Electric Furnace.

Repair question about the

heating elements and sequencers.

Image of Trane Electric Furnace Heating Elements.
Image of heating elements in a GE/Trane electric furnace/heat pump air handler.
The only real way to test the elements is with an ohmmeter or clamp on ammeter (shown below).

Below is the result of your feedback form.  It was submitted by
 on Tuesday, January 2, 2001 at 12:11:19
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Brand: Heil
type: hot air furnace
fuel: electricity
location: basement
Problem: tempurature
problem: uneven, controls
S1: I have a 25 year old Heil 4 element 2 sequencer furnace. it got cold a week ago in the house.
I replaced the fuses to the elemnets and it warmed up. then it never came back on.
the elements would get hot but no blower. I checked the wires to the blower. no power...
through checking the continuity I found a thermal link bad too so I replaced Sequencer number 1 a honeywell r8330c with an r8330d and the thermal link.. this seemed to fix the problem... the blower came back on. and blew hotter air than ever before...but the bottom two element would get hot even when the blower was not supposed to be running... so i replaced that sequencer too... every thing was fine for a few days...but yesteday the furnace did not want to shut off... it kept heating past 75 degrees and the thermostat was set on 68. I shut of the power and inspected the thermosat... I am really not sure what I am looking
for... there is a glass bubble with mercury and it seems to connect two wires if it is hot enough in the house and break the connection when it is too cold??? I cleaned it and kicked the power back on and it is working fine again...
do you have any idea where my problem(s) might be?
thanks
Abe.



    Electric furnaces are relatively simple devices. They are a box with heating elements like those used on electric clothes dryers (I have used electric furnace restring kits to restring an electric clothes dryer). The elements are operated on high voltage 240 volts in residences and 208 if a 3 phase Y system. The elements are easily replaced we can ship you a restring kit if you know the wattage. The restring kits are very inexpensive about $25 with shipping. If you need the replacement thermal fuses we can ship those or you may find them in stock at Radio Shack. There is usually a high limit thermostat with each element.

Top View of High Limit control, Fan control looks the same.Image of High limit thermostat, Fan control looks the same.
Image if high limit thermostat. These devices are used in many other applications.


Image of A.W. Sperry Clamp on Ammeter Avaliable at Lowes and Home Depot for about $80.1819 light bulbs and sockets used to monitor the 24 volt control voltage avaliable from Radio Shack.
I would highly highly recommend getting a clamp on ammeter and checking all the elements to see which ones are drawing current. You may have a stuck sequencers or wired it wrong by accident.
 The first of the sequencers usually turns on the fan too. The sequencers  have little electric heaters powered by the 24 volt ac control voltage from the thermostat that activates a thermo-disk. Use the 1819 bulbs (available from Radio Shack) pictured above to monitor the 24 volt control voltage on the sequencers. They have a timing sequence that takes so many seconds to turn on and so many to turn off. they are usually staggered so that all the elements don't come on at the same time and shut off at different times. Usually "first on" to "last on" and "last off" to "first off". If the elements are not working once you have determined that the sequencers are making contact you need to check all the components in the heater including the over heat thermostats and fuse able links that are used on some units. You should use an ac volt meter to check for voltage across the sequencers, across the over heat thermostats and fusible links. You should read 0 volts if the item is good and 240 volts across the elements. IF YOU ARE READING 120 Volts YOU ARE USING YOUR METER WRONG! (see prefered light bulb test below). An electric Furnace is another name for a heat pump air handler that has straight air conditioning.
    One of the myths or "wives tales" is that some electric furnaces are more efficient than others. This is not true or more precisely BS. When you are heating with electricity it doesn't mater how hot or not the air is coming from the unit. It still cost the same to heat your house. You can use all light bulbs and get the same results. Many people write me concerned that the discharge air is not as hot as they would like it to be. Trust me as long as your house stays warm there is nothing that you should do to make the air hotter even if the unit seems to run a long time. See electric heat facts.

Special note about thermostats for electric heat:

  Most thermostats for heat pumps turn on the fan by activating the "G" terminal on a call for heating or cooling. Some electric furnaces have a provision to turn on the fan on a call for heat using a second set of contacts on the first sequencer.

Others need a signal ("G" terminal) to activate the the fan on a call for heat. If you are using a heat pump thermostat this is taken care of. If you are using a thermostat for conventional systems. You may have to find one with an "electric" setting to activate the fan or add a relay or double sequencer to make it work.

If you replace your thermostat and it doesn't have this feature the fan will stop working in the "Auto" position and you will need to add a relay or double contact sequencer. You won't find this out until you switch to the heat mode so test for it if you replace your thermostat during the cooling season.

To learn more about electric heat thermostats see the thermostat page.
 Scott Meenen



 
Sequencer Carrier Part Number HN67QC 005 24 Volts Image of heat pump electric heat control
Images of electric furnace heating element sequencers.
The terminals at the base are the 24 volt ac. control inputs. 
Caution: some Lennox Furnaces have sequencers with 240 volt inputs.
The specs on the top of the unit indicate the the first stage is "first on last off" and the second stage is "last on and first off" Some setups the sequencer also controls the blower fan in the unit.
Units with more elements will have additional sequencers that are "last on" first off".

Exceptions:
   Ge/Trane and Rheem/Ruud Do Not use sequencers. They use relays. GE/Trane/American-Standard uses contactors similar to the ones in the outdoor unit, plus they may use a sequencer to activate a second stage. Rheem/Ruud uses a relay that looks identical to a 90-340 but has MUCH HIGHER current ratings. So don't replace one of this with a relay that just "looks" like the original because it will burn up.
I had occasion to work on a Rheem unit that relays were cooked and I replaced these with sequencers which are much quieter than the loud "clack" of contactors or relays.

    The best testing tool for electric furnaces is a 120 volt light bulb and a set of clips, you can use a Square [D] "Wiggy" but a light bulb is easier. You can test every component in the element path with a 60-100 watt light bulb (rough service preferred) and a "contractor socket", this works on 120/208 and 120/240 service only.

    With the unit powered and not calling for heat connect one clip to ground (case or large wire connected to the case). and use the other one to probe all the points on the heating element path, at any point the lamp doesn't light you have found your bad component. To test a sequencer remove one wire (power off first!!!) from one side of the sequencer and connect the clip then make the system call for heat. The lamp should light when the sequencer engages. Do not use a volt meter or ohm meter as they will not test for loose connections or shared circuits and a light bulb and socket costs ~$5-$10. Please do not ask questions about what components may be bad until you have done this test.

    I have over 20+ Years of electronics experience, trust me when I tell you that a light bulb works best.

Scott Meenen N3SJH



RELATED PAGES:
    Trane Thermostat wiring.
    Electric furnace fuse question.
     Heat pump defrost cycle.
     Other Heat Pump Stuff.
    Electric heat facts.
    Control wiring colors.
    How blower and fan controls work.
    Thermostat stuff.
 



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Written By:  Scott Meenen N3SJH of:
G&S MECHANICAL SERVICES.
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, Air-conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation.
Anything having to do with Heat and Energy.
Serving MD, DC, and Northern VA.

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                 This text written by: Scott Meenen * G & S Mechanical

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