Circuit for the boiler

My boiler is a relatively modern Worcester non-combi model. It’s a non-pressurised system with a header tank in the roof. The connections to it are straightforward. They are 230 VAC live and neutral supply, live and neutral to the pump (the boiler takes care of when the pump should run) and a control input labelled “LR”. When this is supplied with 230 VAC live, the boiler runs. (As nearly always there is also a safety “earth” connection.)

Also in the mix is a three port valve which can send hot water to the hot water cylinder or the radiators or both. In addition there is a tank stat and relay and a room stat.

Boiler

Pump

3 port valve

Mains rated SPDT relay

The circuit in a way revolves around the three port, or mid position or 5-wire valve as I have heard it called. The colours, orange, grey and so forth refer to the leads from the valve and seem(?) To be standard. The operation of the valve is subtle and is described in detail later. It’s not the way I would have designed a valve but is a lot cheaper than my over engineered version probably would have been. It has a design flaw which could be the reason for the lack of reliability I have experienced. Pumps (now) seem to be very reliable (as long as they are operated daily if only for a minute or two - non-operation over the summer, say, kills them!) My last pump lasted at least 15 years. However, in the same period I must be on my fourth valve. (Maybe I should avoid buying the cheapest valve available?)

The roomstat could be deleted given that the timer has its own sensors but I like to keep it in so that I can return to my old system in emergencies etc. If kept, it needs to be turned right up to avoid interfering with the new system.

The valve and the relay will probably be close together. A junction box is needed to connect up all the wires. I use a twin-gang surface mount box and lid with a screw terminal connector strip inside and a couple of strips of Tufnol secured with nylon nuts and bolts to act as cable clamps.

The relay is a fairly hefty unit, probably quite old, which came with the house. If I was starting from scratch I would probably make a PCB with a PCB relay and connectors etc custom designed for the connections required. However, you’ve got to stop somewhere!

Ours is a fairly large rambling house and a single thermostat will not do a good job of controlling the system. Radiator stats do a reasonable job but what is really needed is motorised valves on radiators or at least on zones. That way rooms which may be out of use on certain days etc. will not be needlessly heated. This gets fairly complex and expensive but is a project I have in mind. (But I have many such mental projects…)

Next: timer software…

How the 3-port valve works