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electric furnace design flaw or safety hazard?

Hi,

For a Nortron electric furnace, I noticed recently that it seems like the furnace heater element is able to turn on independently of the blower fan. This seems like a safety hazard or at least a design flaw. I noticed this with an infrared thermometer as the furnace was about 20C above ambient, however the furnace fan was not on at any time.

I had heard some thermal expansion noises in the ducting on cold days, where the thermostat would trip, however I had the fan set to off expecting this would fully disable the furnace. After investigating the thermal expansion noises I noticed that the furnace was about 20C above the room temperature, however the furnace fan was not on at any time. I believe the furnace heating element is turning on when the 2 wire mercury thermostat circuit closes, and then it must overheat and a thermal trip cycles the heating element on and off.

The furnace has three easily user accessible labelled switches:

three way "fan control" switch: labelled switch positions: fan,off,on

two way "energy saver" switch: labelled switch positions: mild weather, cold weather

two way "summer switch" labelled switch positions: ventilation, heating

In the case described I have the switches set to these positions:

fan control: off energy saver: mild weather summer switch: heating

As mentioned the thermostat is a 2 wire mercury thermostat, I think I should rewire a 6 wire thermostat in that implements proper logic to control the furnace. But I'm curious if this is a design flaw or potential safety hazard (to overly rely on an internal thermal cutoff circuit)

The electric furnace is on a 240Volt 100Amp breaker.

cheers, Jamie

Reply to
Jamie M
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I'm guesing that this is a pretty old installation. Mercury switches haven't been used for years. Environmental issues of breakage, disposal, etc.

Furnaces pre-heat so as not to blow cold air out the vents at the occupants. The fan is actuated once the heating elements reach a suitable temperature. There should also be a high temp cutout to prevent a fan failure from allowing the heating elements from reaching a hazardous temp. So, neither a design flaw or hazard as long as all the bits are working properly. You might want to check the furnace schematic for these functions because really old installations may have neglected them.

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Paul Hovnanian     mailto:Paul@Hovnanian.com 
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Reply to
Paul Hovnanian P.E.

You need to look up your model number. Looks like these furnaces have an in ternal control board that inserts small time delays (a few seconds) between activating the multi-stage heater strips and turning on the fan, which is multi-speed. For the models shown below, looks they're relying on the exter nal thermostat to enable/disable furnace operation.

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Reply to
bloggs.fredbloggs.fred

most blower controls are on / auto ON = fan runs 24/7 AUTO = fan runs when heating

I have never seen OFF as an option for the blower.

mark

Reply to
makolber

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