Late 1960s day and night Furnace

monty1961

I dont get power through my relay to the high speed (black wire) to blower motor. Power does get to Limit swich and when switch clicks it sends power to slow speed (red wire) of blower motor. Is it the relay?

monty1961

Sorry. I didnt know there was a relay in the mix. Still looking to see if I am going in the right direction with finding the fault. Love learning new things.

old_squid

Monty? Do you have a problem or just a general question? ...... seems like you're talking to yourself outloud while thinking and looking at the furnace.

The high speed is "usually" reserved for air conditioning mode and would be energized by the G terminal on the thermostat connection terminals at the furnace.

If you don't have air conditioning you probably only have two wires at these connections..... "usually" red and white and they would be connected to R and W terminals.

A connection between R and G terminals is what powers the fan's high speed and does it safely as to not power both speeds at the same time......... something that is VERY bad to do.

monty1961

Yes there is a issue with my furnace. I didnt think my fan was working right because the high speed is not operating. so is switched the low speed from fan to the limit switch to the high speed. the high speed of the fan worked so I determined the high speed on fan works ok. The wiring is ok at the furnace and before I go buy the parts Im trying to figure out wich part to buy. Does the limit switch turn on high speed of fan or does it just cool down furnace on low speed when it hits a certain temp. I am leaning on the relay, but Im not an hvac tech.

old_squid

Furnace fans only run at one speed.

The blower motor having multiple speed capability is "possibly" so that it can be used also for air conditioning if that option was installed on your system.

Most modern furnaces today come with blower motors capable of 3 or 4 different speeds, yet they still only use one of them.

From your explanation there is nothing wrong with your furnace that needs fixing as far as I can tell .... from here. If the burner comes on when the thermostat tells it to, the fan blows the warmed air into the home until the thermostat is satisfied and then the burner goes off and then the blower motor runs enough to cool the furnace down and shuts off....... everything is working as it should.

monty1961

The only issue is that the high speed does not go on the fan only runs at a slow speed when the limit switch calls to cool down furnace. The hot air is not being pushed through ducts. Thanks

how

I'm still not clear about your problem.
Do you have a 2 speed motor?
Does the fan normally run all the time on a low speed but raises to a higher speed when the furnace warms up?
In otherwords.. do you have a 2 speed motor that is not switching to the heating speed?

Type of motor and fan/limit control would help. Photos even better.

monty1961

It is a 4 speed motor. when the thermostat calls for heat the furnace flames ignite and after a few minutes the flames go out and the fan turns on for a minute or more to cool down furnace. The blower never goes on high speed to send heat through ducts. it goes through this process ov er and overy every approx 5 minutes.

REP

Sounds like your furnace is running on limit instead of t-stat.This would indicate a bad limit.
Since you like to learn things here are a couple of facts that you might want to know.If it is a 1960's furnace,its way too big and if that isn't bad enough it is probably only 60% effiencent.Installing the proper size 90% furnace would save you probably 50% over what you are paying now.The proper size funace size is not a gussing game,there is something engineerts use callewd a manual "J" .Hope that helps you.3 new things learned today.

how

I've never seen a furnace that old, with that type of motor, have a two speed operation that wasn't a low speed continuous with a higher speed thats operated through a fan / limit control.

It makes me wonder if you've actually got a regular heating speed (Red) and that high speed (black) is a manual summer fan. The relay is just to make sure that only one motor lead can be powered at a time.

Can you post a photo of your fan/limit control (with it's cover off) and the "relay" you mentioned?

The more common causes for the gas going on and off that quickly on a furnace of your age is from

a faulty thermostat or anticipater ( bypass the stat to test it out).
a faulty limit on your fan/limit controller that is triggering at a low temp. or

a seriously cracked exchanger thats allowing flame to cook the fan/ limit control.
That would mean poisonous CO is creating a deadly situation and the furnace should be shut off immediately.

If you are planning on keeping this unit, get it professionally checked out!

old_squid

A model number of the furnace would be a great help here, if you can't find that on a tag inside the furnace than at least a picture of the furnace with the doors off so a person can see all the components. The more detail the better.

It's either the fan control or the control board or motor..... more than likely

how

@ old squid
Minumum of 44 Yr + old unit. What kind of control board are you thinking of?