Independant CH controller for Potterton MF RS 70 Gravity system


5 Jun 2018
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United Kingdom
Hey all o/

Here's the situation:
In our rented home, we have an old Potterton MF RS 70 that has gravity hot water, and pumped central heating. It does not have any zone valves, but has a programmer that allows the CH to operate from a room stat when the HW is on. The trouble with this, is that to have the CH running from the thermostat at a steady temperature, we'd have to have the HW on all day, with it heating up to maximum boiler temperature and then cutting out, constantly.

We recently had a new programmer fitted to the front of the Potterton, a BP2000, but I believe that beforehand we had the option to have the CH on without the HW needing to be running constantly (no idea how, but I'm not happy with the way it is now)

I don't want to have to change the system to a C or Y plan. I'd rather not have to fit a cylinder stat.

Here's the question:
Is it possible to wire the BP2000 so that when the CH room stat calls, it then switches on both the HW and the CH?

And if the BP2000 isn't up to the job, would it be possible to fit a programmer that can turn on the CH and the HW simultaneously when the room stat calls for it?

Hope that makes sense, I'm happy to add any more info about the system if it would help. Thanks!
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Is it possible to wire the BP2000 so that when the CH room stat calls, it then switches on both the HW and the CH?
Yes you could, but it wouldn't be very practical. When it's 24 degrees or more in the height of summer. Would you want to have the radiators on, just so that you could have hot water? In very hot weather, you may even (depending on the range of the thermostat) get to the point where the room thermostat doesn't switch on at all, even on its highest setting and then you wouldn't have any hot water at all.

AFAIK as I know, the BP2000 has two channels one for CH and one for HW (it will have these markings on the front under the display if it does) However, the plumbing is the restriction of your system, not the programmer. As you describe it having gravity hot water, that means that the hot water will heat up whenever the boiler is 'on'. The hot water from the boiler will simply circulate around the hot water cylinder using natural circulation. Hot water rises & cold water falls. So you cannot have the heating on without the hot water heating up also. Hence why most are converted to 'C' or 'Y' plans.

If the hot water is too hot, reducing the boiler thermostat will lower this, but obviously the radiators will be cooler during the winter, so will take a bit longer to heat your home.
Thanks for the reply!

Yes the BP2000 has two channels, but the CH can only come on if the HW is on. This is fine for the summer, when the HW can come on for 15 minutes in the morning and the CH is not needed. In the winter it's going to be a nightmare to have the HW running constantly just so the CH can be run off the thermostat.

The thermostat currently just turns on the CH pump. What I'm picturing is a relay from this circuit that also switches on the HW. (I don't mind that the HW will come on with the heating, that's not such an issue) Is there a way to wire this, or a programmer that has this functionality?
The programmer will be presently set / configured for a gravity system so that the CH will also turn on the HW. This is because; as you have figured, at the moment:

The HW channel will turn on the boiler
The CH channel will turn on the pump via the room thermostat (so the HW has to be on at the same time for the boiler to be on)

If you can change the configuration of the programmer to allow separate time control of the HW and CH, and then add a relay as below, it will do what you want.

Drawing1 Model (1).jpg

1) During the winter when the heating is on, the hot water will be also be switched 'on' and 'off' by the room thermostat along with the central heating under time control from the CH channel at the programmer. (HW is set to off)

2) During the summer when the heating is not on, the hot water can be controlled by times set by HW channel at the programmer. (CH is set to off)

It seems a lot of trouble to IMHO not achieve much. In the cold weather when the CH is on most of the day the situation will be pretty much as it is now, in the milder weather you risk running out of hot water. However converting it to a 'C' or 'Y' plan would be far more beneficial and allow total independent control of the hot water and heating.

However, the Potterton must be pretty ancient now and require replacement soon. When that happens modern boilers can't handle gravity hot water systems, so it will all have to change then anyway.
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Ok thanks for that, it looks interesting.

Just to be clear:
The programmer will be presently set / configured for a gravity system so that the CH will also turn on the HW. This is because; as you have figured, at the moment:
That's not how it works presently. The BP2000 will allow for separate timings for CH and HW, but the CH will not come on unless the HW is actively on. So no, the CH will not also turn on the HW, that's the issue. (Unless there's a way to set this programmer differently??) FWIW I've attached the paperwork that came with the BP2000.

But your proposed idea seems fine otherwise, with the CH turning on the HW also in the winter (that's just how the system works, not very efficient, but better than having the HW on 24/7). And in the summer, having the HW work independently would be fine.

If you have any links to where I could get the parts needed to do this it would be much appreciated.


  • BP2000.pdf
    4.8 MB · Views: 283
OK then, it's not been set up correctly for the system you have now. From what you describe, yours is set for the top option from the manual you posted and copied below, which is actually for systems with motorised valves (C,Y, S plan). As it stands now, if you were to turn CH on, only the pump will come on and circulate cold water through the radiators. The boiler won't come on until the HW is set to be on also.

This is wrong. It should have been set for the bottom option, which reduces the programmers functions to suit your basic system and would have turned on the HW automatically when CH was selected. I said before the programmer can cover all possibilities, it is your plumbing that is the restriction, and the programmer when installed should have been set to 'dumbed down' mode to suit it, but it hasn't been.


The good news though, is because yours is presently set for the top option, then it is actually where you want it to be after the relay is added, so you won't have to change it.

Any 230VAC relay / contactor with a volt free normally open contact will do there's an example here. you would need an enclosure to put it in.
Ahh right! Thanks so much for looking into this for me.

Could you humour me and give me more information about how the 'dumbed down' version would work, please? Would that mean that the room thermostat would turn on both the CH and the HW? And in the summer could I still have just the HW on a timer at that point?
1) The modern setup with motorised valves allows completely separate control for the heating and hot water, so they work independently. The programmer allows either to be 'on' with, or without the other. Provided the plumbing has the facility for this.

2) The restricted version is for gravity hot water / pumped central heating as you have now. The HW can work on its own [Boiler comes on, water circulates through the hot water cylinder by gravity] but the CH can't work alone because the HW also has to be on for the boiler to work. When set to the 'restricted mode' whenever the programmer switches the CH on, it automatically switches the HW on too. The thermostat is a separate item wired from the programmer CH and only connected to the pump. It has no control of the boiler. The HW only has boiler control.
Right, again, thanks for the helpful information. What I really need to do is take this info to the landlord who can hopefully get this set up. (If it was my house I'd be happy to upgrade the system with a better solution, but for now I need to find a way to make it work without too much disruption)
If you pay for the gas,your landlord could have an obligation under the MEES regulation to upgrade the property's efficiency.:sleep:

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