Fan Limit control with heater on Lennox unit

[cook42]

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.
:confused:
Thanks

[tinknocker service tech]

the heat is infact a time delay to bring the blower on within 30 os seconds after the burner fire. It also makes sure the blower does not turn off when the burner cycle is over till it cools down and breaks. Then if the heat is still there the temp limit keeps the blower on the it cools off enough to shut the blower off. All of the older lennox units had a fan limit with a heater in them

te limit will also work by temp if the heater were to fail.

[cook42]

Thank's, but could you tell me some of the ways you know when one is going bad? I was watching the furnace cycle and it look like the blower was coming on to fast. I mean soon as the burners came on the blower came on at the same time. The customer was complaining the the registers was blowing out cool air. But I notice that the cold air return was not connected to the furnace, and that the basement was cold. I would think that this would affect the performace of the fan limit switch is this true? We all know (hopefully!!) that it is very important that the cold air return needs to be connected to the furnace to make it run properly to give it the correct temp rise ?

[Milk man]

The RAD shall be connected to the furnace. This is to prevent the blower from robbing the combustion air required for complete combustion. And the blower not back drafting from the flue.

If the blower is starting when the burners light, are the burners not lighting when they first get a call?

[cook42]

Yes the burners come on and thing seconds later the blower will come on. I also noticed that the burners would short cycle off and on. This a indirect spark igniting furnace.

[tinknocker service tech]

bumb up your post count and apply for pro statis
all the info that can be given has been given
if you are a diy sorry call a pro to fix it

[cook42]

WEll as I stated before I have about 4yrs in the field and this was my first run in with this type of fan limit. I'm no DIY!!! I'm a person just like you who's trying to gain knowlege in the trade to make myself a better tech. I'm sure when you first got into the business you've asked questions about things you ran into that you did not understand! And personally I don't like DIY or hacks!! Mybe again they might be ok because as long as they are around there will be more work for everyone to go behind them to fix their mess to make MONEY!!!

[beenthere]

The burners are short cycling?.
You might have 2 problems occuring at the same time.
Thats an old furnace your working on.
If the burners aren't cycling on high limit, fix the flame sensing problem first.
If the blowers coming on in less then 30 secs, the control is bad.


Thats more info then we are suppose to give on the open resi forum.

Which is why tinknocker said to get your post count up and apply fro pro memebership.

[cook42]

The burners are short cycling?.
You might have 2 problems occuring at the same time.
Thats an old furnace your working on.
If the burners aren't cycling on high limit, fix the flame sensing problem first.
If the blowers coming on in less then 30 secs, the control is bad.


Thats more info then we are suppose to give on the open resi forum.

Which is why tinknocker said to get your post count up and apply fro pro memebership.

Thanks, Beenthere I hope tinknocker and yourself did not take my message in the wrong way:) I just want it to be known that Im a certified tech with a number of awards and certifications but that does not mean a thing unless you are out in the field as long as some of the guy's like maybe yourself and tinknocker has to experience some of the old stuff out there! I know a lot of book smart people and once they are out there in the field they are lost. Also I think its not fair for all of these HACKS & DIY people out here messing up the industry while people who do the right thing get the short end of the stick!! I love this web site and it has helped me a lot to become a better tech and to solve problems with the help of people like tinknocker and yourself. With that said, I would like to say THANKS!!!! :)

[beenthere]

No problem.

but, I warn you, you need a thick skin to be on this site. :)

Take care.

[tinknocker service tech]

sorry wasnt trying to insult or upset you. Just following the rules.

give the model number of the unit you are dealing with. It is important because some of the older lennox units have other things that can cause your trouble. G-12=16=20q

get your count up as fast as you and apply. You will be overwhelmed by the info you can get in the pros section.;)

[Daltex]

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.
:confused:
Thanks

Maybe I'm reading your post wrong but are you asking what a fan limit switch is? I'm not a professional and I know. Seems like kinda basic stuff for a pro not to know. Since some of the pros are buying your story- the limit switch turns on the fan at an adj. temp. after the burner has fired. Then it shuts it off after the burner is off and the temp drops. It has a high temp limit that will shut off the burner if reached. There is a spring wrapping around it that can go bad. This is a replaceable part and insert length (8") if you don't have the numbers off it.

[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.

[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.

[Airmechanical]

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]

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

[mike3]

[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.
:confused:


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]

Its time it was moved. :)

[Daltex]

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.:D

[mike3]

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]

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

[tinknocker service tech]

ok it has been a rough day but what the hell is a fan assist heater. Is this some kind of hard start kit ;):D

[mike3]

ok it has been a rough day but what the hell is a fan assist heater. Is this some kind of hard start kit ;):D

Check post #17, if you want more of an explanation please ask

[tinknocker service tech]

Check post #17, if you want more of an explanation please ask


did you not see the smily face:confused:
i was joking arround :)

[beenthere]

did you not see the smily face:confused:
i was joking arround :)
Your smileys were on the next line.
I missed them too.
:)

[mike3]

did you not see the smily face:confused:
i was joking arround :)

I guess ooops...sorry