Help identifying heating system wiring

[Carl_]

Hi

Great forum you have here.

I’ve not long moved house and hope someone can identify this type of central heating circuit for me. I’m beaten.

The closest I can match it too is a W plan heating system, but this uses a diverter valve which I do not have.

(Standard chocolate block affair – each terminal above is joined)

The system is a vented cylinder and about 30 years old and is made up of:

Boiler: Potterton CF80
Programmer: Landis & Staefa RWB9
Room Stat: Danfloss RX1 with wireless controller.

There is no diverter valve attached (I’ve looked everywhere for one!) and there is no stat on the cylinder. There are new copper pipes in the airing cupboard where a diverter valve could/should go so this looks like a recent bodge.

The system will be replaced next year when we renovate the house, but before it bankrupts me I’d like to replace the programmer with a Nest v3 and add a stat to the cylinder. The boiler sounds like a hot air balloon taking off and currently cycles every 10 mins or so.

I tried wiring the Nest Heatlink to the HW/CH at the programmer end and closed the room stat circuit, but the best I can get is the hot water to fire and the CH pump to work, but the boiler doesn’t the fire..

With the current setup using the RWB9, I can have hot water ON/OFF, but when the central heating is on (it works well) the hot water is on too.

Anyone advise on where I need to add the cylinder stat to the chocolate blocks? I’m assuming I need another terminal next to number 4 to attach a it to only call for hot water when the stat is closed?

Any advice on what terminals the the Nest should go? Ideally I'd like to be able to control both the HW and CH.

Thanks in advance.

 

[GuessMan]

I'll bet you have pumped central heating & gravity hot water and the programmer is acting only as a time switch.

 

[Carl_]

Thanks GuessMan I had another look at the RWB9's manual and I think you are right. I'll check later, but it's probably set to 10 mode.

Where does this leave me? I assume I could still wire up a cylinder stat?

 

[GuessMan]

In the days of the mini minder programers you could. You still can but you must set the HW to come on with the the CH.

Simply wire the 'HW on' to the 'call' of the cylinder stat and the 'common' to switch live on the boiler. The 'CH on' is wired to the 'satisfied' on the cylinder stat via the 'call' on the room stat. The pump live is also wired to the 'call' on the room stat. the smarter people will have realised that there will be no heating while the hot water is being heated; with really old cylinders this can take ages. In winter it's best to leave HW on all day and to come on 1 hr before heating is wanted in the morning.

This won't work with a system boiler or allow pump overrun from the boiler. But that's how we used to do it.

 

[stem]

You can wire a cylinder thermostat into a gravity hot water system with pumped heating as per the drawing below, but the cylinder thermostat will only work in the summer when the heating isn't on. [Your wiring diagram is a bit odd and it doesn't show the pump BTW]

Untitled

• stem
• 29 Oct 2009
• 1

The main problem with gravity hot water and pumped heating systems is that the hot water circuit controls the boiler, and the heating circuit controls the pump. In the summer when only hot water is required the boiler can be switched off by the cylinder thermostat when the cylinder is hot. In the winter when heating is required, the pump is operated, but the hot water also needs to be 'on' to operate the boiler, which will also continue to heat the hot water and so the thermostatic control of the hot water cylinder is lost.

This type of system is not really a suitable system for fitting a Nest to and the Nest wiring will have to be frigged to get it to work at all, and it will only have limited functions. You would have been better advised to spend the money on upgrading your system to fully pumped with a motorised valve control than buying a Nest thermostat.

 

[GuessMan]

Thankyou for the nice drawing of what I said, but the hot water on gravity HW system is via another tapping and the pump has no real effect on the HW flow. So it works perfectly well on old boilers. A Potterton CF is a very old boiler.

 

[stem]

Thankyou for the nice drawing of what I said.

You are welcome. Glad we are in agreement. I hadn't seen your post when I started my reply. There are only 5 minutes between them and I can't type / draw that fast.

 

[Carl_]

Thanks both for your advice, I'll leave things as they are until we replace the whole system in the new year.

The Nest was from a previous property and is a couple of years old so I've had my moneys worth already! Was handy kicking the heating off before getting home and not heating an empty house.