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Tim H.
Tim H., HVAC Professional
Category: HVAC
Satisfied Customers: 3971
Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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Troubleshoot Nortan electrical furnace.

Customer Question

I have an 18 KW-240V Nortron electric furnace with attached central air. For the past few years, there has been a problem with the fan control. There are two main symptoms. #1 - you cannot turn the fan on at the thermostat. #2 - the fan shuts off prematurely when the central air is running (resulting in a frozen unit). I now turn on the fan to 'continuous' using the switch on the furnace itself (the summer switch is on 'heating'). The schematic shows a 24V and 240V transformer and several sequencers. Could one of these parts be blown?
Submitted: 7 years ago.
Category: HVAC
Expert:  Tim H. replied 7 years ago.
I'll need either a complete model number or a wiring diagram off the unit, if you have one.
Customer: replied 7 years ago.
I cannot find a model number and tried unsuccessfully to scan in a wiring diagram. I managed to print a rather poor quality version - can I fax it? I also looked on the internet and found what appears to be the same or similar furnance at - mine would be the 18 KW 240V 1 phase variant. The pictures of the unit are a close match.

There appears to be a more modern version at This has a schematic (for the 21d18/20) which has some similarities with the schematic on my unit, but I cannot tell if they are identical.

One other characteristic: the heating function has been working properly.
Expert:  Tim H. replied 7 years ago.
I'll look at these and see what I come up with.
Customer: replied 7 years ago.
I understand you are waiting for me. What more do you want?
Expert:  Tim H. replied 7 years ago.
If this wiring diagram is the same or similar to your unit, it appears as if the board brings the cooling fan on. If there seems to be an issue with the fan going off prematurely in the cooling mode, this is all controlled by the board. There are small relays in the board that bring the fan on. If you have a good thermostat and positive connections to the board, I'd have to question the board. How old is this unit?
Customer: replied 7 years ago.
I'm not sure what you mean by 'the board'. Presumably its the 24V and 240 V relay attached to the unit face, along with a transformer, line lugs and several sequencers.

I have tried the original analog thermostat, an older digital setback and a new digital setback. With all of them, the fan switch does not turn on the fan (it did at one time) so I suspect the thermostat is okay.

When in 'cool' mode, the fan once came on and stayed on so long as the compresser was running. For a few years now, the fan came on but kicked off early (and the cooling coils froze solid). I adjusted the thermostat today to call for cooling and the compressor came on but not the fan. I therefore suspect a relay, but which one?

The 24 V relay has six contact lugs on the schematic:
- the bottom two go to L1 and the fan
- the middle two go to the switch fan control on the unit and to the 240 V relay
- one of the top two is connected to 'G' and the other to the 24V transformer then 'R' and to the first sequencer heater (I think; the schematic is fuzzy).
The 240V one has only 5 lugs. I think the bottom 3 go to the fan motor but can't yet make out where the top two go.

I suspect that in heating mode the fan operation is controlled by the sequencer but in cooling mode and when called by the thermostate it is controlled by the 24 V relay. Since the fan actually works, and has two speeds, and works in heating mode, I suspect the 24V relay is at fault, but you're the expert (I hope).
Expert:  Tim H. replied 7 years ago.
I am going by the previous reply you sent me and the wiring diagram which shows a circuit board. Are you saying there is no circuit board in your furnace?
Customer: replied 7 years ago.

There is no circuit board in the furnace.

I have attached two photographs (hopefully). One shows the unit itself and the other is a schematic. The furnace model is Nortron Model 21B18 and is about 8 years old. Whoops - looks like the schematic will come in a following reply.

photo of furnaceNortan Furnace Model 21B18

Customer: replied 7 years ago.

I think the schematic will follow!photo of schematicgraphic

Expert:  Tim H. replied 7 years ago.

OK, from this wiring diagram it looks like your blower is controlled by the FR relay. So, the thermostat wire attaches to G on the furnace and then powers up this relay which turns on your blower motor. This is what happens when you turn the thermostat to the fan "on" position.

What I don't see is where the Y wire from your stat attaches for air conditioning. It's not in the wiring diagram, at least not that I see.