On Dec 12, 10:14*am, Home Guy wrote:
wrote:
I have a 50 year-old furnace that is more efficient than some
of the new ones after I modified the flame to use 1/2 the gas
it originally used.
Did you keep the same BTU output? ;-)
I bet he did, because by reducing the flame his duty-cycle is
probably longer (just like a modern furnace).

Reducing the flame is as simple as turning the dial on the gas
regulator in the furnace, or partially closing the gas shut-off
valve on the line going to the furnace.

Increase the efficiency even more ...

Are you actually supporting his theory that he increased his
efficiency ?

By turning down the gas supply, his furnace is reducing it's BTU output
and is also reducing the internal temperature of the heat exchanger.
His fan blower is not changing the amount of CFM of air being pushed
through the exchanger. *

Heat exchanger efficiency (any heat exchanger) is not constant over a
range of differential temperature (ie - the temperature difference
between the incoming house-hold return air on one side of the heat
exchanger, and the combustion air on the other side of the exchanger).

For example, if the combustion-air temperature is 300F, and my incoming
household air is 60F and 1000 CFM, I might get an efficiency of 80% at
extracting the heat of that 300F combustion air and transfering it to my
60F incoming air, boosting it by 20F giving me 80F coming out of the
furnace.

On the other hand, if I turn the burners up so the combustion air is now
500F, my incoming household air is still 60F, it's still being moved
through the furnace at 1000 CFM, and maybe it's coming out of the
furnace at 90F. *But my efficiency has dropped to maybe 70%.

This effect is probably more prounounced in the old-style furnaces with
long horizontal burners where the combustion air really doesn't spend a
lot of time in the furnace and has a low resistance path directly up the
flue.

And no, there is no way that a 50-year-old single-stage furnace can
approach the 90%+ efficiency of a modern 2-stage unit, but that's got
everything to do with a completely different burner and combustion
galley arrangement, a longer combustion path, more surface area, thinner
heat-exchanger walls, etc, and nothing to do with using an ECM motor or
electronic pilot or a computer in the furnace.

The point is that by reducing the BTU output of an old furnace, you are
allowing the heat exchanger to operate more efficiently, and you are
reducing the potential payback in getting a new furnace, and you are
increasing perceived comfort by having a more constant heat-output from
the furnace instead of short spikes of high heat followed by long
periods of cool-down.

I agree with the concept that a furnace will be more efficient if the
temperature
differential across the heat exchanger is less. How much it changes,
ie if you
screw around with an old 75% efficient furnace and turn the burners
down, will
it raise the efficiency from 75 to 76, or 75 to 80, however is an
important part.
You are assuming it's substantial. I'm not so sure.

But the problem here aside from the obvious practical problems of
doing it, is
that you can't undo it on the fly. If you can turn the burners down
so that it fires
at 70% for greater efficiency, then one of two things must exist:

1 - The furnace will now be unable to heat the house to normal temp on
the coldest
days

2 - The furnace was oversized all those years and you now have
downsized it permanently.

Those modern two stage or variable burner furnaces can change the
firing % on
the fly. Using it at 100% when the intelligent thermostat knows it's
needed, or firing
it at say 70%, when it's a mild day.

Another thing strikes me here. You keep pointing out how you can buy
cheap HVAC
online and you apparently believe you could do installs yourself or
pay someone on
the cheap to do part of it for you. With the current incentives, ie
Fed 30% credit,
nat gas utility credit, electric credit, state credit, I can
get $3,000 off the cost of a new system. That means doing it your
way, you could
have a whole new high efficiency 95% furnace and AC system for
$1000. Don't you think that's a
good value proposition, with a good payback period?