Why Is It Wrong to reduce output of furnace by eliminating burners?

I've heard there are reasons not to reduce the output of a furnace by eliminating burners (going from a 100btu down to a 35btu). What are the reasons not to do this?

Thanks,
Bill

Well, for starters there is the "johnny cochran" issue: not legal to modify an appliance like that. You violate it's GAMA/AGA/UL listing, and hence lose the permit it was installed on.
Aside from that, now you don't have the air flow balanced against the heat produced. That can't work out well. \

And most important, you affect the operation of safety circuits like hi temp limit switches, etc. Those were all designed around a certain burner/fan operating scenario, and now...who knows.

If you don't need 100,000 BTU, a whole new furnace is not that expensive, and if you want uber energy efficiency, you get a new furnace which is DESIGNED to operate the burners in stages, and modulate the air flow accordingly. The unit costs more, but in a very cold climate can save a lot of energy. If you are upstate like in Buffalo or something, this would be worth doing.

Messing with a furnace on your own is a bad idea.

It would be easier to change the orifice, and hope it don't blow.

Bad Idea.

DonL said:

It would be easier to change the orifice, and hope it don't blow.

Bad Idea.

Click to expand...

las time I changed an orifice (jet) was when I experimented running my 58 Ford pickup on 100% Ethanol in the late 70's

BobL43 said:

las time I changed an orifice (jet) was when I experimented running my 58 Ford pickup on 100% Ethanol in the late 70's

Click to expand...

I hope you added some Lube with that Ethanol, Sounds like it ran fast for a short period of time. lol

Many gas valves have a hidden screw [under a cap screw] that reduces gas flow to the orifices. That could safely downsize the output slightly.

Most gas burners (at least should be) adjusted for proper air/fuel mix to minimize carbon and CO production. Just changing the gas (fuel) would disrupt that balance, and in itself would be unsafe unless you rebalanced the air/fuel mix. As already mentioned, just arbritraily going in and changing things is likely to make for an inefficient system and not increase your comfort or safety. The heat exchange output temp, drafting in the flue, blower speed, and other things would likely be compromised. The reduced heat output might also lead to condensation and self-destruction of the interior since it would not get as hot. Really not a good idea unless it was designed for it (some are, within limited range).

molo said:

I've heard there are reasons not to reduce the output of a furnace by eliminating burners (going from a 100btu down to a 35btu). What are the reasons not to do this?

Thanks,
Bill

Click to expand...

The massive excess combustion air will reduce the steady state combustion efficiency, but you'd also suffer flue condensation issues unless you narrowed it down with a liner. At that large a reduction you'd end up with laminar flow on fire-side of the heat exchangers (unless it has mechanically drafted venting), reducing the heat transfer efficiency.

Derating a low-mass hot air-furnace has negligible effect on efficiency, even when done in a range that doesn't cause other problems. Derating a high-mass boiler can sometimes lead to fuel savings, even it's steady-state efficiency is slightly reduced, but even then the "comfort & efficiency" money is usually better spent elsewhere.

What are the reasons you're contemplating doing this?

DonL said:

I hope you added some Lube with that Ethanol, Sounds like it ran fast for a short period of time. lol

Click to expand...

Actually, it ran pretty well once it warmed up, which took a while, even in nice weather. I did not run it long enough to damage any valves. Used about 15 gallons of Ethanol in total during my experiments.

BobL43 said:

Actually, it ran pretty well once it warmed up, which took a while, even in nice weather. I did not run it long enough to damage any valves. Used about 15 gallons of Ethanol in total during my experiments.

Click to expand...

Man, That is a lot of drinking. lol

DonL said:

Man, That is a lot of drinking. lol

Click to expand...

Brrrp, you got me pegged.

Serious business. A "furnace" should never be downsized, as the tin heat exchanger will not last long voiding warranty etc. A cast iron "boiler" could be de-rated, but the added stress to cold sections makes them fail prematurely, (so say the manufacturers).

As pointed out early in this thread, all gas-fired appliances are UL/CSA tested and approved in a certain configuration and output. Changing orifices and plugging burners is not good practice and should never be attempted without the aid of an experienced HVAC technician with a current combustion analyser.

molo said:

I've heard there are reasons not to reduce the output of a furnace by eliminating burners (going from a 100btu down to a 35btu). What are the reasons not to do this?

Thanks,
Bill

Click to expand...

I agree, It is better to be safe than sorry.

They do make systems that cut back to save energy.

It you only need 100btu, You maybe be able to get that from a candle, Use a smaller candle and get 35btu.

Usually, each burner has a "path" above it through the heat exchanger. if you remove burners you create an uneven heat pattern and that could cause metal stress and create cracks in the metal.