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Originally Posted by ClimateCntlJohn 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.
Most furnaces with these fan & limits are close to 20 years old and older. One should check the heat exchanger closely.
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.
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?
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.
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.
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
found this and answered my questions. http://www.honeywell.com/sites/servl...4-73BD77880C46
Originally Posted by 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. 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.
Originally Posted by 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. thanks for clarifying it for me, too. I never had it explained like this and now I will look at it differently.
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.
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.
Honeywell makes still makes that fan limit. L4064W. You'll have to go back and find out the insert length.
Originally Posted by VTP99 rundawg, Do you have any info on that heater type limit ? Sent you an email.
Originally Posted by t527ed Attachment 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.
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.
Attachment 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.
Originally Posted by rundawg 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
Originally Posted by georgelass 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".
Originally Posted by georgelass 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.
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