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Tom, HVAC Technician
Category: HVAC
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Experience:  16+yrs HVAC/R service. Residential Commerical Industrial
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Honeywell Chronotherm III problem

Resolved Question:

I have a Honeywell Chronotherm III. I replaced the batteries and now when I turn on the heat the furnace turns on and then stops about 1 minute later. The fan never comes on. On the front of the panel it shows the time and the "AC". Why is my system doing this ? Why won't the fan come on and blow heat in the house? Why does the Panel display "AC" ?
Submitted: 3 years ago.
Category: HVAC
Expert:  Tom replied 3 years ago.

The AC display means that the thermostat is not getting ac power from your furnace...

I would try removing the thermostat from the sub-base (plate that is attached to the wall)...try cleaning the connections between the thermostat and base and also check the thermostat wires for loose or dirty connections...

The wiring is 24 volts...so you can carefully remove it with power on...or shut the power off at the furnace if you like..

Customer :
What should I use to clean it with? Alcohol?

Tom :
I'm trying to remember how it connects...brass pins perhaps..? I would use a pencil eraser on any surface you can...alcohol might work ...I would shut power off if you do that..Shouldn't matter..but you cannot be too careful.

Customer :
It is brass pins. I will try the eraser

Tom :
That should take any tarnish off...even if you don't get inside the plugs..getting one surface clean and replugging it often times takes care of the problem..

Check the stat wires..I think the Chronotherm uses little screw terminals..make sure they are snug..

Customer :
Still getting the same thing - AC. Also, when I turn the Fan to "on" nothing happens.

Tom :
Do you own a voltmeter?

Customer :
No volt meter

Tom :
Ok...is this a gas furnace? And it was trying to run earlier?

Customer :
Yes and Yes

Tom :
Ok..if you take the wire from the W terminal of the sub base and connect it with the R terminal...that will by pass the thermostat and we'll at least see if your furnace runs...If it does..you probably need a thermostat..if it doesn't we have a problem in the furnace.

I would shut the power off before moving the wire.

Customer :
The furnace fires up but shuts off in about a minute.

Tom :
I would try moving the W wire and see if it will stay running.

Customer :
What do you mean move?

Tom :
Unscrew the terminal screw...remove the wire under the W terminal and connect it under the R terminal..... you don't need to put the front of the thermostat back on for this test...just turn the furnace back on...you are bypassing the internals of your thermostat... Essentially 'hot wiring' your furnace to see if it will stay running...

Customer :
There is already a wire on the R terminal.

Tom :
That's the 24 volt power from the furnace...that's ok..I want you to connect the W wire with the R wire....that should make your furnace run and hopefully stay running...

If the furnace is ok..it will run until you unhook it..

If that is the case you have a bad thermostat.

If it still quits...you have a furnace problem..

Customer :
Get the same result. Looks like it is a furnace problem. Fan never turns on.

Tom :
Did it light?

Customer :
Yes

Tom :
What kind of furnace do you have/

I'm not sure what to make of this...the AC means the thermostat isn't seeing power...and now it lights but the fan won't come on...It's almost like you have problems on both ends..

If you need to get heat...... you could unhook the G terminal wire and add it to the R and W...that would give you fan..

Customer :
Bdp508. Looks like it is a Carrier

Tom :
Strictly manual control...but at least you'd have heat..

I would get someone with a volt meter over there to do some testing...I'd hate to tell you to buy a thermostat...and it be something else...but the blower should have started in the heat mode...

If the fan doesn't run by connecting G ... then you probably have a control board issue or a bad blower motor..

Customer :
It is a bdp model 518A060. Do you think it is time to call in an expert?

Tom :
If you can get by without the heat for the night..I probably would. ... It will be difficult to go much further without taking some volt readings to find out what's really going on..

Customer :
Ok. Thanks very much!

Tom :
You're welcome...I'm sorry it wasn't something we could diagnose easily over this venue. Good luck..I hope it's nothing expensive.

Customer :
Me too.....

Tom, HVAC Technician
Category: HVAC
Satisfied Customers: 770
Experience: 16+yrs HVAC/R service. Residential Commerical Industrial
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