# How to Size a Furnace



## super_man4471

Having been doing a lot of reading, Saw this DIY forum. Reading a lot. Some great opinions and experts here. Now I finally have a question to throw out.

Looking to replace an existing 23 year old York gas furnace. Have done the research. Narrowed it down to the top three; Carrier Infinity 96, Trane XV90 and Lennox G61V. All good, but leaning likely towards the Lennox. Well serviced brand where I am, and on paper the better warranty; 10 years and Lifetime Heat Exchanger. FYI I am in the Toronto, Ontario Canada area.

Have focused on good reputable installers for quotes. However most of the installers that have come in to quote, have just looked at the current furnace, and ask how the current heat is. FYI the current York is an 80,000 BTU Normal Input spec, with a 60.8 BTU rated Output.

However two installers did a "heat loss calc".

My issue now is this. I have had two Lennox installer in, and both quoting a different size hi-eff: G61MPV-36B-070 and G61MPV-36C-090. The house is approx. 2,800 sq ft.

Now this is what I am trying to understand. Both did the walk around, measured floor space, counted windows, and then went off to another room to do their calcs. When they came back one said the calculation came out to 62,400, the other 64,000. Close I guess.

What I do not understand, but want to, is what does this mean? Specifically trying to understand how this type of number relates to sizing a furnace.

Can anyone comment on this? I am not looking to become an HVAC engineer, but am very interested in understanding what type of sizing of a furnace we should be looking at. Just want to be more educated, and keep the installers honest. Want to make sure that the furnace is not undersized or oversized.

Also not sure how these type of numbers relate to the hi/low stages and outputs of the current hi-eff furnaces.

Also ifanyone has any opinions on the three considered products, I'd welcome that too

If there is another link I can go to to help learn a bit more, then all comments are welcome.

Thanks to all in advance for your feedback.


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## harleyrider

First off.....Congradulations, when you have the G61V installed you will have purchased the quitest most effecient furnace on the market today, plus you will have all the resourses of America's #1 Furnace manufacture on your side.

Both men are correct and this is why........ The G61V is a 2 stage furnace, it starts the cycle in low fire and when it need to it puts itself into second stage. There is only 20,000 btu's of difference between the two units. 20,000 btus minus effecincy = 14 or 15,000 net. Very miniscule. If you live in a "cold" climate the larger btu may be better.............were as a milderclimate may be better with the smaller unit.

Either way they both will do the job, and again congradulations and welcome to the Lennox family.:thumbup:


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## cornbread

If you run too small of a unit your humidity will drop and it will run a lot more. I'd go with the 2 stage unit. 

My next unit will be a 2 stage heatpump/gas unit. I'm in a different climate than you though.


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## rakes9720

The _Output _BTUs of the furnace in _high_ stage should be close to the BTUs that were calculated, but not any less than the calculated amount.

The calculated BTUs are the amount of BTUs lost per hour at the predicted coldest temperature outside in your area. To be most efficient, and for the furnace to last the longest, you want the furnace to run almost constantly (on high if two stage) at this coldest temperature. For this to happen, you need a furnace that will put the those BTUs lost back into the house at the same rate. As the outside temp gets warmer, the house loses less BTUs every hour, and the furnace runs less. 

In your case, on high, the -070 furnace is 94.1% efficient so:
BTU out = 70000 X .941 = 66000 BTUs
the -090 furnace is 94.3% efficient so:
90000 X .943 = 85000 BTUs
Since the calculated heat-loss for your house is 64000, 66000 is a pretty good size. This also depends on what temperature you keep your thermostat at. The calculation was probably done at 70 degrees (F), but if you keep the thermostat higher, then your heat loss will be higher. 
Also, the furnaces input might not be exactly 70000 or 90000, and could be less. Say the 70000BTU furnace was actually 65000BTUs in (I'm not really sure). Now the output would be less than the heat loss calculated for your home, and it might be why the second installer chose the 90000BTU furnace. 
I would check to see the actual output of the smaller furnace is, and if it is greater than the output of your old furnace (assuming that it managed to keep the house warm, even in the cold), I would go with the smaller one, since it is very close to the values in the calculation.


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## super_man4471

Harley, Corn and Rakes, thanks for the reply. Learning a lot. A lot more fun making educated decisions.

So Harley here is my specific detail.

I have a Heat Loss Calculation Report showing 64,520 BTUH. Total Heat Loss. It is indeed calculated based on a 70 summer and winter temperature. The report also assumes Design Conditions for Toronto, Ontario Canada. The report is run through a proggie called "HVAC-Calc" by HVAC Computer Systems Ltd. Not sure of that means anything.

The current York 23 year old furnace is rated at 80,000 BTU Normal Input, and and 60,800 BTU Rated Output.

The two stage furnaces, variable speed fan models that are being considered are:

1. Lennox G61MPV-070

Input BTUH Low/High 45000/66000
Output BTUH Low/High 43000/61000

2. Lennox G61MPV-090
Input BTUH Low/High 60000/88000
Output BTUH Low High 57000/79000


Leaning towards the Lennox, but I also considered a Trane and Carrier product. Those actually seem a better balanced becuase they both fall smack in between the two Lennox products.

Not too big, not too small, just right????

3. Trane XV90 TUY080
Input BTUH Low/High 52000/80000
Output BTUH Low/High 49000/73000

4. Carrier Infinity 96 58MVB080
Input BTUH Low/High 52000/80000
Output BTUH Low/High 49000/75000

I know it is just an opinion, but based on the above facts what would folks choose. Let's address ideal fit first, and then brand loyalty next.

Comments please.

Thanks to all once again for your opions. This is great.


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## harleyrider

I just left a company that sold and serviced Trane products, trust me......let my experence save you from making a wrong decision..........go with the larger or the two Lennox units. The Traneshave many random issues. Bad boards, poorly designed condensate drain systems, noisy operation just to name a few.

I think that there is a 5000 t0 7000 "fudge" factor on a heat loss calculation..........so the larger Lennox puts you in the ball park.

Take a deeper look at the G61V here..........It is the Bset furnace being made today!

http://www.lennox.com/products/overview.asp?model=G61V


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## rakes9720

I would go with the larger Lennox or the Carrier one.


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## super_man4471

Thanks to all for your feedback. Have decided and signed for a:

Lennox G61MPV-090 Hi-eff
Input BTUH Low/High 60000/88000
Output BTUH Low High 57000/79000

Lennox HC16 Merv16 Media Filter

Lennox Humidifier

Honeywell VisionPRO 8000 Thermostat

We are done! Thanks again to all.


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## williamgeorge

Experts recommend 90% furnaces for most homes. A 90% or 92% AFUE furnace has enough efficiency to merit state or utility energy conservation rebates in many states, even though initial costs can be $1,000 more than an 80% furnace.
----------------------
williamgeorge


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## TazinCR

I would stay away from Trane.
We had one and after 4 years there was something wrong every start up season. Now I don't worry about heaters.


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## dac122

Though the OP has signed effectively making thread dead, I would like to make a few points.

The OP did not mention if the quotes took into account types of windows, doors, skylights; insulation values and siding; and ceiling heights. Perhaps the quotes were based on that, but if not, those will affect the heat loss. 

HVAC-Calc claims to model Manual J

Manual J in one sense overestimates the heat loss because most times the heat loss is much less than that, except for those extreme code days for the inputted locale.

Choosing a furnace that is slightly undersized will just run longer during extreme design inputs. Choosing a furnace that is over-sized will cost your more initially, cost more to operate, and could shorten its lifespan.


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## pearnakata

Whats with the slamming of Trane products? Almost all equip. will last along time if initially installed properly. I have put in Carrier, Lennox, and Trane and by far prefer the Trane label. Lennox would be the bottom of my list I would put Goodman in my house before installing a Lennox in my home. Lennox has poor customer support (in my area) and cheap boards, I carry 3 boards all the time just for 1 model of furnace that we used to put in.


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## tk03

For a good explaination on the importance of heat losses see this web site.
www.comfort-calc.net


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## devlon

*Check you Gas bill CCF Usage from old to New*



pearnakata said:


> Whats with the slamming of Trane products? Almost all equip. will last along time if initially installed properly. I have put in Carrier, Lennox, and Trane and by far prefer the Trane label. Lennox would be the bottom of my list I would put Goodman in my house before installing a Lennox in my home. Lennox has poor customer support (in my area) and cheap boards, I carry 3 boards all the time just for 1 model of furnace that we used to put in.


Buying 1 of the 3 Big Brands on the market today is a Gamble. ANY brand on the market today is a safe bet. They are basically the same on the inside. Honeywell produces 90% of the boards Carrier and Trane use today, as White Rodgers produces the circuit boards For Lennox, Goodman, Amana etc. It really comes down to the details of the installtion. You can take the best furnace in the world and put it in upside down and POOF you are now the proud owner of a large piece of >>>>. I personally think you messed up going with the large Btu capacity. Your old york out put btu was only 60,800. And YOU even said it has worked fine for 23 years. Now you had a furnace installed that has a higher output. Was there any mention from this contract about the size of your duct work. If you can not deliver the extra btu capacity you willl end up running the furnace Hot. Causing the limit switch inside the furnace to start cycling the furnace on and off( 0nly 1 possibility) Not to mention if you forget to change that air filter often enough you will end up changing out that heat exchanger in a few years. My second point is that a manual j load calculation software has a safety margin built into of 15-25%. After all the people publishing the software dont want to get sued because there software said you only need a 45000 btu furnace instead of the 60000 btu furnace. Really its common sense, You had two contractors that had obviously educated sales people that spent more then 10 minutes with you to do the manual J load calc. But if your load calc said you needed 64000 btu's output per hour to keep your house at 70 ffor the coldest day of the year in ontario and your old york (which is not my favorite brand) worked fine with an output of 60800 btu, WHY DID YOU HAVE THE 90000 Btu furnace installed. How much did you cubic footage of gas usage change? Did the installer run new thermostat wire and install a two - stage thermostat, or is the circuit board controlling the stages based on time? I do apologize if I am coming accross as being a D##K. I am just trying to stress the importance of having a good contractor or the guy that got the job because he was 50 bucks cheaper. My best recommendation is call the local heating suppliers and ask them who they would want to put their furance in. And as far as the furnace brand the big three a very expensive, not beacuse the components are different or better or worse. the Big Brands Have to pay for the TV ads, The lonely heating sales person sitting at the local home supply center, and dont forget the phone book and bill board ads they help their "contractors" pay for through co op programs. And look out for the guy that will beat any one else price, They are cheating some were to save that extra money they are not charging you. Good luck with you purchase and call a York and Lennox and Trane dealer and ask them How much it will cost to replace the circuit board in their respective brands.


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## stepaq

Do you mind telling me how much you pay for the furnace part only?
Just got a quotation here in Ottawa for $5400 G61MPV-36c-090, plus gst.


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## beenthere

If you think your quote is too high. Call other companies.


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## Marty S.

Costs have gone up a bunch since the original post 3 years ago so what they paid isnt relevant now. Get another quote or two for the same equiptment and you'll know if that's the going rate in your area.


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## beenthere

He'll begin to get an idea.

Some contractors cut so many corners when they give a cheap price.


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## veesubotee

90,000 BTU plus MERV 16--holy ductwork.


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