Please clarify what you mean by "control module." Is that within the heater
chassis? Or is this something akin to a thermostat, as found in the living
space?
The "control module" in my forced-air heater is a complex thing. I suppose
I could simulate a closed thermostat circuit, at the control module. IT
seems your repair man did something like this.


"Bruce" wrote in message
7.136...
I've got a 9 year old natural gas York downflow furnace, with a set-back
thermostat. I don't have the capacity in front of me, but it heats a 5
bedroom house.

It's been working just fine, until a few months ago. When the thermostat
calls for heat, sometimes it won't go on. Nothing. If I take off the
blower cover panel, and press and hold in the momentary blower lockout
switch, it will go on. When I release the switch, it goes off, as it
should, since this switch is inteded to turn the furnace off when the
blower cover panel is removed. I then put the panel back on, which
depress and holds in the switch (as long as the panel is on - which is
during normal use), and the furnace goes on fine and heats the house to
the set tempeerature.

When the house tempereature drops below the set point, and the thermostat
calls for heat, 'usually' the furnace goes on. But, when it doesn't, I
repeat the steps above, and it's comes back on. Sometimes it'll go for a
week without any failures, but sometimes it will fail many times in a
row.

I jumped the thermostat, and tried a new thermostat, and have determined
this is not the problem.

To take the blower lockout switch out of the equation, I installed a new
one, and wired it for always closed, but the same problem occurs. If I
push and release (sometimes it has to be done repeatedly during a
failure) the switch, opening and closing the circuit, the furnace will
come on. It will always come on; sometimes one push, sometimes many.

I had a repairman out (very reputable company), and he suspected the
control module. A new module was ordered, and a week later another
fellow brought it out to install it, only to find it was the wrong part.
Not all bad though, because this fellow isn't all that sure that the
control module is the problem.

"Thermostat at control module" is unclear to me. I think you mean
"thermostat connection terminals at the control module."
Well, that tells me that the wiring between the thermostat, and the module,
is bad. After all, your actions upon the thermostat, in the living space,
were consistently ineffectual. Yet a similar action, with the lengthy
wiring between thermostat and control module REMOVED from the process, is
effective in consistently activating the heater.
You might have also had a bad control module; see below.

He jumped the thermostat at the control
module,
THAT'S THE NEW CONTROL MODULE, RIGHT?
and every time he jumped it there, the furnace came on. He
thinks maybe a gas control valve, but says that the first repairman
should be able to nail down the problem. Is this true, or is this a
'replace parts until it's fixed' job?

One more bit of info: I don't remember for certain, but when the first
repairman was out, I do think there was one or two times when he jumped
the thermostat at the control module and the furnace didn't come on.
That evidences a fault in the OLD control module, too.

The next time the furnace doesn't come on (tonight), I am going to jump
the thermostat at the control module and see if it does come on.

What really has got me stumped is that when it fails, one or more pushes
on the (new or old) blower lockout switch will make it work. Everytime.
So you've got either a flaky interlock switch, or it has shifted in its
mount, so that it does not get the right "signal" to change state when the
panel is closed. Is this switch a momentary, spring-loaded switch? Is it
Hall Effect (magnetically actuated)?
So it sounds like you have, or had, three problems.
Isolate the interlock switch by lifting its wires from the control module.
Then check its continuity, very carefully, under various conditions of panel
closure. There should be an unambiguous state change between panel *in
place* and panel *removed*.
The purpose of the interlock is to prevent operation when the unit is not
sealed up.
My residential forced-air heater was incapactitated by a panel, slightly
ajar. I had to drill and screw, so the panel would not vibrate away from
full closure.

Any ideas?