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Setting or Replacing a Honeywell L4064B fan/limit switch for oil furnaces

JBrant
JBrant Member Posts: 4
My oil furnace fan turns on only after the burner shuts down, but if adjusted to lower fan On/Off settings, the fan never shuts down. It uses a Honeywell R8184G relay, L4064B fan/limit switch, and CT87N thermostat. All is wired correctly, and replacing the old fan/limit switch made no difference.

With the preset fan settings on the fan/limit switch, Off at 90F, On at 110F, the fan turned on only after the thermostat shut down the burner. Lowering the settings as low as Off at 60F, On at 80F allows the fan to turn on before the burner turns off, but it is still running a half hour later with 70F air in the plenum.

As the plenum cools, the fan/limit switch rotor turns extremely slowly in small jumps, and cannot turn past the On lever, to reach a temperature at the Off setting, and is stuck there until touched, when it turns off the fan. This is a new unit, so the design appears to depend on tiny forces and friction. When set to higher temps, the fan/limit switch is able to turn the fan off but then cannot turn the fan on until the burner turns off, or not at all.

Is there a better setting method, or an electronic replacement for this that works properly?

Comments

  • mattmia2
    mattmia2 Member Posts: 3,458
    edited September 2020
    It is working properly. Your thermostat is either calling for too short a cycle or the furnace is grossly oversized.

    Set the t-stat to run longer cycles because if you don't heat the hx long enough to get up to temp, the condensation will kill it.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,120
    edited September 2020
    Yeah, it’s broke, and/or wired wrong, and/or probably something else. Call a competent tech to fix it it. I don’t know what @mattmia2 is talking about, especially regarding condensation.
    steve
  • mattmia2
    mattmia2 Member Posts: 3,458

    I don’t know what @mattmia2 is talking about, especially regarding condensation.

    I'm thinking convective heat from the furnace is satisfying the t-stat before the hx gets hot enough to cause the fan control to turn the blower on. My old furnace was a gas gravity furnace that was installed with a separate blower cabinet in about 1950 and it would do that, especially in the spring and fall when the load was small. The combustion products will condense on a cold furnace hx the same way they will on a cold boiler until it warms up. if it doesn't run a long enough cycle to warm up, that condensation won't evaporate. Maybe it isn't as bad with a power burner.

    If you turn the thermostat up like 5 degrees, does the blower come on after a few minutes? I'd watch and make sure the dial doesn't go too high, like over 180 or so without the blower coming on.

    I suppose the burner could be underfiring for some reason too. Actually this would be why I would strongly recommend you have a good tech look at it.

    Is this a new problem or was it always like this?
  • HVACNUT
    HVACNUT Member Posts: 4,020
    edited September 2020
    Is it a multi speed blower motor? If there's an open winding on the heating speed, the burner circuit will open on limit and kick in high speed fan to cool the HX. Is there a circuit board?
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,120
    You’re thinking to hard and abstract. These are simple mechanical components.
    steve
    SuperTechmattmia2
  • JBrant
    JBrant Member Posts: 4
    If the solution is to increase the burner on-time, I am not sure which thermostat setting would do that. The thermostat (Honeywell CT87K) has four Cycle Rate settings: (1) Steam or gravity, (2) High efficiency warm air (90% plus) hot water, or heat pump, (3) Gas or oil warm air (factory setting), and (4) Electric warm air.

    Two technicians have looked at it in two years without solving the problem. One set the thermostat cycle rate switches for Electric warm air instead of Gas/Oil warm air, which ran the burner for even shorter cycles, so I set it back to Gas/Oil warm air. The fan limits may work better in Winter with longer burner runs, than in Spring and Fall.

    The furnace heats a large room without ducting, with the thermostat about ten feet away. There is a partial plenum for the fan/limit switch, which seems to hold the warm air around the fan/limit switch properly.
  • JBrant
    JBrant Member Posts: 4
    When the thermostat is turned up 5 degrees, the burner continues running long after the fan has turned on. With the thermostat setting normal, and the present fan/limit switch settings, the fan turns on slightly before or after the burner shuts off, and the fan turns off about 8 minutes after the burner, although the air from the heat exchanger has been at about 70F for five minutes. Perhaps that is simply an inefficiency in Spring and Fall that I should expect.
  • mattmia2
    mattmia2 Member Posts: 3,458
    Steam is the longest setting and electric is the shortest cycle setting. Not sure about the 2 warm air settings. My guess would be the higher the number, the more cycles per hour. Could also try locating the t-stat further away or more out of the path of the outlet of the furnace.

    It takes some amount of time for the hx to heat up and some more time for that heat to heat up the probe for the fan control. If the t-stat is calling for very short cycles, under several minutes, the fan control isn't going to heat sufficiently to turn the fan on until the cycle has ended if at all.
  • JBrant
    JBrant Member Posts: 4
    Thank you all very much! This has been one of life's great open question for me for quite a while now! Best wishes to all.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,429
    Could be a burner problem as well. I would suggest calling a competent technician
    mattmia2
  • JUGHNE
    JUGHNE Member Posts: 8,557
    You say no ductwork on the furnace?