There is no stat on my hot water cylinder. Result is almost bolin water coming out the hot taps which is both dangerous and expensive. I would fit one and run the cable but there is no wiring centre, al the wiring from boiler, pump, room stat run into back of programme (potterton Ep200).
There's no evidence of any 2 or 3 port valves so I can't work out ho the system works i.e CH and HW independent or together as required.
Can I still fit one? If so how and is there anything else I shoul check out first
I would guess that you have two virtually independent pairs of pipes connected to your boiler (although they may possibly combine close to the boiler so as to have only two physical connections to it).
One pair will be 28mm pipes connected to the coil in the hot water cylinder, and water in this circuit will circulate by "gravity" (natural convection). The other pair (probably 22mm pipes) will be for the central heating - and will use a pump to pump the water round to the radiators.
In the absense of any control valves, whenever the heating is on water will continue to circulate round the HW circuit and will raise the temperature of the stored hot water to almost that of the boiler output.
The cure for this problem is either to convert the system to a fully pumped setup with two zone valves or a single 3-port valve (S-Plan or Y-Plan), OR to convert it to a C-Plan system. A C-plan requires the least changes - simply requiring a cylinder stat, and a zone valve in the HW circuit. This will have the effect of stopping the hot water getting too hot, and of shutting down the boiler when both HW and CH are up to temperature. For details of all these options, have a look at
Another solution if the boiler is the type that can't support valving on the gravity circuit (only solid fuel or really old boilers might be like this), is to:
(a) Put a cylinder thermostat inline with the HW On output from the programmer. This will limit temperature and improve fuel efficiency when on HW only mode. When the CH is actually on, it will still overheat, though.
(b) To solve that problem, stick a TMV (thermostatic mixing valve) on the output. This will mix the very hot water with cold, to bring it down to a safe temperature (i.e. 55C). You may need to add a pump to get good shower performance.
Obviously, a better solution would be to replace the boiler in this sort of situation. If you have a boiler that can support valving, or, better still, capping off the gravity circuit, then go with Set Square's suggestion.
Wot Mr S Square said. I'm familiar with the Potterton EP2000 programmer and this has enough terminals inside to act as a wiring centre for most standard central heating wiring arangements, or at least with only the addition of a choc block 2 way connector.
Thanks all for advice so far. As suggested both HW and CH are independent and connected to the boile (potterton neteheat 16-22 Mk2 F) with 28mm pipes. The boiler supports valving according to tech help at Potterton so th easiest option till I replace the boiler is to fit a 2 port to the H system. Please excuse the dimwit questions but is this a major job, and cappin off the gravity circuit means what exactly
You might want to stick a pump on there too. You may need a relay or two so that you can run the pumps independently. Alternatively, don't use a 2 port valve at all, just use a pump, an antigravity valve and a relay for boiler interlock.
An alternative is to cap off the gravity circuit and run the hot water cylinder from the pumped central heating side with modern format controls. The gravity circuit is usually then used solely as the vent and water feed, with any actual heating appliance connections capped off. The advantage of doing this is that it simplifies the installation of a new boiler in the future, as the pipework layout and heating controls are to modern standards.
When making modifications to old boilers, you must consider the vent/feed paths. The vent path must be unvalved all the way to the exit. Unless your boiler has a secondary lockout, it must also not be possible to block the feed path to the boiler. The feed and vent may not share pipework, as the escaping gas in the vent will block the ingress of quenching water. These requirements don't apply to boilers that have an overheat cutout, as such boilers don't rely on the quenching effect of new water introduced into the system.
It's not a *major* job, but there are a few things you need to bear in mind. Firstly, there must be a clear - unvalved - path from the boiler to the point where the fill and expansion pipes are connected. This usually means that the 2-port valve needs to go very close to the HW cylinder, after these connections. If the expansion pipe is connected too close to the cylinder to allow this, you will need to re-jig the pipework a bit - and move the expansion pipe connection a bit further away.
The valve must have a set of volt-free contacts which close when the valve is open, and it must be wired up *exactly* as shown in the C-Plan diagram in
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in order to ensure that the boiler only runs when either the HW or CH (or both) are demanding heat, and that the HW circuit is closed off once the HW demand is satisfied.
You will, of course, need to partially drain the system in order to fit the valve - and then re-fill and bleed it - and top up the inhibitor - afterwards.
I'm not quite sure what Christian was referring to by "capping off" the gravity circuit. I think he probably meant that if you converted it to a fully pumped Y-plan ot S-plan system, the CH and HW would largely share one pair of pipes, so you could blank off the other pair.
Thanks again, one final question. Have had a look at the schematic a the above web site, the valve is shown on the return and from your inf I'm assuming it should be on the flow side after the vent connection? Cheers
No, the return side is fine if that's easier and - I think - provided the boiler has an overheat trip. See Christian's post for a bit more detail on this. The absolute rule is that the valve *mustn't* be between the boiler outlet and the vent pipe.
Almost certainly the system is a so-called "gravity" type, where the cylinder is heated by the convection of water from the boiler to cylinder coil. The clues are normally that there are four pipes from the boiler - two 22mm for heating are typical and two 28mm for the cylinder. Basically then, the boiler turns on and off with its internal thermostat and the water in the cylinder will be limited in temperature only by that and the timer. The CH works simply by having the room thermostat operate the pump.
Normally in these systems you can have HW only or CH and HW but not CH without HW.
There are four main solutions:
1) A Honeywell C Plan arrangement. This uses a zone valve on the HW part of the circuit controlled by a cylinder thermostat. You would need to put in a wiring centre of some sort and wire the controls and timer to do the job. From the plumbing perspective it is easy, but you need to take care on siting the valve and not put it between the boiler and vent or feed pipes to the loft header tank.
2) A Honeywell W plan where the system uses a diverter valve and you change operation to fully pumped. This is more plumbing, but the advantage is that the HW will heat much more quickly.
3) A Honeywell Y plan which is a similar technique to W plan but will give CH and HW simultaneously. Whether this is worth it is debatable if you are pumping the system.
4) A Honeywell S plan where there are two zone valves.
Details of these at
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alternative, which is conceptually like C-plan is to use a second pump on the HW circuit, plus either a zone valve or a flap valve which opens when the pump runs. This may be easier to plumb but may require a relay to do the switching because you don't have the auxilliary contacts of a motorised valve if you have pump only.
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