Checking combustion efficiency on 95% furnace


Old 01-06-10, 11:15 AM
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Checking combustion efficiency on 95% furnace

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 01-06-10, 05:52 PM
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A high efficiency furnace be out of adjustment like any other furnace, but that's pretty rare.

Odd odors in the combustion gasses suggest the equipment is out of adjustment and should be inspected.

Some of the things that could be inspected or be a problem would be having the wrong sized burner orifices or burner manifold pressure, inadequate or tainted combustion air, motor speed too high or too low, poorly adjusted primary air shutters.

The rating plate on the furnace lists some specifications, such as heat rise, BTU input, static pressure, parts per million CO in the combustion gasses, indications of cracks in the heat exchanger and so on. Other specifications will appear in the installation manual.

Checking out such things is not a good DIY job, in my opinion. There are LOTS of things that could be a problem, and a DIYer, even with some training is not going to be able to recognize more than a few actual or potential problems.

A skilled repairman can size up a furnace pretty rapidly. It would take a lot longer to go through the actual testing for each of the parameters listed above. More than likely doing that isn't needed on a number of those unless there is an indication that there is a problem some place.

Explain your concerns to the repairman and let him be your guide as to what should be checked and tested.
Old 01-06-10, 07:32 PM
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Combustion Efficiency

To answer the question of components of combustion efficiency testing: Oxygen (or CO2), CO, intake air temperature, exhaust temperature, & btu input.

Something of which you should be aware: Combustion efficiency will always be higher than AFUE rating. Other factors come into play for the AFUE rating.
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