Sizing of a furnace for a house

[EatenByLimestone]

Earlier this year I was wondering if a newly installed space heater was large enough to heat the main room of our uninsulated family cabin. I was given a figure of 45btu/sq ft to heat the place. Using this figure I needed 14,400btu to heat the 320 sq ft. I had 22,000 available. I tested it later during the winter and the place heated up slowly at first when the wind was blowing hard, but when the wind went down over night it heated up fast. I woke up to a room in the 70s.

So fast forward to today when driving home from work. I was thinking about this number and wondered how the numbers worked on my house. The house is just under 1600 square feet. 1600*45=72,000. Using the 45btu figure above a 72,000btu furnace should heat my house if it wasn't insulated. But it is insulated. Not the best, it's a 90 year old home with original windows. But I have R50 in the ceiling and under the siding is a bit of styrofoam. The windows have triple track storms.

The furnace is a 100,000 btu forced hot air. I understand the need to over size to a point since one really doesn't know the temp increase a furnace will have to cope with in the future, but is my furnace grossly oversized?

It was only one year old when we bought the house, so it's only about 5 years old now. If we are still in the house when it finally wears out, should we look at a smaller one? When looking at furnaces, is it better to size one that will fire and stay on for a long time like a wood or pellet stove or one that starts for 10-20 minutes and then shuts back off? Is the on/off expansion/contraction thing something to be avoided if possible? Is metal fatigue an issue in the heat exchanger? Would it be cheaper all the way around to do something like choose a 50,000 btu furnace and then supplement the heat with electric baseboard when there is a real cold snap?

This is all a hypothetical situation since I don't see myself changing my 5 year old furnace. With the stove going it's not used hard so it will probably last longer than most. But if it ever comes time to replace it or we move to a house in need of a new furnace...

Matt

 

[mayhem]

Not sure of the correct answers to your questions, but here is my two cents:

I would imagine that the btu output of the furnace would more realistically be reflected by the cubic feet of your home, not the square feet. In many cases sq feet can be reliably used, but in my house for example its going to toally throw the equation off. Why? Because I have a log cabin with a great room in the front, so that one room has 26 foot peaked celings, which is pretty close to 3 usable stories of air volume that needs to be heated by one zone. My house has about 1750 sq feet of living space, but with a footprint of 28'x40', if it were a conventionally floored house it would have about 2300 sq feet to heat up.

My boiler is 180,000 btu and on cold nights in January when the wind was blowing and we didn't have the wood stove, it would struggle to keep the front room in the low 60's...thats with double height baseboards running the perimeter of the great room and a pair of fan forced kickspace heaters running full blast.

So in my opinion, if you have a "regular" house with conventional 8' ceilings and no unusal amounts of glass or doors, then you're probably fine with square footage calculations...but if you have high ceilings, lots of glass and other possible considerations, it may not give you an accurate result.

PS, I did a quick google search and this was the top hit.

(broken link removed to http://www.acdirect.com/new_faq/info_sizingas_furnaces_1.php)

 

[EatenByLimestone]

I read up more on the subject after I posted and realized I didn't include a figure for the efficiency of the furnace, plus duct loss. The furnace seems to be sized correctly if I didn't run the stove. I have regular ceilings, maybe they are a bit low.

Matt

 

[OldDedHed]

I would suggest doing a heat loss calculation on your home to determine what the maximum Btu loss could be. Then it will be a no-brainer to size your heat appliance. Here's an example:

http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

 

[woodgeek]

I suspect that you are are oversized--most systems in the US are oversized 50-150% according to the DOE.
The installer don't care about efficiency--they do care about angry calls from cold customers, so many use a
big 'safety factor'. we could save a lot of energy as a country if we DIDN'T do this.

You can figure your load pretty easily next season by turning off any supplemental heat sources and timing how
often/long the furnace is on at a given outside temp (averaged over a few hours). I have a 134kBTU/h (nominal
output) oil burner and an old 2200 sqft house, and when it was really cold outside my boiler ran about 40% of
the time. I'd call that oversized! I have my max demand at ~65kBTU.

I think this furnace timing is more reliable and easier than a heat loss calc.

As for your other questions, hard to say without more info--furnaces are designed for thermal cycling, but
that can still be what does some of them in. More often corrosion, though, I think.

The efficiency factor WILL take a hit from short cycling. If you research it, there are a lot of numbers out there
which go from large to small. In my case, I think it not such a huge effect, because the stored heat in my
boiler when it cycles off leaks into my finished space, not up the chimney.

B/c I had an oil burner, I had the option of downsizing the nozzle, dropping the output from 134k to 110kBTU/h

Less heat into the same heat exchanger = more efficiency, about 2-3% according to the manufacturer data.
I'm still almost 2x oversized, but I can't go to a really small nozzle, b/c the unit would reach the flue condensation
point, which it is not rated for (cast iron rather than stainless combustion chamber).

So, my roundabout answer is that you can nail your heating demand next time it gets cold, and if you are oversized,
a heating tech might be able to 'adjust' your existing furnace to run more efficiently at a lower heat output, or maybe not.
I wouldn't expect more than 5%.

 

[EatenByLimestone]

Would a NG furnace be able to be downsized by adding a smaller jet?

Matt

 

[Gooserider]

Second the start with doing a heatloss calc comment - it will at least get you in the right area. If you search in the "Boiler room" you will find several links to different sources, many of them freebies. There is one that does a really nice job, as it lets you work on a room by room basis. It's done by one of the boiler companies, so it comes with appropriate suggestions for what size boiler and baseboards to use, but the numbers still work.

On the timing issue, a properly sized system will need to run almost continuously during your coldest weather, especially if you aren't running any supplemental heat. If it's undersized you'll be running constantly, and still not getting up to the desired temp, but that is rare in the US.

Of course insulating more, replacing windows, etc. will lower your heat loss, and make you even more oversized, but it's probably still worth doing.

Gooserider

 

[EatenByLimestone]

I ran the program above with a few estimates such as window and wall R value which I don't have numbers for. It gave me a loss of 61K btu. Which shows me way over sized, but it also took into account the efficiency of my furnace.

Even when we were pushing 10 below this winter the furnace wasn't running the entire time.

Matt

 

[EatenByLimestone]

To be fair, I had the woodstove going balls out when it was that cold and was blowing as much heat into the house as I could.


Matt

 

[Gooserider]

To be fair, I had the woodstove going balls out when it was that cold and was blowing as much heat into the house as I could.


Matt

That makes a HUGE difference - in our house, even if the wood stove can't keep up, it still cuts the run-time on the furnace by 70-80%

Gooserider