Fan Limit control with heater on Lennox unit

**beenthere** · 03-24-2008, 07:31 PM

I'm not a professional and I know.

Thats must be why the info you posted is incorrect.
You have no idea what type control he saw there.

**Airmechanical** · 03-24-2008, 07:37 PM

either way this stuff should not be discussed in a public forum

i can visualize some diyer reading this and thinking this may be his problem

so he changes a part incorrectly and blows himself up!

i need moderator status so i can remove this thread from here!

.

**beenthere** · 03-24-2008, 07:47 PM

You can click on the triangle, and send an email to a mod, expressing your concern and thought on that.

**mike3** · 03-24-2008, 07:55 PM

[QUOTE=cook42;1805246]Hello,

I'm new to the forum and I've been in the field for about 4 yrs now but I ran into something that I never seen before. On this lennox unit there was this fan limit switch with a heater. I call lennox tech support and the guy on the phone act like he did not know what it was and what it was used for. Could some one please explain to me what's its function and is it me or is lennox tech support the most complicated people to deal with for information? And also do anyone think that they over control their units?

PS:

This was a 80% upflow 100,000 BTU furnace. I can't find the model # but I know the control was a old Honeywell part and the insert size that goes inside the furnace was 8" long.

Heat assist fan switch: Probably the best thing about this type of switch is it's ability to control blower short cycling. When the t'stat is energized it also energizes the heater portion of the fan switch. As it heats a bi-metal strip, the strip warps and closes the fan switch prematurely. As long as the t'stat is calling for heat the blower will stay on. Lets say the return air is pulled from a tile floor area. Since that area would be cold in the morning the air from that area would also be cold and as it passed over the fan switch it would cool it down and it would open, hence short cycling. IF you had a conventional fan switch.

As stated before: please sign up for pro

**beenthere** · 03-24-2008, 07:59 PM

Its time it was moved.

**Daltex** · 03-24-2008, 09:31 PM

Originally Posted by beenthere

I'm not a professional and I know.

Thats must be why the info you posted is incorrect.
You have no idea what type control he saw there.

I guess you missed the "Maybe I'm reading your post wrong but are you asking what a fan limit switch is? "

Do you not know what a fan limit switch (with assist heater) is? I think from your prior post you are smart enough. I know there are different kinds but the OP seemed to imply he didn't know.

Just thought it was funny. Sorry to upset your domain.

**mike3** · 03-24-2008, 10:27 PM

Originally Posted by Jay 41

Also, maybe I missed it, is it the old cam-stat style, or honeywell? If it's the cam-stat, it's time to upgrade to the honeywell.

PS:

This was a 80% upflow 100,000 BTU furnace. I can't find the model # but I know the control was a old Honeywell part and the insert size that goes inside the furnace was 8" long.

Thanks

**beenthere** · 03-25-2008, 07:09 AM

Originally Posted by Daltex

I guess you missed the "Maybe I'm reading your post wrong but are you asking what a fan limit switch is? "

Do you not know what a fan limit switch (with assist heater) is? I think from your prior post you are smart enough. I know there are different kinds but the OP seemed to imply he didn't know.

Just thought it was funny. Sorry to upset your domain.

Maybe, if your description of how it works was correct, I wouldn't have told you, that your wrong.