There must be something snagging. It's never been removed before. I had to remove plaster from the groove/slot of the two screws from when the house was built (9 years ago). I also don't really understand how each of the two-way valves operate - whether the timer "job shares" between the two zones and whether two NEST thermostats can replace that functionality. Or if each thermostat demands heat and the timer is on for each zone, both valves just open and they get served with hot water equally. I have got a small problem with my programme, I think, when it's set for CH it comes on but when it's on for HW ( which is what it it is currently set during summer) it

Here's another version, pipes and wires: http://content.honeywell.com/uk/homes/Catalogue/Sundial Plans/3.1 C Plan.pdfFor instance, I *hope* the cabling to each thermostat allows for me to install the NEST power supply (supplies?) in the hot tank cupboard (where the timer currently lives) and power each thermostat (remotely) that way. This would not allow your system to work properly on the CH, so I have to assume you have already 'had a go'.

The HORSTMANN (SECURE) 425 TIARA PROGRAMMER - Now use 4210051 or 4210056 is a genuine, brand new Horstmann Controls product, covered by a Horstmann Controls manufacturer's guarantee. If this isn't sensible for somebody with little experience like me, what should I do - cut my losses and buy a real H27XL? Or is there a cheaper alternative? The heating controller was held in place by nothing more than the electrical blade connectors. But it need that leverage - very surprising. Obviously this isn't ideal, you get a real mess if you try and use the timers, as each pin does something it isn't supposed to. As for your easy to understand explanation about my system, that was great, I'm a lot happier now I understand what is actually happening in my home.Have you also tried using central heating controllers that does not have a worthy display or are difficult to adjust? Hmm..., sounds a bit strange for a primatic system. I always thought that pumped primary circuit (flow to hot water cylinder) was not recommended. What I really want to do is to install a NEST in place of the two thermostats, and a single timer for the water but I don't yet understand what wiring my system employs and how I might convert that for NEST.