Fan/Limit switch heater?

[georgelass]

York furnace PCMD LD 16N095B, EMAM 409779

This is one of those old-style limit switches that also
turns on and off the fan via temperature. Fan on/off
temps are adjustable. Limit switch is anchored, I think
at about 150. Supply air just on the other side of the furnace.
I didn't have much choice about where to take it as the air
handler was right up against the floor. Anyway, when
the supply air was about 107, the limit switch opened
once or twice. Other times, it ran for a long while without
opening.

My BIG question is the limit switch heater. That's what
it says. There are two terminals in the middle of things
just to the side of the dial, and they are run to the gas
valve. In the schematic, it says the heater is only on some
models. York says the particular one is obsolete and
wanted to know the lenghth. Well, I didn't remove it
to measure because I thought York would have a replacement
so I didn't know the length. But I have a couple of sizes of
Honeywell ones on my van but they don't have heaters.
So what is the purpose of the heater and can I likely
use a fan/limit withOUT a heater with no ill effects?
(The Honeywells also allow adjustment of the limit temp.)

Thank you.

[t527ed]

purpose of the heater is to speed up the fan coming on, they are still available as far as i know.
did you check temperature at limit location? plugged up a/c coil or any restriction in airflow could cause the problem.

[georgelass]

purpose of the heater is to speed up the fan coming on, they are still available as far as i know.
did you check temperature at limit location? plugged up a/c coil or any restriction in airflow could cause the problem.

No, I didn't check at the limit location because it didn't seem consistent.
I didn't think about being able to leave the limit switch installed AND
put a thermocouple in there. Maybe....

I will do further checking.

But do you think the heater is important, that IF I replace the fan/limit,
I need to get a switch with a heater? As I said, the fan on/off temps
are adjustable.

[VTP99]

[QUOTE=t527ed;15051551]purpose of the heater is to speed up the fan coming on, they are still available as far as i know.


t527ed,
I have never seen or heard of these types of limits. I'm confused as to why one would not just set the fan limit at a lower temperature to achieve the same result. What am I missing ? Can you explain the reasoning ? Thanks

[2sac]

There are 4 wires going into the limit. 2 are for your control circuit and 2 are the fan delay. Those limits are available aftermarket

[georgelass]

There are 4 wires going into the limit. 2 are for your control circuit and 2 are the fan delay. Those limits are available aftermarket

Yes, the Honeywells in my truck are like that. But this one has two more for the heater.

[pacnw]

the heater is there to bring on the fan with a call for heat.

it is wired off the "W" and "C" and when a heat call is initiated the heater begins and then brings on the fan- kind of like sequencer in an electric furnace.

it is a safety to assure the fan comes on in case the mechanical portion fails.

I do not install replacements with this, only use the HW without it. never had an issue without it and do get the comment about it blowing cold at first when it is wired to work. they usually comment after you disconnect it as to how it feels better without it.

[georgelass]

When you say it is a safety to assure the fan comes on in case the mechanical
fails, do you mean it is like a heads-up (as I think of it) to tell folks that something
is amiss? So it is more of an alert than a safety feature? Or am I confused
about why furnaces typically run their ibm when something faults?

Thank you for the encouragement to install one of my Honeywells, if they are
the same length.

[VTP99]

When you say it is a safety to assure the fan comes on in case the mechanical
fails, do you mean it is like a heads-up (as I think of it) to tell folks that something
is amiss? So it is more of an alert than a safety feature? Or am I confused
about why furnaces typically run their ibm when something faults?

Thank you for the encouragement to install one of my Honeywells, if they are
the same length.

Here's my take on what was said. If the cork screw was to break lets say. You could lose the high limit safety. Yes,No or Maybe ?

[2sac]

Here's my take on what was said. If the cork screw was to break lets say. You could lose the high limit safety. Yes,No or Maybe ?

It's sounds like a redundant delay

[2sac]

I'm not familiar with the heater in the limit. It sounds like when the gas valve is energized, the heater in the limit also energizes? and according to the schematic this was an option? If that's the case, it's my guess it was used for efficency purposes. I don't believe there would be any ill affect to using your truckstock limit

[georgelass]

I'm not familiar with the heater in the limit. It sounds like when the gas valve is energized, the heater in the limit also energizes? and according to the schematic this was an option? If that's the case, it's my guess it was used for efficency purposes. I don't believe there would be any ill affect to using your truckstock limit

The schematic said the heater was not on all units. But it didn't say it was optional on
the units that had it. Because in my experience my assumptions have often coincided
with screw-ups, I did not want to assume that the heater was optional for the furnace
models that had them installed at the factory. Often what sounds logical to me
usually has some loophole that I didn't consider. I try to be more cautious about
winging it these days.

[VTP99]

The schematic said the heater was not on all units. But it didn't say it was optional on
the units that had it. Because in my experience my assumptions have often coincided
with screw-ups, I did not want to assume that the heater was optional for the furnace
models that had them installed at the factory. Often what sounds logical to me
usually has some loophole that I didn't consider. I try to be more cautious about
winging it these days.

Sounds like someone speaking from experience.

[rundawg]

The schematic said the heater was not on all units. But it didn't say it was optional on
the units that had it.

I emailed you the install manual on this unit. It states the approx. temps the fan should turn "ON" and "OFF".

[VTP99]

I emailed you the install manual on this unit. It states the approx. temps the fan should turn "ON" and "OFF".

rundawg,
Do you have any info on that heater type limit ?
Thanks,
VTP

[rundawg]

rundawg,
Do you have any info on that heater type limit ?

Sent you an email.

[VTP99]

Wow.... just when you think you have seen them all. The only difference I have found with them is the manual fan button option.

[t527ed]

this was the last fan limit used on Lennox furnaces before they switched to blower control boards in the G20 furnaces.
heater brought the fan on sooner than the normal fan setting in the control, more for efficiency than safety.

[georgelass]

this was the last fan limit used on Lennox furnaces before they switched to blower control boards in the G20 furnaces.
heater brought the fan on sooner than the normal fan setting in the control, more for efficiency than safety.

Yep, that's what it looks like.

[mike3]

Wrong terminology used here. It is not a heater for the LIMIT. Camstat was one of the mgf's. They had 6 wire controls and 4 wire controls. Lennox and DNP used them several years ago, before boards were common. 4 wire was fan only, 2 wires were 120 volt for blower, other 2 wires were 24 volt heat assist. When the burner came on the heat assist was energized and the small resistance wiring around the bimetal got warm and closed the switch before the furnace temp would. These especially good for an area that was cold and would pull in thru the return.. That would prevent any blower short cycling due to cold air over fan switch. The other control had 6 wires, the 4 mentioned above and 2 for the limit.