On Sat, 04 Dec 2010 11:47:47 -0500, Home Guy wrote:



That's why I'd preferr to retro-fit the furnace with standard technology
and do away with all the sensors.

It's crazy that they removed the standing pilot light (which uses maybe
$10 worth of gas all year) and they went with electronic ignition, and
by doing so they had to add all sorts of temperature sensors to know if
the main gas supply should be shut off in case the ignitor doesn't
work. Talk about over-kill to save $10 worth of gas (even less if you
normally turn off your pilot light in the summer).


I don't like all the fail safe points on the new furnaces, because any
one of them failing can leave you without heat.
But you've got your work cut out to modify as you want to.
First, toss the motherboard.
Toss the ignitor.
Toss the flame sensor.
Toss the roll out sensors.
Toss the inducer, or unplug it.
Find a thermocouple controlled gas valve that will fit in place of the
electronically controlled valve that came with the unit.
Rig a pilot light.
Then figure out what to do with the many loose wires, how to relay the
main fan speed (you tossed the existing relays with the motherboard),
decide whether to rig an over temp sensor, etc.
You might need to rig a new transformer if that was on the
motherboard. I don't know.
I briefly considered replacing the motherboard on mine when I was
mentally shotgunning parts (I only wasted 6 bucks on an inducer
diaphragm valve.)
The mass of wires connected to the motherboard scared me off.
I always got it going again by cleaning the flame sensor.
Even if I had just cleaned it a couple days ago.
Main fan was always inconsistent too, sometimes running for a minute
after flame off, sometimes not.

When the main fan relay (on motherboard) stuck so there was only low
speed - not enough to heat/cool - I called in a pro.
He had it going in 10 seconds by flexing the motherboard.
Said it could stick again, so I had him put in a new motherboard.
Took the pro about 15 minutes to replace the motherboard.
He only cussed twice.
No problems for the 3 years since.

That motherboard was flaky from the getgo.
I think flaky/failing motherboards are the biggest problem with
modern furnaces, not the sensors.
Heat, undersized relays, etc.
A flame sensor problem is easy to diagnose - gas valve opens, then
closes. Cleaning sensor with steel wool always fixed that.
Ignitor failure is easy to diagnose - inducer clicks diaphragm switch,
and no glow within 15 seconds. Replaced one ignitor.
MB leds give that info too.
Sensors and ignitors are cheap and easy to replace.
Motherboards cost hundreds - think mine was $320.

Came across a post somewhere where a guy had 2 Carrier motherboards
fail in 4 years, so he wired external relays to take the load and the
3rd motherboard has lasted 6-7 years so far.
Next time I get a furnace I'll call up the same pro and pay for a
service call just to pick his brain on this type of thing before I
make my decision.
Problem is, as somebody else said, models are always changing, and
some reliable models get discontinued.
Almost like buying a first year model car - you don't know what bad
was engineered into it.

Then, if you talk to a pro repairman who also does installs you have
to be wary of his prejudices. It's human nature.

Anyway, I wager you won't try to do the modifications I've mentioned
when you replace that old furnace.
I'm a gambling man (-:
Wouldn't mind being wrong on that bet though if you come back and tell
us how you did it.

--Vic