# Sizing a furnace - can't find one small enough



## Jim W (May 10, 2009)

New to this forum but not new to doing everything myself - I am working on sizing a new natural gas furnace for my home.
The original house was only 560 sq ft and was heated with a wall furnace. I have added another 1000 ft and am sizing the new furnace based on results from the HVAC Calc - Residential program.
My results show the total (added 20% to calculated results) heat loss to be 29,198 BTUH. This may seem low but the house is well insulated and 500 sq ft of the addition is ICF walls (R54). Either that, or the program results are incorrect.

The problem is, I can't find a forced air gas furnace that is small enough! 

All I can find are furnaces that start at 45,000 BTUH - this seems like overkill. Everything I have read warns against getting a unit that is too large because it will cycle on/off too ofen and wear out.

Anyone have some direction on this?


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## beenthere (Oct 11, 2008)

40,000 is the smallest you'll find.


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## Jim W (May 10, 2009)

beenthere said:


> 40,000 is the smallest you'll find.


What do you reccomend I shoud do? I'm wondering if several ductless wall mounts would be a better solution. I just don't like the thought of having these things hanging on the walls and would much rather have one central unit.

The mini split systems are fairly nice and would also give me AC (not that I really need it here in San Diego) Problem is, they heat with electricity - not natural gas and could be expensive to use for heating - my primary reason for a system.

Another thing is that the smaller forced air units I have found don't seem to offer a variable speed blower which would be best in this situation. This would "optimize" the system for best performance by allowing it to run almost continuiously (as reccomended) but at very low speeds.

Your opinion is appreciated.


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## yuri (Nov 29, 2008)

A variable speed air handler with 8-10 kwatts of electric strip heaters is about all you will find. No demand for furnaces that small. Lennox makes a CBX32MV with 8/9/10 kw but they don't sell to the public. Need a Lennox dealer.


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## beenthere (Oct 11, 2008)

Use a 40,000 BTu.
Just size the duct work for it to be quiet.


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## Home Air Direct (Jan 6, 2009)

Our line of Whirlpool Furnaces offer a 40,000 btu 95% Gas Furnace, 2-Stage, Variable Speed. The first stage of heat operates at an output of 26,600 btu., with blower definitions from 564 - 1180 cfm. This could be an option. And as Yuri said, a VS air handler and Heat pump could be an option as well.

I am sure other brands offer the same options within their lines. Keep looking.

Good Luck
Jay


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## Jim W (May 10, 2009)

*Thanks Yuri and Benthere*

I'm disapointed that no one makes a smaller forced air unit - especially in this "green" era. I guess a 40,000 BTU unit is my only option.

I did find a 30,000 BTU but it is for big motor homes and probably only runs on LP. Besides, I doubt if they are built to last more than a few seasons.

Your feedback is very much appreciated!

Jim W


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## Home Air Direct (Jan 6, 2009)

Jim W said:


> I'm disapointed that no one makes a smaller forced air unit - especially in this "green" era. I guess a 40,000 BTU unit is my only option.
> 
> I did find a 30,000 BTU but it is for big motor homes and probably only runs on LP. Besides, I doubt if they are built to last more than a few seasons.
> 
> ...



The reality is that until now, the need for furnaces smaller than 40,000 btu has probably been less the 1% of the market. And, no matter how precise you figure your heat load, HVAC is not yet a micro technology. At this stage of the design game of HVAC, we still have to work with ranges of acceptability.

But I do not quite understand your disappointment, as there are several models of furnaces that are 95%+ in efficiency, that have a first stage firing capability of under 30,000 btu with variable speed drives? That is about as "Green" as you are going to get in the fossil fuel world.

Good luck to you in your search. Bring your unanswered question back here. There are some excellant field pros here that can give real world answers that go beyond the manufacturers hype.


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## Jim W (May 10, 2009)

I guess "dissapointed" is an overstatement - and you are right about the high efficiency furnaces currently availiable.
Thanks again.
Jim W


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## sktn77a (May 11, 2009)

Have you considered a regular split system heatpump and air handler - these are available in smaller sizes. I don't care for the ductless mini-splits either.


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## Jim W (May 10, 2009)

sktn77a said:


> Have you considered a regular split system heatpump and air handler - these are available in smaller sizes. I don't care for the ductless mini-splits either.


I really haven't looked into a heat pump because AC is not really needed here (unless you ask my wife in a few months). As far as I know, heat pumps use electric to heat with which would be expensive when natural gas is availiable. I could be wrong about that though (heat pumps only using electric for heating).

I also wouldn't feel comfortable installing a heat pump myself but a gas furnace seems do-able in my estimation. The ducting will be the biggest challenge but I'm commited to doing it right so it's gonna take some time.


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## yuri (Nov 29, 2008)

A heat pump gives you more heat per dollar than straight electric heat strips down to 32deg F or so. Co-efficient of performance theory. I would check with your insurance company and local gas utility. Where I live NOBODY other than a licensed gas fitter can install a gas furnace as a permit and inspection have to be done. Failure to do so and you have no insurance and need a good lawyer.


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## Jim W (May 10, 2009)

yuri said:


> A heat pump gives you more heat per dollar than straight electric heat strips down to 32deg F or so. Co-efficient of performance theory. I would check with your insurance company and local gas utility. Where I live NOBODY other than a licensed gas fitter can install a gas furnace as a permit and inspection have to be done. Failure to do so and you have no insurance and need a good lawyer.


 Thanks Yuri, I have and will continue to work closely with my local inspector (the current addition is under permit). If he wants a licenced HVAC person to install the furnace, I will definatly comply. The requirement is understandable for obvious reasons but, whomever I hire will only do what is absolutly required to comply, and I will do the rest. Don't mean to offend the licenced folk out there but I've had my fill of poor workmanship and high prices. Sorry, but a licence means you are capable of passing a test. It doesn't necessarily mean you are capable of performing the task.

I had two HVAC contractors give me bids several months ago and they both insisted that I get AC as well (didn't listen), never did a complete load calc but used a "rule of thumb" based on square footage and quoted 12K installed. I can't figure out where they get these huge numbers but the brand new SUVs they're driving give me a clue.

BTW I hold a valid California Class A Contractors licence but I'd never say I'm capable of reading grade stakes tomorrow morning - I am "licenced" though - want me to build a road for you?

I'll get off my soapbox now - go ahead and flame me if you choose

I realize I may have a tough time finding someone to only install the furnace and let me do the rest.

I will look into the heat pump as well and appreciate your imput.

Regards,
Jim W


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## yuri (Nov 29, 2008)

As long as the unit is installed properly and a fitter hooks up the gas line without doing some crazy bending of a flexible gas connector etc I would recommend you go for it. What scares the hell out of me is the thought of my neighbor buying a furnace at HDepot and hacking it in. Pay careful attention to the venting tables, slope/grade. Some things that look easy tend to be more complicated than they look. As Beenthere said you need to have proper sized ductwork or it may be noisy.

Good Luck


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## Jim W (May 10, 2009)

Yuri, I agree and am extremly aware of the doing it right, or not doing it at all approach. Sometimes I wish I didn't have this do-it-myself attitude. I has left me with no time for the fun stuff in life. It's always "as soon as I'm done with this project I'll take that vacation.

The projects never end and the vacation never begins.

Thanks so very much for all your advice, it is truely appreciated.

BTW, I'm ready for my plumbing inspection and I'll talk to the inspector about the furnace while he's here.

Jim W


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## yuri (Nov 29, 2008)

"The projects never end and the vacation never begins". 

I know, I have the same problem. Always trying to get "ahead". Of what I don't know, supposedly the next guy. 

Cheers:thumbup:


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## sweaty (Jul 18, 2008)

Best bang for the buck- Seal and Insulate everywhere, put a radiant heat barrier in the attic to cool down the wife, put in ceiling fans, super-insulate the attic.


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## Yoyizit (Jul 11, 2008)

Jim W said:


> heat loss to be 29,198 BTUH.


29,000 BTU/Hr = 8.5 kW heater = ~ 9 toasters.
The average house elec. usage is 1 kW and this mostly goes into heat so go with 8 toasters. If you have 10 sedentary people in the house you could go with 7 toasters!

A heat pump would take about 3 kW, 10A @ 240v.
I'd go with elec. zone heating.


Now I have all of three more-or-less valid data points

SW Ontario, Canada
398.7 cubic meters = 140 therms of NG in 28 days = 5 therms/day
2900 sq. ft. gives 5/2900 = 170 BTU/day/sq. ft.
664 HDD/28 = 24 HDD in one day
170/24 = 7.1 BTU/day/sq. ft./HDD

Wash, D.C.
838 therms of NG in 90 days = 9.3 therms/day
3100 sq. ft. gives 9.3/3100 = 300 BTU/day/sq.ft.
2658 HDD/90 = 30 HDD in one day
300/30 = 10 BTU/day/sq.ft./HDD

San Diego, CA
29,000 BTU/hr = 21 therms/30 days = 0.7 therm/day
1560 sq ft gives 0.7/1560 = 45 BTU/day/sq.ft. 
270 HDD/30 = 9 HDD in one day
45/9 = 5.0 BTU/day/sq.ft./HDD

27 more picked at random and I'll have a statistically large sample!


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## beenthere (Oct 11, 2008)

Yoyizit said:


> 27 more picked at random and I'll have a statistically large sample!


Of useless bad rule of thumb sizing.


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## Yoyizit (Jul 11, 2008)

beenthere said:


> Of useless bad rule of thumb sizing.


Humor me:laughing:

Somebody probably has published heat gain for an average house vs. solar insolation, which will be the next refinement to this method. 
Insolation for cities is already on the Web. 
Perfect absorption of sunlight by house walls and roofs would give 90w for each sq. ft..


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## beenthere (Oct 11, 2008)

There is no average house.

A 2400 sq ft rancher usually has a higher heat loss an gain then a 2400 sq ft 2 story.
While a 3000 sq ft 2 storu can have the same load as the 2400 sq ft rancher.

What size is an average house?
How many windows does an average house have.
What color are the shingles of the average house.
What color is the siding of the average house.

The word average, is misused when it comes to many things.


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## Yoyizit (Jul 11, 2008)

beenthere said:


> There is no average house.
> You're right. This is a mathematical model of an average house, just like 5'9" was the average height when I was born.
> 
> A 2400 sq ft rancher usually has a higher heat loss an gain then a 2400 sq ft 2 story.
> ...


I intend not to misuse it! 

The average for these three houses is 7.4 BTU/day/sq. ft./HDD, so you can already figure out if your house is above or below this "average" just using readily available data from your utility bills, the Internet, and some calculations.
If I still had Manual J from the library I'd use their house data to make a fourth sample.

Your spreadsheet may calculate Standard Deviation but not all do. Two standard deviations for a normal distribution take in ~95% of all data points.
If this heat loss is Normally Distributed and this sample is representative, then ~95% of the houses in North America are between 3.4 and 11 BTU/day/sq. ft./HDD.
An average is a Point Estimate. The 3.4/11 is an Interval Estimate, to some Level of Confidence.

100x (7.4-3.4)/7.4 = 54%, so it's 7.4 +/-54%, a pretty wide tolerance. In this tolerance are all the things you mentioned; style, # of windows, color, prevailing winds, insolation, snow cover, landscaping, all of it.

My 1964 house doesn't do so well by this standard, so I guess I should also ask for the age of the houses. I probably have infiltration due to settling.


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## beenthere (Oct 11, 2008)

Might as well stand on the sidewalk across the street. Hold your hand 6" infront of your face. However many fingers it takes to cover the house, that how big of a boiler you need in sections to heat the house.

The old saying is: There aren't to many houses a 10 section boiler won't heat.


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## Yoyizit (Jul 11, 2008)

beenthere said:


> Might as well stand on the sidewalk across the street. Hold your hand 6" infront of your face. However many fingers it takes to cover the house, that how big of a boiler you need in sections to heat the house.
> 
> If it works better than +/- 54% I'm up for it, and I don't even need a calculator. :thumbsup:
> 
> ...


I hope there are more houses like mine in North America; right now I'm at the extreme end, and not the good end.

Always a pleasure conversin' wit' ya', Mr. There


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## beenthere (Oct 11, 2008)

Depends on brand and model.
500,000 plus BTUs.

Or 5 therms plus an hour.

Lots of houses have boiler that big. And don't need that much heat.


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## Jim W (May 10, 2009)

"29,000 BTU/Hr = 8.5 kW heater = ~ 9 toasters."
I dunno Yoyizit, seems like running 9 toasters all day could get expensive.
- unless I had natural gas toasters


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## Yoyizit (Jul 11, 2008)

Jim W said:


> - unless I had natural gas toasters


I guess they use those little canisters, like the lighters that run on butane.

Depending your local prices, take your pick

1 Therm of energy
=~0.9 gal of fuel oil
=~70-100 cubic feet of natural gas
=~1.1 gal of propane
=~29 kwh of elec heat
=~8.4 kwh of heat pump heat
=~14# of wood

And, average solar insolation in kilowatt-hours per square meter per day for CA is 6.


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## beenthere (Oct 11, 2008)

Oil would be .71 gallons to equal a therm.


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## Yoyizit (Jul 11, 2008)

*correction*



beenthere said:


> Oil would be .71 gallons to equal a therm.


1 Therm of energy can be had from approx. 0.71 gal of fuel oil, 100 cubic feet of natural gas, 1.1 gal of propane, 29 kwh of elec heat, 8.4 kwh of heat pump heat or 14# of wood


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## Home Air Direct (Jan 6, 2009)

Yoyizit said:


> 1 Therm of energy can be had from approx. 0.71 gal of fuel oil, 100 cubic feet of natural gas, 1.1 gal of propane, 29 kwh of elec heat, 8.4 kwh of heat pump heat or 14# of wood



This is a great statistic. Thank you!!! :thumbsup:

I have several areas of my website and daily calls that this info will come in VERY handy. I knew that I had this given to me several years ago in training, but had forgotten that comparison and have since misplaced.


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## Yoyizit (Jul 11, 2008)

Home Air Direct said:


> This is a great statistic. Thank you!!! :thumbsup:
> 
> I have several areas of my website and daily calls that this info will come in VERY handy. I knew that I had this given to me several years ago in training, but had forgotten that comparison and have since misplaced.


110# of TNT will also give you 1 therm, but the neighbors will probably complain. :confused1:

The clear winner for 1 therm of clean energy is 2 billionths of a pound of nuclear material.


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