Two Stage Furnace Efficiency Compared

[ChuckHVAC]

For my own information and education, I decided to try to test the actual efficiency of a typical 2 stage residential gas furnace. There has been much discussion and disagreement about the actual efficiency of these units.
The bottom line is I recorded a steady state efficiency of 95.3% high stage, and a 93.6% low stage from a furnace rated at 95.5% AFUE.

If you care here is a little information:
I wanted to use a “value priced” furnace rather than a premium furnace. I used a Goodman GMV95070, 2 stage furnace with an ECM motor.

I tried to eliminate as many variables and potential measurement errors using the equipment I have available. For all the key measurements, I kept experimenting with my test stand and test equipment until I could get consistent and repeatable numbers.

The setup I settled on included converting the furnace to LP gas so that I could weigh in the amount of propane I used per hour. I trust that my fuel use numbers are very close.

I could write a several page report about the difficulty in obtaining consistent air flow and temperature readings in the supply duct. But after much experimenting with air mixers and duct setups, I have absolute confidence in all my temperature readings.

Total air flow was my biggest challenge. I used a “Duct Blaster” as a powered capture hood. I tried a balanced pressure method and I tried a static pressure matching method. I used a vane anemometer as a comparison, I watched the feedback from the ECM motor using a “Variable Zebra”, and I calculated CFM based on multiple pitot tube readings.
I trust my air flow readings are within 3% of absolute, and any error should be relatively consistent between the two stages.

For my purposes, I used the standard 1.08 x Delta T x CFM. I saw to need to correct for altitude etc., as those should be consistent for both stages.

I am going to try to paste my test data into my next post.

[beenthere]

Gee. you hardly tried to be accurate. :D

Cool, nice going, waiting to see data.

Oh, and thanks for posting it.

[ChuckHVAC]

My condensing furnace efficiency calculations using propane gas
Goodman GMV95070 furnace set up within manufacturers
recommended specifications.

Low Fire
Volts 116.5
Amps 3.74
Watts 344
Volt Amps 440
Power Factor 0.78
KW per hour 0.344
Manifold pressure "wg 6
Lp use, oz per hour 33
BTUH input based on orifice size and pressure 42,000
Actual BTUH input based on fuel use 44,443
Air delivery, rated in CFM 920
Air delivery actual CFM 985
Return temperature F 58
Supply temperature F 97
Delta T 39
BTU output, rated 40,110
BTU output, actual 41,595
Calculated steady state thermal efficiency 93.6%

Elect. Cost per therm @.10 per KWH $0.08
Gas cost per Therm @ $1.29 lb $1.50
Total Cost per Therm. $1.59

Combustion Analysis:
O2% 12.5%
Excess Air % 148.0%
CO PPM 3
CO2% 5.5%
intake temperature degrees F 59
exhaust temperature degrees F 74
delta T degrees F 15
Combustion eff. Per analyzer 99.40%

Flue BTU loss calculation, sensible heat only
flue gas F - combustion air F 15
exhaust gas CFM 121
BTUs lost out exhaust 1,957
BTUs not accounted for 891

High Fire
Volts 117
Amps 6.45
Watts 585
Volt Amps 760
Power Factor 0.77
KW per hour 0.585
Manifold pressure "wg 10
Lp use, oz per hour 44.5
BTUH input based on orifice size and pressure 60,000
Actual BTUH input based on fuel use 59,930
Air delivery, rated in CFM 1,325
Air delivery actual CFM 1,315
Return temperature F 59
Supply temperature F 99
Delta T 40
BTU output, rated 57,300
BTU output, actual 57,092
Calculated steady state thermal efficiency 95.3%

Elect. Cost per therm @.10 per KWH $0.10
Gas cost per Therm @ $1.29 lb $1.48
Total Cost per Therm. $1.58

Combustion Analysis:
O2% 10.3%
Excess Air % 97.0%
CO PPM 0
CO2% 6.9%
intake temperature degrees F 59
exhaust temperature degrees F 78
delta T degrees F 19
Combustion eff. Per analyzer 99.40%

Flue BTU loss calculation, sensible heat only
flue gas F - combustion air F 19
exhaust gas CFM 129
BTUs lost out exhaust 2,639
BTUs not accounted for 99
These numbers were carefully obtained but no one has verified them but myself.
They are only for informational purposes!

[2old2rock]

That's a lot of work there Chuck. I've always been skeptical of the claims that 1st stage is way less efficient.
I guess we now have some data to show the real comparison.
I'll gladly give up that 2 % for the low stage quiet and comfort.

[energy star]

Do you think your return air temp is low? Was that temp taken at the plenum? Do you have a return duct in attic or crawl space?
Lots of info, nice. What was the static pressure.

[ChuckHVAC]

Do you think your return air temp is low? Was that temp taken at the plenum? Do you have a return duct in attic or crawl space?
Lots of info, nice. What was the static pressure.

This is a furnace setting on a test stand in my 55-60 degree shop.
The TSP is about .2

[amd]

I'm surprised at how low the power factor is - expected 0.9+ for an ecm

[dan sw fl]

For my own information and education, ...

The bottom line is I recorded a steady state efficiency of 95.3% high stage, and a 93.6% low stage from a furnace rated at 95.5% AFUE.

For all the key measurements, I kept experimenting with my test stand and test equipment until I could get consistent and repeatable numbers.

The setup I settled on included converting the furnace to LP gas so that I could weigh in the amount of propane I used per hour. I trust that my fuel use numbers are very close.

I could write a several page report about the difficulty in obtaining consistent air flow and temperature readings in the supply duct. But after much experimenting with air mixers and duct setups, I have absolute confidence in all my temperature readings.

Total air flow was my biggest challenge. I used a “Duct Blaster” as a powered capture hood.

For my purposes, I used the standard 1.08 x Delta T x CFM. .

Using enthalpy to determine efficiency, I guess you might be at 95.7%?

You even addressed Power Factor!
[ which seems just slightly low and is still reasonable given differences in / uniqueness of shop set-up ]
_ G R E A T W O R K in paying attention to ALL Details !

I guess you have provided a real Boost to sales and use of 2-stage equipment. ! !! !!! !!!!

[ampulman]

Any commet(s) how the experiment (data) would correlate (if, at all) with modulating furnaces (Rheem, York, Trane).

In the case of the Rheem, the temperature rise remains virtually constant (my findings); don't know about the others.

Amp

[energy star]

Could you post a pic of the test station, and the use of the duct blaster.

[sktn77a]

Did the test system include any ductwork (ie did it include ductwork losses) or is it just looking at combustion efficincy?

[beenthere]

Thats a unit combustion and bonnet efficiency he posted.

System efficiency would have required measurements to be made at all the supply and return grilles.

[energy star]

OP said the system was set up at in his shop. So I do not know how many supply and return grills could have been tested. He also mentioned he used a duct blaster as a power hood, this I would like to see or hear more about. The return air temp is low 59 degrees is low. 10" w.c. could that be 11" for LP? The supply air is heat pump warm.

[beenthere]

He has a high air flow. But temp rise is still within allowable rate for many units.
If his shop was warmer. The supply temp would be warmer also.

[ChuckHVAC]

I'm surprised at how low the power factor is - expected 0.9+ for an ecm

I haven't broken it down but there is a transformer and a shaded pole inducer motor to consider. Also the ECM motors really mess with the sine wave.

[ChuckHVAC]

This data was from a preliminary test designed to answer the question " If I buy two stage furnace, and it is installed by a decent contractor, will it produce the promised efficiency on both stages?" Goodman specs call for Manifold pressures of 6' and 10", temp rise of 30-60, TSP of .1-.5, negative ID blower pressure of .5 low, .95 high, etc. I just hooked it up and turned it on and it was running pretty much within specs.
I purposely made as little modification or adjustment to the furnace as possible. Now that I have a test stand set up which produces fairly constant and repeatable results, I might start playing with air flow, vent length, manifold pressure, etc. (If I stay interested and find time)

[ChuckHVAC]

Did the test system include any ductwork (ie did it include ductwork losses) or is it just looking at combustion efficincy?

I have about 14 ft of insulated supply duct because that is what it took to get consistent face temperature and velocity. My duct losses are minimal, but probably account for some of the missing BTUs that weren't in the exhaust. Of course a furnace installed outside the envelope, or with ductwork outside the envelope, would have duct losses to consider. In our area most furnaces and ductwork are inside the envelope.

[dgruber]

ChuckHVAC,

I have posted this in the Commercial Forum as well. You are doing some good research, so I hope you can shed some light on this issue.

I need some clarification on the Aflue Ratings and overall system efficiency.

I am a contractor.Currently I have a job that I will be replacing the aged Carrier 4 ton package system with a existing single stage gas heat 115,000btuh. Cracked heat exchanger. This system serves a Church Chapel.

This current system is undersized, should have been at least a 5 ton.

A 6 ton is a good fit, with dual stage heat.

All sheet metal ducting and static pressure being created by the increased air volume will be acceptable. We may change the grills to keep some noise down to a minimum. We are still looking into the throw and decibel levels vs. a few grill manufactures.

MY question:

The Aflue ratings on Commercial 3 phase equipment is not a good as a residential furnaces? 80% seems to be the best.

Overall efficiency SEER is also not as good? 13 or above almost impossible to find.

My customer is comparing via internet efficiency ratings between residential vs commercial.

So, this is where my study and education on this comes to play.

I would assume since we a dealing with three phase motors and no variable speed blowers, that has some effect on efficiency. I thought Trane or American Standard was putting some single phase variable blowers in some three phase models. Please remember we are taking about package systems.

I do not know why the Aflue ratings are not the same.

My first guess is more pressure is put on the residential market for more efficient equipment with government regulations and less on the commercial equipment side.

Can anyone help with my continued ongoing education of 25 years in this always changing and exciting industry.

[energy star]

He has a high air flow. But temp rise is still within allowable rate for many units.
If his shop was warmer. The supply temp would be warmer also.

yes I understand, but would not a higher return temp change the results?

[beenthere]

yes I understand, but would not a higher return temp change the results?

Might change it a little.

But, increasing air flow more would bring it back.


But. For the test he is doing. Its not a real concern. Since a single stage in his same set up would be pulling in the same return air temp.

His test is only to compare first stage and second stage efficiency. Since they used the same air temp. It makes no difference.

[ChuckHVAC]

http://hvac-talk.com/vbb/picture.php?albumid=292&pictureid=6822
This is my "test stand" in it's current configuration.

[ChuckHVAC]

Could you post a pic of the test station, and the use of the duct blaster.

The following link is the Duct Blaster Manual, see chapter 13, it will do a better job than I will.
http://www.energyconservatory.com/download/dbmanual.pdf

Also here is a link to a study on measuring air flow where I first got the idea that I should use my Duct Blaster for this purpose.
http://efficientcomfort.net/documents/Measuring_Total_System_Airflow_in_Residential_HVAC _Systems.pdf

Personally my Duct Blaster results were not perfect, the numbers when compared to other methods I was using seemed to run about 3% high. My final numbers are actually a extrapolation using all my sources. I used the same method of calculation for all test so that it should not have much effect of the accuracy when comparing low to high stage.
If I stay motivated, my next idea for calibrating the Duct Blaster, at similar air flows, is to set up an air handler with a electric heaters and use the temp rise method to compare against.

[energy star]

I own a duct blaster. Just did not understand the configuration. Thanks for the pic. I also like the well designed supply plenum. System effect is nice to see. I also like how you compensated for flue length.

[CynicX]

http://hvac-talk.com/vbb/picture.php?albumid=292&pictureid=6822
This is my "test stand" in it's current configuration.

I'm going to start doing this on clean and checks....

[Phasewolf]

I would love to see a Rheem Mod hooked to your stand and tested at 40% 60% 80% and 100% just to see where it really stands.

[openclasspro]

i would love to see an american standard 3 stg modulating 96% er/60k btu with 900 control operating it hooked to your stand

[beenthere]

Everybody that wants to see certain unit hooked up and tested.
Send him the unit(his to keep after testing), and $2500.00 for his time and test equipment use, and posting of the results. :D

Chuck. I'm trying to get you some money. :)

[ChuckHVAC]

Everybody that wants to see certain unit hooked up and tested.
Send him the unit(his to keep after testing), and $2500.00 for his time and test equipment use, and posting of the results.

Chuck. I'm trying to get you some money. :)

Thanks beenthere, I'll accept any submittals under beenthere's terms! I'd even print you up a fancy report!
I do enjoy this stuff, but I do have to make a living most days.
If someone gave me a grant and a pile of new furnaces to test, I'd be in hog heaven.
I'd much rather stay in the shop and experiment then be out on a rooftop in the cold. :D
Seriously when I'm done playing with this one I would like to test something with a ECM motor on the inducer.

[EmilG]

I'm surprised at how low the power factor is - expected 0.9+ for an ecm

An ECM doesn't have to have a PFC front end as shown in this case.

Actually correcting for power factor is something the power company wants but it may actually make the ECM controller less efficient. There are losses in the PFC circuitry. The electric company likes PFC because it reduces the amount of current flowing in their wires which is a loss for them.

It can be confusing.

[jerryd_2008]

Everybody that wants to see certain unit hooked up and tested.
Send him the unit(his to keep after testing), and $2500.00 for his time and test equipment use, and posting of the results. :D

... :)

Is that cheaper than AHRI? Maybe the big guys like Trane, York, Rheem, etc. might bite.

[arc8]

Maybe we can all donate money for a certain type of equipment/system.

I'm interested to see how and where this goes!

[gary_g]

Where is Jim Davis when you need him? :eek: