Cylinder Thermostat

[APE]

Hi, I hope someone can help. I have just fitted a Salamander shower pump and the instructions recommend having a Cylinder stat fitted (also in interests of saving energy/money). I am an electrician by trade so wiring it is not an issue but does anyone have a wiring diagram of where it should fit into the circuit?
My existing system comprises of a Boiler, pump, zone isolation valve (I think thats what its called), room stat & a time clock (for separate CH & HW control).

I hope I have given enough info for someone to help, any help will be much appreciated.

Cheers

Andy

 

[corgiman]

do you have a part gravity system?

 

[Space cat]

It would help if you told us what that zone valve was doing. Is it in the supply to, or return from, the cylinder heating coil? This is a common place to put one. It's purpose is to stop the boiler water from going through that heating coil when the tank water is hot enough. If you don't do this your tank water will get as hot as the boiler water, which is at least 20° too hot! In your case there is no cylinder stat (yet) so there is no way of knowing when the tank water is hot enough. You might still have a zone valve in there controlled by the programmer (time clock).

A typical cylinder stat is a changeover switch. A logical way to use this is to turn a demand for HW from the programmer into its opposite, ie a demand for No-HW. This assumes that your programmer also contains a changeover switch because No-HW is a real wire with volts on it, not just an absence of HW demand.

The wiring is straightforward:

1) Find the HW demand output on your programmer, disconnect it from wherever it goes and connect it instead to the COM terminal on the new stat.

2) Connect the closed-when-cold terminal of the stat to the point from which you removed the HW demand.

3) Find the No-HW demand output on your programmer. If it goes somewhere, connect the closed-when-hot terminal of the stat to the same place.

Your stat will now work in conjunction with the programmer such that a demand for HW will become a demand for No-HW when the water is hot enough. Whether this achieves the desired result or not will depend upon what that valve does.

PS: The demand outputs from programmers are usually mains voltage. If yours are low voltage you'll need a stat with gold plated contacts - but you knew that already!

 

[APE]

Corgiman:
Hi thanks for replying. i have a full gravity system i.e. two tanks in the loft.

Spacecat:
Thanks for your info, but unfortunately I don't think it is wired as you assumed. The zone isolation valve appears to isolate the rads. i.e. I have just had a good look and the boiler & pump only come on when the HW is on, (on the time clock (BTW its a digital time clock if that makes a difference)), then when you turn the CH on the ZI valve opens (I proved this by turning off the HW and just turning on the CH, you can here the ZI valve open but no pump or boiler).
To try and make sense of this, basically the CH will only work when the HW is on, therefore if I put the cylinder stat in line with the HW supply, when the water is hot enough the CH will go off too.
So back to my original question, any ideas how to wire a cylinder stat?????

(Heres a rough sketch I scanned in)

 

[gas4you]

You're going to have to add another zone valve for the dhw that the cylinder stat will control but this needs to be fitted after the T to the indirect coil inthe flow, or before the T in the cold feed, in the return from the coil.

Must not be fitted in the main f & R gravity primaries.

 

[Space cat]

To try and make sense of this, basically the CH will only work when the HW is on, therefore if I put the cylinder stat in line with the HW supply, when the water is hot enough the CH will go off too.

Sadly, your diagnosis is spot-on. A cylinder stat serves no useful purpose unless you alter the plumbing. Are you up for it?

First off, you don't have a full gravity system; you have a fully pumped system. It's also open vented rather than sealed and that's where the small tank in the loft comes in. A gravity system has no pump and the flow is driven entirely by convection. (Hot water is less dense than cold so it rises.) Old CH systems - the ones with massive cast iron radiators and pipes to match - worked this way. Modern slim-line radiators and small bore pipes need a pump.

To get independent control of CH and HW in a fully pumped system you need two zone valves, often combined into a single, three-port valve. Since you have one zone valve already, the sensible choice is to add a second one but, as gas4you says, you have to be careful where you put it. The location of two pipes you haven't put on the drawing is critical. You have a feed/expansion pipe coming down from the bottom of your small tank and a vent pipe looped over the top of it. On a fully pumped system these must be attached very close together and NEVER on opposite sides of a valve.

WARNING! The vent pipe MUST be open all the way UPWARDS with NO VALVES from the top of the boiler's heat exchanger to the open end.

Are you still up for it? The wiring will depend upon what kind of valves you use. Zone valves may be driven both ways or spring loaded shut. Your pump and boiler won't be switched directly from the programmer either but by switches on the zone valves. I'll be watching this space.

 

[APE]

I'll be back shortly with a more comprehensive drawing, and you can hopefully tell me where to put the ZI valve. I now think I understand the wiring aspect but a wiring diagram/schematic would always come in useful if somebody has one.

 

[APE]

Here is the drawing I promised. Can anyone show me where i might install a ZI valve for the HW.

(I'm not 100% sure on the exact connection point of the loft tank supply, i.e. if it connected to the flow or return side. I is under the floor and not easily accesable, although i will have a look if it's crucial.

Cheers

Andy

 

[Space cat]

I had to look at that drawing twice to see what was going on. You have two vents. I'm thinking it might have been modified a few times in the past! No matter, the good news is that your primary vent and your feed are connected very close together, so close that they become the same pipe! That's a foolproof arrangement. There's even a little dip in the feed pipe to keep air and steam out. That's a neat touch!

The one thing that looks wrong is that your cylinder coil is in backwards. It doesn't make a huge difference on a pumped system but the heat transfer would be more efficient if the boiler water came in at the top. You could correct this by crossing over the two pipes leading from the kitchen to the airing cupboard. This would also raise the pressure in that secondary vent and thus reduce the risk of sucking air down it.

Since the existing zone valve is in the return pipe from the radiators, I would put another one in the return from the cylinder coil. That's in the pipe just to the right of the pump as you've drawn it. This will keep everything close together.

Got to go now. Watch this space.

 

[APE]

Got to go now. Watch this space.

I'm still watching but nout's happening!!!!

 

[Balenza]

Im surprised your CH works at all if that diagram is correct as it seems to show open circuit to the HW which I would have thought will short circuit your CH flow.

 

[Space cat]

Im surprised your CH works at all if that diagram is correct as it seems to show open circuit to the HW which I would have thought will short circuit your CH flow.

A lot depends upon the flow resistance of the HW coil and its pipework. It will certainly work once the HW zone valve is in.

APE, I take it that you are prepared to tangle with the plumbing. From your first drawing, it would appear that you have a spring return zone valve in there already. That's fine but you need to find something out. In the new system with two valves, power for the boiler and pump can no longer come directly from the time clock. It must come from switches inside the valves which close when the valves are fully open. Does your existing valve have such a switch?

 

[APE]

It does appear to have a switch, but I will meter it out just to check it operates correctly.

 

[mikely]

Your circuit diagram suggests a very strange way of wiring the system. It must mean that your pump is running continuously whether heat is required in CH or HW or not. As already stated, there is no way of adding a cylinder stat without altering the plumbing. The standard wiring for S plan and Y plan systems are given here: http://content.honeywell.com/uk/homes/systems.htm

 

[Space cat]

It must mean that your pump is running continuously whether heat is required in CH or HW or not.

I agree. It's a very simple system in which the pump and boiler are on whenever the programmer (time clock) demands HW. The cylinder water will come up to boiler temperature and the boiler will go on and off on its internal stat but the pump will keep running.

Fitting the new cylinder stat and zone valve will make the whole system more efficient. The pump and boiler will run only when they are needed.