Fan Limit control with heater on Lennox unit
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?
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.
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.
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 ?
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?
Beware of advice given by some guy on the Internet.
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.
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
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!!!
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.
but, I warn you, you need a thick skin to be on this site.
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.
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.
Originally Posted by cook42
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.