Testing the old limit control that the furnace guy said was bad

[Lisa]

 I measure the limit control resistance at room temperature - the multimeter reads very low  (1.8 at the most sensitive setting, a length of wire is 0.3)  so the current should flow and allow the gas valve to open and ignite the burners.

Now I test to see if it failed at turning off the gas in case the temperature had exceeded the upper limit, allowing the thermal fuse to open.  I set the upper limit to 100 deg.  I put it in the oven at 140 deg.  I measure the resistance and the ohmmeter reads "I" (this is I for Infinity, not a number 1, correct?)  So the switch is open, there is no current, and I know the upper limit did not fail.

The old fuse also has a resistance of I.

Is this method correct?  What setting should I use on the ohmmeter - it goes from 200 to 2000K.   It's a 29V circuit, not sure what current it draws.

I'm definitely asking the heating service company for my money back.

The repairmen coming to my house have been batting .400 on diagnosis.  That's great - for baseball.

[hvacdrd]

You are correct in your testing.:clapping: When the circuit is closed resistance should be at or near 0, when open (sensed temp is greater than setting) circuit is infinity.

Is it possible the "furnace guy" called the thermal fuse a limit or did he specifically point out the fan/limit control? Just wondering where he was headed in his diagnosis since the fan/limit is such a common part and would not require "ordering".

[Lisa]

No, he called it a limit control, held the limit control,  cursed at the limit control while detaching it.

He said "This side (the blower side) is 120V and this side (the valve-igniter side) should be 24V and it's 0."

You know what?  It still is right now, in my working furnace.  The blower side is 120V because that switch is open, and the valve-igniter side is 0V because that switch is closed, waiting for the thermostat to complete the circuit OR for the upper temperature limit to be exceeded, and then the switch opens and the gas valve shuts off.   And when the burners are on and the blower is running, both sides will be 0V.

So he got the numbers and interpreted them wrong - they actually meant that the limit control was working and he should look elsewhere.

He did not pull the sheathing off the fuse, but I did - and the voltage there was 29V meaning that fuse was open and I needed a new fuse.

[hvacdrd]

Sounds like you should be the new "furnace guy" or bring him back and show him how to use a meter...

[Lisa]

Just found where my husband put the receipt - says "Limit cot. is out - they will call if they want us to fix it."

Ha.

[That Guy]

My only question is why did your thermal fuse "blow"? It has a temperature range on it and they only blow for one of two reasons. The area it is mounted on exceeded its designated temperature or it handled excessive amperage and burned out (unlikely but possible).

You need to find out why it "blew".

[Lisa]

I don't know - I didn't observe any flame roll-out while I was watching.

Could it just get old and glitchy?  About 10 days before the fuse shut down for good, the furnace stopped working, and started again for no reason I could figure out after I poked around a while.

The furnace has had no problems at all since I put the new fuse in Thursday.

[AccApp]

[user=20480]birdg[/user] wrote:

Could it just get old and glitchy? 

No, these thermal fuses are a one-shot deal. They don't sporadically fail.

[jahjahbinks]

i belive the correct  term would be (fan/limit) control  is it a (Camstat , or a Honeywell) does it have 4 wires or 3 ?

[Lisa]

Honeywell, 4 wires - 2 high voltage for the fan circuit and 2 low voltage for the gas valve- igniter circuit.