# Thermostat apparently blowing low voltage fuse



## Homefiddler (Jan 17, 2014)

I am in Florida.

Please as if more info needed but I will start this generally.

- Upstairs and downstairs units, identical. 
- Bryant tstats on both (TSTATBBNB001).
- A/C and heat pump
- Purchase Honeywell tstat (RTH221B)
- Story begins: Downstairs tstat display blank.

Ok,

1) Noticed Bryant thermostat display off, checked overflow switch & breakers & transformer (230 in 28 out ??). They were good.

2) Have identical upstairs and downstairs units, so put upstairs tstat on downstairs (one not working) to see if worked; it worked, seemingly telling me tstat was bad.

3) Could not find same one at Home Depot (was installed 10yrs prior) so got a Honeywell RTH221B. Seemed to work at first (on A/C anyway) but then stopped at some later point, seemingly when heat was called on (cool night).

4) Checked things again, turn out the 5A low volt fuse blown.

5) Put new fuse in, checked using good Bryant tstat again to make sure it did not blow fuse, fine.

6) Put Honeywell back in and blew fuse. This time seemed like AC would not work either, and first time, it might have stopped sooner then I realized as I cannot remember the exact events as it looked like was working and I probably stopped paying attention until it got cold in morning.

7) Put new fuse in and put working Bryant tstat back on and has run since, BUT I still need to replace as I have 2 units.

What might cause this? Is it a tstat thing, or something else that just makes it look like it. Also, the Bryant had automatic switching from AC to heat pump, Honeywell has a mechanical switch I have to select heat or cool with. Also, Honeywell had B terminal for I assume the blue wire but Bryant had C for where the blue wire was connected. Also, Honeywell had jumper preinstalled from R to RC AND Honeywell had a selector on the back for Gas or Electric.

Thank you.


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## beenthere (Oct 11, 2008)

The B of the Honeywell thermostat is not the same a C for the Bryant thermostat. Don't connect any wire to B. Where did you connect the orange wire.


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## Homefiddler (Jan 17, 2014)

*......*

Where should the blue wire that was connected to Bryant "C" go then?


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## Homefiddler (Jan 17, 2014)

Apologies, did not see the Orange wire Q. Not ini front of me, but I believe there was an O that I used for Orange. By the way, thank you for replies.


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## Homefiddler (Jan 17, 2014)

Where should the blue wire that was connected to Bryant "C" go then?

Not in front of me, but I believe there was an O that I used for Orange. By the way, thank you for replies.


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## beenthere (Oct 11, 2008)

If the Honeywell has no C. Then that wire does not get connected. Which means your thermostat is a battery operated only thermostat.


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## Homefiddler (Jan 17, 2014)

Ah .... and it is battery op. What exactly was it then that was going on to blow fuse? I assume the blue wire has something to do with powering unit then. What is the B terminal on HW? And yes, as you already know, the Bryant was not battery and the HW is.


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## beenthere (Oct 11, 2008)

The B terminal is energized on a heat call. So it was a dead short with the wire from the common of the Bryant.


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## rodkahn (Aug 13, 2012)

I had the same experience installing a new HW thermostat. 5 wires in the wall, blue apparently not used. HW rth2300b has contacts for O and B, no common C. It runs on batteries. So, initially I thought B=blue=common, and hooked up blue to B. blew some fuses. 

My voltmeter showed me that upon switching to heat, it connects red to W and B. Back on the furnace board, I see that the blue wire is connected to a C terminal. So I just connected 4 wires in my HW, leaving the blue unused. The common wire blue may be useful in the future for wifi/smart thermostats.


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## mark256 (Jan 22, 2021)

For non-heat pump applications, do not connect the black wire to the thermostat. The thermostat shorts the black and red wire terminals together when heat is called for which will blow the fuse every time. The thermostat does not require a ground wire to operate, in a simple furnace system only the red (24vac) and white (heat call) wires are needed.


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