View Full Version : Furnace Efficiency

09-27-2005, 07:18 AM
I have an OLD Lennox Furnace from 1956. There is a sticker from insection at that time. It's getting replaced on Thursday. Everyone has looked at it has said 'Boy it's old' but no one can tell me anything about it. There is no model or serial number on it that I can find. Its clean and good shape but it rivals NASA's fuel bill for the space program (well...). It's only 110K BTUs. (Heat calc showed 63K BTUs needed). It is forced air natural gas, but it doesn't force much air. Blower is slow.
The heat exchanger looks like a barrel. A friend of mine has an average model from 1982 that's 60% AFUE which makes me guess this is probably in the 50% range.

Because I just bought the place, I don't know the PO heating habits, but my budget is based on them. Would have been $165/mon, year around.

Any ideas what the AFUE of furnace from this era would be?


09-27-2005, 08:06 PM
Somewhere on it, it should state the input and output btu...you can figure the efficiency from that. Look near the gas valve.

09-28-2005, 06:51 AM
When tearing it apart to get it out, I found that it said 110000BTUs and Bonnet Capacity 88000 BTU 88/110 is 80% but this isn't an 80% AFUE furnace.

Did a little research and found that older models were rated according to bonnet capacity. "...Bonnet capacity is the actual amount of BTU's that you get from a furnace during the run cycle. AFUE takes into account bonnet capacity, vent loss and off cycle loss during a period of one year.(1)" "There is no formula (to convert between the two.) The AFUE would actually be quite low since with this rating the manufactures must consider the warm up period and the pilot operation over a complete year (none because my old one was upgraded to no standing pilot) as a complete loss on the efficiency, even though the actual operating efficiency, bonnet capacity, may be 80 percent. (2)"

So, want to wager a guestimate? :) This thing had a pretty big flue (6in) and like I said the heat exchanger was like a large upside down barrel. I can see the flame would go right in, and heat out the flue keeping the chimney nice and warm. Heh



10-12-2005, 06:19 PM
We had a 1952 model Lennox furnace until just this summer when we replaced it. We only lived in the house one winter with it, but it really wasn't much of a (natural) gas hog.... The house is 1900 sq ft, and our winter bills averaged about $100. Considering this is North Dakota and it gets really cold here, that wasn't too bad.

When we were getting estimates for a replacement all of the companies were amused that we had such an old furnace. In fact the company we went with kept the old original thermostat for thier 'museum'! We are hoping that the new furnace will be much more effecient, but were told by the specialits that the one was probably running at about 70%. I don't know what the BTU specs were on it, but just wanted to share. Those old Lennox's were made to last! :)

10-12-2005, 07:52 PM
70%? Wow. Hope my old one was like 50% so I see a difference in the bills! Old budget was around $165/mo. I went w/ a new Lennox 80% (couldn't afford the 90+) so I hope it was a good decision.


10-13-2005, 05:08 AM
lakee911; when sizing a furnace 50 years or so ago it was the "norm" to oversize it considerably. Fuel way back then was very inexpensive, so oversizing it was of no real concern.
And the rated efficiency of some furnaces was only in the 50-60 % range.
You should feel fortunate that this old unit lasted this long.(they made them of good quality back then)

Enjoy your new model.