Replacing central heating controller

MarkSmith · 30 Mar 2017

Hi,

I'm renovating a few rooms, one of which contains a very old central heating controller:

I'd like to replace this. The wiring looks fairly normal, with one output for "heating on" and another "hot water on".

Can I put a new controller on in place of this one? My concern is that this one only allows "water" or "water and heating" -- not independent control of water and heating as most do, even though the wiring looks like it treats them separately. Can all systems support independent control of water and heating, or do I need to check? If I need to check, what do I need to check please?

Normally I'd just try it and find out -- but I can't because there's no power, and I need to make the decision before getting the plastering done, only after which the power can be reconnected.

Thanks!
-Mark

Mottie · 30 Mar 2017

Ok. Ah, you've posted more now!

I had one similar to what you had and that had a 3 port valve like the lower picture in the post below. I recently had it changed to a digital one when I had my system changed to an unvented hot water cylinder. The 3 port one was changed to two x two port ones like the top picture in the post below but I don't know whether that was to suit the cylinder or the new programmer.

45yearsagasman · 30 Mar 2017

You need to check near your cylinder for similar to these or better still post pictures of your systemView media item 44939 View media item 38425

MarkSmith · 30 Mar 2017

Thanks. There are neither of those near the cylinder (in a bedroom, immediately above the boiler), nor visible in the living room where the back-boiler is, nor in the cellar below the boiler where the central heating pump is.

If it helps at all, the cable going to the boiler has four cores: live, neutral, water and heating. A second cable (neutral and heating) goes to the central heating pump.

Any idea where the valve might be located on a back-boiler system? (Sorry, probably should have mentioned that it was a back-boiler system.) The only place I can imagine it could be, if there is one, is behind the fire with the boiler. Does that seem likely?

ianmcd · 30 Mar 2017

you more than likely have a gravity hot water system so no matter what you do with the controls, you will not get CH without the HW being on unless you completely alter the pipework, a simple pic of your HW cylinder will confirm

DP · 31 Mar 2017

Why not leave the programmer as is and fit a programmable room thermostat, wireless option if cabling back to programmer an issue

Thus you will have tighter control of central heating without relying on the existing programmer to start things rolling. You will with suggestion option be able to fire the heating ( though HW will also heat during that time) for frost protection, background heat as well as for comfort level.

MarkSmith · 31 Mar 2017

Thanks all - I'll get more photos.

MarkSmith · 2 Apr 2017

More pictures as requested:

I think I may have answered my own original question, though: I had a closer look at the wiring, and only one "signal" goes to the boiler: hot water. The other control, central heating, goes only to the pump. So my guess is that there's no valve at all, and it selects between water and water+heating simply by running the pump or not. Does that sound like a reasonable understanding?

If so, two questions:

1. Can I get a more modern controller which supports this same mode of operation?

2. Is this (the heating being invoked only by running the pump, no valve) a good, normal, sensible way of doing things? The radiator in the top bedroom gets reasonably warm when the water is on and central heating off -- could this be a side effect of this arrangement?

Thanks!
-Mark

Mottie · 2 Apr 2017

That setup is what I originally had. I converted it to a fully pumped system. It involved a bit of pipework but then my tank was almost next to the boiler. Yours are far apart so I'd imagine a fair bit of new plumbing would be needed.

stem · 3 Apr 2017

MarkSmith said:
Only one "signal" goes to the boiler: hot water. The other control, central heating, goes only to the pump. So my guess is that there's no valve at all, and it selects between water and water+heating simply by running the pump or not. Does that sound like a reasonable understanding?

Yes, that's how the old systems used to work. When you select 'hot water' it turns on the boiler, and the hot water cylinder is heated by natural circulation. When you select 'central heating' the pump will operate and circulate water around the radiators, however, the 'hot water' must also remain 'on' to keep the boiler running.

MarkSmith said:
Can I get a more modern controller which supports this same mode of operation?

Yes, but you must choose a programmer which is specified as suitable for 'gravity hot water systems'. Many still are suitable, they have the facility to be selected for this mode of operation so that when central heating is selected, the hot water is automatically turned on also.

MarkSmith said:
Is this (the heating being invoked only by running the pump, no valve) a good, normal, sensible way of doing things?

No it's an old way of doing things, wastes energy and doesn't provide thermostatic control of the hot water. Systems like this haven't been installed for 30 / 40 years. Modern gas boilers cannot support this method of operation anymore, so when you replace your boiler, the piping arrangement will have to be changed.

MarkSmith said:
The radiator in the top bedroom gets reasonably warm when the water is on and central heating off -- could this be a side effect of this arrangement?

Quite likely. If the system was originally fired by solid fuel, it was customary to have one radiator connected to the hot water system as a 'heat leak' to effectively waste heat, to prevent the boiler actually erm...boiling. Usually, but not always (especially if the bathroom wasn't near the hot water cylinder) it would be the radiator in the bathroom.

Mottie · 3 Apr 2017

Yep. When I had that type of setup, the bathroom rad would get very hot, even in the summer!

MarkSmith · 3 Apr 2017

Thanks Stem, very thorough.

The whole house has plenty of "legacy" considerations of all kinds. Although I doubt the boiler is 40 years old, I can well imagine the rest of the system hasn't been properly reworked in that long. I'll bear in mind that I've got a significant job ahead of me when the boiler finally packs up, not least because replacing the plumbing will probably require taking up at least two laminate and one tiled floors.

stem · 3 Apr 2017

You may not have to do as much as you think. I converted my system from what you have to a fully pumped system and only had to lift two floorboards on the landing; and that was for the new wiring.

Your radiator pipework could well be OK as it is, most of the work is usually around the boiler and airing cupboard.

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