Question about Honeywell Programmable Thermostat Heater

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Honeywell ST6400C with no display

Hi, I just returned from holiday to find no display on my Honeywell ST6400C. When i went on holiday I turned the heating off but have done this before. Is this an issue with a fuse? The gas is working along with all the electrics in the house. I followed the instructions and pressed the reset button but nothing worked. Thanks

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  • gmelville Nov 17, 2008

    Thanks for your help

  • Anonymous Jan 03, 2009

    yes we had the same problem over a holiday. The elctricity supply to the programmer was switched off. Bad idea! The solution proposed seems not appropriate. The st6400 is the programmer rather than the thermostat. The usual solution is to press the RESET button. But if this doesnt work how about replacing with the newer st9400? Circa £50-£65.. Same backplate and wiring.. (how about leaving connected to mains over holiday..?)

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I pressed the reset button in error and the display just would not return o no heating or hot water. Nothing tripped on the board and all wires and the unit itself showed power coming through.I got it working again by sheer determination.

What I did
I took the unit from the base plate and opened the front panel to see circuit board. It has 4 clips and it all goes back together easily.

What I did.Time taken 15 minutes.

The tiny rubber reset button was collapsed, perhaps maintaining the 'off' situation.

I moved a tiny wire from touching the side of the battery.Perhaps not important but it didnt look right.

When i remounted the unit it lit up beautifully.

So the solution lies in 1 or all of these 3.
If it happens again I will try

1. simply remove the unit from its base plate and remount to see if that is all it takes.

2. Open the front and check the reset button isnt collapsed and stuck.

3. Just move that little wire from leaning against the battery -(least sure of why this might help)

hope this might help someone else

Catherine - a determined blonde lady

Posted on Mar 04, 2011

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I am not familiar with this particular model but in general all digital thermostats are either powered by batteries or by the 24 volt transformer from the furnace. Usually the standard heat/cool type that you may have is powered by batteries and will show a low battery display for a few days before they go out entirely. You may have missed that warning because of your being away.
There will either be a small door on the front of the thermostat or you will need to pull the thermostat from the subbase and the batteries will be inside. Usually 2 AA size....replace with the good kind.

Good luck

Posted on Nov 14, 2008

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Not sure that the advice above is appropriate. TheST6400 is a programmer rather than a thermostat.. we had exactly the same problem after a holiday exceeding the specified four day back up so the usual solution of pressing the RESET button didnt work. We are replacing ours with ST9400 honeywell computer that has the same backing plate and wiring. The answer seems to be, once installed do not switch off at the mains.

Posted on Jan 03, 2009

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