What size BTU?

[joenic27]

I am getting bids to replace my furnace and ac. One contractor is talking of upgrading the furnace from the current 75,000 BTU unit to a 100,000 BTU. The others have recommended the opposite, saying the larger unit will actually run more and use up more energy and I would be better off staying with 75,000 BTU. The home has about 12-1300 square feet of living space. I am interested in opinions from those with more experience than me in these matters. Thanks.

[ambarton]

do yourself a favor and find a contractor with some idea of heating and air conditioning fundamentals and ask them to do a load calculation and to let you see the results or better yet purchase the program from this web site and do it yourself. too big a system CAN be a big problem

[lvieira]

I agree with ambarton, find a contractor that will run an acca manual J and acca manual D.

[turtle]

This is Turtle.

1] A 100K BTU furnace will run less than a 75k BTU furnace , but the 100K BTU furnace will burn more fuel than the 75K BTU furnace and who knows what fuel cost difference would be by using either one. the only thing your looking for here is if the 75K BTU furnace is big enough and if the 100K BTU furnace is too big for the application. In some cases being too big on the gas furnace is as bad as not being big enough. You can always add a little heat to suppliment the 75k BTU but it is hard to take away from a gas furnace ability for the application.

So , You have not give me enough of info of your house , area of country, how the 75K BTU furnace worked in the past, Heat load data to calculate with, and any modifications to your house made that would change the size of furnace used.

So, with present data i would have to say use a Heat load calculations run on the house and use the correct size that away. now if you answered the above i could maybe guess at the size thought about.

TURTLE

[NormChris]

Only a load calculation can give an accurate answer. If your contractors ran one they should be able to show you a copy.

Read this:
http://hvac-talk.com/vbb/showthread.php?threadid=33866

[joenic27]

I appreciate all of the input, and will look at that calculator. This is the first house I have owned, so buying a central unit is a new experience. To answer Turtle, I live in Missouri where it can go to extremes in summer and winter, but rarely stays severely cold (teens or lower) for extended periods--as it might a little further north in Iowa. The old furnace is 20 years old, and actually worked pretty well considering that fact, perhaps night quite as efficient as we would have liked. The blower unit went out, which prompted the necessity to shop for a new HVAC system, and we had been considering it anyway. Is there a chart, listing design temperatures by state or region, or is this more or less a personal figure based on one's own comfort level as well as region? Thanks.

[turtle]

This is Turtle.


Well with this info and what you said before. Go with the 75k BTU furnce and leave the 100k BTU furnace alone. You said the old 75k BTU furnace was fine for keepng up but just cost too much to run. if something is doing fine , don't change it.

i'm in Louisiana and have 2250 sq. ft. and which that i would have never went with the 100K BTU furnce in it. with you in Ms and only 12 to 1300 sq. ft. you don't need not 100K in there.

Now as to the cooling size of the unit. It is hard to tell about it guessing like this so i say go by the cooling load calculations of the dealer and install that size cooling system. Any dealer that is in on the know of the hvac business can get you a Cooling load calculations for your house.

Now there is a few sites to see about this but none will take the place of a good cooling load calculations to tell you what exactly what size cooling system you need. Don't guess at cooling loads for they will bite you later on in down the road.

TURTLE