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Old 11-12-2011, 07:47 AM   #1
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Confused about propane vs natural gas AFUE ratings


HVAC contractor left me some brochures for the Goodman gas furnaces he sells, and I went to the Goodman site for more tech info (I like to read the fine print ).

I'm looking at the GKS9, GMH95, and GME95.

Something that confuses me is that both the GKS9 and GMH95 show the output BTU's significantly lower for propane than for natural gas at any given furnace size.

As as example, the GKS9 70,000 BTU unit shows:
input BTU of 69,000BTU
NG output of 64,400 BTU
LPG output of 57,753 BTU
If this is correct, then the unit would have an AFUE of 93% on NG but only 84% on LPG????

However, the GME95 80,000 BTU unit shows the output BTU's being the same for propane and natural gas:
input BTU of 80,000 BTU
NG output of 76,000 BTU
LPG output of 76,000 BTU
Both which would be an AFUE of 95%.

Can anyone explain this discrepancy to me? I thought that any gas furnace had the same BTU output using NG or LPG as long as the conversion orifice and pressure was correct.

Thanks

Michael

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Old 11-12-2011, 08:25 AM   #2
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Confused about propane vs natural gas AFUE ratings


probably a misprint. goodman should have a 1-800# or contact us link on their web page. try those.

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Old 11-12-2011, 05:59 PM   #3
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Confused about propane vs natural gas AFUE ratings


Interesting question and you should get an answer to your question.
That said I guess I would question what your house needs output wise.
If your contractor has done a manuel "J" and a manuel "D" then you would know what the answer is,if not then he is just making a wild guess with your utility bills for the next 20 years.Its not his utility bills so why should he care.
Some contractors make me laugh until I consider what it costs people when they are wrong ,then it makes me mad.
They want your hard earned money for a furnace but they don't want to do the work to get your money in an honest way.
If they are going to take short cuts at this point what makes you so sure they won't take installation short cuts installing the furnace.
Your money is valuable ,make them earn it,don't just give it to them and hope you get a good job.
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Old 11-12-2011, 08:40 PM   #4
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Confused about propane vs natural gas AFUE ratings


"Manual"
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Old 11-15-2011, 08:25 PM   #5
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Confused about propane vs natural gas AFUE ratings


Well talk about frustrating....I emailed Goodman asking for clarification of if/why some but not all of their gas furnaces had lower BTU outputs for LPG compared to NG, and I got a 1 line response from a customer service rep saying "there is a misprint in the spec sheets, all our gas units have reduced output when operating on LPG".

That's great, but that doesn't tell me why, but more importantly it doesn't answer my main question asking if the AFUE rating is the same when burning LPG compared to NG.

I called back to speak to a tech support person but the customer service rep won't allow me to because I am not an HVAC tech. I emailed again asking for more information, and they sent me copies of the spec manuals that I already downloaded off their site. This really upsets me as I'm a tech in another field (telecom) and like to spec out my own stuff, and not having access to the info I need to make an informed decision drives me crazy. This isn't a slam against Goodman, at least they have some "tech" available for download to the consumer, as the other big names seems to offer even less info, but regardless there is a discrepancy I need to resolve.

Can anyone tell me if they've ever heard of the AFUE begin reduced in a gas furnace when using LPG? I've searched the web and cannot find anything about this.

Thanks

Last edited by mrennie; 11-15-2011 at 08:37 PM.
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Old 11-15-2011, 08:43 PM   #6
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Confused about propane vs natural gas AFUE ratings


I looked at the specs for a Lennox G61V and they don't show any difference in AFUE between nat gas and propane. Not sure why Goodman does. Tried to post a link but they locked down the site to techs only or the server is offline now.
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Last edited by yuri; 11-15-2011 at 08:47 PM.
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Old 11-15-2011, 09:53 PM   #7
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Confused about propane vs natural gas AFUE ratings


Hi Yuri,

Thanks for the feedback.

Just to be clear, I'm not saying that Goodman shows the AFUE as being lower for LPG compared to NG, but that their documentation shows the BTU output is lower for LPG compared to NG, and that because their documentation only lists one input BTU (and does not specify whether this input BTU is for NG or LPG units), I'm left to wonder if the input BTU for LPG is the same as for NG (which if so, means the AFUE is lower for LPG units), or if the input BTU is actually lower for LPG and the corresponding lower output BTU for LPG still calculates to the same AFUE.

Hope this makes sense?
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Old 11-15-2011, 11:19 PM   #8
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Confused about propane vs natural gas AFUE ratings


Does the manual say what is involved in the conversion process? There is a different pressure used for LPG than NG and it may have to do with thier conversion process.
Some manufacturers use a new orfice kit and new spring in the gas valve to get the same input on LPG, others just adjust the regulator pressure in the furnace and use the same orfices. Just wondering?
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Old 11-16-2011, 05:51 AM   #9
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Confused about propane vs natural gas AFUE ratings


LP has a different BTU content than NG


1 Cu. Ft. of Natural Gas 1,030 BTU
1 Cu. Ft. of Propane 2,500 BTU

found this page also...
http://www.human.cornell.edu/dea/out...e&pageid=47808
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Last edited by hvactech126; 11-16-2011 at 06:25 AM.
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Old 11-16-2011, 06:15 AM   #10
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Confused about propane vs natural gas AFUE ratings


Quote:
Originally Posted by HVACDave View Post
Does the manual say what is involved in the conversion process? There is a different pressure used for LPG than NG and it may have to do with thier conversion process.
Some manufacturers use a new orfice kit and new spring in the gas valve to get the same input on LPG, others just adjust the regulator pressure in the furnace and use the same orfices. Just wondering?
The manual shows the orifice being changed, as well as readjusting the pressure.
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Old 11-16-2011, 06:20 AM   #11
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Confused about propane vs natural gas AFUE ratings


propane has a slightly higher heating value / cubic ft or meter. not sure if that has anything to do with it.

http://www.engineeringtoolbox.com/gr...ues-d_420.html

gives off more gases/quantity and the heat exchanger is a fixed volume/size so that is why the output is SLIGHTLY lower. should not make a HUGE difference in AFUE IMO but I have not studied exactly how they determine AFUE. too much trivia/info causes my old bean to ache.

google: propane heating value for lots of interesting info.

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Last edited by yuri; 11-16-2011 at 06:24 AM.
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