I have a International Comfort Products heating and AC unit. The problem I am having is that the blower runs constantly and there is no power to the thermostat. I can find no fuses on the unit. None indicated in the schematic either. The transformer checks out OK, the wirng checks out OK also. Any suggetions would be appreciated. I am unsure if it could be a limit or pressure switch somewhere or if they could cause this type of problem.

May be 2 separate problems. If you have 24 volts on the secondary of the transformer and no power at the thermostat, you may have a bad connection. Some thermostats must have a battery.

The fan running running constantly likely is a stuck relay. Before coughing up for a new circuit board, locate the high temperature limit switch and disconnect it. If the fan still runs, likely it is the relay.

Do you have 24 V at the furnace R and C terminals? If so the wire is bad between the furnace and the stat. If you are sure you have 24 V at the transformer and not at the thermostat terminals in the furnace, you need to look for a blown safety device, e.g.. Fusable link, roll-out limit, main limit etc. Be aware that if one of these safties trip there is probably a good reason, they are after all safety devices.

I do not have any voltage across R and C. I do have 29.5 volts at the transformer. The limit swictes disconected it still runs. I can not find a fusible link. But, on that I am not sure what I am looking for. I also tapped on the relay to no avail. I do have four wires to the thermostat. R,W,Y,G. On the board I have C,G,Y,W and R. The wires are connected with G,Y,W,R to the thermostat, A wire is connected to C and Y that runs out to the condenser which is currently disabled (Main Pulled).

Disconnect the wires between where you measure the 29 volts and the R & C terminals where it is dead. If one is open, carefully follow it. If it has a fat place molded into it, that would be fusible link. I would cut it out and replace it with an inline fuse rated the same as transformer output. Otherwise look for a flexible place in the wire where the conductor may be broken under the insulation.

The transformer connects to the control board. I can find no anomalies in the wiring. I did just find out that the AUX limit and Pressure switch are not getting power. Should they be passing 24 volts?

After further review... The pressure switch is fed by the Gas Valve. I have power to the gas valve and no power coming out.

Do you mean the transformer is soldered to the board? If so, look for a fuse on the board somewhere. Otherwise, check between where the transformer connects to the board and the R & C terminals.

Usually one terminal of the switches will connect to the R terminal.

It has wires connected and it is all pretty wide open and in good view. I even pulled the board to check behind it. No luck. How do these gas valves work? Could it be faulty?

The way the gas valve works is that power flows from the transformer to the R terminal to the thermostat which switches it to the W terminal. The control board then switches power to the gas valve once it verifies other things are ready. That isn't your problem. Your problem is between the transformer and the circuit board. Is it possible you have a second transformer, and the one you are checking isn't the one that supplies power to the board?

Thanks for the explanation. No for another transformer.

Thanks for the info. I only have one transformer.

You need to find the open device between the +24 V from the transformer to the R terminal at the furnace. I don't see how you are getting 24 V to the gas valve if not at R or W in the furnace. You will not measure voltage coming out of the gas valve since that would be C (common)(Ground). I still need to know more about your furnace to help much. How many wires go to the gas valve, not including the green wire hooked to the body of it?

The gas valve is a honeywell SV9501. It has a connector with four wires(to the board) and a connector with three(ignitor). The transformer at the board is OK. There is no power at the board to the blue wire that goes to the limit switches and then the gas valve.

I reread your previous post. I assume from what you said that I should get no power to the blue wire at the board.

If you have 24 volts going to the board from the transformer, and no voltage at R you have a bd board or you are overlooking a fuse. It could be an automotive type fuse or an in-line type fuse.