I am wondering if the stand-off comes away from the main unit. I can pull the main unit away from the stand-off enough to get a screwdriver into the gap but it's not coming away consistently - only where I exert pressure so much that the plastic deforms. 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. I haven't touched the wiring at all - in fact, while waiting for a reply I wanted to switch the water on and figured I could work out for myself how to turn it on using the controls on the H37XL unit in place - by switching "Zone 1" Heating on the unit to "ON", it switched the water on as I had hoped - you can see this quite clearly if you compare the wiring diagrams, pin 1.

A piece of plastic which may act as some sort of stand-off or interface - it's the piece that has the 2 screws in the top (not any more though). I suspect this is integral to the main unit - I only mention it because it is a separate mouldingI can get a screwdriver between the wall box and the stand-off but there's no way they are going to separate without my breaking one of them. Not what you're looking for? Try our unique Spares Finder tool or you can view all Horstmann Controls products. By pulling outward and generally downward I can clearly see the single wall box because I am dragging that towards me too. It is not separating from the main unit or the stand-off. The Horstmann 425 Tiara Mechanical Programmer offer a easy to use and simple way of controlling hot water and central heating systems, it uses a 24-hour rotating dial with 2 on/off periods indicated by the red and blue pins.

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. For 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. 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. 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. Hey guys, it seems like you are all very helpful, and as I know very little about central heating/boiler systems (I still can't figure out whether I have a "Fully Pumped" or "Gravity Fed" system), I thought I would ask for help.

3 – NVent

The 425 Tiara is a two circuit programmer for hot water and central heating and is ideal for use with gravity systems and fully pumped systems with mid-position or zone valves. It has 16 programmes available for fully pumped systems, including independent programming of heating and hot water as well as 10 programmes for gravity hot water/pumped heating systems.