Repair question about the
heating elements and sequencers.
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
type: hot air furnace
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?
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.
Image if high limit thermostat. These devices are used
in many other applications.
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
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
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
Images of electric furnace heating
The terminals at the base are the 24 volt ac. control
Caution: some Lennox Furnaces have sequencers with 240
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".
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
furnace fuse question.
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any of the means below. good luck Scott.
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Written By: Scott
Meenen N3SJH of:
This text written by: Scott Meenen * G & S Mechanical
G&S MECHANICAL SERVICES.
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and Electrical Modifications Of
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