View Full Version : Help with Understanding "delta t"
04-13-2012, 03:43 PM
Hi this is my first post and I apologize if it not in the right place. I am upgrading my ten year old gas furnace to a more efficient model. I had 4 contractors come and give me quotes. Out of the four one said I would need some new ductwork. My current furnace is 75,000 btu and the new will be 80k variable speed. The concern I have is. He says that my current ductwork is sized too small for a new larger btu system because of the Delta T as he calls it. I've never had any overheating problems. He says to get the warranty I need to make a few 1,000$ changes. How can I tell I'd he's being honest? I'm a single mother of three and every $ counts to me but... Of the four I like him the best.
Any help or questions I can ask him to help ease things would be great. Oh all he would be adding is a floor duct 6x18 and changing the plenum from 8x12 to 8x16.
04-13-2012, 05:07 PM
Is old furnace low efficiency? Are you looking at a new 80% or 90%?
Today's furnaces do need more airflow than the old ones due to efficiency. So a duct system that worked for an oldie could be too small for a new one. So he could be right that it needs more air capability.
Pretty rare we replace a 75K with an 80K. Most of the time we find the old one oversized. Wonder if he did a calculation or just put the closest thing he had to what was there? Certainly not good if he's replacing an lower efficiency 75K with a 95% 80K.
04-13-2012, 05:34 PM
Did any of them do a load calc? Is your current furnace a 80 or 90plus% furnace. What efficiency will the new furnace be.
Does your house need a larger furnace? Could a 60,000 BTU VS furnace heat your house.
04-13-2012, 05:59 PM
My old furnace is a 90% new I think is 95%. It's a trane xv95 if I'm correct, don't have papers in front of me. He said there is no 75 that's. Why we went 80 btu. My house is two story open loft about 2300 sqft.
04-13-2012, 06:38 PM
Trane doesn't have 75,000s. other brands do have 70,000s though, and a new 70,000 95% would give you 66,500 BTUs output, which is only 1,000 BTUs less heat then you have now.
Adding a return may get you enough return for the furnace, but did he check your supplies if you have enough of them.
04-13-2012, 07:26 PM
Without a sketch/ pics And the fact that the old system didn't have high temp air I would be inclined to suspect the guy you like is exaggerating the potential "problem"
A nice way to say he is applying a "rule of thumb OR he knows he is exaggerating ( lying)
Send a pic get a 2nd opinion BEFORE you pay the extra $
04-14-2012, 05:01 PM
Well, my home had a 140,000-Btuh Input Oil furnace, replaced it with a 60,000-Btuh Input 95% propane furnace & it handles the heating perfectly no matter the SW WI cold temps.
They need to do a Home Energy Efficiency Audit & duct system checked for tightness & efficient airflow; plus 'after' doing weatherization work plus reducing infiltration leaks, have them do a Manual J room by room heat-loss/heat-gain calc.
Willing to bet a 60,000-Btuh 95% would handle that new heat-loss situation & keep you very comfortable.
You don't want an oversized furnace & short runtime cycles, also get a Room TH with a SWING Setting so you can adjust runtime cycles...
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