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Old 09-29-2011, 10:17 PM   #1
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Furnace size selection


My home is 3,000 sq ft. - 1,500 main level, and 1,500 finished basement. I've had 5 estimates, and 4 reps conducted load calcs, but didn't look at as much detail as the other rep. I think the load calcs from the 4 reps were in the slightly less than 60,000 range, but they all recommended going up in size to the 80,000 BTU unit.

One rep conducted a more detailed load calc , measuring all rooms, windows size and direction, and ceiling height, age of house, etc, which resulted in a load calc of 56,000; therefore, he recommended a 60,000 BTU furnace. He did a calculation at -20 degrees outside, and 75 degrees inside, and indicated that 60,000 should be fine. I live in a colder climate, and he recognized my concern about not having a big enough unit (and I told him the other reps all recommended 80,000), so he wrote up the estimate for an 80,000 BTU unit, even though I think he was leaning toward the 60,000.

I'm going to have the final rep do the installation, as he was by far the most knowledgeable; however, I'd like feedback on size. How do I know what's best and what's the risk if we go too big or small?

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Old 09-29-2011, 10:34 PM   #2
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You need to have load calculation done.

http://hvac-calc.com/main.asp

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Old 09-29-2011, 10:41 PM   #3
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Originally Posted by JJboy View Post
You need to have load calculation done.

http://hvac-calc.com/main.asp

Uhm, did you not read the first post? JP's had 4 load calcs performed already.


I think the question is what are the concerns of each size unit offered, the pro's and cons.

I'd have to think that if everyone came up with 60,000 btu but just to be on the safe side said to go bigger than the only thing I can come up with is your home will heat up really quickly and possibly feel uncomfortably hot while on. You might end up putting more heat per hour into your home than it's losing.
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Old 09-29-2011, 10:44 PM   #4
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Furnace size selection


What size is in there right now?
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Old 09-29-2011, 10:53 PM   #5
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What size is in there right now?
Currently 90,000, but that's clearly oversized. My home is small for the neighborhood, so I think the builder just had a big order of all the same size. House was built in 2000 and I've owned it for two years and had constant furnace issues, so I'm replacing.
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Old 09-29-2011, 11:46 PM   #6
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Originally Posted by Doc Holliday View Post
Uhm, did you not read the first post? JP's had 4 load calcs performed already.


I think the question is what are the concerns of each size unit offered, the pro's and cons.

I'd have to think that if everyone came up with 60,000 btu but just to be on the safe side said to go bigger than the only thing I can come up with is your home will heat up really quickly and possibly feel uncomfortably hot while on. You might end up putting more heat per hour into your home than it's losing.

I need to stop drinking beer when I'm posting. I would ask why 4 load calculations? He could confirm the values of the load calc with the site I posted ......How about the duct width?
.
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Last edited by JJboy; 09-29-2011 at 11:53 PM.
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Old 09-30-2011, 12:37 AM   #7
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Furnace size selection


Well without looking at the load calcs I would have to say that the 60M would be the right choice but if you don't have the right size ductwork it doesn't matter.
A manuel "D" would tell you that info.
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Old 09-30-2011, 08:01 AM   #8
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Furnace size selection


What type of furnace are you contemplating buying? If you are going to buy a single stage high efficient, then I don't have much prefference over which model I'd use (other than if cooling fan cfm/ductwork sizing is a concern) If you are looking at multi stage then I would prefer the 80M.

The reasoning would be- if you are tight on the load calc at 60M your furnace would be running on high most of the time using a 60 which means you will hear it a lot more than if you had an 80 which will run on low a lot more of the time and be much quieter.

The big fuss about heat load calcs and sizing and such is really a bit of a mute point in my opinion with the multi stage equipment. (providing you are sure you have enough capacity to heat the building) If used with a multi stage stat or a comunicating stat then you aren't really using a certain size of heating unit any more anyways, and the efficiency remains the same no matter which firing rate the unit is running at, so there isn't as pronounced of a concern as there used to be by having an incorrectly sized unit.
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Old 09-30-2011, 12:17 PM   #9
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Dave - Yes, I'm looking at two stage - Trane 95% XV95 Two Stage Variable Speed Furnace. I'm thinking the 60,000 might be able to do the job, but based on what you're saying it seems the 80,000 would also be fine, and without any risk of being underpowered for those harsh Minnesota days. I'll probably go with the 80,000.
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Old 09-30-2011, 12:34 PM   #10
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A two stage furnace is designed for comfort. On a mild day it will run in low stage with a longer cycle to heat all rooms more evenly. If you oversize it you would lose this benefit and the unit will short cycle and not heat evenly.
I think that proper sizing is actually very important with two stage equipment. What is your reason for going with a staged furnace?
What kind of calculation did the first folks do if they didn't measure things?
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Old 09-30-2011, 02:02 PM   #11
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hvac122[QUOTE]A two stage furnace is designed for comfort. On a mild day it will run in low stage with a longer cycle to heat all rooms more evenly. If you oversize it you would lose this benefit and the unit will short cycle and not heat evenly.

Just for clarity

You are suggesting since he has a heat load calc of 56,000 based on a temp of -20 (and since he is in Michigan you know he will see -20 or colder, most every year for a couple weeks) that he should install a 60,000 btu unit @95%eff. which will just handle his 100% heat load at that condition, so that it will run in high fire the entire time, at high heating fan speed, instead of installing an 80,000 which would run at low fire and low heating fan speed a far greater portion of the heating season at the same efficiency?

You do realize that we are talking about less than a 10,000 btu difference at low fire. Just wondering how this will make his system short cycle when he is running at 40,000 btus in low with an 80M and 60,000 btu"s with a 60M in high fire?
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Old 09-30-2011, 05:28 PM   #12
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What I am saying is that a load calculation was done and there is absolutely no reason to oversize the equipment. The load calculation already has a fudge factor built in.

Properly sized equipment will give better comfort. Otherwise why even do a manual j.
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Old 09-30-2011, 07:48 PM   #13
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Furnace size selection


Better to have a 60,000 BTU running 24/7 on the coldest days, then a 80,000 nosily going in and out of second stage. The 80,000 will want tom move 320 CFM more air in second stage then the 60,00 will in second stage. The 60,000 will be more efficient in second stage then the 80,000 will be in first stage. Since the 80,000 would be in first stage more then the 60,000, the 980 will use more gas.
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Old 09-30-2011, 11:22 PM   #14
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I need to stop drinking beer when I'm posting. I would ask why 4 load calculations? He could confirm the values of the load calc with the site I posted ......How about the duct width?
.

It's all good, bud.

I'll try and make it up there and we'll hit Sixth Street, bar hopping, and then post drunk together from our cell phones.

Our posts will look like this: eYe wUv yUu AnD uU aNd Uu and geT a JumPer WirE, yUu niInNcoMpoOp.

I think that four is much better than one. Out of those the OP felt really comfortable with one, as she had comparative contractors doing the same thing and learned a little bit. Good for him/her if you ask me.

See you bud.
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Old 09-30-2011, 11:43 PM   #15
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Originally Posted by Doc Holliday View Post
It's all good, bud.

I'll try and make it up there and we'll hit Sixth Street, bar hopping, and then post drunk together from our cell phones.

Our posts will look like this: eYe wUv yUu AnD uU aNd Uu and geT a JumPer WirE, yUu niInNcoMpoOp.

I think that four is much better than one. Out of those the OP felt really comfortable with one, as she had comparative contractors doing the same thing and learned a little bit. Good for him/her if you ask me.

See you bud.

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