Problem with honeywell st6400c??

[cap1000]

Any advice on ST6400C programmer behaving erratically. The heating switch often doesn't come on (red light) at the set time in the late afternoon. or if it does come on, it often doesn't switch off at the set time. Normally there is no problem in the morning. It's the late afternoon settings that are erratic.

 

[cap1000]

I should perhaps add to the above that everything works if I do it manually. I switch on when I get home (but that means coming home to a cold house) and switch off before going to bed (but that means having to remember!). What i want is for the automatic on/off settings to work.

 

[aggis]

Should be able to get a direct replacement

 

[cap1000]

Thanks. But still baffled. The programmer ALWAYS works as it should with the morning settings. It's the later ones that are erratic. What would explain this discrepancy?

Is this a battery problem? If so, how to replace it? I read somewhere the battery is soldered to the guts of the device, so a new one can't just be slotted in.

If I need to replace the programmer itself, should I replace with 9400C? Can I do the replacement myself, or do I need a plumber or electrician?

 

[Mike1234]

I have replaced the clock memory battery in a Honeywell 6400C. This component is soldered to the circuit board.
The original battery is a GP Batteries GP80BVH X3. This has an 80mAh capacity however, I followed a lead found elsewhere and fitted a Varta Mempac 3.6V 150mAh. The Varta battery is larger and there is a need to fold over another component (SW15) found on the circuit board. There is a risk of breakage here so if you want to play safe go for the original part. Both can be sourced if you do a Google search.
You will need:
1. New battery (obviously)
2. Suitable soldering iron for circuit board work and some solder wire.
3. A static protection wrist strap and connection to ground is a good idea. Otherwise be careful what you touch on the circuit board.
4. Two flat bladed screwdrivers and a pozi-drive.
STEPS
a) Switch off power to unit at mains.
b) Slacken two screws on underside of controller then hinge upwards to release from the upper tabs in the base plate and remove controller from wall.
c) There are 4 plastic tabs holding the housing together, look to rear face. Using one flat bladed screwdriver in the housing split line and one on the tabs work your way around the 4 tabs to separate the housing front and rear.
d) The circuit board can now be lifted out. Look around perimeter for small plastic tab preventing vertical lift out of the board, hinge from one side to get past this.
e) Desolder old battery.
f) Solder new battery to circuit board. May not need much additional solder, just reflow original.
g) Re-assemble in reverse.
h) I left on the mains for 24hrs to charge before attempting a simulated power cut.

If this fails to solve your problem I have read on the Honeywell website that the 9400C is a direct replacement that will fit the 6400C baseplate.