where are high limit and fan limit sensors on ancient Rheem

Motorapido

Where do I find the high limit switch and fan control switch on a 1973 vintage Rheem 3040 -110 gas hot air furnace, standing pilot, natural gas.

I'm guessing the fan control temperature switch is inside the jacket-surface-mounted galvanized box with the white push button for manually turning on the fan.

Before I start removing panels from the outside to get a look inside, I'm seeking more info. Would both the high temperature limit switch and the fan switch be inside of or behind the manual fan box with the fan push button, which is mounted on the furnace jacket, up high and near the top of the furnace?

My huge local plumbing/HVAC supply desk professionals can't seem to find schematics or parts data on this 1973 Rheem furnace, which seems strange for such a large brand name. I'm assuming that after I locate and remove the high limit switch and the fan sensor switch that I will find numbers imprinted on those two parts so that my HVAC parts counter can sell me replacement control switches.

Please advise on the location of those two control switches, and many thanks.

Motorapido

JUGHNE

That is probably a combo high limit/fan switch. Maybe 4 wires into the bottom. 2 control the fan (120 volts) and 2 control the gas valve (24 volts).

The cover comes off and there may be 2 screws holding it in place.

There is an element 7 to 9" long that inserts into the air stream between the heat exchangers.

You can match this by the length of the insert and the ratings on the dials you will see.

Probably no one is alive at Rheem that has ever seen one.

Motorapido

Motorapido

Thanks Jughne. Here is a photo of my manual fan control box with the cover removed. Is this a combo high limit/fan switch?

JUGHNE

Yes, Rheem never made them. Honeywell or White Rodgers item.

You will have to pull it out to measure the senor.

Why do you think you need one?

EdTheHeaterMan

There are at least 3 different lengths.  and you DO NOT need to get one with the white push button, unless you use it to operate the fan in the summer, (since you are just finding this part, I guess you do not do that). 

That white button has caused more nuisance service calls after homeowners try to push it to reset the furnace.  Then the technician needs to come out to find why the fan will not shut off, and all he does is pull the white button back out.   $$$ for a simple pull of a button. and then everyone is unhappy.  The customer says "Is that all?  …and how much does it costs?"  Technicians are uncomfortable because they agree with you but need to collect.   The boss is unhappy because the tech didn’t charge enough for additional repairs.   That white button is a No Win situation.

Ask me how I know this?

JUGHNE

I have an 8" insert fan limit I would like to get rid of. Made in Canada by Honeywell.

New in the box. It even has the white button. PM me if interested

HVACNUT

@Motorapido , That is the Fan/Limit Control. That model does not have the manual/auto fan switch.

Is that control mounted in the furnace, or in the supply duct plenum? I'm hoping plenum, otherwise the tape needs an explanation.

Its also definitely not factory wiring, as White and Green are being used as Hot legs (I love you honey!).

Is the only issue no manual fan control? You want to be able to run fan only without heat? If you can show the wiring diagram (if it's there), and if there's 3 wires at the thermostat, it's doable.

EBEBRATT-Ed

There are two switches in that control. Some furnace mfgs use 3 wire and some use 4. Usually, the bottom two terminals are connected together with a common jumper which is used for a 3-wire system and removed if you have 4 wires.

SuperTech

1973 furnace? I would be concerned about the safety of that thing, even if it was installed in Texas or San Diego that's an awfully long time to keep a furnace operating.

Motorapido

Here's why I will be removing/replacing the combo fan/limit controls.

Upon the very first use of this year's heating season, the furnace ignited the gas flame properly. Normally, the blower fan turns on 45 seconds or so later. But this time, the blower fan did not turn on. The thermostat was set for AUTO fan control the whole time during the test. After waiting a few minutes with full flame but zero fan, I turned off the furnace.

I checked out the fan motor to make sure it wasn't seized up. The fan motor is just fine. It was replaced around 8 years ago. No humming or buzzing from start capacitor, and the shaft turns freely and smoothly on its bearings. The pulley is fine, the belt is fine, and the bearings on the pulley end are also fine. So no problem with the fan motor.

I then set the thermostat to call for heat again, and again with the fan switch on the thermostat set to AUTO. The furnace flame ignited properly but the fan did not start to run after the heat started to build.

As an experiment, while the flame was still burning, I pushed in the manual fan control white button, the fan came on, and when I pulled the manual fan button back out, the fan continued to run. The fan kept running after the call for heat ended, and a few minutes after the burner flame was turned off, the fan also automatically turned off.

So I've concluded that either the fan control or high limit or both are failing/failed, and I'll replace both.

SuperTech

Those fan and limit controls are prone to failure from age and wear. Eventually they won't bring the fan on at the correct temperature. Certainly after 50 years that one is probably overdue for replacement. Again, everything on that furnace is well past the time that it was designed to last and is prone to failure due to age and wear. The integrity of the heat exchanger is the greatest concern.

Motorapido

I'm surprised that the control is not designed to shut off the gas if the fan control fails to turn the fan on. Seems like it would be safer if fan failure would shut the gas off

JUGHNE

The limit part of the control will shut down the gas valve, on temp rise because of lack of air flow.

The fan on and off are adjustable by pointers, hold the wheel if you move them.

The limit has a max adjustment.

Look the control to see your settings.

EBEBRATT-Ed

The high limit is usually set to shut off the flame at 160-180. The fan should come on at 125 ish and off around 100.

Those are round #s that can be tweaked

Motorapido

Can anybody help me figure out how to trigger the release mechanism for the wire push fittings on Honeywell fan controllers? I don't want to get brutal and try to tear the wire out of the push fit connection if there is a release button. But my old eyes in the dim light can't see a release tab.

ratio

IIRC there's a slot right beside the round hole where the wires stab in, that gives access to the retaining spring. Use a small screwdriver etc. to push in on it & that should release the wire. Alternately, twisting & pulling usually gets the wires out as well.