Intertherm Heat/AC

Old 10-08-00, 11:04 AM
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Have Intertherm Electric Heat/AC central unit, and think have problem with fan switch?? When set to heat, elements come on but no blower, runs fine in manual on or a/c position. Any Suggestions?? How can you check a sealed fan switch? Thanks
Old 10-08-00, 01:54 PM
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Hi Rl:

If you are familiar with electrical testing and can trace some wires, we should be able to walk through this. If you can folow the wiring diagram, this is even better. Electric furnaces use sequencers to stage the fan & elemnets on & off. They're either going to look like switches stacked one upon another or they'll be rectangular with a lot of terminals on them. You want to find the one that controls the fan motor. Usually, it will operate the fan and one element, sometimes the fan and 2 elements. You need to figure out which wires go to the fan motor, which go to the elements and whicn are the control power from the thermostat and transformer. Have someone turn on only the first stage of heat [one mercury bulb] at the thermostat and see if you're getting 24 volts on the control terminals. If not, you'll have to trace the wires down and see why. Maybe a limit is open. Generally, sequencers are wired so that if the first one doesn't pull in, the next one won't pull in. In other words, they are electrically interlocked. If you are getting 24 volts to the control terminals, then see if you have power at the fan motor terminals. If you hear the sequencer pull in and you have voltage ACROSS the fan terminals and the fan motor doesn't start, then the sequencer is bad. If you have 24 volts at the control terminals and the sequencer won't pull in, then it's bad. If it does pull in and you have voltage going to the motor, [240] then there is something wrong with the motor wiring. Most electric furnaces with a/c use high speed for cooling and a lower speed for heating. Both speeds are wired throught he fan relay to keep them separate and the problem may be there. You should find that the lower speed goes through a set of NORMALLY CLOSED contacts on the fan relay. My money says yuo've got bad fan contacts on the first stage sequencer unless someone has been in there messing around with thew wirng. Electric furnaces can get pretty complicated, you have to break it down into one circuit at a time. Let us know how you make out. Thanks. john.

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