Thread: A few questions for those in the know

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Originally posted by troyorr
Go back to your fourth grade math class. 125,000 in and 100,00 out. Sounds like 80%.
Well gee, I'm not sure if this is meant for me or not. Prior to the post saying I could divide the numbers, they meant nothing to me, a homeowner. Then I had that post saying 80, which I went by. Until, that is, my local professional who is *bidding* on the job and actually looked at the furnace posted on this forum that its estimated efficiency is 60-63 based on the Preston guide. That means troyorr, you are calling one of two of your colleagues a liar, and me an idiot. I guess I can live with being called an idiot, but I'd be very interested to know which of the hvac pros are lying! Or "confused" if you prefer? Or is it a question of efficiency as new, versus efficiency at 30 years of age. What does *your* Preston guide say?

[Edited by uncleblaine on 10-20-2005 at 03:24 PM]

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125k in, 100k bonnet capacity means 80% efficiency running at steady state under perfect conditions. Calculating from bonnet capacity is by no means equivalent to AFUE, which considers standby losses, startup losses, etc. In other words, real world conditions.

Hell, I'm not even a pro and I know that. So maybe somebody should go back to fourth grade furnace class

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Originally posted by wyounger
125k in, 100k bonnet capacity means 80% efficiency running at steady state under perfect conditions. Calculating from bonnet capacity is by no means equivalent to AFUE, which considers standby losses, startup losses, etc. In other words, real world conditions.

Hell, I'm not even a pro and I know that. So maybe somebody should go back to fourth grade furnace class
Dude, sounds like you qouted right from GAMA.

But you are perfectly correct. There are many other things to consider than just dividing input by output.

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Thanks wyounger and curry. That makes me feel better. Being called an idiot is not nearly as distressing when the source is a 4th grade math whiz.

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I will second that thought uncleblaine. Talk to you tomorrow

Travis

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"Go back to your fourth grade math class. 125,000 in and 100,00 out. Sounds like 80%."

Wow, is it that simple?!? I've been waiting to hear that!That means my 50+ year old furnace that has a stamped metal plate saying 150000btuh in and 120000btuh out also has an 80% efficiency - cool!!!

"And old furnaces never lose efficiency. Or do they?"

I hope not, cause now that I know mine is an 80% I don't need to get a new one...

KJ

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I reread my post. I must have missed where I called my collegues liars or you an idiot. Go back and reread your original post. You ask about input and output. You didn't ask about anything else. If you divide output by input, you get a percentage. Now, try asking an informed series of questions on how to determine the btu requirements you need based on a specific set of guidlines we use to perform a heatload calculation and you'll find an answwer that will determine what size equipment you need. You can ask from now until hell freezez over about input/output of your furnace and the answer will always be 80%. Can't make it any clearer than that.

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Hope the smoke clears soon.

Still would like to know what the book says about the estimated AFUE (not efficiency, eh?) for my 1953 vintage Premier GA150.

KJ

[Edited by kjones on 10-20-2005 at 09:08 PM]

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Originally posted by troyorr
I reread my post. I must have missed where I called my collegues liars or you an idiot. Go back and reread your original post. You ask about input and output. You didn't ask about anything else. If you divide output by input, you get a percentage. Now, try asking an informed series of questions on how to determine the btu requirements you need based on a specific set of guidlines we use to perform a heatload calculation and you'll find an answwer that will determine what size equipment you need. You can ask from now until hell freezez over about input/output of your furnace and the answer will always be 80%. Can't make it any clearer than that.
I'll grant that you did not use those words. But based on posts by others that followed, I obviously was not the only one to interpret them that way. My original post is below:

My 2-story home in central Minnesota currently has a Ruud HR-125JD gas furnace. Does anyone have any idea what kind of efficiency this model has? I've owned the home for 9 years, but I've no idea when the furnace was put in, though I doubt it is an original 1958 model.

It's rating is 125000 BTU/hr with 100000 BTU bonnet capacity. Can I use that as a wild guestimate for proper sizing of a new unit? Also, I intend on replacing the current junk US Steel windows as part of the entire project. Is it safe to assume my BTU needs should be less with energy star windows? I've got calls to set up estimates with the local Bryant, Trane, and Rheem dealers but haven't heard back yet. Given the approaching cold and rising NG prices, if they get back in the next few days I'll be pleased. I'm thinking I want to go 90+ efficiency on the furnace and replace my 3 ton(?) AC unit at the same time. I have a Trion electronic air cleaner and a Skuttle humidifier that I would like to be able to keep using as they are under 3 years old. Is this realistic or a pipe dream?

Where do I ask input/output questions? I do ask if I can use those numbers for a wild guess at sizing of a new unit, so perhaps you took that to mean output? And remember, I read those numbers off a plate not having any idea what meaning they held. That's why I was here in the first place. Of course sizing of a new unit even if based on 100K output is dependent on the efficiency of that new unit, is it not? By the time you added your post alluding to my mathematical abilities, the discussion had evolved a bit. There is several pages in the thread...

And if it matters, based on a heat load calc I received today, a furnace with 125K input would be *perfectly sized* for my house if it was 36 percent efficient given the heat load of approximately 45K. See, I really CAN do math!

And if your point was semantic in my use of the term "efficiency" rather than AFUE, then you were obviously just itching for an argument and I'm glad I could help.

[Edited by uncleblaine on 10-20-2005 at 09:17 PM]

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Thank you for clearing that up and pointing out my short comings. Just want you to know, I took a call from MENSA and they told me you weren't being considered for membership. However, I also took a call from Sesame Street and they told me they were considering you for the assistant position to help the Count. For your information, if you replace the windows, insulate the floor, walls and ceiling and install true thermally efficient doors, you could heat your house with a match.

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Theory of Relativity Explained! !!

Originally posted by troyorr
For your information, if you replace the windows, insulate the floor, walls and ceiling and install true thermally efficient doors, you could heat your house with a match.
How many matches do YOU use to heat Your house each winter?

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Originally posted by troyorr
Thank you for clearing that up and pointing out my short comings. Just want you to know, I took a call from MENSA and they told me you weren't being considered for membership. However, I also took a call from Sesame Street and they told me they were considering you for the assistant position to help the Count. For your information, if you replace the windows, insulate the floor, walls and ceiling and install true thermally efficient doors, you could heat your house with a match.
Thanks troyorr. This really clears up the value of your opinions for me.

If energy efficiency in home heating is really as easy as you say, perhaps you should be in charge of national building codes.

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