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 York says the particular one is obsolete and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.

[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]

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]

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

[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

[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

[t527ed]

349051

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.

[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.

[georgelass]

349051

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.

[rundawg]

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

Sent you an email.

[beenthere]

Honeywell makes still makes that fan limit. L4064W. You'll have to go back and find out the insert length.

[doc havoc]

If your supplier does not have the Honeywell, Camstat still makes them. I personally feel the Honeywell product is the better option though. I don't see any safety concerns with using a 4 wire F/L control without the heat assist. I say use what you have. As mentioned earlier, the customer may even be pleased with the warmer air coming out of the registers when the unit first starts.

[jetstream]

We called those "assured fan controls". As said they will bring the blower on quicker, and will maintain heat in the control through the run cycle so the blower will run a bit longer on shut-down and not recycle back on from residual heat. I recall a couple of furnaces that used them had drum style heat exchangers and the control was located where it would be very slow to pick up the heat when the burner was on no matter how finely you'd try to adjust the control, unless you used the "assured" control. You might wait several minutes for the blower to come on. Also one Lennox horizontal model in particular would trip on limit immediately after the blower came on if you didn't use the "assured" control. You can probably get away with not using the proper one without problems, but it's there for a reason. And I recall several times upgrading to an "assured" fan control as a way to solve nuisance cycling problems.

[pacnw]

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.

thanks for clarifying it for me, too.

I never had it explained like this and now I will look at it differently.

[pacnw]

We called those "assured fan controls". As said they will bring the blower on quicker, and will maintain heat in the control through the run cycle so the blower will run a bit longer on shut-down and not recycle back on from residual heat. I recall a couple of furnaces that used them had drum style heat exchangers and the control was located where it would be very slow to pick up the heat when the burner was on no matter how finely you'd try to adjust the control, unless you used the "assured" control. You might wait several minutes for the blower to come on. Also one Lennox horizontal model in particular would trip on limit immediately after the blower came on if you didn't use the "assured" control. You can probably get away with not using the proper one without problems, but it's there for a reason. And I recall several times upgrading to an "assured" fan control as a way to solve nuisance cycling problems.

so does the assured control act like a sequencer when the call for heat is lost, in other words after X amount of time that portion of the control is open and now just residual heat activates the fan?

if so, does it really help on the cool down or just for initial start for those cold short cycle instances?

thanks.

[pacnw]

found this and answered my questions.

http://www.honeywell.com/sites/servlet/com.merx.npoint.servlets.DocumentServlet?docid=DB6 EDE2B5-389C-8487-0644-73BD77880C46

[dave sulz]

Im old enough to have worked on that control 20 plus years ago. The purpose of the heater in the fan control is when you start up heat in morning with cold ra temp the fan control will cool off enough to stop fan.the heater keeps the fan on with a w call

[georgelass]

After reading all of this, I'm left wondering how other furnaces using
the more common, no-heater fan/limit don't have short-cycling on
occasion. But it also makes it more clear why everybody now uses
timed fan-on, fan-off.

[ClimateCntlJohn]

A pic of it would be great. At first I thought you were talking about one of the fan limits with an adjustable high limit, fan on and off tab that rotate when the HX gets warm but now I'm not sure. Either way, I'd do like T52 and check the temp at the limit and check your airflow. Its possible it could be bad but I'd confirm first because its not exactly common. You don't want to install a new one and have the same problem ya know.:deadhorse:

[ClimateCntlJohn]

:censored: Nevermind, I didn't realize so many people had posted, and with pics....... lol Anyways, seems like alot of mobile home units with gas or oil use them. Some I've noticed get "hung up" when rotating and don't always turn blower on at right time. I'm guessing because someone in the past had screwed with the dial and got it out of whack?

[mike3]

Nobody has mentioned energizing the tstat and have the blower come on after a few seconds when the gas is off. That is a sure indication there is a "heat assist" fan switch (control) installed.

[Milk man]

Most furnaces with these fan & limits are close to 20 years old and older. One should check the heat exchanger closely.

[mason]

:censored: Nevermind, I didn't realize so many people had posted, and with pics....... lol Anyways, seems like alot of mobile home units with gas or oil use them. Some I've noticed get "hung up" when rotating and don't always turn blower on at right time. I'm guessing because someone in the past had screwed with the dial and got it out of whack?

No that's just a really common failure on the fan/limit combos. They get weak over time from rotating and can't push through the set point or snap back through it(so the fan either won't come on or won't turn off), don't try to wd-40 or oil them because it doesn't help, the metal is just fatigued.