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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8443
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Bryant/Carrier #F4DNF005, typinmg, thermostat is out, found

Customer Question

Hi there. Bryant/Carrier #F4DNF005
JA: I'll do all I can to help. How long have you been dealing with this?
Customer: typinmg
JA: Do you plan on doing the work yourself?
Customer: thermostat is out, found blown 5 amp control board fuse, replaced fuse. turned breakers back on,. fuse blew, breakers off, replaced fuse, turned breakers on, fuse fine, no thermostat.
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: I don`t know. One place said to jump G and R wirtes together to start blower, but that didn`t do it. Thermostat still out.
JA: Anything else we should know to help you best?
Customer: 5 amp fuse was in the furnace control board
Submitted: 6 months ago.
Category: HVAC
Expert:  Phil replied 6 months ago.

Hello, you have an intermittent short circuit either inside the control board itself, or in one of the peripheral items that plugs into it.

You will need a multi meter so you can take various voltage reads as I ask for them, tell me what voltage read you get from the red thermostat wire to the cast metal body of the gas valve (a ground)

We can go from there

Customer: replied 6 months ago.
zero VDC from red wire off transformer to pc board (to ground of course) I disconnected red wire to read it. [electric heat pump system]
Customer: replied 6 months ago.
gottra splitto rehearsal. will have to reconnect later. sorry.
Expert:  Phil replied 6 months ago.

Thanks, ***** ***** in that furnace are in volts AC ...repeat the test with the meter set to volts AC...

you do not have to disconnect the red wire to read its voltage to ground (should be 19 vac or so).... if you still get zero volts then check the primary side of the transformer, if that is getting the required voltage, probably 240 volts, and you are still not getting 24vac across the two 24vac output terminals, look for a fuse or tiny breaker built into the transformer, if there is none then the transformer is bad,

We can go from there when you get back

Expert:  Phil replied 6 months ago.

Reply in THIS thread, no need to open a new question as you did in order to reply. Just reply in here by clicking on your email notice that you got an expert response... it will take you here.... reply here

We can go from there

Customer: replied 6 months ago.
I think I had the breaker off; I now read 27 VDC at the secondary. Tried another fuse and it blew when I turned on the breaker. I assume you`re beddie-bye by now. Will check with whomever picks this up in the AM. Wife wants me to call HVAC in AM, which I`ll do, but there is HVAC company in town which has no problem in selling parts to consumers to do their own repairs if they are able after describing the symptom.
Customer: replied 6 months ago.
Rats, I meant 27 volts AC...AC.
Expert:  Phil replied 6 months ago.

Hello again, locating the short that is causing the problem can sometimes be easy, other times, if it is intermittent can take a good long while.

The outfit that sells parts will not know which one is shorted until and unless you do tests for it with an ohm meter, power off, test probes on each peripheral such as the gas valve and various sensors, looking for a zero resistance reading.

Customer: replied 6 months ago.
indeed. As a homeowner I search for the simple way first and if that fails, the professional is called, which I ended up doing. It turned out to be a barely bare wire on the defroster line going into the outside unit near its door somehow. It was out of sight of our HVAC tech. the first time, but he did further checking and found what he thinks was the probable cause. It`s been working fine now. Well, thanks anyway. (I wouldn`t have known what on earth to look for; he did.)
Expert:  Phil replied 6 months ago.

Good work, keep that guys card. he is smart and honest tech.

If you want to thank me for pointing you at an electrical short, just rate my work...I will appreciate it

Expert:  Phil replied 6 months ago.


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