I'm concerned that first stage efficiency on 90% furnaces is less than 90%. On my own 90% furnace I have compared second stage increase in temperature rise and the cfm increase against the first stage. I am not a pro, but my tests indicate that the furnace is actually delivering 53% more heat on second stage while consuming only 47% more BTUs.(68,000 to 100,000).This plausible only if the first stage is performing less than 90%, more like 78-80%.
Since manufacturers state that 90% furnaces run on first stage 80% of the time,I called the furnace manufacturer's technical department and inquired about efficiency on first stage. The answer was that they don't know! Wanna bet? Their concern is meeting 90% efficiency on second stage only under laboratory conditions.
So, based on my time consuming, but not necessarily laboratory level testing; are professionally sized 90% single stage furnaces that are always on high stage the most efficient?
What were your tests?
You would need to know the actual BTUH input versus the BTUH output in both cases. You would need to know the actual manifold pressures, gas values, airflow and some other stuff.
Can you explain how you got to your numbers?
I haven't heard of any manufacturer stating that the low fire stage on a 90+% is 90+%, they carry that rating only for high fire.
As doc points out, your furnace could be underfired in first stage, and over fired in second stage.
Did you clock the meter for both stages.
Depending where you had your probe in the supply, it may not have been good air flow on first stage.
Did your installer set it up when they installed it, or just turned on the switch and left.
Fla cracker here so I dont get the good fortune of too many gas/lp/oil furnaces down here. I have had the good fortune of getting on some 80 and 90+'s here and there on occasion...Seems kinda common that the installers just hit the switch and call it a start up. I have come across more than an acceptable amount of units that havent had the gas valves set up properly or that couldnt have been checked on start up for adequate pres. cuz they were so far off. Thats pretty sad considering I dont see that many for service. I can see the efficiency dropping like a rock on a 90+ if the gas pres. were never dialed in right on start-up. I guess "Start-Up" is sometimes taken a bit too literally.. Combine that with the abundance of ductopi,(plural for ductopus I think), that we have here in the sunshine state and they all become 30+'s......hahahahahaha....l8r
Answering some of the questions:
Temperature rise was taken 2 feet from the plenum in a short duct. High fire maxed at 58 degrees and low fire at 45. I expected heat rises to be closer since the furnace has a vs motor. I have read on this forum that lower heat rise results in a decrease in efficiency.
Air flow was measured unprofesionally at the duct outlets (registers removed) numerous times with a flow meter purchased for that purpose. Also placed the flow meter stationary at some ducts to measure flow change with an identical flow meter setup.
Duct size, cfm volume change and the difference in heat rise allowed me to calculate at least an approximate delta % change in relative heat delivered at second stage over first stage. This calculated change was compared to the difference in btu inputs stated on the furnace "boilerplate . The calculated % heat increase delivered on second stage was higher than the boilerplate % increase in btu. A surprise.
When my furnace was installed, the installing company rep said that the furnaces were factory set up for Denver area altitude and the installers did a simple "plug and play" installation.
I called the manufacturer to see if there was a need to have a professional furnace inspection to determine if my furnace is setup properly. The response was your furnace heat rise is operating within spec and we don't know what our first stage efficiency is.
Does the fact that my tests were not entirely scientific mean that its acceptable to label furnaces as 90% efficient without verifying the efficiency at first stage which is the operating mode 80% of the time.
Shouldn't we be more concerned about first stage efficiency?
Since furnace efficiency is factory engineered for high stage only does anyone know what first stage efficiency really is compared to second stage?
The thing that you must, or will assume for the sake of argument you already do, is realize that the efficiency ratings are based on IDEAL conditions. If any of the condtions...(ie, ductwork in all respects, gas supply to name a couple) arent met, then your 90+ efficiency will drop and be sub par not to mention unacceptable in more severe conditions.
I agree that attention must be given to first stage performance. Given the fact that the number of demanding days requiring second stage are less, it would stand to reason why first stage performance would be more important from an efficiency stand point....
The fact that it was "set up" for Denver leaves me to beleive that was a nice way to say that they didnt check **** and they assumed everything was correctly adjusted.
My recommendation is to hire a quality hvac professional to go thru the sys properly like someone that takes pride in their work should. Providing that all the ideal conditions are met, you should have a sys that is capable of exceeding your monetary expectations.....l8r
I think this is much to do about nothing. First, there is no evidence to suggest that your furnace is not operating at the highest efficiency. Not to bash you, but your 'test' is meaningless.
You can always have a 2nd opinion on your installation done, but you haven't provide anything that warrent any concern from what I can see.
I haven't done a energy balance on a residential furnace but IF I would have to do one as an academic project in a home setting I would:
* Install temperature and flow meter at the flue this will provide enough data to calculate energy lost through flue.
* use the existing gas meter to calculate btu in.
And *assuming* all other energy lost are minimal when compared to flue.
Efficiency = (btu in - btu flue) / btu in
It seems to me that anyone doing these "tests", especially as anal as they're being, is looking to sue someone with deep pockets.
I smell Loi-yah. Probably from NYU.
Live for yourself and ask no one to live for you.
Low fire is not as efficent as high fire. Two stage furnaces are for comfort, you give up a couple percent or so in low fire.
The installer should have checked gas pressure's and clocked the meter as in the instuctons. This is important on a 2 stage.
The furnace isn't set up from factory for any one area.
They didn't check, so you don't know if its set up right.
A combustion eff test, and clocking of the meter neeeds done to know what firing rate each stage is at.
Your test are good enough to tell you the approx ouotput, but you don't know what the input is to tell eff.
You low fire input could be 7,000 btu low, and high 5,000btu high.
Have a combustion test done, and have them set up your furnace.
I'm a little puzzled.
I have a 2 stage high efficiency furnace and my input/output values as provided by manufacturer stating the following.
1st stage input 45,000 BTU's output 43,000 BTU's (95.55%)
2nd stage input 66,000 BTU's output 61,000 BTU's (92.42%)
Taking the percentages it tells me that the furnace is more efficient in the first stage. I know that in all likelihood these numbers are rounded up and/or down but it can not be off by that much.
If the second stage is relly more efficient, which I find hart to believe, that would be one more strong argument for t-stat setbacks.
I do not agree with the statement that 2 stage furnaces are for comfort and not for efficiency. I believe they are for both.
Because of the two stages you can get away with a smaller furnace... reduce cycling. Cycling will make you more uncomfortable therefore you need a higher t-stat setting. This combined with a larger single stage furnace will use more natural gas. Or not?
What do I know? I'm just a Homeowner.
I don't think that's necessarily true. Assuming no stage change, if I increase the blower speed on my furnace, the temperature rise will go down, but the efficiency will go up, because more heat is being extracted from the heat exchanger due to more air blowing around the exchanger. Less heat is being lost out the stack. There will also be less heat lost from the plenums and ducts through conduction and radiation, since they are cooler and heat loss is a function of the temperature difference between objects. Some furnaces therefore have an "efficiency" setting with a high blower speed and lower temp. rise, and a "comfort" setting with a lower speed and higher rise. Also, at a lower stage your blower arguably will run more efficiently because you have lower static pressure and less leakage.
Originally posted by whitedog
Answering some of the questions:
I have read on this forum that lower heat rise results in a decrease in efficiency.