Honeywell ST7100 - farewell to an old friend?

[purplemoon]

Woke up this morning to find that the HW indicator light on my ST7100 was out and that I had no hot water despite normal operation of the heating system. Pressing the 'Change' or 'Extra Hour' HW buttons had no effect - dead as a Dodo.

After opening up the programmer I detected a faint whiff of electrical 'burning' and could see that the Takamisawa LZ 48H power relay associated with the HW circuit had melted slightly on one side. (As a matter of interest I've had my ST7100 for around 20 years and in the last couple of months changed from HW 'on' 24/7 to two timed on/off periods - maybe this switching demand was too much for it?).

It looks as if the new ST9400C will do the job as a replacement but, given the time of year with deliveries etc., I'm not hopeful of getting this in a reasonable period of time.

My question is - can I link the HW 'Com' (6) and 'On' (8) terminals on the ST7100 backplate to give me what would effectively be a continuously on HW circuit as before (ie bypassing the timer), with the 3-port and cylinder stat looking after the control side of things until I get my hands on the replacement ST9400C?

Regards

 

[simond]

Yes.

You should find the ST9400C on the shelf in most merchants, or delivered tomorrow if you are quick.....

http://www.screwfix.com/prods/88608/Plumbing/Central-Heating-Controls/Honeywell-ST9400C-Programmer

 

[purplemoon]

Many thanks Simon - tried it and the HW cylinder is now heating up nicely. I've kept the ST7100 body disconnected whilst I have the link in place to avoid any possible interaction with the 'dead' relay etc. I'll remove it once I have a full tank of HW and refit the programmer to get the CH back on.

As a matter of interest I've just received this from Honeywell ...

You may be able to link out the timer, but be aware that there may be a fault on the system that has overloaded the relay which is 3 amp only, so do some current checks before doing any bypass. It may be there is an intermittent short somewhere. ; certainly check the cylinder stat for ‘good condition’.

Thanks again. Your input to the Forum is much appreciated.