Furnace wont do anything

ruwilde59 · 06-20-2006, 04:57 AM

I was wondering if anyone had some idea why my heating and central a/c wont work. No Fan. Thermostat does nothing but a month or 2 ago the heat worked fine and now that I try to set it for A/C, Nothing! I even set the thermo to fan and nothing... Thermostat bad? It's the standard Honeywell round turning one and probably 30 years old. Any suggestions would be greatly appreciated.. Thank You

redline · 06-20-2006, 07:24 AM

Have you checked to see that the power has not tripped to this circuit?

MAS2006 · 06-20-2006, 11:49 AM

Ditto Redline. Also check for sevice disconnect not turned off. If system uses a low volt trans it may have failed.

ruwilde59 · 06-20-2006, 06:29 PM

Checked Main Breaker, No Problem there, Not sure what a service disconnect is, I did check the on of switch o the furnace and it was on, I could hear a slight hum which swiched on from off. Replaced the Thermostat as I thought that was it but still no blower or fan from furnace. Any other suggestions? Thank you

MAS2006 · 06-20-2006, 11:07 PM

http://home.howstuffworks.com/home-thermostat3.htm

Take a look here, a place to start. Double check connections on thermostat, check that 24 volts OK.

ruwilde59 · 06-21-2006, 01:49 AM

Got a Mulitmeter, set it to V AC and 200v and also 20V and I got nothing on any terminals touching the ground probe to a mounting screw . I would still get power readings even if the thermo was hooked wrong, right? Am I using the multimeter correct and why would there be no power to the Thermstat at all? Everything worked fine during the heating season... What do ya think and thank again.

MAS2006 · 06-21-2006, 10:32 AM

Find the 24vac transformer, usually on or near furnace. The on off switch is your service disconnect for heating system. One side of transformer wired to 120 vac. Other side should have screw treminals check for 24 vac between them. Make sure meter working test an outlet for 120 vac. Mounting screws are not and should not be normal part of circuit. The 24 vac has to get from the transformer thru the thermostat to a relay and return to transformer, a complete circuit.

redline · 06-21-2006, 05:08 PM

If you bypass the thermostat then you will be able to tell if it is the thermostat.

Jump (connect) the wires from the transformer to the relay.

MAS2006 · 06-22-2006, 12:15 AM

http://www.hometech.com/learn/hvac.html

Something for your reading pleasure.

ruwilde59 · 06-22-2006, 02:48 AM

Hey Mas2006

I had a friend who is an electrician come over, he checked the LV Transformer in the furnace and it had 117 Volts coming in and 28 Volts going out..But there was no power in the circuit board or the thermostat connectors coming off the circuit board. Do you think that a 4 volt overage could have blown the circuit board causing no furnace activity? My friend was not sure if 28 Volt reading being not 24Volts and could have damaged the board so I would have to replace both transformer and board or just the board? Thanks a Million for the insights here.

MAS2006 · 06-22-2006, 01:16 PM

Most older style thermos are just switches, pretty tough to trash, it can be done if you try hard enough. Like Red Green says "If it aint broke your not trying hard enough."
The 28 vac should be fine. You need to find why its not getting to blower and AC control relays. Understands (draw it out for yourself) how you system is wired and try to bypass thermo. Jumper 28 vac to control side of blower relay. What brand are we talking about?
Check site on thermostats in earlier post. Broke or miswired most likely.

lindamdesign · 07-15-2007, 03:26 PM

Sounds like you need a spark to get your fire started..!

MechanicalDVR · 07-15-2007, 11:36 PM

Voltage close to 30 vac is very common and won't hurt the board. Often a problem with the blower shorts out the board and the blower motor and board need to be changed. What brand furnace do you have?