Blower runs continuously furnace heats OK


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Old 01-24-09, 11:44 AM
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Blower runs continuously furnace heats OK

I have a Hiel model: EPD30B15C2 this is a split unit with the gas furnace in the attic.
It seems to heat fine, the gas cycles by the temp settings but, the fan never shuts off. Even if the thermostst is completely removed. Opening the blower door tripping the saftey switch does shut it down but thats the only way. I beleive it is a H.E. unit. I do not see a fan limit switch.

I suspect the control board ST9120c 4057 is this the likely cause? If so, will any ST9120c board replace it or does it have to be with the 4057 only.
What else could cause this problem?
The unit seems to be working fine but the air gets very cold between heating cycles , so I need the fan to cut off too.

One other thing, with the blower door off I could press the saftey switch, then I hear movement sounds like freon or sometihng moving through some pipes then the blower fan kicks on . takes about 5 sec or so between pushing button to fan kick on. This is when the thermo is set to off or removed. But like I said when the thermostat is set high the gas cuts on and furnace heats fine even though the blower is already running.

Thanks for any suggestions to my problem.
 
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Old 01-24-09, 12:15 PM
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Often furnaces have a separate "summer fan" switch or other electrical switch which will run the fan continuoiusly as you describe. You might look around for that kind of switch which may inadvertently have been turned on.
 
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Old 01-24-09, 12:21 PM
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The model number you provide is for a Heil Evaporator coil, not the furnace.


You'd need to provide the make and model of the furnace, usually found in the burner compartment of the furnace on the rating plate.

The make and model of the ignition control module the thermostat wires attach to would be useful as well.

Usually there would be thermostat wires attached to the R and W connections of the ignition module --- if any others are attaches please list which connections they go to.
 
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Old 01-24-09, 12:55 PM
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Some mighty strange things can happen. And without trying to figure out how right yet, make sure the A/C is not coming on when the heat is coming on. Check where the compressor is.

Have you tried to stick a thermometer down in the register when you say you feel cold air when just the blower runs? Is it colder than room temp down at the floor level? That be another tip off about the A/C coming on.
 
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Old 01-24-09, 12:58 PM
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correct model #

Yes I realized that I gave the wrong model number .....The right one is FBF075B12A4
It has a Honeywell ignition MPLS MN 55422 but the thermostat wires don't go there they go to the ST9120c board located in front of the blower motor there are wires at G, Y ,W, R connections on this board to the thermostat and two wires one from C and another from the Y terminal looks like they lead to the outside compressor.

There is no summer fan switch but there is a switch that looks like a regular light switch that cuts power to the furnace

The fan switch on the thermostat is set to auto there is no off setting but the fan runs even with the thermostat completely removed. I have a digital thermostat that is easily removes from the base where the wires are attached.
 
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Old 01-24-09, 01:04 PM
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Checked and the AC is running!!

Thermostat set to heat but AC is running all the time!!

Heat does kick on also as it is supposed too
 
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Old 01-24-09, 01:15 PM
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You could have malfunction stat or short in wires in wall or at sheetmetal in furnace that bridge 2 of the wrong color wires or defect in board be my guess. I think Y is the terminal for A/C. So you could disconnect Y wire at terminal strip in furnace for now, as you certainly don't need A/C, right? And see if that stops it. If so, the issue most likely would be in the stat per reason given below.

What kind of stat do you have, by brand and type? Type would be say a mechanical type or digital type? What MIGHT be happening is 'set temp' for COOL is too close to 'set temp' for hot, with not enough "swing" number degrees separating them. And when you get some over-temp going on with the heat, that then triggers the A/C to come on. If all this is simply stat related, you may be able to fix this by adjusting that spread in temp differential inside the stat.

If I remember correctly, I think this year in the building I take of HVAC, the Honeywell digital-programmable combo A/C-Heat stat did just that because of the set temp between each (the stat gives you option in number of degrees)was too close, and when heat came on and also people were around, adding to the heat, and lights came on causing aditional heat, then the A/C came on. So I had to make the correction.
 

Last edited by ecman51; 01-24-09 at 01:34 PM.
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Old 01-24-09, 01:49 PM
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I disconnected the C and Y wires that run down the freon lines , looks like they go outside to the compressor .
This did stop the AC

The blower motor still turning

The stat is a digital made by Hunter I've been using it for 9 years it seems to be ok though...

As I stated before the blower/AC would run even with it totally removed from circuit.....

Now the Blower will start to run after about 30 to 40 seconds from when the blower door saftey switch is held in and runs continuously even with the stat in off position or removed.

My digital stat has a base that attaches to the wall and wires then the digital portion just plugs onto this base. Making it easy to remove.

Atleast its not as cold now

Should I still suspect that the ST 9120c 4057 control board is the problem ....Can it couse this to happen? or is it somthing else?
 
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Old 01-24-09, 01:59 PM
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My stat is digital but like the older ones I have to change it from AC to Heat manually it will not do that on its own...

Also I have not changed any of the timing or temp settings ....They are the same as they have always been for years

Any other posible causes of the AC running all the time other that the stat and wiring?
 
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Old 01-24-09, 03:21 PM
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Try disconnecting the G wire from the ST9120 board and see if the blower stops. Also, before you do that check to see if there is 24VAC between C and G. This will determine if you have a short in the thermostat wiring. G terminal controls your fan, but the fan will still work in heat mode because the control board will time it on and off.
 
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Old 01-24-09, 05:01 PM
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There is one way the circuit board can be operating normally and leaving the fan to run continuously:

Here is the operating manual for the circuit board:

http://customer.honeywell.com/techli.../69-2252EF.pdf


Page 9 of the manual shows a diagram of the connections on the board.

At the bottom is a triangle enclosing a 1 denoting that the fan can be made to run continuous by connecting a wire from the "cont" contact to one of the fan speed connections. A dashed line indicates the connection from "cont" to the "Fan Lo" motor speed contact.

I'd locate the "cont" contact and check to see if it's connected to anything, and if so to what.

That may not be what's causing the problem, but as far as I can see it's about the only way everything can be operating normally and properly and still be operating the fan continuously.
 
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Old 01-24-09, 11:12 PM
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There is no wire on the "cont" terminal

There is not 24v AC between the C and G terminals there is between the R and G terminals with the thermostat wires disconnected.

Disconnecting the Green wire from the G terminal stoped the fan

checked Thermostat wiring for shorts with thermostat removed and wires disconnected from ST9120 control, found the Red and green wires were infact shorted!

Lucky the installer used a 6 wire harness with a spare brown and blue wire.... checked them and no shorts. Used the brown for the R at both ends and Blue for the G.

Reconnected the AirCond wires to the C and Y terminals

Everything seems to work fine now!!
And I was about to buy a $70 control board on ebay and waste time too.
I would never had suspected the wiring.

THANKS GUYS for all your great advise and help tips!!!
Beer 4U2
 
 

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