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Old 01-06-2010, 01:19 PM   #1
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Testing combustion on 95% furace?


A few question re combustion efficiency.

1) What is the best way check combustion efficiency on a high efficiency furnace?

2) A licensed furnace guy told me that if the flue gas has little or no smell, it could mean that the combustion efficiency is pretty good?

3) Is it possible that the vent motor can bring in too much air and cause the some of the gas to go below the LEL in the primary Hx tubes?

4) What are the main elements of combustion efficiency?

5) The power company here, will loan the customer tools (combustion gas analyzer, manometer, data loggers, thermometer, etc) if you take a couple energy classes (free) from them.

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Old 01-06-2010, 03:47 PM   #2
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Testing combustion on 95% furace?


There is nothing you can change to affect the combustion efficiency. It is determined by the design of the heat exchanger etc. The gas pressure and efficiency and readings continuosly modulate by the vacuum/draft in the burner box anyway. You should check the temp rise thru the furnace to make sure it is not too high or low. Read the specs by the model # info ie; 45-75F. Check the supply temp a foot downstream from the plenum and out of sight of the heat exchanger and subtract the return air/house temp.The gas pressure should be checked on both firing rates and set according to the manufacturers procedures (specialized on some units). That info is in the installation manual. All this info is in the installation manual under the startup section.The smell idea is useless.


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Old 01-06-2010, 03:59 PM   #3
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Testing combustion on 95% furace?


Quote:
Originally Posted by okrite View Post
A few question re combustion efficiency.

1) What is the best way check combustion efficiency on a high efficiency furnace?

With combustion annalyzer, manometer, and temp probes.

2) A licensed furnace guy told me that if the flue gas has little or no smell, it could mean that the combustion efficiency is pretty good?

It could mean its burning pretty bad also. Do you go around sniffing your cars exhaust pipe to see if its running right.

3) Is it possible that the vent motor can bring in too much air and cause the some of the gas to go below the LEL in the primary Hx tubes?

Yes it is possible. And there are several things that can be done to correct for it.
Not well known that it can be controlled.

4) What are the main elements of combustion efficiency?

Wow. that gets so involved.
So, just the rudiments. Fuel to air ratio, and amount of turbulance in the combustion cells/passage ways.

5) The power company here, will loan the customer tools (combustion gas analyzer, manometer, data loggers, thermometer, etc) if you take a couple energy classes (free) from them.
Are these classes on proper testing and burner set up?
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Old 01-06-2010, 05:19 PM   #4
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Testing combustion on 95% furace?


I agree with all the above. The big thing is gas pressure and temp rise through the furnace. The only change is in the vent temperature which is related to fan speed. If the O2 stays the same but the temp rise is high due to not moving enough air than the vent temp goes up which lowers the efficiency and visa versa.
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Old 01-06-2010, 07:28 PM   #5
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Testing combustion on 95% furace?


thanks beenthere and tk03

3) Is it possible that the vent motor can bring in too much air and cause the some of the gas to go below the LEL in the primary Hx tubes?

Yes it is possible. And there are several things that can be done to correct for it.
Not well known that it can be controlled.


I don't know what the flue temp is yet, kind of scared to drill the PVC.

The temperature rise is approx 50F (manufacturer 35F-60F)
I poked a hot duct and also measured in front of the RA grill.

could you point out some of the "several things that can be done to correct for it."

Am getting the CGA and mano tomorrow.
I took 5 weeks of HVAC classes at the power company in 2008.
I have worked on a few furnace installs 2 NG, 3 propane.
I was a GC, admit I am pretty green on HVAC.
Seems to be a little more to it than I thought.

Maybe not good to bring up on DIY site but I have to ask. Have you ever de-rated a H.E. furnace for size (not altitude)?

The DOE talks about de-rating and says its pretty tricky and you probably will need an old pro if you can find one who will do it due to liability and warranty issues.
Change it out? No. Will be adding more load as soon as in-law unit is vacant.
Need to add a few ducts to existing living room.
Yea, yea, yea go figure, right?

Last edited by okrite; 01-06-2010 at 11:01 PM.
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Old 01-06-2010, 07:41 PM   #6
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Testing combustion on 95% furace?


Changing how much air the intake/inducer has. is done more on a case by case. Bad idea to just try different things.
Report results back. And I'll tell you what should work best, and safest.

Temp rise is higher then need be(of course youo didn't say if thats with a new clean unrestrictive air filter, or a dirty one)

Silicone will fill your test hole.
When you get the CGA. If it has 2 external temp sensors. You need to drill a hole in both pipes.
If it has only one external temp sensor. You need to remove the intake pipe to test efficiency.

Furnace derating because of oversizing is a bad thing to do. And can cause more troubles then it fixes.
Along with decreased efficiency(loss of some of the radiant heat from the flame heat).
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Old 01-06-2010, 10:57 PM   #7
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Testing combustion on 95% furace?


couple years ago i looked at a small church complex for a retro-cx class. the building manager (volunteer parishioner) was old retired engineer. he had the original plans from the 1960s. included was a set of mechanicals, four or five pages.

furnaces were janitrol and the plans specified a certain size and then a certain de-rate. i can't remember exactly but it was something like 200,000 btu model # xxx-xxx-xxx de-rate to 165,000 btu.

the plans were done by a reputable mechanical engineer, they were wet stamped and signed; and approved by the building dept.

do you special order the furnace that way or does the contractor de-rate it?
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Old 01-07-2010, 05:04 AM   #8
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Testing combustion on 95% furace?


You need the manufacturer's approval.

If you don't have it. Its not allowed.

Those plans may have had a stamp. But, if the manufacturer didn't approve it. All liability was on the engineer. Since it would not have had a UL approval.

Back in the 60's, lots of dumb things were done. They didn't know all the problems that could crop up.

Induced draft units are not as forgiving as old natural draft units were either.
An old nat draft unit didn't lose control of the combustion air if you derated it. An induced draft unit does.
Furnaces 40 plus years ago ran high temp rises, so condensation in the heat exchanger was not common. Even on a derate.
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Old 01-07-2010, 09:58 AM   #9
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Testing combustion on 95% furace?


beenthere,

thread getting complicated
I top-posted a verification of what you said from DoE website.

thanks


Last edited by okrite; 01-07-2010 at 10:01 AM.
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