Fan keeps running, no heat

[dfyoung10]

I have a Carrier model 396JAW 036045. the pilot and main burner shut off but the fan just keeps running indefinitely until I open the upper door that releases the main power switch. Nothing I do at the t-stat seems to make any difference. The limit switch was recently replaced, and jumping the leads doesn't make any difference, so I don't think it's the limit switch. Maybe the relay or the control * Any convenient way to tell the *

there are two relays on the circuit board; are these replaceable separately from the board *

Thanks for your help!

Doug

[Big]

If the fan keeps running and you have no heat , There is a safty open in the system

[how]

Fans keep running on because they are wired that way, the stat has been switched from auto to "on", the "G" stat wire has been powered by a short or crossed stat wire, anything causing an opening on the safety limit circuit, (on some furnaces, tripped roll out switches) and frozen fan blower relays on the board.
If you are needing to kill the power just to get the unit to fire up again then the limit circuit is the thing to check as "Big" says.
Otherwise...
Check the stat fan switch, disconect the "g" stat wire from the furnace, Isolate and ohm the entire safety limit circuit & or roll out switches circuit and you can try tapping the fan relay on the board to see if it unfreezes.
I'd leave the field replacements of fan blower relays on furnace boards for the electronic whiz kids. The rest of us HVAC tech's just replace the boards.

[dfyoung10]

How and -

Thanks for your thoughts. I am tending toward the fan relay (on the board) because there is continuity in the limit circuit. Tapping it doesn't help, so I think a replacement board is probably in order.

Doug

[dfyoung10]

Well, I did a little more research and analysis before buying or installing anything, and it turns out the transformer is bad. No continuity on the primary circuit; no voltage on the secondary low circuit, so no voltage to the board. Ergo the fan just keeps running and running and running.

Just thought this might be of interest to someone facing a similar problem.

[heatseeker]

Wrong answer, something else is wrong a bad transformer will not make your blower run nonstop.

[dfyoung10]

To quote from another board (*.*/forum/gas-oil-home-heating-furnaces/373358-bdp-carrier-furnace-20-yr-old-faulty.html)

Quote Originally Posted by mbk3 View Post
There a four things that make the blower go into operation and no heat with this -

1. bad transformer (loss of 24 volts)
2. open limit
3. open fuse link
4. bad board


Good list. And of course a transformer failure would prevent the relay from closing when the power is turned on, too.


So maybe it can't be true, but somehow it is.

[heatseeker]

regardless a fan that is running nonstop generally is a problem a young tech could figure out in a 15 or 20 minute period on you average resi. air handler, it is not really that difficult, surfing the web will get you canned answers like your list above. the true and only way to repair it is get in their up to your elbows, I find it works for me.

[dfyoung10]

Canned answer or no, the problem turned out to be the transformer. No power output, no continuity on the high circuit, therefore no voltage to open the blower relay.

A canned answer might also consist of "a bad transformer will not make your blower run nonstop." That was clearly wrong in this case.

Getting in up to your elbows is a great idea, after you have some idea of what you are looking for. That can come from experience, like you or the young tech, or it can come from research and careful inspection, in my case. This isn't a vacuum cleaner; it's a complicated and potentially dangerous large multi-functioning appliance.

Thanks otherwise for your obviously extensive contributions to the forum.

[heatseeker]

needless to say you got lucky. Not very common to have fan run continuously with a bad transformer must be manufacturer specific. I am not a Guru and sometimes might get it wrong.