Boiler wiring question

[ccrowe]

A customer has had a new electric central heating boiler installed and wants me to wire it up. The customer has already got the time programmer but the plumber has told the customer that I'll need to use a relay as well.

For those of you who regularly do CH wiring, is a relay a plumbers term for a bit of electrical kit in a plumbing system, or does he actually mean a contactor to allow the programmer to switch the high loads of the boiler?

If the latter, it seems odd to me that this boiler, Dimplex Ascari, which is supposed to be a direct replacement for standard gas boilers needs such a piece of kit.

Thanks in advance.

 

[ericmark]

In the past I have struggled to work out a method to wire up a system without a relay. Then I realised if I used correct motorised valve the optional built in switches did the work of a relay.

So the demand for room heat opens the motorised valve which in turn turns on boiler.

As far as I can see other than having a 63A supply it is very like a gas boiler the Instructions are on line and no relays are shown.

Not fitted one mainly gas in this area.

 

[bernardgreen]

Dimplex Ascari, is an electric boiler. So NOT a direct replacement for a gas boiler.

It is rated at 4 to 12 kilowatts.

So a contactor capable of switching a 12 Kw load ( 50 amps ) is definated required. The timer/controller can switch 1amp maximum for reliable operation and long life time.

Is there a suitable electrical supply installed. Otherwise it will be necessary to design and install the correct electrical supply for it.


Update... the boiler has 4 internal 20 amp contactors ( one per element ) so an external one is not needed for normal control of the boiler.

 

[ElectricDunc]

In my experience with these electric boilers the only time you need a contactor or relay is if there is an electric shower in the property as well. The idea is you use the same supply for both shower and boiler to ensure you can't use them at the same time and take out the supply authorities main fuse. The boiler will need it's heavy mains supply to run the elements then the controls are usually the usual low current connections or sometimes even volt free as you'd find in any gas or oil fired boiler.

 

[ccrowe]

Thanks for the replies.

So NOT a direct replacement for a gas boiler.

I should have chosen my words more carefully. It can be used "in place of a gas boiler where gas isn't present". All other external controls programmer, room stat etc, "should" work as with a gas boiler. However, this mention of a relay has thrown a spanner in the works. The plumber has gone awol so I can't ask him what he meant.

This install is to be used for central heating only. There is no need for an external pump as the internal boiler pump will do the job AIUI.

From ericmark's link, page 24 onwards show the wiring diagrams. Page 24 shows operation from a switched live. I would have thought this is the most appropriate as between them, the programmer and room stat will provide a switched live to the boiler. However, it also shows an "External command relay" across terminals 1 and 2.

Page 25 shows a directly attached room stat (which this one won't be) and a cylinder stat providing a switched live. It also shows an "external command relay" across terminals 5 and 6.

I think the switched live configuration, as shown on pages 24 and 26 are the correct option but I'm confused by what this "external command relay" is for, what it is, where do I get one, and what spec it should be.

I haven't done CH systems before so am not prepared to continue without understanding what is required up front. I guess a call to Dimplex tech support will help.

 

[ElectricDunc]

Okay, reading the instructions i just want to clarify a few things. If the boiler is for central heating only then i assume there are no motorized valves in the system (is this correct?). Secondly, what make of room stat have you got and do you have a separate programmer? Does the programmer need a mains voltage supply or does it just have a battery in it to operate it. Soon as i can confirm this i'm pretty certain i'll know what you need to do.

 

[ccrowe]

i assume there are no motorized valves

Correct. The room stat is a Drayton Digistat+RF (RF601) and the programmer is a Drayton LP241.

I've been reading the manual to the room stat and it occasionally refers to itself as a relay. I'm thinking that what I know as a "room stat" is actually the "relay" refered to by the plumber. The stat provides volt-free connection when calling for heat. I think this goes to terminals 1 and 2. The switched live will come from the mains powered programmer when timed to do so.

 

[ElectricDunc]

Okay, you'll need to find some way of getting power to the programmer and RF unit for the room stat. Preferably you want to take it from a supply within the boiler but if it hasn't got a low current mains voltage output you'll have to install a fuse spur from another source. This will only be for the power to the programmer and RF unit. You then need to take a cable from terminal 1 of the boiler (remember this is a voltfree contact so must never have 230V fed on to it) to the Common terminal of the programmer. You will then need to connect the CH ON terminal on the programmer to 1 on the RF unit. You then need to connect 2 on the RF unit to 2 on the boiler. I've done i diagram below which will hopefully help. You can do all volt free connections in flex but make 100% certain that you do not put 230V down 1 or 2 on the boiler as you'll blow control fuses and worst case damage the electronics.

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• ElectricDunc
• 21 Apr 2009
• 2

With the control wiring sorted you just need your high current supply 16mm T+E by the looks of it from your consumer unit. I recon a 63A rotary isolator would be your best bet for terminating a cable that size then another piece of cable in to the boiler. That should be all you need to do for a central heating only setup.

 

[ccrowe]

Thanks for all your help with this. I've been doing a bit more digging and to me it looks like it should be wired thus...

Untitled

• ccrowe
• 22 Apr 2009
• 1

Any thoughts would be most welcome.

 

[slup]

That's not tight. The RF receiver needs a permanent unswitched supply.

The first drawing was correct.

Steve

 

[ccrowe]

But the boiler also needs a switched live. How about this?

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• ccrowe
• 22 Apr 2009
• 1

I'm not disagreeing with you, but as I said earlier, I haven't done CH wiring before so I'm just trying to get a full understanding. If I'm wrong then please tell me, I'm just trying to get it right.

 

[slup]

It doesn't. The swiched live is optional.

You just connect your thermostat / time switch combo across pins 1+2 on the boiler.

Steve

 

[ccrowe]

Wonderfull. Thanks all for your invaluable help.

 

[ElectricDunc]

As Slup said my first drawing was correct the switched live is optional, the instructions state that it's only used when the boiler is heating DHW as well as CH. Your only using it for CH so terminals 1 and 2 are the only ones that need to be used. All things considered it would be far simpler having a wireless programmable roomstat but if what you've got is all you've got just follow my diagram and it'll work. Just remember (and i can't say this enough times) do not put 230V down pins 1 and 2 of the boiler otherwise you will have problems.

If it makes it any clearer there is no physical connection between live and common on either the RF unit or the programmer. The only thing that is switching within these units is a relay which changes between common and on and common and off.

 

[gas4you]

Can't see any need for a relay in the MI's .

I would assume the plumber is thinking of a relay to connect the switched live from the heating controls to volt free as indicated in the MI's for a volt free room stat, but I cannot see how this is needed at all on this boiler

Any external heating controls will still only be rated at 3 amp.

There seems to be a terminal to connect a 240v switched live already provided, no 25 I think.

You will have to change the link wire and tell the boiler what it is connected to according to the instructions.

As said above, wire the controls as a normal heating system, noting the terminals supplied for connection.