Rheem furnace specs

[ampulman]

Copying from the specification sheet (.pdf) for Rheem furnace RGFD-07EMCK as follows:

Input-Max BTUh 75,000
Heating Capacity Max 72,000
Input-Minimum 30,000
Heating Capacity Min 28,200
AFUE 92.8%

I was under the impression that:

Heat input X AFUE = Heating Capacity

If this is true, then there are discrepancies in the document:
75,000 X .928 = 69,600, and
30,000 X .928 = 27,840.
Also, there are discrepancies listed in other sizes.

If I am mistaken, please explain how the capacities are calculated.

Thanks.

AM

[htrguy]

I don't have an exact answer but since no one else is chiming in here's my opinion.

Every time I question manufacturers on these kind of things/or read on the rules it seems to be a statistic's thingy that the Federal Registry (laws) control. AFUE is a cycling on/off equation that accounts for flue losses while the unit is off and jacket losses where heat escapes while the unit is idle and equations that are derived from the unit turning on and off. The actual running efficiency is different, probably higher. Then a certain number of units have to be tested that each have a different efficiency level when cycling and when running steady. Then you have to statistically average those #'s to say that 90% of the time the unit will be this efficient and deliver this many BTU's.
If you ask your question with the assuption that they don't know what's up you may be supprised at all the guidlines that are in play here and how often they change.

Or they made a mistake.

[frankt]

I don't think I want my Manual J cut so close that this matters anyway.

Frank

[ampulman]

Thanks for the response--makes sense.




I don't have an exact answer but since no one else is chiming in here's my opinion.

Every time I question manufacturers on these kind of things/or read on the rules it seems to be a statistic's thingy that the Federal Registry (laws) control. AFUE is a cycling on/off equation that accounts for flue losses while the unit is off and jacket losses where heat escapes while the unit is idle and equations that are derived from the unit turning on and off. The actual running efficiency is different, probably higher. Then a certain number of units have to be tested that each have a different efficiency level when cycling and when running steady. Then you have to statistically average those #'s to say that 90% of the time the unit will be this efficient and deliver this many BTU's.
If you ask your question with the assuption that they don't know what's up you may be supprised at all the guidlines that are in play here and how often they change.

Or they made a mistake.

Actually, looking over the entire Rheem furnace line, there are many 'mistakes'.

AM

[ampulman]

I don't think I want my Manual J cut so close that this matters anyway.

Frank

Good point, it's not a question so much of cutting it close, but rather to find a unit which will provide what I consider to be a reasonable safety factor over OD design temp.

Most manufacturers specs that I've seen are either too low (output) or a bit high (this is what they sell), and since none to date was willing to do a manual j, they couldn't back up their recommendation. Had to do my own HVAC-calc.

Besides, I like the Mod concept. Rep coming today to take measurements (Rheem dealer).

AM