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Furnace size question
Old 10-13-2009, 12:13 PM   #1
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Furnace size question

Well I have 4 quotes in hand to replace furnace and a/c.
I've narrowed it down to 2 companies. Carrier and Bryant.
They did their ACCA Manual J calculations, and came up within the 85,000 to 87,000 btu range.
The Carrier guy says that a 80,000 btu furnace is plenty.
The Bryant guy says I should go with a 100,000 btu furnace.
Live in NW Pennsylvania. Get a lot of snow (144 inches last year), and winter temps mainly fall in the 10-30 degree range. We have minus degree days, but not many.
Trying to get a handle on whether the 80,000 is enough furnace.
Or if the 100,000 is too much.
Any help would be appreciated.
Rob
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Old 10-13-2009, 12:41 PM   #2
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What did the other two guys say? Maybe that would give you some idea as to the proper size.
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Old 10-13-2009, 12:42 PM   #3
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Grew Up in NW PA...

...so I know all about the snow and cold that you get, from living near Lake Erie. I suggest you take your question(s) to the HVAC-Talk website - they have a Residential board where they allow people to post their HVAC issues and the pros respond - as long as you don't discuss pricing or any Do-It-Yourself-type issues. Here is the link: Residential HVAC - HVAC-Talk: Heating, Air & Refrigeration Discussion

The pros usually need information such as the size of the house, its age, and then the model numbers of the equipment you got bids on - just no prices. By the way, they will be happy to see that the potential HVAC companies did Manual J calculations - that is one item they seem to always recommend be done.

Good luck.
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Old 10-13-2009, 12:48 PM   #4
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I've heard that if you're within 10% of the BTU rating, you can go smaller. Otherwise, get the next one up. That would say 80,000 would be ok. Either should work.
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Old 10-13-2009, 12:49 PM   #5
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First, congratulations on finding anyone that would actually do the Manual J calculations. Business must be slow, maybe the HVAC guys are now willing to go the extra mile.

You don't want a unit that is too big or too small. Do you know (or can you find out) what desired internal temperature they modelled? If they chose 65 deg F and you like it at 72 in the winter, then that's something to consider.

Also, what is the rating of your present furnace, and is it sufficient? The ratings on most furnaces are the input BTUs (i.e. the BTU value of the natural gas going into the unit) not the output, so you have to multiply by the unit's efficiency to find the otput BTUs. If your present furnace puts out 72,000 BTUs and you are happy with it, then the 80K BTU 95% efficient new furnace should do the trick.

Too small and you'll be cold on a few days per year. Too big and the unit will cycle on and off too often and reduce the life of the ignitor and efficiency of the unit overall will be less. If the bigger unit also has a bigger fan motor than you need, it can make your AC less effective (will leave too much humidity in the air in the summer).
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Old 10-13-2009, 06:41 PM   #6
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I replaced my furnace this spring. I could not get anyone to do a real manual J calculation so I did my own, then cross checked it by comparing my actual gas usage vs heating degree days for the previous year. The furnace contractors all used some rule of thumb and recommended way too big a furnace. For them, it is no lose to recommend too big. If too small, you might complain on a very cold day, but too big and you never know the difference, except for the higher bills and frequent cycling.
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Old 10-14-2009, 04:32 AM   #7
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What did the other two guys say? Maybe that would give you some idea as to the proper size.
One guy said 75,000, and other guy said 100,000.
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Old 10-14-2009, 05:00 AM   #8
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I will check out that HVAC forum.

In regards to my current furnace/ac unit, it is 30 yrs old and the guys who came in say it is 150,000? btu, if I recall correctly. Obviously way too much furnace. They said it is about 50-60 percent efficient.

They used 71 degrees for the internal temperature model.

Thanks for the all the input.

Rob
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Old 10-14-2009, 08:46 AM   #9
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If the other guys actually did proper Manual J calculations, it's hard to understand why their estimates of the needed equipment size varied by 25%.

50% to 60% efficiency sounds about right for a 30 year old furnace in typical condition. If that's right and if it is rated at 150K BTU (it will tell you on a data late attached to the furnace), it is delivering 90,000 BTU of heat to your home. According to the Manual J calculations, that's just about right. This is surprising, since builders tend to install oversize furnaces, people tend to replace them with the same (oversize) equipment, and you've probably made improvements to the energy efficiency of your home in the last 30 years that would reduce your max heating demand. Here's the test: If this current furnace remains running on a nearly continuous basis (more than approx 80% of the time) on your very coldest days, then it is probably sized about right and in your shoes I'd be reluctant to install an 80K BTUH 95% efficient furnace (that will actually put out 76K BTUH).

On the other hand, if your present furnace is only running 50% of the time on your coldest days, then the 80K BTU furnace would be fine.

Other observations:
- As you probably know, Carrier and Bryant are made by the same company and the equipment is virtually identical.
- The 71 deg F internal modeling gives you some wiggle room. If you'd be alright with an internal temp of 65 deg F on the very coldest days, that might alow you to use slightly smaller equipment. Likewise, if your ductwork has sufficient capacity, closing the registers in one or two unused rooms could help you on the very coldest days.
- A single case study, FWIW: I did a heat load calculation on my house and insalled a furnace that was about 10% larger than called for (it was the closest size available and much smaller than the old furnace). In the five years since, it has never run more than approx 40% of the time, even on the coldest days.
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Old 10-14-2009, 09:10 AM   #10
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If the other guys actually did proper Manual J calculations, it's hard to understand why their estimates of the needed equipment size varied by 25%.

The guy who came up with 87,000 from the Manual J calculation took extensive notes while at the house. He felt more comfortable with the 100,000 btu furnace. And I felt more comfortable with him.
The guy who recommended the 80,000 btu furnace, said that would be sufficient. He took notes, but didn’t seem as thorough as the guy above.
The other two guys I eliminated were more like salesmen, did not take many notes and I did not feel comfortable with their overall presentation.


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Originally Posted by samclem View Post
50% to 60% efficiency sounds about right for a 30 year old furnace in typical condition. If that's right and if it is rated at 150K BTU (it will tell you on a data late attached to the furnace), it is delivering 90,000 BTU of heat to your home. According to the Manual J calculations, that's just about right. This is surprising, since builders tend to install oversize furnaces, people tend to replace them with the same (oversize) equipment, and you've probably made improvements to the energy efficiency of your home in the last 30 years that would reduce your max heating demand. Here's the test: If this current furnace remains running on a nearly continuous basis (more than approx 80% of the time) on your very coldest days, then it is probably sized about right and in your shoes I'd be reluctant to install an 80K BTUH 95% efficient furnace (that will actually put out 76K BTUH).

Just added an additional 6 inches, which gives me 18 inches in the attic now. Walls have the normal amount of insulation. Windows are 30 yrs. old.

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On the other hand, if your present furnace is only running 50% of the time on your coldest days, then the 80K BTU furnace would be fine.
Quote:
Originally Posted by samclem View Post

Other observations:
- As you probably know, Carrier and Bryant are made by the same company and the equipment is virtually identical.
Yes, I was aware of this fact.

Quote:
Originally Posted by samclem View Post
- The 71 deg F internal modeling gives you some wiggle room. If you'd be alright with an internal temp of 65 deg F on the very coldest days, that might allow you to use slightly smaller equipment. Likewise, if your ductwork has sufficient capacity, closing the registers in one or two unused rooms could help you on the very coldest days.
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Originally Posted by samclem View Post
- A single case study, FWIW: I did a heat load calculation on my house and installed a furnace that was about 10% larger than called for (it was the closest size available and much smaller than the old furnace). In the five years since, it has never run more than approx 40% of the time, even on the coldest days.
I have my house at 65 degrees most of the time. I do bump it up higher when feeling cold.

Thanks for all the insight.

Rob
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Old 10-14-2009, 09:13 AM   #11
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I have decided to go with the Bryant installer. He is stopping later this week, to go over everything again. I may make some adjustments to his original estimate.
If anyone is interested, I will post the different components and final price to me.
Thanks everyone.

Rob
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Old 10-14-2009, 10:00 AM   #12
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I popped for a 95% efficient furnace to get the federal tax credit and picked up two stage operation as a plus. So 99% of the time my furnace runs on low which is about 35,000 BTU output. This makes the furnace run more often, which is more efficient and also keeps the temperature extremely even.

My calculations came up with only about a 40,000 BTU heat loss for my home, verified as I mentioned, with actual usage. I'm suspicious that most installers who really do a Manual J calculation toss in a fudge factor at the end to come up with an extra large furnace recommendation.
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Old 10-15-2009, 02:30 PM   #13
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Would you mind sharing about how much such a project costs? I am in the same boat soon...need a new furnace/air for a house in the northeast. About 2,700sqft.

Thanks.
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Old 10-15-2009, 04:16 PM   #14
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you might consider a HEIL of similar specs for less $$ than carrier...they are made by carrier and in my experience, are every bit as much a quality product as carrier


so you have an idea, an 80k 92% heil furnace costs me about 800-900 bucks,.
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Old 10-16-2009, 05:05 AM   #15
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Would you mind sharing about how much such a project costs? I am in the same boat soon...need a new furnace/air for a house in the northeast. About 2,700sqft.

Thanks.
I will try to get the specs and pricing up on Monday. Kind of busy at work today, and out-of-town this weekend.

Rob
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Old 10-16-2009, 09:00 AM   #16
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Would you mind sharing about how much such a project costs? I am in the same boat soon...need a new furnace/air for a house in the northeast. About 2,700sqft.

Thanks.
My advice is to get at least 3 to 4 estimates. I had a huge variation in price for comparable furnaces / installers. Look at available Federal energy efficiency tax rebates, too, as a factor in pricing.
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Old 10-16-2009, 09:23 AM   #17
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My advice is to get at least 3 to 4 estimates. I had a huge variation in price for comparable furnaces / installers. Look at available Federal energy efficiency tax rebates, too, as a factor in pricing.
In addition to the 3 to 4 estimates, I would ask family, friends, and neighbors if they have had new furnace/ac units installed recently, and what their experiences with them were. That is the first thing I did. A lot of different input supplied by asking.
Rob
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Old 10-16-2009, 01:39 PM   #18
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In addition to the 3 to 4 estimates, I would ask family, friends, and neighbors if they have had new furnace/ac units installed recently, and what their experiences with them were. That is the first thing I did. A lot of different input supplied by asking.
Rob
Me too, and after they left I still spent a couple of hours properly sealing the new duct work and plenum, remortaring the hole that they hammered through my brick veneer and sealing it, properly attaching the chimney liner to the vent cap, rewiring the humidifier outlet and putting a proper slope on the condensate and humidifier drain pipes.

Not worth calling them back for this stuff and most people would never know the difference.
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Old 10-19-2009, 05:06 AM   #19
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I will try to get the specs and pricing up on Monday. Kind of busy at work today, and out-of-town this weekend.

Rob
Here goes:
Furnace-Bryant Evolution 355CAV 100,000 Btu 3-state Modulatiing Variable Speed 95% AFUE
A/C-Bryant Evolution 187A 3 ton-2 speed 17 Seer
Air Cleaner-Bryant High Efficiency
Thermostat-Evolution Control
Humidifier-Bryant LFP Automatic(upgraded this to one with a fan-don't have the model number.)
Extended Warranty-5 years labor
Total Cost-$8,600.00
Less Carrier rebate-$1,030.00
Less Federal tax credit-$1,500.00
Final Cost-$6,070.00
Hope this helps others.
Rob
p.s. I am having the ductwork cleaned professionally before installing the new equipment.
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Old 10-19-2009, 07:02 AM   #20
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Very helpful, thanks. How big is the house approximately?
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