8 replies to this topic

[jimj]

Hi,
I have an older Bryant 75000 BTU Natural Gas furnace built in 1986 with a HH84AA 011 board. The thing started acting up where after the furnace would cycle the blower would come on briefly for a couple of seconds then turn off about 3 times within about 10 seconds. This had gone on for about a month.

Now the blower will not turn off at all. We have to turn the power off to the furnace to get it to shut down. When it gets cold in the house we flip the switch and after the flame goes out and HX cools we flip the switch again.

I’ve searched the forum and found a few instances where the blower stays on for this type of system. Here is what I’ve learned and done so far…

I was looking for a bad Transformer which is working and putting out 24V.

Checked for power between SEC 1 and SEC2 and get 24v.

Checked the voltage between R and C and I get 24 volts as well so the board seems good.

I cleaned the contacts on the relay and getting a spark when switched on.

Checked the upper limit and it seems to be working fine along with the fuse on it is OK too.

I disconnected the T-stat wires and tested everything again and everything seems fine.

BUT the blower still will not turn off!

I started tracing looking for any bare, worn or broken wires or loose connections and everything seems fine as well.

I thought I'd have this fixed by now with the info I found on this site (you guys give great info) But this thing is driving me nuts!

ANY HELP would be appreciated!

Thanks!
Jim J

[RegUS_PatOff]

the Blower Motor is controlled by a solid state time delay circuit on the (bad ?) Control Board

[jimj]

So you think it is the control board?

[RegUS_PatOff]

So you think it is the control board?

could be...
it's 26 years old ?

[jb8103]

I agree the board is suspect. Usually if a blower won't turn off, it's a stuck relay, but

the blower would come on briefly for a couple of seconds then turn off about 3 times within about 10 seconds.

...stuck relays don't act like this. Maybe a short somewhere but most likely it's the board.

[Shootist]

It's the fan limit. The fan limit is the little silver box with a round dial inside. On an upflow furnace its mounted about a foot above the burners and on a horizontal furnace it on the side of the furnace near the discharge end. If you snap the little silver cover off you will see a little round dial with 3 little pointers on it, Typically set at 100F, 150F and 200F. When the furnace starts to heat up the little dial turns clockwise and when it gets to 150F it clicks on the low-speed blower. Once the thermostat is satisfied it turns back the other way as the blower cools the furnace back down. Once it gets to 100F, the blower clicks back off (normally). In your case the bi-metal spring that turns the dial is weak and it's no longer going back far enough to click the fan off. You can get more time out of it by carefully moving the low pointer. Unplug the furnace first and then keep the dial from turning with one hand and move that little low pointer from 100F to about around 120F. You should be able to adjust it just enough so the furnace will operate the blower normally. If the fan turns off and back on during the heat-up period you can adjust that middle temperature up a hair as well. If all else fails you can replace that fan limit. They are pretty much universal now as furnaces that use them are no longer made; you just have to remove the screws, slide the limit straight out and measure the length of the probe rod behind it.
Here's what the fan limit looks like:
http://inspectapedia...LimitSwitch.htm

Good luck!

Edited by Shootist, 01 February 2012 - 10:09 AM.

[RegUS_PatOff]

... It's the fan limit. The fan limit is the little silver box with a round dial inside.

I don't think this Furnace has that type of Fan Limit.

Relay Board
HH84AA011
replaced by
HH84AA020
click on picture

[jimj]

Sensei is correct, this furnace does not have that kind of limit switch on it. I wish it did. Been wanting to replace the furnace anyway but not this soon, wanted to waid to the summer. So I guess it is the board now and furnace later.
Thanks guys!
Jim J

[jimj]

Thanks to everyone who took the time to respond.

I just replaced the board HH84AA 011 with the new HH84AA 020 and she is now humming along!!! Heating the place fine and blower turning off as designed!!!!!

Arigato Gozaimasu