Changing a Central Heating Programmer - How difficult?

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Hi

Our central heating system is currently controlled with a Honeywell ST699, which appears to not allow us to control the time of the heating & Hot Water separately, or have a boost function.

We've thought about changing it for something like the Siemens RWB29, which seems to have some reasonable reviews and would allow us to do these things.

Just wondered, is this likely to be a nightmare job and only one for an expert?.... I consider myself to be a reasonably competent DIYe'r and have in the past done some basic wiring etc but am not that familar with how the wiring of central heating works

Thanks

Brittas
 
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Changing a programmer is reasonably straightforward, provided you note which wires connect to what before removing anything.

That Honeywell programmer can do separate switching for heating and water - however if yours only does hot water / hot water & heating, then this suggests your heating system can't do heating separately.
What type of heating system have you got?
 
Thanks for the quick response Flameport.

I guess it's what you'd call a standard gravity fed system. The water is heated in a cylinder and we have a header tank in the loft. The boiler is a potterton condensing boiler.

I'll have another look at the programmer but I thought you could only have two programmes per day with a choice of on,cont, one or twice for both H/W & Heating, but I might be mistaken.

Thanks for your help

Brittas
 
You are quite correct - the ST799 programmer does not allow the CH and DHW to be run at individual times. I changed mine for a Honeywell ST9400C last year for exactly this reason - the ST9400C allows individual time control of CH and DHW; and also allows up to 3 different switch-on an switch-off times each day. A much more flexible unit, and not difficult to change over.

There is a very useful document which shows exactly which wire goes where when swapping from the ST699 or ST799 to the ST9400c on the Honeywell website - here's a link: http://www.honeywelluk.com/downloads/Useful-Documents/

Download the 'Wiring Guide Issue 16' and scroll about half way down the PDF file.
 
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the ST799 programmer does not allow the CH and DHW to be run at individual times
Yes it does - but this depends on what system is it connected to.
They are supplied with a label which states 'heating and hot water' which is fixed over the 'heating' logo on gravity hot water systems.
http://www.honeywelluk.com/Documents/Reference-Guide/pdf/899.pdf page 2

If brittas2009 has a gravity hot water system, then changing the programmer will achieve nothing, since the hot water will always be heated when the boiler is on.
 
The term Gravity Hot Water is probably being misused here as the system has a Potterton condensing boiler fitted. How can it be GHW?
GHW refers to the way the water in the cylinder is heated. This has caused much confusion in the past. Posters do not understand the industry terminology.
 
Flameport - I think we're at cross purposes here. Firstly, sorry about my typo - the OP has a ST699 and I mistakenly typed 'ST799'.

I agree that the ST799 is a 7-day programmer - as per your PDF link - but it's still not possible AFAIK to program the CH and DHW to operate at separate times - ie. quite independantly of each other. This is something that the ST0400C does achieve, as well as offering 3 separate on-off periods each day.
 
All of you, firstly thank you very much for your help.

As you point out, I think I've probably confused the situation by not fully understanding what I'm talking about. :oops: I thought If your system was fed by a header tank in the loft, it was gravity fed but I feel now that may not be the case?

Brittas
 

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