Posted: 11/18/2007 8:33:18 AM EDT
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I have a American Standard unit, around 3 years old. edit (YC model, gas/electric) Thermostat will make the air conditioning go on but the heating side does not respond. On manual the light goes on but the unit on the roof does not kick on, nor does any fan. Fuse is fine, both circuit breakers are fine. Robershaw 9615 Digital Thermostat. This more or less just stopped working as I know we used the heat a few times in the last month. |
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Go up to the unit, Take off the main burner control panel. Have some one select "heat" and set the T-Stat to 100. watch the igniter...does it glow? or if its a spark ignition does it spark? My bet is that it is NOT the T-stat, but lets take it 1 step at a time. |
The thermostat works the AC side of things. You can hear the thump of the startup and fan noises. In the Heat mode, there is a click and the green light goes on as normal but nothing happens, like the signal is sent but no burners or circulation fans go on. I see it has a 5 year warranty on parts but not labor of course. |
Haven't got anyone to help with this right now but I hear what you're saying as I've replaced thermocouples on water heaters a few times. The owner's manual states 3. As the thermostat calls for heat, the system cycles as follows: 1. Combustion blower is energized 2. The Pressure switch is closed 3. The gas valve opens and the ignitor lights the burner The IGN Diagnostic LED indicator manual mentions System Lockout Pressure Switch Problem High Limit (TC)) Open Flame sensed with Gas Off Flame Rollout Switch (FL) Open I can see how it could be a few things, ignitor most likely. |
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Go down to the furnace and look for the diagnostic LED, it's inside the blower door on most models. DO NOT TAKE THE DOOR OFF, it will reset the CPU because of the dead mans switch, look through the little round plastic window they installed on the door. Count the flashes, record it on a piece of paper and then remove the door and on the inside there will be a shackle sheet for the codes Example: 1 flash - Open limit switch 2 flash - DP switch problem 3 flash - Ignition failure etc...... Get back to me with that info. |
Ok you do need to read that code flash. System lock out means just that...something was wrong so the system locked itself. Pressure switch, is the small fan that pressurizes the vent. if the vent doesnt pressuize properly the system will lock out. the pressure switch is the round sensor with two tubes going into it. that is what tells the fau the vent is under pressure. High limit is the internal senor that checks to make sure the heat exchanger isnt over heating. flame sensor is sorta tricky, it will will trip if either the flame is sensed, (sensor shorted) when no flamne is there, or will trip if the gas valve is open but the sensor doesnt heat up. Roll out sensor is a small disk like sensor in several places, if any of thoses get hot, it will lock out the system...think of them as small fire detectors. My bet is on either a bad ignitor...a bad vent switch...or the flame switch doesnt sense the flame or the flame isnt carring over to the sensor.....but you really need to watch the system cycle. 1)call for heat....click at Tstat, heat light on at tstat. 2)click at gas valve... 3)3 seconds after GV click, small vent fan turns on. 4)2 seconds after fan turns on, pressure switch tells GV vent is ok. 5)Ignitor starts up..2 seconds after that gas flows. 6)Gas is lit, sensor tells GV flame is present. 7)after several seconds heat exchanger gets hot, Blower fan switch says go, fan turns on to circ hot air into house. at any of these points the system will shut down if its a no go. The "clicks" are basicly relays closing telling you the signal was sent and received. |
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Reasonably common for the thermostat to take a dump on one side or the other. [heat cool] All you need to do to test is jump the white wire and the H Red wire connections and see if the unit powers up. If it does it is the thermostat most likely. You could also have a bad furnace transformer on the 24V side, especially if you have a thermostat that actually uses the batteries to power the switch in it. All it takes is a good lightning storm and a close hit to fry the thermostat or transformer. Also, if the thermostat has batteries in it, change them. I love charging people $100.00 for $2 worth of thermostat batteries.  [Have to for the service call]
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Thanks for all the help. It turned out to be a bad blower motor. It was a variable speed (not so typical) one and wanted to turn backwards. A fuse blew too. I guess in reality the AC would not have worked either but with the big condenser fan and compressor on it sounded like it was functioning. |
DC or A/C motor? D/Cs are $$$$$$. So what did it do, wipe out the fibreboard limit that is screwed into the heat exchanger front? |
AC I think, but I watched it as he signaled it on and it just moved an inch or so, bounced and stopped. He said the parts will be around $700 and the motor is expensive. Coming from back east. The unit has a 5 yr. guarantee and was installed in July 2003 so it's still under warranty so the HVAC guy is going to show them the Model and Serial number to establish the warranty claim. About a week without heat but the fireplace did OK last night. |
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I can buy a multispeed A/C for $100-$150 or so, D/C are MUCH more pricy, they are nice as they very quiet and are basically infinite speed [tween max and off] They should last the life of the fnc but the controller seems to cause issues. Glad it's under warrenty. Seen some D/C motors price out at close to a grand, hell I can buy an entire furnace with that with $$$ left over. Plus One can usually wire a genaric A/C one to work in most furnaces, not so easy with a D/C. |