Am I Under-firing my Furnace?

[hvacrmedic]

now your getting it, many of us have been speaking of heat-exchanger efficency. remember hot air heats the house, heat exchangers heat the air, and the fire( proper combustion, this is the only portion a combustion anayizer measures) heats the heat-exchanger.

so physics state there is greater heat transfer when there is a greater temperature differential. therfore a more efficient furnace has a hotter fire

That's incorrect. Energy efficiency and heat transfer efficiency are two entirely different things. If by a feat of engineering prowess a furnace is designed and set up so as to keep the stack temp and excess air constant when switching from low to high fire, then the combustion efficiency will be exactly the same in low and high fire. This is the whole point of O2 trim controls, and in most cases with O2 trim the combustion efficiency will be higher in low fire than in high fire, flame temperature notwithstanding.

[ch4man]

i wasnt speakin of two stage furnaces. only comenting on heat tranfer from fire to chamber to air in response to the OP's question.

i know and agree that low fire usually has a greater o2 content.

excess air ( and therefore o2 ) tends to cool the fire.

i still stand by my staement that a greater temperature difference, the greater heat transfer you have. just ask any wethead about radient panel temp....

[ebierley]

This furnace and its manual make easy to dial in the gas pressure. First you need to check static pressure to verify airflow then you can use the chart that is either in the install manual or the service manual which you can get at goodmanmfg.com. The chart gives you the temp rise +- 2degree for each airflow. You adjust the gas pressure to get the rise. As long as you are between 1.8 -2.2 and 3.2-3.8 you are good to go. Not that confirming by CA is a bad thing.

eric

[perfectionist]

This furnace and its manual make easy to dial in the gas pressure. First you need to check static pressure to verify airflow then you can use the chart that is either in the install manual or the service manual which you can get at goodmanmfg.com. The chart gives you the temp rise +- 2degree for each airflow. You adjust the gas pressure to get the rise. As long as you are between 1.8 -2.2 and 3.2-3.8 you are good to go. Not that confirming by CA is a bad thing.

eric

So if you follow the manual your good to go and if you follow the manual and check w/ CA and your oxygen is 12% at least you checked? I used to do it the same way before I bought my CA and knowing what I know now I wouldn't set up or check a furnace any other way.

[BigBacardi]

been there, you stated earlier that in a 2 stage furnace the inducer doesnt slow down enuf????? and to much excess air.....am curious, cause all 2 stage furnaces have 2 stage inducer motors, wouldnt the mfg install correct motor cfm for each stage?



or a imissing the obvious????

[beenthere]

been there, you stated earlier that in a 2 stage furnace the inducer doesnt slow down enuf????? and to much excess air.....am curious, cause all 2 stage furnaces have 2 stage inducer motors, wouldnt the mfg install correct motor cfm for each stage?



or a imissing the obvious????

The heat exchanger diameter, and the flue pipe diameter are size for second stage volume. So an inducer slowing to 66% of its full RPM will be moving more then 65% of the air it does in second stage. Hard to size an inducer to be able to handle max flue pipe length, and still slow down enough for first stage on on short flue pipe runs.

[perfectionist]

The heat exchanger diameter, and the flue pipe diameter are size for second stage volume. So an inducer slowing to 66% of its full RPM will be moving more then 65% of the air it does in second stage. Hard to size an inducer to be able to handle max flue pipe length, and still slow down enough for first stage on on short flue pipe runs.

I have found that by installing the restrictor in the intake improves combustion quite a bit. The manual says to install the restrictor on vent that is 10' or less, on all 40k furnaces some 100k that meet the right vent conditions and TO IMPROVE COMBUSTION. I wish the manufacture would give us real combustion numbers to work with, it would be a lot simpler for all of us.

[hvacrmedic]

i wasnt speakin of two stage furnaces. only comenting on heat tranfer from fire to chamber to air in response to the OP's question.

i know and agree that low fire usually has a greater o2 content.

excess air ( and therefore o2 ) tends to cool the fire.

i still stand by my statement that a greater temperature difference, the greater heat transfer you have. just ask any wet-head about radiant panel temp....

I was responding to the last sentence in your previous post, which you wrote in the form of a conclusion to the entire preceding argument. The argument was correct, the conclusion wasn't. I apologize for not clarifying.

[hvacrmedic]

This furnace and its manual make easy to dial in the gas pressure. First you need to check static pressure to verify airflow then you can use the chart that is either in the install manual or the service manual which you can get at goodmanmfg.com. The chart gives you the temp rise +- 2degree for each airflow. You adjust the gas pressure to get the rise. As long as you are between 1.8 -2.2 and 3.2-3.8 you are good to go.

eric

Hmmm, no. That isn't what the manual says.

[ch4man]

I was responding to the last sentence in your previous post, which you wrote in the form of a conclusion to the entire preceding argument. The argument was correct, the conclusion wasn't. I apologize for not clarifying.

no problem, many times i forget to form my responses in a way that answers both the OP, the guy im commenting to and to all the other readers.

40 years ago in english class, the public schools never covered " internet forum" composition