How much does an oversized furnace waste? Can downsizing save enough to replace?

[jws3]

Yet another question here that has me stumped despite a fair amount of research. I suspect this issue may help many others, hence my thread.

Background: I have a 2800sf home in NY heated with a Williamson forced air furnace pumping out a full 1 gal per hour (139K BTU input). After deducting 20% for efficiency loses my output is about 111K BTU. I lso have a 2 zone system and a 5 ton Lennox heat pump. My question is only for the furnace sizing.

Of the many contractors I met with, only 2 would do a manual J calc. Both came up with about 75K BTUs as the max load under a worse case scenario. Cooling was measured as needing 56K BTUs.

On the surface it would appear that my furnace is oversized by a full 35K BTU, which is quite alot-- a full 1/3 to big

At first it was suggested that I simply use a smaller nozzle. Can't do that per the Mfgr as such will ruin the unit due to excess corrosion. The engineer was most emphatic on this one.

Both contractors said that the over-sizing is quite significant and results in substantial waste--on the order of 25% or so. They indicated that a "right sized" furnace, especially a variable one, would be far better and result in substantial savings since I'd not be pumping out 111K when all I really need is 50 or maybe 75 under really cold conditions.

The bottom line is thus: How much does an oversized furnace like this waste on average and what might I see in savings should I replace it? If I can recoup savings in a 4-6 time frame I would consider a new unit.

Any thoughts? Suggestions? Ideas?

Thanks in advance-

[jws3]

There has to be a relationship between over sizing and waste- the question is: How much?

[beenthere]

It depends on the ductwork also.
If the duct work is undersized, it uses more fuel.
If the duct work is sized ok, it doesn't waste as much.

Could be as little as 3%, or as much as 50%.

A furnace can be twice the size, needed. With duct work sized for the furnace, and you might only have a 3% exccess fuel consumption.

If the duct is only have the size needed for the furnace, then it will be closer to 50%.

[jws3]

Damn--another variable.

I have no idea if the ducts are good, bad or indifferent. In fact, I had a hard enough time getting a guy to do a "Manual J" calc. That was like pulling teeth. Can't imagine how hard it would be figure out the ducts.

I swear, the hardest thing thus far is finding someone who knows what he is doing and is willing to do it. I've gotten more help and knowledge on this board than from all the contractors I spoke to put together. That's not the way it should be at all..... Oh, well, at least some people know what they are talking about!

I wonder if there is a decent way to figure this all out.....

It depends on the ductwork also.
If the duct work is undersized, it uses more fuel.
If the duct work is sized ok, it doesn't waste as much.

Could be as little as 3%, or as much as 50%.

A furnace can be twice the size, needed. With duct work sized for the furnace, and you might only have a 3% exccess fuel consumption.

If the duct is only have the size needed for the furnace, then it will be closer to 50%.

[beenthere]

I'm sure there is "A" contractor in your area, that can figure out all that you want to know.
He just won't be cheap, because it does take time. And he went to school to learn how to do it.

[zzyzzx]

Depends upon how much you use it. I also have an oversied furnace, but I'm only using it for about 1/2 hour - 45 minutes in the morning after being off all night, and even then only for a few months in the winter. When it gets really cold, I sometimes run it for 1/2 hour when I get home from work. I don't leave it on all the time, cycling on and off like most people would. It's oversized because when the furnace was put in (well before I bought the house) the house had no insulation and crappy windows. I figure I can go down at least one size if I ever want to have it replaced, but I really see no need to replace it just because it's oversized.

[big johnson]

Oil furnaces are rated for output, not input. Is it short cycling, like less than a 10 minute run time for the burner?

[firecontrol]

The waste if any is up the chimney and in the shortened life of the furnace heat exchanger if the air flow can't maintain sufficiently low temperature drops across the funace. One is immediate the other comes out of your wallet when you have to prematurely replace the furnace.

If someone was to do a complete combustion analysis and set up on your unit it would give them all the information to bounce against "best case". The difference would be how much you're loosing. Of course this would have 3 parts to it.... zone 1 calling alone, zone 2 calling alone and both zones calling together.

As stated by others... the biggest factor would be if the ductwork was sized correctly for the capacity of the furnace factored against the zone sizes.

[jws3]

Oil furnaces are rated for output, not input. Is it short cycling, like less than a 10 minute run time for the burner?

I never really knew what the definition of short cycling is!

[big johnson]

I never really knew what the definition of short cycling is!

Short cycling is when the equipment has such a short run time that it never approaches what is called a "steady state." When a tech does a tune up or cumbustion test, they let the unit run for at least 10 minutes, usually 15. That gives the furnace time to "warm up" so to speak. Manufacturers rate their equipment for steady state operation. An oil furnace is most efficient at it's max rated output. During spring and fall months when the heating load is light, you will have shorter run cycles and there's not much you can do about that. However, in the dead of winter if your furnace runs less than 10 minutes per cycle, there needs to be some investigation into why and it could be a number of things, over-sized is one.

[da_poppa]

I believe that the oversize efficiency loss is considerably less in furnaces than boilers since the typical boiler just shuts off at the end of a call and the energy left goes up the chimney. A warm air furnace purges the heat out and usually brings the furnace temp down to near ambient so most of the energy that was put into it goes to the area where it was wanted.