Need some help - Not Shopsmith related but HVAC

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db5
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Need some help - Not Shopsmith related but HVAC

Post by db5 »

Need some help about HVAC

This is puzzling. Replaced a faulty thermostat on 9-7. I switched the thermostat from cool to heat last week. No heat. Turned on the fan. No fan. Replaced the fuse, turned on the fan and it ran. Switched to heat and it blew a fuse. Replaced that, pulled the thermostat from the terminal block and used a probe to connect W(hite) – Heat to R(ed) Power. Heat came on. I reasoned that the short was in the thermostat and not down line. Jumpered from W to R to turn on the heat and when it had warmed the house removed the connection from R. I did that twice. Replaced the thermostat yesterday. Same problem this morning. Jumpered again to turn on heat. Replaced the thermostat (3rd time) and it blows a fuse when switched to heat. FYI it is a programmable thermostat but I cancelled all programming so it would not be controlled by any settings other than manual.

My neighbor owns an HVAC Mechanical company that he ran with two sons (now he’s ready to retire and is winding down) and he concludes it MUST be in the thermostat. Any suggestions?
Thermostat.JPG
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Thermostat.JPG
Thermostat.JPG (17.13 KiB) Viewed 1144 times
RFGuy
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Re: Need some help - Not Shopsmith related but HVAC

Post by RFGuy »

Which fuse are you replacing? Some t-stats have a fuse inside them, but I assume you are meaning the high voltage fuses e.g. at the quick disconnect to the HVAC. T-stat wiring does go bad. In fact, I had to replace mine on one of my systems about a year ago for this very reason. If the wiring is old and the insulation is starting to break apart, there could be a short somewhere along the length of the cable that is causing this. Also, I assume you double-checked that all connections are proper, i.e. when you switched thermostats you didn't swap any of the wiring between t-stats by accident?
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db5
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Re: Need some help - Not Shopsmith related but HVAC

Post by db5 »

This is a simple Honeywell T-stat and has no fuse, so far as I know. I'm referring to the 3-amp fuse on the circuit board (control board) on the HVAC. All connections are proper - double-tripple checked. Shown in the photo. You stated, "If the wiring is old and the insulation is starting to break apart, there could be a short somewhere along the length of the cable that is causing this." I think that means the wiring from the thermostat to the control board. If it is defective why would the fuse not blow when W (heat) is directly connected to R (power)? Thanks for taking the time to think about this and reply.
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twistsol
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Re: Need some help - Not Shopsmith related but HVAC

Post by twistsol »

The fastest way to test for a wiring issue is to take the thermostat to the furnace and wire it up with a couple of feet of known good thermostat wire. If it works properly, you know the wiring in the house is bad. If you have the same result, it is probably not the house wiring, but still a possibility if you have multiple issues going on.
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DLB
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Re: Need some help - Not Shopsmith related but HVAC

Post by DLB »

db5 wrote: Wed Oct 06, 2021 8:10 pm This is a simple Honeywell T-stat and has no fuse, so far as I know. I'm referring to the 3-amp fuse on the circuit board (control board) on the HVAC. All connections are proper - double-tripple checked. Shown in the photo. You stated, "If the wiring is old and the insulation is starting to break apart, there could be a short somewhere along the length of the cable that is causing this." I think that means the wiring from the thermostat to the control board. If it is defective why would the fuse not blow when W (heat) is directly connected to R (power)? Thanks for taking the time to think about this and reply.
The thermostat function is a little different depending on what kind of system you have. It sounded to me like yours is controlling heat and fan independently. So I'd think when you jumper it you need to jumper both heat and fan to hot to simulate the full load.(?) They could both work by themselves but together create too much load for the fuse. If it is standard I think green is the fan. IIWM I'd measure current and voltage if possible since you've essentially eliminated the most likely cause. Voltage should be around 24 VAC. I'm not so sure about amperage, I've read about 1 Amp per control is normal with some systems drawing way less, in the low milliamps. For sure, not more that 3 Amps for the two combined. ;) If you jumper both and this is the problem, it will probably blow your fuse again so maybe measure them independently first if you have a multimeter that measures AC Amps.

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Re: Need some help - Not Shopsmith related but HVAC

Post by RFGuy »

So, you have a heat pump right? I say this because I see that the "B" terminal is connected on the t-stat. IF the 3A fuse on the heat pump control board is blowing, then it is likely a short in/around the relay for the heat pump. I would think it has to be this, or possibly a wiring error/shorted wiring in the walls (t-stat wire between t-stat and heat pump unit). So, if you bypass the t-stat completely and just jumper to turn the heat on (with fan) does it stay on and NOT blow the 3A fuse? That is the part that is kind of confusing to me is that you got it to work where it didn't blow the fuse, I think, but then when connected to the t-stat the fuse blow? Is that correct?
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Ed in Tampa
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Re: Need some help - Not Shopsmith related but HVAC

Post by Ed in Tampa »

Start at the beginning of the problem.
What was wrong with the original thermostat?
What happens if you get an exact duplicate of the original thermostat?
While almost anything could be causing your problem start with what you did first and move forward.
The problem with the original thermostat may be the key to figuring out what is going on.
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db5
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Re: Need some help - Not Shopsmith related but HVAC

Post by db5 »

Got it fixed. Thanks. I installed the previous t-stat according to written directions. It stopped working on cool this summer. I pulled it, all the wires were connected so I just reconnected them to the new t-stat terminals. Cool worked again. I think the previous blue wire connection was to a null terminal.
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Re: Need some help - Not Shopsmith related but HVAC

Post by JPG »

Brand and model# of both thermostats???

FUSE blowing = control transformer primary?

or

= ?
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