Yet another furnace upgrade question...

[Gerry Gardiner]

I live in MD and have a 30+ year old gas furnace that soon needs replacement. BGE is the local company and their man recommended a two-stage Carrier unit exactly matching my old one's BTU rating. In the last 2 years we have added insulation, replaced all doors, (particularly a sliding door that might as well have well been a hole in the wall!), and put in really great windows. With these upgrades, do we have to match BTU for BTU with the old and new units? From some other posts here, I wonder if it's necessary. Also, can anyone give me some feedback on BGEs estimates and savings with upgrades? Gerry

[bill allemann]

that's quite a lot of difference in heat load. you should get a new calculation done. the new furnace can no doubt be significantly smaller.

bill

[m Ransley]

Im sure a much smaller unit will work , you didnt even refer to Output btu, output btu will be much more with a 90%+ Carrier unit. Your sales guy is taking the typical lazy ass way out. First you may be oversized now, but with more insulation and more efficient units there are 3 reasons to demand to see a written load calc. Never replace till you get the load calculation.

[Bob]

The guy is most likely just a salesman who gets paid a percentage of the sale. I doubt if he did a heat loss calculation. Call around for more estimates until you find someone you can trust.

[barry]

Then "their man" is an incompetent or a liar. Didn't do load calc.

Tell BGE "no good, try again." And why.

DAGS on this; it is NOT news.

J

[Bubba]

He's done it all wrong. He needs to give you TWO furnaces that size...................NOT just one. It will heat your home much faster so you wont be burning that expensive natural gas long periods of time. This will relate to a huge savings. Bigger really is better, you know? I'll be over your way next week. I have a slightly used bridge that I need to sell and Im sure I can get you to buy it. Bubba

[Mark]

I thought furnace BTU ratings were output BTU or Bonnet BTU....

BTU per Hour actually.

And to the OP... look at the duty cycle (% of on time to off time) of your present furnace on the coldest days. If it is say 50% and you get a new furncace that is 50% BTU rating then it will run 100% of the time i.e. continuously on the coldest day...

Using the operation of your present furnace is probably the most accurate way to judge your BTU requirements.

Mark

[Speedy Jim]

That's correct. And you don't need to wait for the coldest day to make measurements. Pick a day when it's overcast, so there is little heat gain from the sun. Let system stabilize for say, 4 hours. Measure duty cycle. Now, if the outdoor temp has been steady 35F and indoor is 70F, the diff is 35 degrees.

Take the furnace "output" BTU/Hr rating and multiply by the %hour running time. That gives you BTU/Hr. Divide that by 35 to give BTU/Hr/Degree diff.

If the design temp (minimum) you want is, say, 10 below zero multiply by 85 (70 to -10) to give the BTU/Hr required. Add another 10% overcapacity if desired.

Jim

[Bubba]

Holy Crap, Jim. Ive never heard such silly shit out of you in my life. The guy needs a Manual J load calculation done on his home. PERIOD! That's THE ONLY way to know. Bubba

[Wayne Whitney]

Why should a theoretical estimate be better than a direct measurement?

Cheers, Wayne

[trader4]

"Why should a theoretical estimate be better than a direct measurement?

Cheers, Wayne "

That's exactly what I was thinking. Any inaccuracies in back calculating it from the duty cycle of his existing furnace is likely to be less than a theoretical calculation.

[Bubba]

Because a manual J load calculation done competently is not theory. It is based on actual numbers, data and fact. Its also standard of the industry. Your way is kind of like lighting match sticks until you have enough of them lit to warm the house sufficiently. Bubba