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Phil, Mechanical Engineer
Category: HVAC
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Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a thermostat code -ac on my Honeywell thermostat how

Customer Question

i have a thermostat code -ac on my Honeywell thermostat how do i repair this
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: i lost power today briefly for just minutes ever since then the code is present
JA: Do you plan on doing the work yourself?
Customer: if i can yes
JA: Anything else we should know to help you best?
Customer: no it was working fine power lose was brief only 5-15 minutes power came on the code was there has not worked since
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the HVAC Technician about your situation and then connect you two.
Submitted: 5 months ago.
Category: HVAC
Expert:  airheatman replied 5 months ago.

THe ac normally means we have lost power from the furnace or air handler.Do you know model on thermostat?

Expert:  airheatman replied 5 months ago.

Let's do this. Locate the breaker for the indoor unit.Turn breaker off and back on and see if the AC will go away.If not we will go from here.

Customer: replied 5 months ago.
turned breakers on and off left off for 2 hours code still present
Expert:  airheatman replied 5 months ago.

Do you have a volt meter to troubleshoot? If so we want to go to thermostat, remove cover exposing wires and check between R and C terminals on thermostat for 24 volts.We will go from here.

Customer: replied 5 months ago.
no my dvom is not here with me
could issue be cause by low or weak batteries?
Customer: replied 5 months ago.
replaced batteries turned breakers off and on again checked anticipator shows 2 green lights control ok and power applied
Expert:  airheatman replied 5 months ago.

Did the AC go away and does thermostat work now?

Customer: replied 5 months ago.
no change
Expert:  airheatman replied 5 months ago.

Ok we will need to get that meter to troubleshoot.

Customer: replied 5 months ago.
ok i can so after checking for 24 v what the next steps i must follow and what are the possible causes for this code
Expert:  airheatman replied 5 months ago.

The code is telling us thermostat is not sensing the 24v from furnace.

1.If there is no 24v go to furnace or air handler and open it

2.Look for a small auto type fuse they may have blown.

http://i.imgur.com/kYe4Epz.png

3.Check for high voltage into and low voltage out of transformer..

You can leave the question open until we have solved the issue.

Customer: replied 5 months ago.
is that 1 amp fuse on both air handler and furnace checked furnace fuse is good shows power applied control ok
Expert:  airheatman replied 5 months ago.

That s a 3 amp fuse. Tell me the model on your furnace

Customer: replied 5 months ago.
crown model jbf-52spd
Customer: replied 5 months ago.
i can not reach air handler the space is not large enough for me
Customer: replied 5 months ago.
i can have my dvom tomorrow and a test light and or a short finder any other test equipment i should bring??
Expert:  airheatman replied 5 months ago.

Meter should be good.Check the 24 volts out of transformer on unit and we will go from there.

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