How to wire an interlock?

[nedflanders]

The room thermostat and the hot water thermostat should be wired up to the boiler in what is known as an "interlock". This means that if both the house and hot water are at temperature, the boiler will be switched off. If this does not happen, when the water temperature inside the boiler itself falls, an internal thermostat will cause the boiler to fire to heat up this water - a process known as "dry cycling". All the energy used in this cycle is wasted as it is not used for any useful purpose

(This quote from the National Energy Foundation) http://www.nef.org.uk/energyadvice/heatingcontrols.htm

Hi. Is there an electrician out there who can tell me how to wire this 'interlock' as I have this 'dry-cycling' problem, which is very irritating as well as being a waste of gas. I have a room stat and a tank stat, (not currently wired in) and a Drayton Tempus 7 Electronic Programmer. My heating system is pumped/gravity hot water.
I shall be eternally grateful.

 

[Lectrician]

All wiring methods should ensure this doesn't happen.

If the timeclock and room stat are satisfied, then the valve for the cent heat shuts, and removes the trigger voltage from the boiler via the valves built in micro switch.

If the timeclock and cylinder stat are satisfied, then the valve for the hot water shuts, and removes the trigger voltage from the boiler via the valves built in micro switch.

So, as long as both valves are shut, the boiler can't fire.

If it is a mid position valve, the same applies in theory - they are wired a little differently though (requiring an OFF command from hot water and cylinder stat to ensure the valve pulls across when heating is still calling, as the valve 'relaxes' to HW open). The valve still supplies the 'call' command to the boiler.

Sounds like your system is incorrectly wired.

What do you have?

Cylinder Stat?
Room Stat?
Two valves with two pipes each, or one valve with three pipes? Or maybe no valves at all?

 

[nedflanders]

Hi, Lectrician, thanks for your reply.
It is all a bit basic. When we moved in there was no room stat; room and water temp were controlled purely by the boiler stat. I fitted a room stat. The water is gravity fed, and there are no valves in the system.
With the boiler stat fairly high (for the hot water) it all works ok except for the dry cycling.
I also fitted a tank stat, but when I wired it in nothing worked at all, so I obviously got that wrong! At the moment it's disconnected.
The HW side is not connected at all, and we use the immersion heater when the heating is not reqd.
I'm sure you're right about it being incorrectly wired; if you can help, that would be great.

 

[Lectrician]

If the water is gravity fed, you rely on the boiler stat to control the water temp, and it wii cycle.

The cent heating pump can be controled via a room stat, and the hot water must be on for the heating to work.

 

[IJWS15]

I don't see any reason why the boiler stat should not be added - just means that the boiler will cycle if the stat is set higher than the boiler temperature or set too low on the tank. Should be possible to use the stat to stop the boiler cycling.

Note- both the room stat and the boiler stat would provide feeds onto the boiler demand connection so they will also backfeed the outlet connection of the other (i.e. if the room stat is not demanding heat you cannot assume the output is dead and similarly with the boiler stat.

NB I am an amateur but have wired a few.

 

[mikely]

Here is a circuit diagram for a boiler interlock for a gravity system using tank and room thermostats. http://www.gasman.fsbusiness.co.uk/images/grvhw.gif

It isn't very clear but shows the principle. The left hand contact on the Cyl stat is closed when the tank is cold and switches to the RH contact when hot. You can also use separate outputs from your controller so that HW can be controlled separately. The CH output would go to the Room stat as shown and the HW output to the Cyl Stat.

Note that the Room stat will probably also require a Neutral connection for the accelerator heater - see the instructions that came with your stat.

 

[nedflanders]

Hi, Mikely. That is excellent. Thankyou. I'll see if I can work that out in terms of wires and terminals!!!!
Just one thing: what do you mean by 'accelerator heater?' The room stat is a Drayton RTS 1. It has Live and Neutral going in, and a Switched live output coming out, (terminal 3). [that's the one that goes to the pump and right side of cyl stat?]

 

[ebee]

http://content.honeywell.com/uk/homes/systems.htm

Hope this helps

 

[mikely]

Just one thing: what do you mean by 'accelerator heater?' The room stat is a Drayton RTS 1. It has Live and Neutral going in, and a Switched live output coming out, (terminal 3). [that's the one that goes to the pump and right side of cyl stat?]

The accelerator heater reduces the temperature drop needed to switch the thermostat (the hysteresis of the control system). So long as you connect the Neutral as shown, then that's fine. Here is a wiring diagram for the system - I hope it's readable:
http://server6.theimagehosting.com/image.php?img=ch_control.gif

Yes, terminal 3 is the one going to the pump and RHS Cyl Stat.

 

[nedflanders]

Hi, and thanks very much to both of you. I'll have a go at it in the next day or two and get back to you. Thanks again.

 

[nedflanders]

Hi Mikely,
I had a look at this, and found that the tank stat wires have been cut short, so as a temporary measure I did this:
Using the wiring diagram you supplied me: Controller 4/CH to roomstat, then Boiler and Pump feeds connected together to the return from the roomstat. (On the diagram, No 8, direct to the Boiler instead of the link through the tankstat.)
That has stopped the drycycling, but is there anything wrong with doing this that I'm unaware of?
(Previously, the Boiler feed was connected to Controller 4/CH and the pump to the return from the roomstat, which I can now see allowed the Boiler switch in and out.)

 

[mikely]

The only problem is that the system will only heat hot water when the CH is demanding heat. As a temporary measure, this may be OK in the winter when the CH is running a lot, but you can't now separately heat HW.

 

[nedflanders]

That in itself isn't a problem as we use the immersion heater for hot water during the warm weather. But I can now wire in the HW side using the diagram you supplied.
I can't thank you enough for your help; it was luxury last evening not to have that b****y boiler firing up all the time!

 

[nedflanders]

Hello, Mikely, are you still there?
I bought a Drayton HTS3 tankstat, and fitted it, but what happens is this;
it works ok until the tank stat switches out, (water hot), then the boiler won't fire up again. The pump runs, but turning the room stat up and down makes no difference to anything. I am positive I have it wired correctly; I even phoned Drayton's helpline to check. (They are brilliant, by the way, really friendly and helpful).
If I turn the tank stat up a bit, it works again, as though there is no connection to the boiler from the 'satisfied' side of the tankstat? But it is connected as shown in the diagram you sent me.
The programmer has a jumper for 'gravity' or 'pumped' and both give the same result.
Is it me?

 

[Lectrician]

All I can say is double check your connections. The diagram would work if you have followed it to the letter.

For the pump to run all the time, and the boiler not to fire when the cyl stat is satisfied and the room stat calling, you must have wired it incorrectly.

Check the cyl stat first. Double check the connections.