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 moulding The installer will have set the programmer to suit the installed system. If this is a Fully pumped system this will allow independent control of the Central heating and Hot water, however on a Gravity system the Central heating and Hot water are linked so that it is not possible to use the Central heating independently. This will allow only one common time setting for both Hot Water and Central Heating. BATTERY

If a Heating or Hot water period is not required it can be cancelled by setting the ON and OFF settings to the same time. Example ON 10:00am OFF 10:00amSo using the lever will not take the valve the full distance and not far enough to trigger micro switch to cut (c) and make (d) 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. On the ChannelPlus display - Boost 1 is Hot water extension 1 or 2 hour and Boost 2 is Heating extension 1 or 2 hours ADVANCE FUNCTION

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 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. On Sunday morning I was looking forward to a lazy few hours in my PJs. The plan called for a hearty breakfast, freshly ground coffee, some light reading and a hot shower before deciding what to do for the reminder of the day. Regrettably, the unexpectedly cold water from the showerhead put an end to an otherwise agreeable time. c) allows power to PCB to produce and provide modified voltage to hold valve in mid position against the strength of the return spring.More manuals of Thermostats Frequently Asked Questions about Horstmann ChannelPlus H27XL Thermostat