Sizing new furnace based on gas utility bills

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Old 06-18-10, 07:57 PM
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Sizing new furnace based on gas utility bills

I'm preparing to sell the house. Looking to replace my existing furnace before I put the house on the market. The old furnace is a 22 years old, low-efficiency Lennox. 80K input, 60K output. It works, but not perfectly.

I will likely go with Goodman GMV95. 95% efficiency, two stage, variable speed. Mainly because I can cheaply buy an extended Goodman warranty. 10 years parts, 10 years labor, fully transferable to the house buyer.

So far I had four sales reps visit my house to give me a quote. None of them offered to do a heat loss calculation. They just looked at my existing furnace and recommended that I go with GMV95070. 70K BTU model, slightly larger than my existing Lennox. The model one step down is GMV95045, 45K BTU.

I did some googling on furnace sizing, and came across this page:

Replacing Your Furnace | CMHC

The page explains how to calculate the house heat loss from the gas utility bills. I did just that, based on my past bills. If my calculation is correct, GMV95045 would be the right size.

Can anyone comment on my calculation? Does it make sense? Should I trust my own numbers and go with a 45K model? Or should I do what most consumers would do in my position... go with a 70K model recommended by the sales reps?

Thank you all in advance for your feedback.

Heat loss calculation based on gas utility bills

1. Actual gas meter readings

Dec 12, 2008: 8061
Dec 14, 2009: 9925

2. Annual gas consumption: 9925 - 8061 = 1864 m3.

To be safe, round up to 2000 m3.

3. I have two gas appliances: a furnace and a water heater.

To be safe, assume that 100% of gas was consumed by the furnace.

4. I live in Ottawa, Ontario. According to Environment Canada,
Ottawa has 4600 degree days below 18C (64F) annually.

To be safe, I will use 4000 degree days in my calculation.

5. Heating consumption by degree day:

2000 / 4000 = 0.5 m3/HDD

6. The record minimum temperature in Ottawa:

-39C (-38F)

7. The maximum HDD per day:

18 - (-39) = 57 HDD

8. Heating consumption at 57 HDD/day:

(57 HDD/day)*(0.5 m3/HDD) = 27.5 m3/day

9. Gas energy content:

37.5 MJ/m3

10. Existing furnace efficiency:

60K / 80K = 75%

11. Heat loss at 57 HDD/day:

(27.5 m3/day)*(37.5 MJ/m3)*(0.75) = 773.4 MJ/day or 32.2 MJ/h

12. Heat loss in kW:

32.2 MJ/h / 3.6 = 8.95 kW

13. Required furnace output:

8.95 kW * 3,412 = 30543 BTU/h
For the reference:

Goodman GMV95045 output

low stage: 30,800
high stage: 45,000

Goodman GMV95070 output

low stage: 46,400
high stage: 67,000
 
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Old 06-19-10, 11:34 AM
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I'm sorry to state this in the beginning but I have been "burnt" too often by first (and last) time posters asking a complex question and then never returning after I post a detailed answer.

If you are serious about wanting a professional opinion please post back and I will give you several things to consider. It won't be immediate as I am planning on being out most of the day and probably into the evening so it might not be until tomorrow.
 
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Old 06-19-10, 11:39 AM
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Get a reputable person to do a heat loss calculation. Over 22 years you may have added new, more efficient windows, insulation, etc. You may also have more leakage around old windows, doors, etc.
I could show you my gas usage month by month for the 12 years I have lived here. All it shows is milder winters vs more severe winters.
 
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Old 06-19-10, 07:17 PM
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I've never heard of using the gas bill to size the home. Manual-J is wht I've done and had no issues.

How big is the home, and where are you out of?
 
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Old 06-20-10, 10:57 AM
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Originally Posted by furd
If you are serious about wanting a professional opinion please post back and I will give you several things to consider.
Thank you. I'm back.

Originally Posted by goldstar
I could show you my gas usage month by month for the 12 years I have lived here. All it shows is milder winters vs more severe winters.
It's not an issue. See step 5 in the calculation. It divides actual gas consumption in a period by actual number of HDDs in the same period. You get gas consumption per HDD. The rest of the calculation uses this number. It doesn't matter how severe the winter was.

Originally Posted by Jay11J
I've never heard of using the gas bill to size the home.
The calculation is posted on CMHC site.

Replacing Your Furnace | CMHC

CMHC = Canada Mortgage and Housing Corporation, a government agency. You can trust Canadians to know a thing or two about winter heating :-)

Originally Posted by Jay11J
How big is the home, and where are you out of?
I'm in Ottawa, Ontario, Canada.

The home is a 3 level townhouse. Each level is 600 sq.ft. 1800 sq.ft total. 20 years old construction, reasonably well insulated. New energy efficient windows throughout. Middle unit with neighbors on both sides. So two walls have no heat loss. On the other hand, the foundation is slab on grade (no basement). The floors on the first level are really cold in the winter.
 
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Old 06-20-10, 11:39 AM
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Sizing a new furnace from the gas consumption and degree days can indeed be a viable method but ONLY if the furnace is the only gas-consuming appliance. If you cook with gas, heat domestic water with gas, use a barbecue with the same gas supply as the furnace or have a gas-fired clothes dryer the other appliances WILL make using this method little more than a crap shoot.

For best results you need several years worth of gas consumption records AND degree day figures that are for your particular area. Climatic conditions can vary widely with just a few miles difference in sites. The closer you can define the degree day figures to your own home the better the system works.

Replacement furnaces are often sized from the original furnace although this is not the proper method. Estimators who do many furnaces in similar homes can get pretty close with the size of the original furnace and factoring in the energy-saving improvements a homeowner tells them about. That stated, a full "Manual J" heat loss calculation is the best starting point for determining the proper sized furnace. The down side for the homeowner is that a full Manual J takes a few hours to first gather all the data for the particular house and then to enter that data in the computer program. It is not something a contractor can do for free unless they know they are going to get the contract for the furnace replacement. A charge of fifty to one hundred dollars would not be out of line to do a complete Manual J in my opinion.

There are some heatloss calculators available on the Web. Some are simple and free and others are complex and charge a fee for use. I don't have the website handy but there is one that is specifically aimed for homeowners and allows limited usage, I think it is for three months on a single house, for the fee of about fifty dollars. I have not personally used this program but others on this forum have and state it was the best money they ever spent.


All that aside, the sizing of a multi-stage furnace is far easier than for a single-stage model. The first stage of a two-stage furnace is about 60% of the total BTU output and this is the area the furnace will run probably 90% of the time. It is very common, although not entirely correct, to size a two-stage furnace from the original furnace with the thinking that the furnace will be in the first stage most of the time but under worst-case scenarios the second stage will provide all the heat necessary.

I'll add more if you need it but I also wrote a fair bit just a week or two ago and you can find that by scrolling down this topic.
 
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Old 06-20-10, 11:59 AM
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I went from a 115k down to a 60k in my home.. Trane did not make anything smaller, so went with the 60k..

being that you are in a town home, a 45k may be enough for your home, but the Manual-J will give you the answer.
 
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Old 06-20-10, 12:57 PM
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Originally Posted by furd
Sizing a new furnace from the gas consumption and degree days can indeed be a viable method but ONLY if the furnace is the only gas-consuming appliance.
I have a gas water heater that consumes about 15%-20% of gas. I calculated the percentage by comparing my summer and winter bills. When I did my furnace sizing calculation, I on purpose ignored the water heater. I attributed 100% of gas consumption to the furnace. Just to make sure I have some room for error.

Originally Posted by furd
For best results you need several years worth of gas consumption records AND degree day figures that are for your particular area. Climatic conditions can vary widely with just a few miles difference in sites. The closer you can define the degree day figures to your own home the better the system works.
We have two weather stations in town. Both report about the same number of degree days. When I did the calculation, I rounded UP the gas consumption, rounded DOWN the number of degree days, and ignored the water heater. Yet the result came well under 45,000 BTU.

Originally Posted by furd
That stated, a full "Manual J" heat loss calculation is the best starting point for determining the proper sized furnace.
The heat loss calculation depends heavily on correct R values. The R values are not readily available. At best, the R values are estimates. Whereas the calculation based on gas consumption uses ACTUAL observed numbers. So I'm failing to see why "Manual J" would be more accurate.

Originally Posted by furd
The down side for the homeowner is that a full Manual J takes a few hours to first gather all the data for the particular house and then to enter that data in the computer program. It is not something a contractor can do for free unless they know they are going to get the contract for the furnace replacement. A charge of fifty to one hundred dollars would not be out of line to do a complete Manual J in my opinion.
Agree.

Originally Posted by furd
There are some heatloss calculators available on the Web. Some are simple and free and others are complex and charge a fee for use. I don't have the website handy but there is one that is specifically aimed for homeowners and allows limited usage, I think it is for three months on a single house, for the fee of about fifty dollars. I have not personally used this program but others on this forum have and state it was the best money they ever spent.
I tried to do a free heat loss calculation on this web site:

Home Heat Loss Calculator

The problem is, I don't really know my R values. I used the defaults provided by the calculator. So the result I got is really a crapshoot.

Originally Posted by furd
All that aside, the sizing of a multi-stage furnace is far easier than for a single-stage model. The first stage of a two-stage furnace is about 60% of the total BTU output and this is the area the furnace will run probably 90% of the time. It is very common, although not entirely correct, to size a two-stage furnace from the original furnace with the thinking that the furnace will be in the first stage most of the time but under worst-case scenarios the second stage will provide all the heat necessary.
If I follow that line of thinking, I should go with a 70K model. The first stage will run most of the time. The second stage will rarely kick in. The way I look at it, I will be paying a two-stage price for a single-stage furnace.

On the flip side, the first stage of a 70K model should be quieter than the second stage of a 45K model. Right?
 

Last edited by Goldstone; 06-20-10 at 01:31 PM.
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Old 06-20-10, 03:31 PM
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If you counted the gas consumed by the water heater as being consumed by the furnace (did not make a correction on the metered gas to allow for the water heater) then you are already oversizing the furnace.

If your home sees about the same winds and solar gain as these two weather stations then using the averaged degree day statistics from the two stations should be fairly close for your home.

Notice that I did NOT state the Manual J would be more accurate but I DID state it was a better STARTING point. I completely agree with you that not knowing the installed R-values makes the Manual J a crap shoot. There is also the fact that a Manual J will often oversize the heating requirements by 20% or more. I do not think that a Manual J is the be-all / end-all of calculating heat loss or heat gain, ONLY a starting point. Since you have no doubt done a "quick and dirty" Manual J and the calculation based on fuel purchased I agree that the fuel purchased method in your particular case is probably more accurate that the results you got from the heat loss calculation.

Another simple trick is to think about how the furnace behaved during the coldest weather you have experienced. Did the furnace run for long periods of time with fairly short off periods? Or did it run for a short time and then stay off for a long time? The latter is a classic cycle for an oversized furnace. Ideally, in the coldest of weather the furnace would run continuously and possibly even fail to raise the temperature in the house to the desired setting if that setting is above say, 68 degrees F.


When I had the furnace replaced in my home I wanted a two-stage unit with a variable speed blower. I had a half-dozen or so companies give me a bid although I tried to stay with only two manufacturer's to reduce the possibilities of comparing apples to kumquats. ALL of them went by the size of the original furnace but they all also asked me if I had had new windows installed or additional insulation beyond the original construction. They all specified a furnace with maximum output slightly more than what I had BUT the reasoning for this was my insistence for a two-stage, variable speed model. In my particular case there were NO two-stage, variable speed furnaces made that really fit my heating needs. The result is that the ONLY time my furnace has gone to the second stage is when I manually turn the thermostat up more than three degrees such as when I get up ion the morning after turning the thermostat down eight-to-ten degrees before going to bed. If I had a "regular schedule" (I'm retired and live alone) and used the programming function of my thermostat the furnace would likely never go to the second stage.

So, in your case I would be completely comfortable with the decision to go to the 45,000 BTU model.
 
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Old 06-20-10, 06:36 PM
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Originally Posted by furd
If you counted the gas consumed by the water heater as being consumed by the furnace (did not make a correction on the metered gas to allow for the water heater) then you are already oversizing the furnace.
Precisely. I did it on purpose, to give myself an extra margin for error.

Originally Posted by furd
If your home sees about the same winds and solar gain as these two weather stations then using the averaged degree day statistics from the two stations should be fairly close for your home.
Both stations reported about 4600 degree days annually. I used 4000 degree days in my calculation. Again, to give myself an extra margin for error.

Originally Posted by furd
There is also the fact that a Manual J will often oversize the heating requirements by 20% or more.
Interesting. When I did my quick-and-dirty heat loss calculation (using the default R values suggested by the web calculator), the result was almost exactly 45K. Again, I have no idea if my actual R values are lower or higher than the calculator defaults.

Originally Posted by furd
Another simple trick is to think about how the furnace behaved during the coldest weather you have experienced. Did the furnace run for long periods of time with fairly short off periods? Or did it run for a short time and then stay off for a long time?
Hmmm, I don't really remember.

Even if I did... it's a 22 year old furnace. I read somewhere that old single-stage furnaces lose ~1% efficiency a year, because the burners corrode and the heat exchanger becomes less efficient. If it's true, my 60K-rated furnace might be delivering 50K or less.

Originally Posted by furd
So, in your case I would be completely comfortable with the decision to go to the 45,000 BTU model.
To tell you the truth, I'm still undecided. I'm meeting the contractor tomorrow who will likely get my business. I will see what he has to say.

Thank you very much for your feedback. You've been very helpful.
 
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Old 06-21-10, 01:12 PM
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Go to hvaccalc.com....for $49 you can use the same program I have used for 30 years without a problem. Then size your furnace accordingly. You may be able to install a 2 stage unit so you can save even more by burning less when the demand is lower.
 
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Old 06-28-10, 01:50 AM
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Sizing based on heating degree days isn't accurate, because they're calculated based on temperature deviation from 65F/18C.

Most houses don't need heat at 65F by a long shot.

A tried CMHC's method a few months ago and go a rediculously low result.

When was the house built? A 60k input/55-58k output HE furnace is a suitable replacement if the original heated fine.

2. Annual gas consumption: 9925 - 8061 = 1864 m3.

To be safe, round up to 2000 m3.
Based on that, a 45-50k input furnace might just do the job (goodman makes one); 2000 m3 isn't high at all, especially if you didn't subtract DWH tank usage.

Since you're selling the house, consider getting the GKS to save some cash. (single stage, standard blower)
 
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