Blower doesn't come on until limit opens

Blower doesn't come on until limit opens-
replaced honeywell st9120c fan control and still does the same thing. Fan runs on "cool" , or "fan on" setting and also when lead is placed on "cont on" terminal.
No 110v on heat terminal on either fan control module. Of course, could be bad module too, but can't believe they both react exactly the same. Can't see anything else that would do this. thanks, rick

 

To test the module:
Create a call for heat.
Measure voltage between the blower "heat" terminal & neutral until the furnace shuts down on limit.
If at any time during this run you get
120v the module is working & you most likely have a bad motor.

 

Fan runs on "cool" , or "fan on" setting and also when lead is placed on "cont on" terminal.

 

All of that doesn't tell you if the fan is getting power thru the heat terminals at any time.

 

Fan lead is connected to the heat terminal, fan runs when limit switch opens, again while fan lead is connected to heat terminal. Also if "fan on" is selected at the thermostat, again while fan lead is connected to heat terminal. I have 120volt at this terminal and approx 4 amps on this lead during these periods.
My probelm is, this terminal does not come hot, when it should, 30-60 sec. After call for heat with either old or new fan control. St9120c honeywell.

 

When the thermostat is switched to "fan on", the fan usually runs on the cooling speed. Same is true if the furnace goes off on limit. If you are not getting power from the "heat" terminal of the contol, either something is wired wrong (doesn't seem likely) or the control is bad.

 

So when DOES the HEAT terminal have the 120 volts, and when does it not? Until that answer is clear(to me anyway), then you could try this:

Since we know the motor runs at some speed, we then know at least part of the windings are good. Can't you swap around the speed control wires on your board's terminal hookups by say putting the CONT.ON speed wire or COOL speed wire on the HEAT terminal, and then see if it runs then? If it did, that would mean your heat speed windings are bad in the motor. And if it did not run (on AUTO), then the board is likely bad, as stated.

 

ecman & grady, I owe you two an apology, I discovered that the red wire leaving the cool terminal was spliced to a black and I was reading the amps on that black when I thought I was reading amps from the heat terminal. The heat speed does run when placed on the "cont on" or cool terminal. So, I also feel that my new fan control is bad. This house is vacant and winterized and I haven't got back to put on yet another fan control, thanks for your help.
rick

 

I've never seen a furnace/fan control which runs the heating speed of the fan when the thermostat is set to "fan on". Every single one I've ever seen runs the fan in the cooling speed.

 

You're not going to believe this. Got back to furnace today with another new board. Before changing, i checked again, i was not mistaken before, i was measuring amps on the heat lead, it is the one that becomes hot when the limit opens and when the "fan on" is selected. I too have never seen this. It gets better, the 2nd new board does the same thing. The only time i get the hi speed to run is when i put the lead on the "cont" terminal. So, i still don't get the blower to come on until the limit opens, and of course the burner dies when that happens. To get around this temporarily, i put a jumper from the w terminal to the g terminal, which again energizes the heat terminal. Of course now there is no on or off delay with the blower, ( i checked to make sure the heat and cool didn't energize at the same time during several cycles) but they can get by until i, "we" figure this out. The board that was on it when i 1st got there may not have been the right one and i have been matching it. But i don't think any board is going to do what these are doing unless there is something else somewhere that is causing this. Let's is how good you guys are. Thanks for your help , rick

 

Can you please give us the last 4 digits of the model numbers of each board (old & new)? Also just want to confirm that both are ST9120C.

 

Probably too late now.?

Nothing was mentioned if when blower voltage comes out of the COOL connection, if the furnace fires up and heat comes out. If not, I was wondering if the stat is miswired, or shorted. If say Y(AC) was being energized instead, rather than W(heat), and IF the outside AC unit was shut off, then control board COOL terminal would/should become hot. Trouble is, no heat would come on, with this theory. Only the blower.

 

All 3 were:
9120C2004 fan controls

yes, the flame comes on when it should, the blower doesn't.
Actually the thermo is heat only, no a/c on unit
Rick

 

Of the 13 variations of the ST9120C Honeywell lists on their site, guess which one isn't listed. Yep, you're right, the 2004. That being the case, I have no idea as to how long the blower on delay is. One way to test is to turn off the gas & initiate a call for heat. The blower on delay timing starts when the gas valve opens (24 Volts AC applied to valve).

Other than having a weak limit (opening too soon), multiple bad boards (not likely), or having something wired wrong, I'm at a stand still.
The ST9120U has adjustable delays (on & off) to bring your fan on sooner.

 

In review: YOU have no AC. But the way things stand, you do not get power to the blower to show up on the HEAT terminal on the board - but instead you do on the COOL terminal? And you also have heat (burners on) come on when the COOL terminal activates for the blower?

If true, even if the thermostat was miswired to say Y, then you would have the COOL terminal activate but you would not have heat though. But you do. That is what is goofy here. If this was on that radio show's "puzzlers", someone out there would solve it.

Hmmmm. What would happen I wonder if your stat has an Rh and Rc hookup and someone hooked to to Rc, I wonder. ?????

 

If power were supplied ONLY to Rc the burners wouldn't fire on a call for heat.

 

Kinda figured, but not sure for sure.

SOMETHING is causing that weird malady. Unlikely the board since he replaced it and new one also does same thing. That is......if this is a normal "board" as how we think of boards, and not one of those double-wide control modules where maybe retro jobs have jumpers on them that have to be dealt with according to instructions.