SV9541 Valve Bad??

GordPick

Hi,
My furnace has a problem and it won't fire up.

Some background, a couple years ago the circuit card on the SV9541 valve blew up. (black and smoke) I ended up buying a used valve and desoldering/resoldering the circuit card from it onto mine while still in the furnace as to avoid the cost of a new valve and installation.

This past summer the drain for the AC heat exchanger decided to partially plug and flooded the furnace. I ended up replacing the drain as well as the fried main control board. At that point I verified the AC and heat worked. We then used the AC for a few months and here I am with the heat not working just as its starting to get colder.

I've very capable, but lack the information about the valve board itself to confirm what I think is the problem.

Furnace Info:
Keeprite N9MP2050B12A1 (19.5 years old)
Control Board ST9120U (3 months)
Gas Valve SV9541M2094 (valve 19.5 years, board ??? installed 2 years ago)


The Control Board diagnostic shows regular heartbeat throughout.

The SV9541 shows heartbeat until it errors 30 seconds after the heat signal is received. The error is 3 blinks. (Airflow proving switch remains open longer than 30 seconds)


Troubleshooting so far:
Thermostat set to heat 5 degrees above room temp.

Control board:
W to C - 27v

SV9541:
C1-7 to C1-6 - 27v (24 vac)
C1-8 to C1-6 - 27v (R)
C1-4 to C1-6 - 14v (data)

C3-2 to C3-4 - 120v (feed from control board)
C3-1 to C3-3 - 0v (power to combustion blower)

The rollout switch and limit switch are closed throughout (before power and after heat signal).
The air proving switches (x2) are open throughout (before power and after heat signal).
The combustion blower does not operate.

I verified the operation of the air proving switches. Can suck on the hoses and they close. When doing this with the heat signal, the furnace fires up and operates as normal. (quickly stopped to avoid CO issues)

I verified the operation of the combustion blower by wiring it to 120v. It operates and the air proving switches close as expected.
When doing this within the 30s from heat signal, the furnace will fire up and operate as normal.

I removed the cover from the SV9541 valve and there is no discoloration or visible problems. I tripped the combustion blower relay using external 12 supply and it worked.

Everything is pointing to that the combustion blower is not turning on when told to.


One thing that stood out is the C1-4 to C1-6 was only 14v. I disconnected the cable from the SV9541 and it measures 27v. It drops 13v when connected to the SV9541.


Am I correct in thinking the SV9541 is bad?


Thanks for your help,
Gord

EBEBRATT-Ed

Not farmallier with that furnace but whatever circuit board turns the combustion air blower on is the problem

JUGHNE

IIRC, the inducer fan is controlled by the fan board located in the lower compartment.

The inducer fan must come on first to satisfy pressure switches before any ignition should start.

Can you post a picture of the wiring diagram for the furnace?

GordPick

Here is the wiring diagram. 

The fan control board in the bottom supplies 120v to the SV9541 (connector C3). The SV9541 then supplies the 120v to the combustion blower ( connector C3). There is a relay on the SV9541 that controls it.

The only weird thing I've found is the data line from the fan control board to the SV9541 is only 14v when the C1 is connected, but 27v when not. I assume that indicates a failed part on the SV9541 that in turn is not powering the relay for the combustion fan. 


Thanks for your help,
Gord

ratio

The Honeywell smart valve has all the gas train controls implemented internally. Check the line voltage & low voltage while it attempts to start, if they don't sag replace the valve.

BTW, replacing the board on the old valve was a horribly bad idea. Repairs to a listed assembly like that just can't be made, they must be replaced complete. You open yourself up to all kinds of liability. If something has gone south, who knows where the lawyers would've stopped.

JUGHNE

Can you show us the upper and lower compartment?
Especially where the C3 junction is

GordPick

The first picture is the lower compartment. The connector with white, blue, orange, purple, etc. Is what connects to the SV9541. The white and black go to C3, the others to C1.

The second picture is the SV9541. C3 is the connector with only black and white. C1 is the connector with the rainbow of wires.

The third picture is overall of the upper compartment.

mattmia2

So the inducer loops through the gas valve to power the ignitor?

GordPick

Ya, the valve controls the combustion blower, safety switches, and ignitor.

mattmia2

See if you are getting 120v to and from the gas valve on the inducer connector and work in the appropriate direction.

You can probably find some instructions for those control boards that show the sequence of operation to make sure there isn't some step that is failing before trying to start the inducer. Since it looks like the main control board is powered by an off board 24vac transformer, make sure you are getting 120v to the control board too, I think it could do everything you have seen with only 24vac from the transformer.

GordPick

Yep, there is 24v to the control board as well as 120v. 
The 24v and R pins on C1 at the valve have 24v, the Data pin on C1 has 14v. 
The C3 connector on the valve has 120v from the control board. The C3 connector output to the combustion blower has 0v. The relay on the valve does not activate for the combustion blower. It is 12vdc. I am able to activate it by applying 12v directly to it (with the furnace off).

mattmia2

You could try jumpering the limit and rollout on the gas valve and see if the inducer starts in case it is a connection somewhere but it isn't looking good for the control board on the gas valve. Someone else may know what that data is from the main control board to the gas valve control board but it seems like it is just feedback, not control.

JUGHNE

I never have seen this version of SV's.
The ones I have seen do not run the fan, rather the SV waits for a signal from fan board below, initiate and looks for pressure switch closures as needed to ignite.

Someone took a fairly simple gas valve and had to "improve" it by more complications??