Gas and Oil Home Heating Furnaces - Blower keeps running

I have a gas powered, forced air heater (AquaTherm installed in 1998) with a three-wire setup at the thermostat (no a/c). We have changed the filter, changed the thermostat (three times, three different brands and types) and are still having this problem.

Sometimes, when we set the temperature, say at 68, the blower will force heat until 68 and turn off like it is supposed to. Other times (happening more frequently now) the blower just continues to blow hot air. To switch it off, I have to use the on/off switch on the thermostat. Most times now, when I get home from work, it is 80 degrees in the house.

Does anyone have an idea why this is happening. Like I said, we have changed the thermostat several times already, even moved the wires around to see it that would help. Nothing. It started happening after a power outage.

Thanks for your help in advance.

Additional: It is manufactured by First Co. (www.firstco.com) and is a cased-multiposition (hot water w/pump) model #XVAQ

even moved the wires around to see it that would help. Nothing. It started happening after a power outage.
Oh my.

Does that website have something about your furnace?

I believe you have a system referred to as Hydro-air. You have a gas boiler or water heater and it provides the hot water that circulates through the coil in your air handler. What you need to understand is that the blower may be triggered by hot water and is a slave to the other half of the system. Your thermostat may control the pump in the air handler or a zone valve. We will need more info about your hot water sourceto help diagnose the problem much further.

Ken

It is a water heater that is connected/in conjunction with the heater. According to the diagram, there is a isolation valve: supply line coming from the hot water heater and going to the fan coil unit. It then comes out through a pump, cruises by a air bleed valve, then comes out and flows through another isolation valve:return line to the water supply to hot water heater. Does that help?

The pump they refer to, pumps water through the hot water coils.

One more thing, the wiring diagram for our system only has two wires - the wires coming to our termastat has three. And before, the fan would work without heat on hot days to circulate the air.??

You are getting unwanted circulation for some reason. Either the pump is running when you don't want heat or the flow check is not working. There should be a check valve to stop water from flowing through the coil when you don't need heat. I suppose it could be a faulty relay but i don't know where the relay to run the circulator would be in that system. Possibly inside the air handler.

Ken