# sizing furnace



## mindoreno (Sep 7, 2009)

I have a 1978, 1200 sq feet house with a 96K BTU output. I live in Edmonton, Alberta Canada and it gets pretty cold in the winter months. Can I replace this furnace with one that has 80K BTU output? It seems like the one I have in there now is over-calculated as I heard that, that was the practice back then. Please advise. Thanks in advance.


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## beenthere (Oct 11, 2008)

Maybe.

Best to do a load calc, and find out for sure.
HVAC CALC is advertised here. Its a good program. Try it. It will also let you do what ifs.
What if you add insulation to what ever area. Then it will tell you how much that will reduce your heat loss. So you know if its worthwhile doing and has a ROI.


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## mindoreno (Sep 7, 2009)

Beenthere,

Thanks. I realize that that is the most exact way of doing it. I figure, the load calculator will arrive at 2 possible scenarios - it will say that if I replace my windows and add an extra insulation in the attic, etc. I can go down to 80,000 BTU output; if I just leave the way things are, the 96,000 BTU is just right. I plan to fix my windows, replace weather stripping on the outdoor doors and I usually caulk my windows prior to winter ( removable caulking ).
There are some generic calculator here on the internet and when I punch in my numbers, 1500 sq feet and the region of Alaska ( it is asking for USA zip code ), it gives me 64,000 BTU.
I also have a brother-in-law whose 1999 house is 2000 sq ft. 2 storey in the same area as mine and he has an 89,000 BTU output. Mine is only 1075 square feet, but since it is a bi-level I estimated it to be 1500 sq ft. because a bigger part of the basement is elevated from the ground as opposed to a bungalow. 
And when I asked some people in the trades, they say 80,000 BTU output is plenty. I am really in a dilemma so I thought I post it in here and hopefully get lots of replies and lots of different scenarios.


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## Scuba_Dave (Jan 16, 2009)

My 2 floor Cape I did the sizing & came up with 68,000 for the original size before my additions
Approx size was 1640 sq ft - mostly 1st floor


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## beenthere (Oct 11, 2008)

Use the load calc program to see what you need with your homes current construction.

You mind be suprised to find that your furnace is already 40% oversized.

Generic web based programs just use rules of thumb for the area your home is in. They aren't very accurate at all.


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## Scuba_Dave (Jan 16, 2009)

Yes - I did the calc with the additions & the current furnace will heat it all
But its over 21 years old & needs to be replaced
I heat a lot with wood so its hanging in there for now
I wanted to wait until the additons were roughed out before new furnace & piping went in


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## beenthere (Oct 11, 2008)

Sorry. I was replying to the OP, mindoreno.


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## yuri (Nov 29, 2008)

Mindoreno, does your furnace have a pilot light? Give me the make and model and serial# and I can narrow the process down for you. Wpg is much the same climate as Edmonton.


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## mindoreno (Sep 7, 2009)

Yuri,
Thanks. I have a Lennox G8-120-1, serial number 6379A86213. It has 120K BTU input and 96k BTU output and it has a pilot light.
My house is built in 1978. It is a bi-level 1075 sq ft. with original big wooden windows. Again some of my refrigeration friends have said that a 100KBTU input with 80K BTU output is plenty. I am looking at buying a mid-efficiency since this is the last year we are allowed to purchase them. I am planning to fix my windows next year. I have a poured in insulation in the attic with normal thickness I am guessing. I am also planning to replace one of my 2 outside doors soon. Besides the size of the furnace, another question is what CFM should I get, 1600 or 2000. I heard that 1600 is plenty and 2000 can be a little noisier. Please advise and thanks very much.


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## beenthere (Oct 11, 2008)

You REALLY should do a load calc, instead of just wild guessing. And saying good enough.

A load calc can save you money on your heating bill, by allowing you to get the right size furnace.

CFM of the blower is more dependent on the A/C your house needs.

Newer furnaces need to move more air then older ones did. This makes proper sizing more important then it was for the older furnaces.

Why mid efficiency? Why not a 90%plus. And save even more on your heating bill in the future.

Good chance a 70,000BTU input 90% would be plenty for your home, with its current insulation and windows.


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## hvaclover (Oct 2, 2008)

I did a short form for you and subed your Alberta temps for Drtroit MI.

70 or 75k (btus in ) @ 95% will give you more than enough heat with a reasonable amount reserve


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## mindoreno (Sep 7, 2009)

Check out this link

http://www.saskenergy.com/Residential/Appliances/FurnaceSizing.pdf

It has the calculation for the size of the furnace vs the size of the house and it takes into consideration the thickness of the insulation on the walls in the main floor and in the basement and also the thickness of the insulation in the attic or ceiling.


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## beenthere (Oct 11, 2008)

It just a rule of thumb one also.

Read note 4 on it.

You want to use a free garbage one. Go ahead.
That one will oversize you by a lot.

Pay the small fee, and use This One


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## hvaclover (Oct 2, 2008)

beenthere said:


> It just a rule of thumb one also.
> 
> Read note 4 on it.
> 
> ...


 Prob with a house that small is that most of the mfgs smallest furnaces are TOO small.


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## beenthere (Oct 11, 2008)

hvaclover said:


> Prob with a house that small is that most of the mfgs smallest furnaces are TOO small.


Bet checking different brands of furnaces, you can find sizes 40,000, 45,000, 50,000 60,000, 70,000, and 75,000 BTUs.

Without a problem.


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## hvaclover (Oct 2, 2008)

I would not put anything less than a 60k in that home. The amount of reserve would not be that great with the numbers I am seeing for Alberta.

Sometimes it's just better to err on the safe side.


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## beenthere (Oct 11, 2008)

hvaclover said:


> I would not put anything less than a 60k in that home. The amount of reserve would not be that great with the numbers I am seeing for Alberta.
> 
> Sometimes it's just better to err on the safe side.


All the more reason to do a load calc.

His house might only need 31,000 BTUs.

Making a 45,000 90% a furnace that has a 23% safety.
Or a 50,000 90% that gives him 31% safety.

Why put a 2 to 3 times the size needed monster in. When a load calc will save him money both upfront in install cost. And in operating cost.


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## yuri (Nov 29, 2008)

I would pay a few $$ and do the load calc:Load Calculator and let us know the results.

Why would you buy a mid, you would be far better off with a high efficiency unit in the long run. With that home improvement 15% grant a high is a better choice. Mids are almost as expensive as some high efficiency units. Here is what I would recommend for our climate:

I would recommend a Lennox G61V or G71MPP 90,000 BTU model. Possibly a 70,000 if the load calc warrants it. The reason being that if you use a true 2 stage thermostat the G61 will run a LOT of the time at 56,000 BTU's on low fire and the rest 81,000 BTUs when we get the damn -40F stuff. The G71 is even better as it will run at low fire of 34,000 BTU's and will modulate up to the full capacity (81,000) and be VERY comfortable. The G71 is a modulating unit. Both should use a proper 2 stage thermostat and have 6 wires to it. You should go with R40 in the attic and 3 pane low_e argon windows.
The mid will take the combustion air from your house and that is $$ being sucked in and pouring up the chimney. I would like to know the load calc results before recommending a mid. 110 may be too big and 90 too small. These are Lennox units, Carrier and Trane and other may use, 60/80/100 models. Lennox uses 45/70/90/110.

Guys you need to factor in windchill factor, we need some extra capacity for that.


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## beenthere (Oct 11, 2008)

yuri said:


> Guys you need to factor in windchill factor, we need some extra capacity for that.


Around here, we usually use a 10, sometimes 15 MPH wind if the house is in the open.


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## hvaclover (Oct 2, 2008)

beenthere said:


> All the more reason to do a load calc.
> 
> His house might only need 31,000 BTUs.
> 
> ...



May look good on paper, but I know a lot of guys who would have a bad gut feeling about something seemingly as small as that.

I come up with 39900 out by short form. I know this is a good accurate number cause I have had a friend run numbers on his load calc. It's not just ball park . But since a 45k won't have enough juice and a 55k is is not in my equipment line up I would still do a 60k.


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## beenthere (Oct 11, 2008)

hvaclover said:


> May look good on paper, but I know a lot of guys who would have a bad gut feeling about something seemingly as small as that.


Thats the problem.
Guys go by gut. Instead of doing the work to measure and do an accurate load calc. And then recheck there measurements and numbers.

So they never find out how reliable a load calc is. They just take short cuts.


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## beenthere (Oct 11, 2008)

My brand doesn't have a 50,000 either. So I would be a 60,000 also.

My point was when he is looking at different brands. He can choose one that will be sized closer to what he needs if he wants.


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## hvaclover (Oct 2, 2008)

beenthere said:


> Thats the problem.
> Guys go by gut. Instead of doing the work to measure and do an accurate load calc. And then recheck there measurements and numbers.
> 
> So they never find out how reliable a load calc is. They just take short cuts.


You miss my point. When I say the "gut feeling" I am talking about guys who got burned by load calcs for that small a unit.

I did too. Look at the all threads that gripe about the diff in results form soft ware to soft ware vendor.

I put a 60k 2stg 95% VS in a 1200 sq ft brick ranch that never got off second stage below 40*f. And that is mild for around here.


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## yuri (Nov 29, 2008)

Around here and Edmonton we easily get 30 or more mph winds. Until you have lived in a severe climate you don't ever want to undersize a furnace. The other issue is quality of house construction. Older homes don't have the well sealed vapor barrier in the walls etc as the new ones. My mother has a 80,000 Lennox Pulse in a 40 yr old house like his and her bills are cheap, so bigger does not necessarily cost more. That Pulse is 94% efficient, 28 yrs old and still kickin.


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## hvaclover (Oct 2, 2008)

beenthere said:


> My brand doesn't have a 50,000 either. So I would be a 60,000 also.
> 
> My point was when he is looking at different brands. He can choose one that will be sized closer to what he needs if he wants.



Even if I had a broader size choice I would still choose 60k cause I just feel in a home 1000 sq ft up to 1500 sqft that size would be the least likely to give you an unwanted surprise on a unusually cold day.


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## hvaclover (Oct 2, 2008)

yuri said:


> Around here and Edmonton we easily get 30 or more mph winds. Until you have lived in a severe climate you don't ever want to undersize a furnace. The other issue is quality of house construction. Older homes don't have the well sealed vapor barrier in the walls etc as the new ones. My mother has a 80,000 Lennox Pulse in a 40 yr old house like his and her bills are cheap, so bigger does not necessarily cost more. That Pulse is 94% efficient, 28 yrs old and still kickin.


That's why i would go with a 60k. I did not want to speak for a part of Canada I know only from what I read. But being to Toronto one winter was enough to make this Greek want to stay South of Windsor. The wind is a bear there so i can imagine how it must be in the more arctic like regions of Canada.


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## yuri (Nov 29, 2008)

Aw common, you like our beer. :drink:Its a dry cold and you get used to it. Important to have a good sized heater in your house or fishin shack.


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## hvaclover (Oct 2, 2008)

yuri said:


> Aw common, you like our beer. :drink:Its a dry cold and you get used to it. Important to have a good sized heater in your house or fishin shack.


Yeah, get stranded with a butt load of Molson and Labatts and you won't even need embalming when they find you, hell, you won't even be frozen thanks to that good Canadian brew!


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## hvaclover (Oct 2, 2008)

LOL 30 MPH winds.

Bet there were a lot of blown out pilot calls back in the day.


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## beenthere (Oct 11, 2008)

yuri said:


> Around here and Edmonton we easily get 30 or more mph winds. Until you have lived in a severe climate you don't ever want to undersize a furnace. The other issue is quality of house construction. Older homes don't have the well sealed vapor barrier in the walls etc as the new ones. My mother has a 80,000 Lennox Pulse in a 40 yr old house like his and her bills are cheap, so bigger does not necessarily cost more. That Pulse is 94% efficient, 28 yrs old and still kickin.


Higher winds require higher infiltration rates to be used.

Thats what many miss. They use average, or tight construction ratings.
When the home is actually loose. While a .7 may be ok for many homes. 
In high wind areas, a 1, or 1.25 may be more appropriate. And makes a BIG jump in the BTU requirement.


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## yuri (Nov 29, 2008)

Now you got it. The newer homes are a LOT better/tighter. My walls have R20 instead of his R14. My vapor barrier is all glued at every joint in the walls,ceiling. I have close to 1400 sq.ft and easily heat my home with a standard G51 70 which put out 66,000 BTU's but there is a huge difference in house construction in the last 30 yrs. He has a good house but you can only improve so much and then you still need an adequate furnace whith a few extra BTU's. And yes, blown out pilots were a problem, not many left now.


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## hvaclover (Oct 2, 2008)

beenthere said:


> Higher winds require higher infiltration rates to be used.
> 
> Thats what many miss. They use average, or tight construction ratings.
> When the home is actually loose. While a .7 may be ok for many homes.
> In high wind areas, a 1, or 1.25 may be more appropriate. And makes a BIG jump in the BTU requirement.



And that is one of the equations that seem to differ enough from load calc to load calc to make a diff in furnace size and comfort.

i have read enough stuff on load calcs that make me want to just put them aside. I have been told at HVAC TALK that it's all in how you use it or that it is like learning to use any new tool, you have to learn how it works.

In this age of high tech electronics you'd think you could buy load programs that all give the same answer every time. That is not true.

If I have to take time to learn a system that doesn't agree with other programs even though they are derived from Man J, what advantage is that.

Look, I am not bragging but I have compared my short form to bids that had three or four manJ calcs all from diff vendors. None of them agreed.
Most were ball park with one another but with the accuracy computers bring they should do better than Ball park. My shortform was in the middle of the ballpark.

I'll stick to it since it is what I know.


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## beenthere (Oct 11, 2008)

I've seen the same load calc program done by 2 different people come up with a difference BTU.

Its like any other tool. It must be learned. And it does take time.
The more you use one. The closer you can get to the true BTU needed, weather for heating or cooling.


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## beenthere (Oct 11, 2008)

yuri said:


> Now you got it. The newer homes are a LOT better/tighter.


LOL...

Sad thing is.
Houses built in Canada 30 years ago, were often built better, then homes down here in the states are today.


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## yuri (Nov 29, 2008)

You should visit Europe. There are buildings in Vienna and Switzerland I visited that go back to the 1100's. All stone but they sure last. If they last 150 yrs here we will be lucky. Too cold for termites.


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## beenthere (Oct 11, 2008)

Everything is too expensive in Europe.
Even to visit.


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## yuri (Nov 29, 2008)

Not if you have relatives and friends there. European custom is that the host pays for everything ie: meals etc. I couldn't even give them $$ for anything w/o insulting them. If they visit here then I pay. Hope they stay home. LOL:laughing:


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## beenthere (Oct 11, 2008)

yuri said:


> Not if you have relatives and friends there. European custom is that the host pays for everything ie: meals etc. I couldn't even give them $$ for anything w/o insulting them. If they visit here then I pay. Hope they stay home. LOL:laughing:


ROFL :laughing:


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## hvaclover (Oct 2, 2008)

yuri said:


> Not if you have relatives and friends there. European custom is that the host pays for everything ie: meals etc. I couldn't even give them $$ for anything w/o insulting them. If they visit here then I pay. Hope they stay home. LOL:laughing:


Oye!

Don't remind me! My brother in law kept me drunk on Johnny Walker Red so I couldn't complain.I couldn't even buy a coke. He's married but had a girl friend who sang at a club we were at. I got pictures of me breaking plates and Greek dancing. They didn't tell me you had to pay for the plates. 

Woke up the next after noon and looked in the mirrior...I had a black eye!

My brother in law clapped me on the shoulder and said, "You should see the other guy. You left him bleeding in the gutter."

I went back to bed and covered my head with the pillow. Haven't been back since.


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## mindoreno (Sep 7, 2009)

You guys are all great. Thanks for all your replies. I enjoyed reading your comments and the funny stuff that people throw in away from the topic e.g., beer, Europe, etc. There are merits to everybody's opinions. I am leaning towards 100K BTU input and 80K BTU output. This is one size lower than what I have right now which is 120Kinput and 96 output. This 80K output that I am thinking of putting is what 3 of my Edmontonian friends who are in refrigeration trade, have told me. The load calculation has its own merits but my house is old and the insulation is old as well and for this kind of weather (extreme cold and wind chill ), a little over calculation is to be expected. They say that we have two seasons here in Edmonton and the same is true for Winnipeg, winter and July. It's so cold out here, it's not funny. Thanks again everyone. I am glad I put this question out here. It definitely made up my mind.


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## hvaclover (Oct 2, 2008)

Get a two stage furnace with two stage stat.


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## GetYourShineBox (Sep 20, 2009)

my a.c. teacher gave us a great way to check your btu. you multiply the width of the house by the length then multiply the sum by the height of your interior. so for example. 25Wx40L=1000x8H=8000. then you would use a multiplier to determine your btu/hr. this multipliers are determined by your homes construction and insulation. if you lived in a older home with thinner walls that is not as tight you would use 10. a home that has 3.5 inch walls with tighter insulation and better windows you would use 6, and if you have a home with 6.75 inch walls, with the tight insulation and the best windows you use 4.5 so to make this make sense....

25x40=1000x8=8000x10=80000 btu/br
25x40=1000x8=8000x6=48000
25x40=1000x8=8000x4.5=36000

its easy and it works.


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## beenthere (Oct 11, 2008)

GetYourShineBox said:


> my a.c. teacher gave us a great way to check your btu. you multiply the width of the house by the length then multiply the sum by the height of your interior. so for example. 25Wx40L=1000x8H=8000. then you would use a multiplier to determine your btu/hr. this multipliers are determined by your homes construction and insulation. if you lived in a older home with thinner walls that is not as tight you would use 10. a home that has 3.5 inch walls with tighter insulation and better windows you would use 6, and if you have a home with 6.75 inch walls, with the tight insulation and the best windows you use 4.5 so to make this make sense....
> 
> 25x40=1000x8=8000x10=80000 btu/br
> 25x40=1000x8=8000x6=48000
> ...


And its a bunch of garbage.

Learn how to do a proper load calc. Your teacher doesn't know how to do them. Or he would teach you the proper way.

Might as well just sell 120,000 BTU furnaces for any size house as use that method.

Using/teaching those kinds of methods are what makes this industry an industry in which more then half don't know what they are doing.


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## hvaclover (Oct 2, 2008)

Been you are being WAY too hard on the youngster (assuming age here).

Loadcalcs in long form are very difficult because of all the math and voc schools will usually go short form to get the student familiar with the concepts until they can get in the field and start using software.

GetYourShineBox, I use a short form calc myself, but I know my area and how the homes are construction methods used. So I can tweak my numbers as experience dictates. Beenthere is right about doing a proper load calc but I differ with him because the software available will give a different result from each different brand.

That being said I say if you are using a short form that puts you in the ball park with everybody else's software calc then I would stick with using it.

But it's like anything else, you have to refine your technique and know where to tweak you own calculations to meet the needs of the market you you will be working.


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## beenthere (Oct 11, 2008)

Not when he is using such a guess method. With no real logic behind it.

In some areas, a 1000 sq ft house with 8' foot high ceiling might need 80BTUs per sq ft.
If the area has temps below -30°F and a lot of wind, and the house has walls with R values of less then 2.


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## hvaclover (Oct 2, 2008)

beenthere said:


> Not when he is using such a guess method. With no real logic behind it.
> 
> In some areas, a 1000 sq ft house with 8' foot high ceiling might need 80BTUs per sq ft.
> If the area has temps below -30°F and a lot of wind, and the house has walls with R values of less then 2.


That is why you tweak it, like I said, to the point of fittings your market area.

A competent tech can make that equation work by changing the numbers to suit his area.


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## beenthere (Oct 11, 2008)

Real load calc program doesn't need tweaking.

Learn the proper way first. And life is much easier later.


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## GetYourShineBox (Sep 20, 2009)

I just finished schooling and i am not in the field yet so I am open to using whatever is the best method. The method he gave us is something his dad and him made up so obviously its old school and may not be 100 percent accurate all the time but its pretty close. I am not saying that I will be using it everytime I size a home for a furnace, it is just a simple alternative.


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## Scuba_Dave (Jan 16, 2009)

His method would oversize my furnace by more then 50%
Not surprisingly my furnace WAS oversized by 50% over 20 years ago 
I'd say its accuracy is hit or miss....mostly miss


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## hvaclover (Oct 2, 2008)

Scuba_Dave said:


> His method would oversize my furnace by more then 50%
> Not surprisingly my furnace WAS oversized by 50% over 20 years ago
> I'd say its accuracy is hit or miss....mostly miss



Every short form (like I said) needs to be tweaked to the market it's used in.

That's something that will come to any student over time.


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## hvaclover (Oct 2, 2008)

beenthere said:


> Real load calc program doesn't need tweaking.
> 
> Learn the proper way first. And life is much easier later.



Is that why every program out there agrees with one another?:no:

There are threads all over the inter net where so and so load calc came up 10k btu difference than xyz company.

I am sure that GetYourShineBox has been taught a proper manJ and he will be able to apply that to his skill set.


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## hvaclover (Oct 2, 2008)

beenthere said:


> Real load calc program doesn't need tweaking.
> 
> Learn the proper way first. And life is much easier later.



Is that why every program out there agrees with one another?:no:

There are threads all over the inter net where so and so load calc came up 10k btu difference than xyz company.

I am sure that GetYourShineBox has been taught a proper manJ and he will be able to apply that to his skill set.


HEY, GETYOURSHINEBOX, DID YOU GRADUATE NORTHWESTRN TECH'S HVAC COURSE?


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## GetYourShineBox (Sep 20, 2009)

hvaclover said:


> HEY, GETYOURSHINEBOX, DID YOU GRADUATE NORTHWESTRN TECH'S HVAC COURSE?


yes sir


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## hvaclover (Oct 2, 2008)

GetYourShineBox said:


> yes sir



Email me

[email protected]


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## GetYourShineBox (Sep 20, 2009)

hvaclover said:


> Email me
> 
> [email protected]


done.


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## beenthere (Oct 11, 2008)

Results of programs vary with the user also.

Along with weather its a ACCA version 7 or 8 program. And weather its an ACCA approved program or not.

I'm sure you can even find a program that actually uses ASHRAE data directly, instead of shortened version of it that Manual J is.

Then check Canaca's F280(Think thats the right number) it an even more accurate load calc program for heating.


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## beenthere (Oct 11, 2008)

If someone pays good money to go to school.

He/she should be taught the proper method. And not some method that the instructor and the instructors dad fudged to work years ago.

Shinebox was ripped off.

Now if a customer asks him if improving the attic insulation from R29 to R50 will save much money and allow him to use a smaller furnace.

He can only answer. I'll use a multiplier of 3.5 instead of 4. And we'll see if it saves you money and can heat your house.


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## GetYourShineBox (Sep 20, 2009)

this isn't all i was taught when it comes to sizing furnaces. i know how to do manual j and use software. it was just a simple way i threw out there. like i said it isn't always accurate. dont jump to conclusions saying i was "ripped off". i had 8 teachers, not just 1.


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## hvaclover (Oct 2, 2008)

GetYourShineBox said:


> this isn't all i was taught when it comes to sizing furnaces. i know how to do manual j and use software. it was just a simple way i threw out there. like i said it isn't always accurate. dont jump to conclusions saying i was "ripped off". i had 8 teachers, not just 1.



You don't have to make apologies ShineBox. I know the curriculum at Northwester, I contributed to it. I know you got a solid foundation in hvac.
You can make that short form work in homes upto 2500 sq ft with a little reworking. I live in your area so I know where fore I speak.


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## hvaclover (Oct 2, 2008)

Hey andrew you got mail!


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## beenthere (Oct 11, 2008)

GetYourShineBox said:


> dont jump to conclusions saying i was "ripped off". i had 8 teachers, not just 1.


I'll jump to conclusions if I want. :laughing:

Once you have several types of homes set up as templates in a program.

It takes very little time to do a load calc.


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## hvaclover (Oct 2, 2008)

Andrew you got mail!


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