Three Way Valve, No Dhw Off On My New Controller

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freezergeezer

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Hi,

I've just bought a modern Heatmiser WiFi Heating/Hot Water programmer for my Heating System which I was hoping would be a simple replacement for my Honeywell ST7100 (which controls the Hot Water Only) and a Drayton SC3 Digistat (which controls the Heating).  I've managed to replace the Digistat which had simple L/N/Heating On connections, but am having trouble with the Honeywell which has L/N/DHW ON/DHW OFF connections and they're all wired.

Problem is that I've got no DHW OFF terminal on my new Heatmiser programmer (just a DHW ON).  

I've emailed Heatmiser who have told me this...  "You will need to introduce a relay with a set of changeover contacts. This will provide a 230 volt supply for both power open and power closed for your motorised valve." which seems to make a great deal of sense to me from a theoretical standpoint.

It seems I have a single three-way valve on my system which, I guess, makes my system a Y-Plan system.  I've been reading this forum and other places and have concluded that I really DO need to have a solution for this DHW OFF before I look to get rid of the Honeywell programmer otherwise I guess bad things might happen.

Few questions for the forum, please...

1.  Am I right in that I DO need to wire up the DHW OFF somehow ?

2. What's the practical approach to doing this ?  i.e. where would I get a "relay with set of changeover contacts" ?  is this a Maplin thing, or something made specifically for Heating Systems ?  Is it something that is wise to get a qualified electrician to do, or can a competent DIY'er do it ? (i.e. competent enough to add sockets in a house etc.) 

3. Any advice on wiring ?

Thanks a lot.

 
For a 3 port valve you NEED a DHW OFF signal, because the quirky way 3 port valves work, they need one of the terminals to be energised to move the valve away from HW over to CH.

I had exactly the same problem for a friend recently, and I had to fit a relay to create a HW OFF signal from the only available HW ON signal.

I suggest in your case simply get a different programmer that does have the HW OFF output

 
Thanks ProDave,  Getting another programmer isn't an option - I've already bought the new one.  I need to go down the relay route, I think.  Any advice on where to get the relay, how to wire it up etc from a single HW ON signal ?

Thanks

 
Any reputable electrical stockist should be able to source most types of relays. The key things you will need is the correct operating voltage for the coil & suitable amp ratting of the switch contacts. I assume your new controller outputs a 230V DHW on, so you would need a 230v rated coil. Most central heating systems a happy working off a 3A or 5A fused spur, so providing the switching contacts can operate a 3A load you should be OK. It will all depend upon how the existing system is physically routed and wired as to the best location for this relay, but it will most likely need a separate enclosure to mount it into as most standard CH wiring boxes barely have enough room for the cable terminations let alone adding relays.

It could be argued that this work is notifiable under part P, as central hearting wiring is within the scope of part P. A straight like for like swap of a controller is not notifiable, but some would interpret your proposed modifications as being not direct like for like replacement. You will need to make your own decision on this aspect. Ref info can be found here,  page 9 bullet point n. http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_P_2010.pdf

Doc H.

 
I've attached a drawing of the wiring I did for a similar thing a few weeks back. In that case it was to allow an under floor heating system to control everything, but the controller only had "HW on"

Not all of it will be relevant, but it should give you some ideas.

The relay is switching only a very low current. I used an octal relay which plugs into a base, mainly because that's what I had in a box of bits. It needs a 230V ac coil.

John_1.pdf

 

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Thanks for all your help so far.  More investigating and apologies if this is a stupid question now....  

So, with the relay.... when the HW ON signal goes off at the programmer (and "turns off" the relay) it'll create a new 230V HW OFF signal which will go to the valve etc. in place of the old programmer's HW OFF signal.  That's all making sense to me.  This means that the HW OFF signal will stay energised at the valve until the HW ON signal comes back on at which point it'll turn off the HW OFF signal.  I presume this is what the valve is expecting (i.e. a constant HW OFF signal to keep the valve in the HW OFF position) ?  My worry is that a constant HW OFF signal (while the Hot Water is off) isn't going to damage the valve is it ?

Hope that made sense.

Thanks

 
I presume this is what the valve is expecting (i.e. a constant HW OFF signal to keep the valve in the HW OFF position) ?  My worry is that a constant HW OFF signal (while the Hot Water is off) isn't going to damage the valve is it ?
That's exactly what the valve's expecting, and no, it won't damage it.

 

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