I have a “Day & Night” BDP (Carrier) furnace that had been working fine. BDP 395AAW036055. The furnace had been providing heat and then I noticed one day that the blower motor kept running and it was only blowing cold air. Now if I turn it on the blower just keeps running and does not respond to the thermostat. The control board had smoke marks so I replaced the board. I also replaced a limit switch (with a manual reset) which had an open loop. However, the blower still keeps running and won’t stop. Also, I am unable to get any voltage at the control board terminals that head out to the thermostat. The transformer seems ok. I get 117 on on one side and 27v on the low side. Am I missing another switch? Can a limit switch or pressure switch deny voltage the terminal? Any help would be appreciated.

Thanks!

Furnace pic

 

Welcome to the forums.

There are two items in the 24vAC line. It goes to the board first via SEC-1. Then it goes out pin 5 to TH1 switch. Then to 11c link and then back to terminal 1. You need to find out what is open from pin 1 to pin 5.

 

The only thing I've ever had go wrong with these is the dedicated heater breakers next to the unit and the run capacitor if you have the outdoor unit. Safety before you work on or test anything make sure all of the sets of breakers are off to these then can test between connections with ohmeter.

 

Thank you. I have been lurking on these boards for a while. I really appreciate the help and advice that you guys provide. Thank you for pointing me in the right direction.

So I followed the red wire from pin 5 and it goes to the limit switch (TH1). Limit switch looks good. Into one side of the the limit switch and out the other side to another red wire that leads back to Pin 1. There is a fusible link (11c) on the red wire headed back to pin 1.


When I tested the two red wires I wasn’t getting any voltage. But when I tested under the fusible link and the other red wire I get 27v. I tested the fusible link and got an Open loop. So the busted fusible link is preventing the voltage, correct?

Also, the fusible link has to be model specific, correct? I only ask because it’s hard to see a part number on the fusible link.

 

That is a microtemp. It's rating is printed on it in degrees c. You need to match that temp.
It should be 144°C.

That is in series with the thermal limiter so that if the thermal limiter malfunctions or the heat goes to high.... it blows open. It's very important to find out why the furnace is overheating. Make sure there is no visible flame rollout. You should also consider having the heat exchanger checked for cracks/rust thru if it hasn't been checked recently.

Carrier safety limiter 307566-751

 

Thank you! The filter needs to be replaced and the blower motor needs to be cleaned out as well. Is there anything else I should look at that would make the furnace overheat?

I’m also going to check out the rest of the furnace for any other issues.

is the link you provided for the microtemp I need for my furnace?

 

The heat exchanger should be checked.
Yes....that is the device for your furnace.