Greetings,

I have an old electric furnace located in my attic. I cannot determine the mfg/model, but estimate the unit to be ~20+ years old. We have owned the house for 2 1/2 years and have had heater problems each season. In the last two years we have replaced burned out heating elements and the heating sequencers. This year we called a service company and they diagnosed a burnt out element and a bad sequencer. The element was replaced, but I decided to do my own diagnostics on the sequencer before paying (again) for another one. I'm not an HVAC guy but very handy with a voltage meter and home improvement/repairs. Here's what I've found and it doesn't seem correct to me. Can someone provide an opinion on the findings.

Electric furnace info:
Location - Attic
Mfg / model - Unknown
Age - Estimated 20 - 25+ years
Heating type - Electric
Heating element wattage - 5Kw
Number of elements - 3
Sequencer - "Quad"
Number of sequencers - 1

Sequencer diagnostics and wiring info:
Sequencer - two 2-stack units mounted on a base plate. On the base plate is this information:
15SH22 309576
H1-110 C1-110

Sequencer #1 information (from top of sequencer):
T-O-D
12SH22 H24v
308771 A0546

Sequencer #2 information (from top of sequencer):
T-O-D
12SH22 H24v
308734 A0548

Sequencer 1 = M1 & M2, M3 & M4, 24V + / -
Sequencer 2 = M5 & M6, M7 & M8, 24V + / - (the 24v terminals are jumped across pos -> pos, negative -> negative between sequencers)


Diagnostics results with the unit cycling on / off:
With the unit powered off (not calling for heat) M1 / M2 and M3 / M4 carry 240v. When the unit powers on these go to 0v and do not carry voltage until the unit cycles off again.
With the unit powered on (calling for heat) M5 / M6 carry 0v and ~90s later carry 240v. M7 / M8 carry 120v then go to 0v ~90s later, then a little longer go back to 120v.

Question: Is it normal to have sequencers "normally closed" as with M1 / M2 & M3 / M4? Also, is it normal to have a mix of 240v & 120v running across a sequencer stack?


Here's the wiring. The colors are as best as I can determine. Most wires are faded and hard to distinguish the original color:
Note: High temp thermos & heating elements are designated 1 - 3 for information only. I'm presuming they are wired correctly.

There are two (double) breakers at the unit. Breaker 1 = 40amp, breaker 2 = 30amp.
Breaker 1 - Hot (black) -> M1
Breaker 1 - Hot (black) -> M5
Breaker 1 - Hot (black) -> Wiring harness
Breaker 1 - Common (white) -> Wiring harness
Breaker 1 - Common (orange) -> High temp thermo #1
Breaker 1 - Common (orange) -> High temp thermo #2

Breaker 2 - Hot (black) -> M7
Breaker 2 - Common (orange) -> High temp thermo #3

M2 -> (purple) Heating element #1
M3 -> (blue, maybe violet) Wiring harness
M4 -> (brown, maybe violet) Wiring harness
M6 -> (purple) Heating element #2
M8 -> (purple) Heating element #3

High temp thermo #1 -> (red) Heating element #1
High temp thermo #2 -> (red) Heating element #2
High temp thermo #3 -> (red) Wiring Harness

Sequencer 1 - (yellow) 24v+ -> Wiring harness
Sequencer 1 - (violet, maybe gray) 24v+ -> Wiring harness
Sequencer 2 - (purple) 24v- -> Wiring harness
Sequencer 2 - (purple) 24v- - > Pigtail from 24v-

Question: Is it possible that the 24v wires are simply reversed, in turn causing the normally closed condition on sequencer #1?

Thanks in advance for any comments and feedback.

Sincerely,
Greg Dotts

http://www.white-rodgers.com/wrdhom/..._06_pg0096.pdf

http://www.white-rodgers.com/wrdhom/.../0037-5224.pdf

http://www.white-rodgers.com/wrdhom/.../0037-5224.pdf

Here is some info for you.

Quote:

---

Originally Posted by hvac1000

http://www.white-rodgers.com/wrdhom/..._06_pg0096.pdf

http://www.white-rodgers.com/wrdhom/.../0037-5224.pdf

http://www.white-rodgers.com/wrdhom/.../0037-5224.pdf

Here is some info for you.

---

Hello HVAC1000.
I'm not an electrician but have reviewed the documents that you suggested.
My main question is, is it normal for this sequencer (White/Rodgers 24A34-6) to have "closed" switches when the thermostat is open (off)? If not, does this indicate a bad sequencer or improper wiring?

Thanks for the help.

This diagram might help you to better understand sequencers.

Part number Q106 is the same as 24A34-6

http://www.supco.com/images/pdfs/Man...s%205-1-07.pdf

Sequencers are designed as a time on time off device.

Problems happen with electric heaters when

The connections are not tight on all the electrical components and the wire terminal allows the wire itself to deteriorate a inch or so under the wire jacket. (You cut off the old terminal and attach new but the wire is still bad in the new attachment area.) THIS IS A COMMON PROBLEM SEEN ON PREVIOUS REPAIRS.

The age factor (parts are only designed to last so long)

The wire heating element sag factor and breakage. This is also a problem of age but also a stress factor from not enough air movement to help keep them cooler over time. (less air over coils allow them to heat up more and in turn this stresses the element more over time)

Quote:

---

Originally Posted by hvac1000

This diagram might help you to better understand sequencers.

part number Q106 is the same as 24A34-6

http://www.supco.com/images/pdfs/Man...s%205-1-07.pdf

---

Thanks hvac1000, this helps a great deal. I've been in the attic again with some new info.

With the thermostat off (not calling for heat)...
M1/M2, M3/M4, M5/M6 are closed circuit drawing 240v across the switches.
M7/M8 is closed circuit drawing 120v across the switch.

With the thermostat on (calling for heat)...
M1/M2, M3/M4 are open circuit drawing 0v across the switches.
M3/M4 is open circuit drawing 0v then ~90s later it closes drawing 240v.
M7/M8 is open circuit drawing 0v then closes drawing 120v, then opens drawing 0v.

This all seems completely backward from Q106 in the document you suggested. I have checked the wiring against the schematic on the unit and it all seems wired correctly to me, but I can't really read schematics. I just follow the lines around. So the only conclusion that I can come up with is that the sequencer must be bad. It's a $30 part so I'm just going to replace it and see what happens. Hopefully the darn thing will start working again.

Thanks for your help.

I'll post my results after the sequencer change for future users benefit.

Also, sorry for the original double post. I tried to edit my original post and it updated as a new post.

As per the diagram those units are supposed to be open and close on a timming basis to activate the heat. Post back with results.

Follow-up post

After all my diagnostics and troubleshooting I'm not sure I ever isolated the problem. However, the replacement part was cheap ($20.00) and took about 15 minutes to switch out. The new part fixed the problem and the heating unit is now producing warm air as it should.

The best I can conclude is that one of the stages of the 4-stack sequencer was bad.

Thanks for your assistance HVAC1000!

Sequencers can be difficult to trouble shoot if you are at the job but almost impossible over a message board. Glad you got it going.