get the furnace to run constantly will likely be in the -15f to -20f range for a 25k stage. I'm not actually looking to get a 2 stage though but will get a variable speed blower with a condensing furnace to remove the exhaust and subsequent backfill infiltration needed to cover the flue losses. This should reduce the heat load even more. Longer cycles are a positive thing though, very nice for comfort and might be better on efficiency depending on how long the cycles are in each stage. For me, a 40k would draw the 12 minute cycles with 10 minutes of gas burn to about 14-15 minute cycles in moderate weather and 20 minute cycles on days like these to about 26 minutes or possibly longer versus my current 57k natural draft furnace.

01-22-13, 09:05 AM	#9
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	MN is exactly correct on gas burner efficiency. I am including the energy necessary to run the higher fan speed in the stage 2 setting (less efficient) together with the efficiency of the burner (higher) in stage 2 together with the more rapid cycling in stage 2 (lower efficiency). In general, the longer a gas burner runs, the more efficient it is. But MN is correct that other issues are factors as well. Some of the variable gas inlet values do allow for adjustments so that you can tweak the settings for maximal efficiency. Don't know if this gas fired unit has that. Gas fired units that have an external temperature sensor do use the delta T (inside to outside temp) to maximize efficiency, use long run times and allow the unit to provide enough heat for the outside conditions thereby minimizing cycling. The variable scroll Copeland compressors on heat pumps do exactly the same thing. Their advantage for dehumidification is superb as run time is long thereby maximally reducing the latent humidity (dewpoint) while minimally lowering the inside temperature. Hope this helps . . . __________________ consulting on geothermal heating/cooling & rational energy use since 1990