Intertherm Furnace Malfunction

[gary15thomas]

I have recently purchased a singlewide mobile home we use on the weekends. It is about 15 years old and has an Intertherm Furnace. When we go up for the weekend and turn up the thermostat the furnace seems to come on and work fine for a while. It brings the house up to the temperature we set on the thermostat, shuts off until the temperature drops down a few degrees and comes back on to bring the temperature back up. It continues cycling properly like this for several cycles. However, some time during the night it stops working and we wake up to a house that is COLD. At this point it does not matter what we set the thermostat to the furnace will not come back on. However, If I go to the main circuit breaker panel and turn off the power to the furnace momentarily then turn it back on the furnace starts to cycle properly again for several cycles. This procedure of momentarily turning off the power then turning it back on seems to be the only way to jump start the furnace back into operating properly. Have you ever heard of this symptom? Do you have any suggestion what might be wrong and how to fix it?

Here is some information from the unit:
Model: FEH-012HA-04
Serial Number: 9008-01598
Single Speed Motor
Total Amps: 51.3
BTUH: 41,000 (KW 12)
Thermostat Heat Anticipator (.02 Amps) ...Actually in one place it says .02 Amps and in another place it says .2 Amps.

I would appreciate any help you could give me.
Thanks.
Gary

[Robert]

Would need to know where power interruption is at when it quits.

The very bottom terminals of sequencer are the heater coil terminals.

Place one meter lead on front H1 and the other lead on back H2, do you read 24vac for entire heat cycle ?

When it quits, check and see if ALL elements are off. One element is energized with blower. The others are energized by themselves.

Need to know if ALL is off or if maybe one or two elements are energizing, but not the one with blower.



Thanks,
Robert

[gary15thomas]

Thanks Robert

I will be up there this weekend and will try to get the information you asked. Unless the heat has been off all week and the plumbing has frozen. Then I will be spending my time cleaning up a big mess.

[Robert]

You're very welcome, happy to help. Just let me know.


Take care and best wishes,
Robert

[gary15thomas]

Robert

I was not able to get readings while the system was in its non-responsive mode. It was in that mode when we arrived at the house but in order to probe the circuits I had to open up the cover which meant removing the safety disconnect shorting bar. Once I replaced the bar the furnace worked properly for the rest of the weekend. This is similar to what happens when I cycle the power at the breaker panel.

I was able to take some measurements while the system was working “properly”. However one observation seemed strange so I wanted to pass it by you. With the system off and the thermostat not calling for heat, I had my wife turn up the thermostat and tell me when she heard the “ping” of the thermostat contact closing. I had two voltmeters so I was monitoring the heater terminals of the sequencer and the second element, the one that is not switched with the fan (my furnace has only two elements).

The sequencer heater coil got energized immediately after the thermostat started calling for heat (the delay was much less than a second). Then there was about a 10 or 15 second delay followed by the fan and second element starting up at exactly the same time. Again if there was a delay between these two it was much less than a second.

The same thing happened in reverse as the thermostat was turned down. As soon as it stopped calling for heat the voltage across the sequencer heater coil went to zero. About 10 or 15 seconds later the fan and second element shut off at the same time.

Just to make sure I wasn’t missing something I connected my two meters to the two elements and listened to the fan while we went through the steps with the thermostat once more. Again, both elements and the fan came on at exactly the same time and they all went off at exactly the same time. I thought the sequencer was supposed to create timing delays for these switching events.

I also noticed that the fan seems to have a little trouble starting up on most of the cycles when it does work. I typically hear it go “wah, wah” a couple times before coming up to speed. Perhaps the failure mode happens when the fan is trying to come on at a particular phase of the AC and just can’t start up due to the extra load associated with both elements being energized at exactly the same time as well. Does this sound like a plausible explanation? If so does this mean I need to replace the sequencer?

[Robert]

Hi,

They have a variation as to whether there is a delay and when there is, it isn't that much.

Yours being a two element on same heater coil, would be same or very close.


When the furnace does not operate or is unresponsive, do you have 24vac at H1 and H2 ?


If so and you have 120vac at either M1 or M2, depending on which way it is wired, then the element and blower for M1 and M2 should be running.

M3 and M4 element should be also or shortly thereafter.


IF the t-stat delivers the signal to sequencer and you have line voltage (120vac) present and waiting, then the sequencer should close contacts and send voltage to elements.



Thanks,
Robert

[gary15thomas]

I was finally able to do some measurements when the furnace was malfunctioning.
The thermostat was set for 68 degrees but the house was at about 50 degrees.
There was 28.5 VAC across H1 to H2.
There was 240 VAC across M1 to M2 and also 240 VAC across M3 to M4.
There was only 2.7 mVAC (millivolts) across each of the heating elements.
There was 2.7 mVAC (millivolts) across each of the safety switches in line with the heating elements.
2.7 mVAC was probably just noise pickup. You get similar reading if you short the probes of the meter together.
The fan was not running.
I waited about 10 minutes but nothing changed.
I flipped the circuit breaker off and on at the main panel. Within 15 seconds the furnace came on and cycled properly.

Seems to me like the sequencer is bad since sometimes there is power to the heater and voltage to the switches but the switches are open. Do you agree?

Gary

[Robert]

Hi Gary,

Yes, the sequencer is bad.

Sequencer # 901075 can be seen and ordered through PayPal from

right side of page here :

http://www.mobilehomerepair.com/NordyneElec.html


Thanks,
Robert

[gary15thomas]

Hi Robert

When I first realized the sequencer was bad I ordered #621451 from your web site. It says that part is a replacement for #6205191 which is the number written on the old part I took out from the furnace.
After I ordered it, I saw your response which indicates a different part #901075 so I was wondering if the problem is really that the wrong part was installed all along.
When the part I ordered arrived I went ahead and installed it in the furnace. It seems to have worked properly all weekend long so it is definitely an improvement over the one I took out. Is the one I bought also a suitable replacement for this furnace or is there something else wrong with it?

Gary

[Robert]

Hi Gary,

The sequencer itself was bad. However, it was also the wrong sequencer and evidently had been used as a replacement earlier.

Sequencers each have a certain timing sequence and often when a hvac tech does not have correct one, they'll use what they have .

That's fine for short periods, but it should have been replaced with correct replacement as soon as possible.


901075 is the oem replacement for your furnace.


Thanks,
Robert