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Old 12-22-2010, 10:07 PM   #16
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Derating a multi-stage furnace


I wasn't going for effenciency, I was looking for total comfort, but the side effect of full modulating is less overall energy consumption. I only tested HX temps when setting up the controls not true combustion efficiency.

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Old 12-22-2010, 10:22 PM   #17
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Derating a multi-stage furnace


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I wasn't going for effenciency, I was looking for total comfort, but the side effect of full modulating is less overall energy consumption. I only tested HX temps when setting up the controls not carbon output.
Modulation doesn't decrease energy consumption. It increases comfort.

Still takes the same amount of BTUs to heat the house to the same temp, weather firing at 40% or 100%. The only savings modulation can provide. is if the modulation allows you to set the thermostat setting to a lower temp. that requires less gas to maintain.

Often. The mod furnace is a higher efficiency furnace that was in the house(EG: the old furnace was an 80%, or a 90% and the new mod is a 95%) . And the customer confuses the fact that its a higher efficiency furnace, with it also being a mod furnace. And doesn't realize the fuel savings is just because its a 95%, not because its a mod.
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Old 12-22-2010, 10:23 PM   #18
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Derating a multi-stage furnace


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I wasn't going for effenciency, I was looking for total comfort, but the side effect of full modulating is less overall energy consumption. I only tested HX temps when setting up the controls not carbon output.
So you could be running at 280% excess air, and not getting full benefit of the higher efficiency of the furnace.
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Old 12-22-2010, 10:30 PM   #19
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Derating a multi-stage furnace


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Modulation doesn't decrease energy consumption. It increases comfort.

Still takes the same amount of BTUs to heat the house to the same temp, weather firing at 40% or 100%. The only savings modulation can provide. is if the modulation allows you to set the thermostat setting to a lower temp. that requires less gas to maintain.

Often. The mod furnace is a higher efficiency furnace that was in the house(EG: the old furnace was an 80%, or a 90% and the new mod is a 95%) . And the customer confuses the fact that its a higher efficiency furnace, with it also being a mod furnace. And doesn't realize the fuel savings is just because its a 95%, not because its a mod.
Ahh I agree to an extent.

The gains in running a full mod system they run longer at lower outputs which results in not having to reheat the equipment and ducting there-by gaining in overall system efficiency.

That being said, I built mine for overall comfort and cleaner air as the fan runs longer.
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Old 12-22-2010, 10:52 PM   #20
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Derating a multi-stage furnace


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Condensation only happens if the HX reaches dew point, which only happens if the air velocity across the HX is too high for the BTU output of the burner.

And no you can't just change the orfice, nor did I ever suggest that.
And what the Sam Hill velocities do you think most ECM motors are operated at?
You live Mi fer cryin' out loud and unless you have never done a resi insall
you know this state is horrendous for under sized duct work in almost all areas. which means working the VS motors harder than it would in properly sized duct work.

I'v seen 1200 sq ft homes with the and the VS set at 1200cfm to stop the furnace from hitting limit.

That is a perfect scenario for a derated furnace to cause dew point temps.

And if you have never seen there are several 60btu furnace/four ton drives. Some wacked-out contractor was derating these 80% furnaces to
50btu and during wet-time (the time from burner ignition to the time the flue gas gets the chimney hot enough to draft) the flue gases were falling below dew point in the heat exchanger.

Hell I even had a customer who had his stat set to 62* and the new furnace we installed showed signs condensation in the heat exchanger on the first year inspection. We had to take extra ordinary steps in order to shorten the wet-time by getting the supply air temp up.

Dew point corrosion occurs more often than you realize.
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Old 12-22-2010, 10:56 PM   #21
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Derating a multi-stage furnace


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Modulation doesn't decrease energy consumption. It increases comfort.

Still takes the same amount of BTUs to heat the house to the same temp, weather firing at 40% or 100%. The only savings modulation can provide. is if the modulation allows you to set the thermostat setting to a lower temp. that requires less gas to maintain.

Often. The mod furnace is a higher efficiency furnace that was in the house(EG: the old furnace was an 80%, or a 90% and the new mod is a 95%) . And the customer confuses the fact that its a higher efficiency furnace, with it also being a mod furnace. And doesn't realize the fuel savings is just because its a 95%, not because its a mod.
~snicker~ Fire one!

And Been didn't even go to Ferris
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Old 12-22-2010, 11:00 PM   #22
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Derating a multi-stage furnace


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So you could be running at 280% excess air, and not getting full benefit of the higher efficiency of the furnace.
When I set up the system I used 100% output (120,000 btu/hr) as the basis for combustion air and circulation air.

100% out = 120,000 btu's/hr - 100 cfm combustion air -2000 cfm circ air
90%= 108,000 btu's/hr - 90 cfm combustion air - 1800 cfm circ air
80% = 96,000 btu's/hr - 80 cfm combustion air - 1600 cfm circ air

All the way down to 50%, full modulation changes made in increments of .01%, so to say there was excess air, I tried not to.
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Old 12-22-2010, 11:08 PM   #23
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Derating a multi-stage furnace


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~snicker~ Fire one!

And Been didn't even go to Ferris
Neither did I
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Old 12-22-2010, 11:15 PM   #24
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Derating a multi-stage furnace


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Neither did I
obviously


don't mind me. just bustin' yer chops...it's a Mi thing
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Old 12-22-2010, 11:25 PM   #25
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Derating a multi-stage furnace


You should beware, arguing with an engineer is like wrestling a pig in the mud, chances are you won't win and the pig loves it.
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Old 12-23-2010, 02:55 AM   #26
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Derating a multi-stage furnace


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You should beware, arguing with an engineer is like wrestling a pig in the mud, chances are you won't win and the pig loves it.
Thats ok. Both my kids are Engineers. One heads a dept at Bosch .
The other one is a GM design engineer.
Guess what Dad's full credentials are?
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Old 12-23-2010, 04:49 AM   #27
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Derating a multi-stage furnace


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Originally Posted by Jackofall1 View Post
When I set up the system I used 100% output (120,000 btu/hr) as the basis for combustion air and circulation air.

100% out = 120,000 btu's/hr - 100 cfm combustion air -2000 cfm circ air
90%= 108,000 btu's/hr - 90 cfm combustion air - 1800 cfm circ air
80% = 96,000 btu's/hr - 80 cfm combustion air - 1600 cfm circ air

All the way down to 50%, full modulation changes made in increments of .01%, so to say there was excess air, I tried not to.
And how did you measure your combustion air. That would come out to roughly 357% excess combustion air for all of the inputs you posted.

Yes. Long run times eliminate some of the start up losses. Since the modulated firing rates under 100% are at a lower efficiency then the 100% input rate. It becomes more of a wash. And sometimes a loss. Then an increase of total efficiency.

Manufacturer's Mod furnaces slow the inducer down more percent wise then the gas input percentage. Your furnace is not set up right for modulation.

You may have condensation in the primary heat exchanger where you don't want it. And it is operating at far less then its 95% rating.

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