Ancient furnace alternating between flame and fan

model number, sequence of operation, type (natural draft standing pilot, mid, high efficiency etc)

1725 rpm
1/6 hp
115 volts
service factor 1.35

Click to expand...

sounds like a belt drive based on the rpm.

don't worry about the lower amp draw - 1/6hp won't draw much current. rla is only the max before the thermal overload trips with a factor built in.

So when the indoor blower starts, it blows the flame out?

It is a belt drive.

The reaction is exactly like the flame was being blown out. I hesitate to say thats exactly whats happening, because nothing directly blows at the flame, though i suspect that the movement of air is extinguishing the flame.

Which makes me think some air flow is blocked, because it shouldn't pull air that strongly from the burner enclosure.

i checked the filter but it was fine.

Thoughts?

Is this a standing pilot furnace?

Does the pilot go out when the blower starts?

Is the blower access panel properly in place?

A standing pilot furnace can't have the flame blown out and shut down from that unless the pilot also gets blown out.

user_12345a said:

Is this a standing pilot furnace?

Does the pilot go out when the blower starts?

Is the blower access panel properly in place?

A standing pilot furnace can't have the flame blown out and shut down from that unless the pilot also gets blown out.

Click to expand...

standing pilot, remains lit.

the door... doesn't really seem to fit well no matter what i try, but it's on.

Monitor for power 24v ac at the valve when this happens. The valve is losing 24v, can't have a flame get blown out on a standing pilot without the pilot going out too. Flame can get blown around if the heat exchanger is badly breached but won't go out.

Most of these old furnaces to my limited knowledge don't have rollout switches or other fancy safeties - just a high limit switch and vent spillage cutout on the draft hood.

Could even be a marginal low voltage connection - vibration when the fan motor on may break the circuit.

Check the all the low voltage connections, jump out the thermostat.

Just a thought.

I would checking if that furnace is limiting out and shutting off the flames. (like when a newly installed motor is not the same rotation direction as the last motor)
You probably have a fan limit control with a mechanical dial which can be watched to see if the temp within a furnace is getting up to 200F when the gas is shutting off..

how said:

Just a thought.

I would checking if that furnace is limiting out and shutting off the flames. (like when a newly installed motor is not the same rotation direction as the last motor)
You probably have a fan limit control with a mechanical dial which can be watched to see if the temp within a furnace is getting up to 200F when the gas is shutting off..

Click to expand...

I poked around a bit regarding the limit switch, though what I saw looked too modern for this furnace. My tenant moved in in 79' and no matter what the furnace predated him. I didn't see anything likely when looking on it, is there somewhere in particular i should look? I couldn't find a diagram.

You need a multi-meter to troubleshoot it.

visual means nothing.

Need to know if the burners being blown out or shutting down on limit.

main burner gas valve will stay open on a standing pilot furnace as long as there's a call for heat reaching the valve and a pilot on.

user_12345a said:

You need a multi-meter to troubleshoot it.

visual means nothing.

Click to expand...

Well I appreciate any opportunity to improve my multimeter skills, but where do I even start? Also, if visual means nothing... is that to say there is no limit switch?

I think How may be onto something with the motor going the opposite direction. Am I correct that reversing the lines powering the motor will make it spin the other way?

Time to consider sending us some photos.

Your fan cage may have an arrow on it showing the direction that the fan should be turning towards. The cupped fan fins should be moving in a direction that allows the cups of those fins to scoop the air along with them.

The box containing the fan is also offset so that in the correct direction, the fan can push the air directly into the exchanger, where as in the wrong direction, only some of the air would enter the exchanger area and at an angle.

Check first on the fan rotation to see if the motor direction is an avenue worth exploring.


If you bought a motor that can run in either direction, changing that direction will require either a re connection of the wires within the motor
(as shown on the schematic on the side of the motor) or it will have an extra two external lead wires that are spaded together that need to be switched with each other.

DIYHero said:

Well I appreciate any opportunity to improve my multimeter skills, but where do I even start? Also, if visual means nothing... is that to say there is no limit switch?

I think How may be onto something with the motor going the opposite direction. Am I correct that reversing the lines powering the motor will make it spin the other way?

Click to expand...

Check for 24v at the gas valve at the time that the burners shut off.

Have to see where the circuit is being broken - voltage drop across the safeties.

so many things can cause this, if u have the ability to take a video and post, do it.

If your fan/limit control looks like user 12345a's picture, then by squeezing and removing that outer cover, you'll expose a by metal controlled dial within that will turn according to the heat measured in the house air stream of the heat exchanger. The temp is measured on that dial according to the center line of the fan /limit control.
There will be three nibs on that dial that control when the fan turns off, when the fan turns on and when the gas should be turn off because the temp is too high. If the control is working properly and it's getting up towards 200F...the furnace is getting too hot and the reason why that is happening needs to be tracked down.

Welp, hooked up the old motor and discovered the new one rotate a different direction. The link to the motor specs is below. Am I able to change a motors rotation?

https://www.grainger.com/product/CE...a8293b823ae56b3d06362523d606e39e0&origin=prod

According to the specs on the motor it is reversible.

There should be 2 orange/yellow/(maybe with 2 brown/purple or 2 more orange and yellow) wires that connect to each other and return to the motor. They would have been connected out of the box. Switch them. The motor label will confirm the colours.

Cheers!