Separating CH and HW

[Dan Smithers]

We have gas central heating with a Potterton EP2001 controller.

Currently the controls are linked so that CH requires the HW to be on. I would like to separate them as the hot water gets very hot since we had a new insulated tank installed (approx 75 C) and I want to fit a thermostat to the tank.

The controller can be set up so that the two are independent, but when I have tried this the boiler pump doesn't come on.

I expect that I will need to change some of the plumbing too and fit motorised valves.

How do I go about this?

thanks

dan

[Roger Mills]

I suspect that you have a gravity HW and pumped CH system. If so, the room stat will only control the pump, and the programmer has to force the boiler to fire when CH is required - with the result that the HW gets hot whether you want it or not.

There are *two* basic solutions - convert to a fully pumped system, or convert to a C-Plan[1] system by fitting a cylinder stat and a motorised valve in the HW circuit. The second is far simpler - but you have to be careful where you fit the valve so that there is always an open connection from boiler to vent pipe even when the valve is closed.

Going fully pumped is a better solution in many ways, but will require more extensive changes to the pipework. You have the option of an S-Plan[1] system with two 2-port valves or a Y-Plan[1] with one 3-port mid-position valve.

[1] See for details of the various control plans.

[Dan Smithers]

The pump comes on for HW alone, and I think that it is unlikely to be a gravity fed system as the tank is a long off being above the boiler.

None of this was clear from the original post, but I was thinking of a tank stat - a room stat would probably be a good idea too.

[Roger Mills]

So what controls *have* you got? What stops the radiators getting hot in summer when you only want HW?

[Dan Smithers]

The controller has a timer in it and two sets of controls: HW off- timed - once - continuous CH off- timed - once - continuous

In the current mode the two are linked so the CH control must be no higher than the HW. Currently CH is off and HW is timed. In the winter both are set to timed.

In the winter I would like to be able to stop heating the HW when the water in the tank is hot enough while still allowing the CH to be on.

There is an alternative mode that controls the two independently but when I tried it the boiler fires but the pump doesn't run when CH only is on.

I hope this explains a bit better.

thanks

dan

[Roger Mills]

What I'm really trying to get at is what does the programmer actually control? For example, are there any motorised valves? Is there more than one pump? How many water pipes are connected to the boiler?

If both HW and CH are pumped, and if there is only one pump which runs whenever either HW or CH is selected, and if there are no valves to allow water to go to (say) the HW cylinder whilst preventing it going to the radiators, you will get *both* HW *and* CH whatever the programmer is set to.

If this is not the case, you must have some components that you haven't yet told us about!

Starting at the boiler, can you draw a circuit diagram of the pipework - and scan it and upload it somewhere with a link here - so that we can get a better idea of the layout of your system.

[I still have a sneaking suspicion that you have a gravity HW system despite your earlier comments. If this is the case, there will be 4 pipes (or maybe only 3 if there's a common return) connected to the boiler - 2 big ones going to the HW cylinder without a pump in them, and 2 smaller ones going to the radiators - with a pump somewhere in the circuit.]

[Tim Downie]

Although its hard to be sure, it sounds very much like you got a basic gravity HW/pumped CH system. Does the pump run when the hot water is on without the CH? If not, the HW system must be gravity.

As others have said, a bit of replumbing and conversion to fully pumped HW and CH would be desirable, not just to stop your hot water getting too hot, but because it also greatly reduces the time needed to reheat a cylinder of HW.

Personally, I prefer systems with a separate motorised valve for each circuit as fault diagnosis & repairs are a bit easier (IMO).

Tim

[ARWadworth]

Like a hidden 2 port valve.

I have seen the following setup many times (a set up that could be like the OPs system)

The HW and CH are fully pumped and where the pipework splits there is a 2 port valve on the CH side and no valves on the HW side. The programmer is set for a gravity system with the HW output from the prog calling for the pump and boiler and the CH output opening the 2 port valve to allow the CH to get hot.

Adam

[Ed Sirett]

Read, faqs and wiki.

[Roger Mills]

That's certainly a possibility!