Switching CH/HW programmers

[JAD]

I have recently replaced an old analogue room stat with a programmable Honeywell CM907 on a standard open vented fully pumped central heating/hot water system. My plan was to set my British Gas EMP2 timer so that the CH was on 24hrs (letting the CM907 switch the heating on and off) and leave the HW on the twice-a-day setting. Unfortunately I did not figure that the HW comes on with the heating with this controller! I am now planning to replace the timer with something like a Siemens RWB29 Programmer (which should be a straightforward switch having an industry standard back plate). My questions are: will this work, is the switch really an easy swap-over, is there a more appropriate timer to use and am I being dense and missing something?
Thank you in advance.

View media item 19151 . . . View media item 19152

 

[JohnD]

Edited:

I think the correct answer is:

you say "HW comes on with the heating with this controller" which sounds like it is set to HW Gravity. It may have a switch to set it to "HW Pumped" (assuming that is what you have got) which will make HW and CH independently switchable. On the BG controller IIRC there is a switch on the back (when you take the programmer off its backplate by undoing the pinch screw underneath). This depends on you having at least a Cylinder Stat, and preferably also a motorised valve to turn the HW circulation and boiler on and off. If you do not have a motorised valve it can be done, but is a non-standard circuit since it does not meet energy-saving standards.

My original answer is probably giving a more complex solution than you need, but here it is anyway...


If you email the makers of your new programmer, and tell them what the old one was, they will very likely have a conversion chart that shows you how to install the new programmer on your old wiring.

ACL/Drayton certainly provide such charts (the BG one is just an ACL/Drayton SM2 with a BG badge on it so maybe you could tell then you are changing "from" the other one if not?)

It might have been a bit easier to change to a ACL Drayton (Invensys) Lifestyle programmer such as an LP722 since they fit on the same backplate you already have, and will need minimal or no wiring changes.

http://www.draytoncontrols.co.uk/TimeControls.aspx

don't get the LPSi which is designed to stop working properly when the date comes for a service

If you find the wiring a bit fiddly, you can buy a "wiring centre" for a few pounds (it is just a glorified junction box really but has terminals for the boiler, pump, motorised valves, thermostats and everything so it is easy to see what you are doing)

I am a householder not a pro

BTW the old analogue programmer is in demand among elderly or technophobe householders, as it is so easy to understand and and intuitive to set. I fitted an SM2 for my old mum to replace her digital LP722 which she couldn't set. It was easy.

 

[JAD]

Thank you for your information JohnD, I didn't realise there was a switch on the back. I haven't had a chance to take the programmer off to check the setting but I see from looking on-line that there is such a switch on my model (if its identical to the SM2). If its set to 'gravity' (as seems likely) perhaps I do have a gravity fed HW cylinder after all! How can I check? Is it the use of 28mm pipe or is it more complicated than that? If I simply flick the switch on the back to P, and my system is not arranged to suit this, will I be 'courting calamity"?
Thanks again.

 

[JohnD]

28mm is typical of gravity HW. But the real determinant is the route the pipes from the boiler take, Pumped HW means that the flow from the boiler goes through a pump before forking off to (or from) the rads and the cylinder route, There will also be a motorised valve to turn the flow on and off the different routes depending on need, and there must be a thermostat strapped to the side of the cylinder

some photos of pipes around the boiler and pump will show us.

 

[JAD]

To be honest, I'm talking about my Ma's heating and it will be a couple of days before I can go and check for valves etcetera. I'm fairly sure that its a fully pumped system. If this is the case, what sort of motorised valve will allow HW and CH/HW options only? Perhaps it is simply that the 'link' on the back of the controller is pointing to G and by switching it to P will allow fully independent HW and CH? I have read that some systems will favour the HW when the stat on the cyclinder (there is one by the way) calls for heat but I can't say that I have ever noticed that the heating has shut off after somebody has a bath.
Thank you for you time.

 

[Whitespirit66]

perhaps I do have a gravity fed HW cylinder after all! How can I check?

Turn the HW cylinder stat up to max, make sure heating is off, then turn the hot water on , on the programmer.

Does the pump come on? If it does, you have a fully pumped system.

 

[JAD]

MAGIC! I followed your advice Whitespirit66 and discovered that the system is fully pumped! So took the timer off and removed the link from the back to make it suit the pumped system and hey presto the HW and CH seem to operate independently. Interestingly the pipes coming out of the boiler are 28mm and the motorised valve is tucked (almost) out of sight, so I'm guessing that the BG installers either forgot to remove the link from the timer or mistook the system for a gravity feed cylinder. I think when it was originally installed it might well have been gravity fed and then upgraded at a later date. I do know that there are 2 types of 3 port valves so I might find that the CH cuts out when the HW stat is calling. Would that be right?

 

[Whitespirit66]

so I'm guessing that the BG installers either forgot to remove the link from the timer

I mean, never, they would never do such a thing, these BG chappies.

How many wires go into your motorised valve. If its 5, you mave a mid position 3 port (the best), ie. either, HW, heating or mid position = HW and heating.

Fewer than 5 wires = plain Jane 3 port valve, ie. either HW or heating, but not both.

 

[JAD]

Thanks for that last snippet Whitespirit66 and thank you JohnD. It appears we have the mid position 3 port valve and all my tinkerings have worked out as I hoped - independent HW & CH and CH controlled by a fancy room stat.