DIY Doctor

How to install - Honeywell ST6400S Heater Timer

Postby worm » Mon Dec 01, 2008 11:25 am

I have a combi boiler central heating system, about a year old now. All the rads are thermostatic, I want to install a digital programmer in my kitchen- Honeywell ST6400S Heater Timer.

Are these easy to install, I assume they need a main supply 240V that I can spur of from the closest socket, and do they also require a flex to the boiler, I have a glowworm boiler!

Is that job done, or is there more to it than that.

I presume that as all my rads are thermostatic, I can set them all individually 'by hand' therefore no need for a room thermostat??? Then once the programable timer is installed I can set it to function as desired over a 7 day period....hols etc etc???

Many thanks
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Postby ericmark » Mon Dec 01, 2008 8:45 pm

Seems an odd programmer to use with combi boiler where hot water is not stored so normally would require no control.
Something like the Horstmann DRT2 see ... Thermostat would seem more likely option mounted in coldest place in house which would then both set time and max temp.
Instead of turning central heating off and on they change the temperature according to time of day so if you set to low temperature while away from house if it gets really cold then heating will cut in to stop ice problems and in the summer it will turn off heating when warm enough.
I am sure the one you quote would do the job but like using a sledge hammer to crack a nut.

Postby worm » Tue Dec 02, 2008 12:44 pm

Hi Eric, I understand your comment.

The combi boiler does indeed provide hot water on demand, this boiler has a timer/thermostat built in underneath...

I have the honeywell digital programmer, all I want this for is to be able to program in heating requirements with the advantage of 7 day programming. Rather than having to change them from day to day with the fiddley little dial that is in a awkard place too on the boiler.

The honeywell would make life easier, and may be a bit of a sledgehammer approach.

Is it easy to install/wire? Does it simply require a main supply 240V that I can spur of from the closest socket, and do they also require a flex to the boiler????

So I presume as all rads are thermostatic, I don't require a room thermostat or do I?

I can individually set each room rad to the desired temp I always want, and the honeywell will allow me to program this as and when with the added advantage of 7 day programming in advance.

Cheers Eric if you can advise
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Postby ericmark » Tue Dec 02, 2008 8:46 pm

1) For a timer to work on a condensating boiler the lock shield valves need to be carefully set as when it switches on after being allowed to cool the thermostatic radiator valves will all be wide open and unless the lock shield are set correctly than you will get one room at a time heating up. With pre-condensating boilers this is not so much a problem.
2) Without a thermostat as winter sets in and as spring arrives the boiler will cycle to test if radiators need any heat so a thermostat that measures nearly outside temperature to switch off the heating completely on warn days is an advantage of course in mid winter it will do nothing only as summer approaches will it do any good stopping the central heating cycling on warm days it would normally be fitted in coldest room often placed where it will be warmed by morning sun so as summer returns on prospective warm days it will stop central heating cutting in. With the type which includes a timer it can have two separate temperatures for morning and afternoon so does not need to feel morning sun.
3) Yes the timer you have can be used to control just the central heating. The type you have is mains powered and the same power feeds to programmer and central heating and has to take its power from same source as central heating. Some boilers require volt free contacts on the programmer and if yours is this type then you can’t use this timer.
4) The thermostat / timer I listed does have volt free contacts it is powered by two AAA cells which are easy changed and even cheap ones last a couple of years. So just one pair of cables required from it to boiler.
5) I have seen radio control thermostats but not timers nothing to stop a radio controlled timer from working but never seen one.
I am not a plumber I am an electrician and you may find advice on how to connect up boiler on heating and ventilation forum for me to answer any questions on boiler I would need to down load instruction manual first and some makes will only make them available once you enter Corgi reg number which I don’t have.
The instructions for your programmer are available on screwfix web site which is what I used to find out what type it is.

Postby griffo » Tue Dec 02, 2008 10:14 pm

Hi the programmer you are looking at seems to be the wrong one for a combi boiler (Could only find the 6400c not s) a twin chanel programmer like this one is for a standard tank type system. You need either a single channel programmer, if you want time only or a better option to be building regs and part L1 efficiency regs compliant would be a Honeywell cm907 in either hard wired (2 wire) or wirefree options depending on how easy it is to cable to boiler - spot on suit any combi.
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