First off efficiency ratings, and capacity ratings are for SEA LEVEL. The air in Denver is much thinner to begin with. Even blower performance charts are somewhat inaccurate because they too are based on the air density at sea level. Assuming 5,000 - 6,000 feet. In some areas (per the amana i/o) the gas companies in high altitudes, may artificially derate the gas to compensate.
I'm more than a little concerned with "plug and play", Depending on the mfg, you likley need to have had an oriface change along with an adjustment for manifold pressure. The ECM motor software is programmed for sea level density.
I can safely say, that if your furnace produces water in low fire, its firing at over about 88% minimum.
you still need to clock the meter, know your gas value, and know your vent temps at a minimum.
Thanks to all for constructive responses.HVAC tech is coming tomorrow to check it out.
No, if a house needs 60,000 btu's output at design outdoor and indoor temp, it needs that much weather a single stage, or 2 stage furnace.
Originally posted by nord 64
I'm a little puzzled.
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.
Don't undersize or over size 2 stage firnaces.
The HVAC tech checked out my furnace today and found nothing wrong. Last night the condensate was collected in a pail and the furnace produced quite a bit of water running on first stage. I'm going to assume that the furnace efficiency is as good as it can be on both stages.
However, the tech recorded the mainfold pressure as 3.5 inch w.c., but the manual calls for an adjustment to 3.4 for altitudes 4501 to 5500 ft which includes the Denver area.
What is the impact on furnace operating conditions and efficiency with the higher than spec manifold pressure?
.1"wc won't affect it much.
What was the manifold pressure for first stage?
When I asked the tech about the first stage pressure, he replied that his testing equipment "Wouldn't measure that low".
Which leaves me in a quandary. It seems that neither the initial installing tech nor today's tech took the effort to ensure that the furnace was setup for altitude. Do I call the company's manager and report the problem, which may put the young fellows job at risk, or should I call another company and try to ensure that they have the interest to do the job completely?
I doubt it will put his job at risk.
I don't know what manometer he would be using that it couoldn't read 1.6"wc.
I'd call some one.
How is this company going to fix a low fire problem, if they can't check low output.
Yeah, But ...
It's Not Rocket Science, But It is SCIENCE
with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
This is the first 90% configuration furnace that I've had. The selling point is high efficiency, which costs a premium compared to an 80 percenter. Pros in this forum frequently state that the installation is more important than the brand.
How can I tell if it is heating "properly" and efficiently? Heat rise is within mfg's specs but isn't there more to a proper installation.An installation done to the manufacturer's spec would increase my confidence that the furnace is most likely functioning at its best.
Low fire is always less efficient than High fire when it comes to BTUs delivered to the air stream versus gas input. Your temperature rise is better than most but the blower could be running too slow. It really takes someone with a combustion analyzer to evaluate system performance along with measuring airflow. Running the blower too fast would also make the furnace less efficient because the air doesn't make enough contact with the heat exchanger to absorb any heat.
Originally posted by fat bob
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.
"Low fire is always less efficient than High fire when it comes to BTUs delivered to the air stream versus gas input."
If that is true, how come that the manufacturers state the opposite?
For example, on my 2 stage Lennox the %age of input BTU's versus output BTU's (delivered to the air stream) is higher in low fire.
Why would Lennox state wrong information in the manual and on the furnace with regards to input/output BTUs, and therefore efficiency?
I'm not convinced that low fire is less efficient just because a "pro" is writing it.
What am I missing?
I don't see how this can be true. With any hot object sitting in front of a fan, increasing the fan speed will increase the transfer of heat from the object to the surrounding air. It's just basic thermodynamics that heat transfer increases with the temperature difference between the air and the object. When air moves more slowly across the object, the air heats up more and the heat transfer slows down. But maybe I'm missing something.
Originally posted by Jim Davis
Running the blower too fast would also make the furnace less efficient because the air doesn't make enough contact with the heat exchanger to absorb any heat. [/B]
[Edited by fat bob on 02-10-2006 at 04:10 PM]
Now I'm getting more confused on first vs second stage efficiency. My non-pro tests on my Lennox 90% furnace indicated that high stage was more efficient than second stage. A Lennox factory rep said that efficiency was measured at second stage and they did not know what the first stage eff was. They are interested in second stage eff because that is the stage where the government specs are.
Now I'm reading that Lennox may also state that eff is higher in first stage. There is no output info on my Lennox.
And more efficient heat exchange at second stage's higher cfm makes some sense because we personally experience it as "Wind Chill" factor.