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Old 12-22-2010, 11:02 AM   #1
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Derating a multi-stage furnace


I few months back I asked a question in a thread about derating a furnace that was slightly larger than was needed. The consensus from the pros was that doing so was a bad idea.

I did not think of it then, but generally speaking, what is the difference between running a 3-stage furnace that has been slightly derated in its high stage when the same furnace may run for weeks or months in its low or medium stages when the demand is low?

Does it have to do with somehow matching the blower cfm/rpm for high fire with the assumed, correctly set output to keep the temp rise within the proper range?

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Old 12-22-2010, 12:16 PM   #2
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Derating a multi-stage furnace


The reason is the systems are design tested for the firing rates they use.
Too lower that could cause a multitude of problems. You could cause the inside of the ht ex to rust if the lower burner temp of the products of combustion fall below due point. Soot could form. And the pressure switches would no longer work like they were intended to,

Too many other things that only a design engineer could tell you.

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Old 12-22-2010, 12:44 PM   #3
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Derating a multi-stage furnace


If you are talking about a 3 stage or say a closed loop controlled burner with matching speed ranges to burner output percentages then I believe you are correct, there would be no difference, as the equipment is designed to run at the different outputs, why would one want to use equipment to large for the job.

If you are talking about a standard single stage controlled furnace then I would be very wary on trying to derate as you will not be running the HX in its designed temp range.
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Old 12-22-2010, 01:16 PM   #4
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Derating a multi-stage furnace


I fully understand the potential problems with a single stage furnace.

My question is specifically about a 3-stage furnace where the high fire could be reduced from, say, designed output of 76k to a 70k output. How is the derated high fire output of 70k different or worse than the 2nd-stage output of 52k? In other words, how is a reduced high fire of 70k harder on the furnace and component than the designed, 2nd stage output of 52K?

Thanks.
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Old 12-22-2010, 02:12 PM   #5
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Derating a multi-stage furnace


The only difference would be fan speed, match the fan speed with the output and there should be no problem.

In your example you are only talking an 8% change, hardly worth messing with.
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Old 12-22-2010, 02:32 PM   #6
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Derating a multi-stage furnace


Quote:
Originally Posted by civiltoatee View Post
I fully understand the potential problems with a single stage furnace.

My question is specifically about a 3-stage furnace where the high fire could be reduced from, say, designed output of 76k to a 70k output. How is the derated high fire output of 70k different or worse than the 2nd-stage output of 52k? In other words, how is a reduced high fire of 70k harder on the furnace and component than the designed, 2nd stage output of 52K?

Thanks.
The burner orifice size dictates the btu out put. Derating the orifice would decrease the out put of all stages...not good for reasons you already have been advised of.
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Old 12-22-2010, 02:34 PM   #7
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Derating a multi-stage furnace


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Originally Posted by Jackofall1 View Post
The only difference would be fan speed, match the fan speed with the output and there should be no problem.

In your example you are only talking an 8% change, hardly worth messing with.

lot more to be considered than just changing the fan speed.
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Old 12-22-2010, 03:36 PM   #8
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Derating a multi-stage furnace


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Originally Posted by hvaclover View Post
lot more to be considered than just changing the fan speed.
Yes, not really alot more, but let me rephrase,

The fan speed needs to be matched to the delta T across the HX to stay within the allowable ranges of the HX design parameters. hvaclover, that was implied mentioned in my previous post.

I am also inclined to say in the example given 76K to 70K or 8% of design output wouldn't make a hill of beans difference.

There is greater allowable tolerances in HX design given many people don't change filters, contractors install with poor duct conditions, etc, etc... which dramatically reduces air flow across the HX thereby increasing Delta Ta.

I would still like to know why an individual would use a larger furnace than needed, bigger is always more $$.
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Old 12-22-2010, 06:03 PM   #9
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Derating a multi-stage furnace


Regarding Jackofall1's question about oversizing: I was more interested in getting a better match for the high output stage as compared to a Manual J calc for heating loads that fall between two furnace sizes. If your MJ is 65k (and don't want to undersize at 60k in, 57k out) and you purchase a furnace at 80k in and 76k out for the high setting, it would be a better match if you derated the high stage to 68k or 70k out.
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Old 12-22-2010, 06:12 PM   #10
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Derating a multi-stage furnace


On a 3 stage furnace. The high fire is NOT derated.

It has 3 firing rates. And each rate is accompanied with the inducer running at a speed for that firing rated.

Carrier did a lot of testing before they released their 3 stage. They didn't just haphazardly throw a 3 stage valve on a furnace and say we have a 3 stage furnace.
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Old 12-22-2010, 06:13 PM   #11
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Derating a multi-stage furnace


As Clover said, the orifices would derate all 3 stages if smaller and at the lowest stage that would result in an unstable flame, condensation inside the primary heat ex and other BAD problems.
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Old 12-22-2010, 06:28 PM   #12
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Derating a multi-stage furnace


You are taking man J as religious Gospel. It is far from a perfect science in it's application.

The man J calcs result in sizes that seldom match exactly with any mfgs eqquip ratings.

I had a small home that I put a furnace in. I had to use a 75KBTU furnace.

The home needed a 60Kbth. But since the customer did not want to pay the price that the 60k cost (it was available only in higher end line) I was stuck going with a middle of the road furnace. That furnace was a 75Kbtu and the next available size down was a 50kbtu--too small for this home.

We as contractors have to make the decision whether or not to go with a slightly higher rated furnace because the customer may not want another brand except what you sell. Or he may have a price that he can't afford to exceed. And the result is always a little bigger furnace than man J calls for.

So you see worrying about 6kbtu is just a non-issue.
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Old 12-22-2010, 08:26 PM   #13
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Derating a multi-stage furnace


civiltoatee - Try looking at a Rheem Prestige it is full modulating from 40% to 100% in 5% increments.

The Rheem Prestige Series™ 90 Plus Modulating Gas Furnace equipped with the Comfort Control² System™ is seldom “off” and seldom “on” at 100%. Your Rheem Prestige Series™ 90 Plus Modulating Gas Furnace equipped with the Comfort Control² System™ is so quiet, you may never know it’s on. And it’s so frugal with both gas and electricity, that it has perhaps the highest overall efficiency* of any furnace.

I built one of these for my home, off a 95% effecient furnace, long before they ever came out. Just took the gas output from a standard valve through a full modulating 4-20 ma motorized valve controlled by a Allen Bradley SLC processor motors driven by small VFD's to vary speeds of the inducer and recirculating fans according to burner output. Home is wonderfully controlled temperature wise, and may have an additional $350 in equipment to run it. I would bet that the control of the system I built outperforms the Carrier or the Rheem as I am controlling the output of the burner and fans with a true closed loop controller.

So to those Neh sayers, you have to think outside the box
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Old 12-22-2010, 08:29 PM   #14
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Derating a multi-stage furnace


Quote:
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As Clover said, the orifices would derate all 3 stages if smaller and at the lowest stage that would result in an unstable flame, condensation inside the primary heat ex and other BAD problems.
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.
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Old 12-22-2010, 08:59 PM   #15
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Derating a multi-stage furnace


Throw a CA on your furnace. And see how efficient it really is. Check it in several firing rates.

The Rheem 90 mod, is the lowest efficiency mod furnace made.

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