Programmable Thermostat and Gravity fed hot water?

[RufusA]

We have an older Worcester gas fired condenscing boiler with pumped central heating and gravity fed hot water which has given us years of faultless use.

Currently the boiler is controlled by a "Grassun Towerchron QE2" digital timer which provides a useful level of control over on/off times of the heating and hot water. We have TRVs in most radiators, but the weak point in the system is a Honeywell analogue thermostat, which I have to adjust serveral times a day.

Due to work patterns it would be great to be able to programme differening temperatures throughout the day/week and have some of the "energy saving" features like start optimisation etc.

Having trawled this forum, conventional wisdom seems to be that it would be a minor task to swap the existing thermostat for a modern programmable one. However I'm left confused about what to do with the existing controller/timer.

If I put the CH on constant, and control it via a programmable thermostat, then by design the QE2 sets the Hot Water to also be on constant. Wouldn't this result in loads of wasted energy keeping the water tank at a constant temperature 24 hours a day.

Do I need therefore to replace the timer also? Can a programmable thermostat also control Hot Water in a gravity fed system, and if so how?

Should I be looking at doing something different with the Hot Water i.e. add a themostat, or a pump etc.

Are there any recommendations on a suitable CH thermostat which offers the maximum flexibility in terms of manually overriding/boosting, has a full 7 day control and optimum start and is reliable etc.

Many thanks - Rufus.

[John Phillips]

No need to replace the existing timer for this reason. If you swap the current thermostat for a thermostat/controller (e.g. a Honeywell CM907), then you can re-connect the thermostat's switched/timed live ("call for heat") lead at the existing timer to a permanent live terminal instead.

Then leave the existing timer on timed just for the hot water and the time controller in the new thermostat takes over for the heating.

Actually, not having a timed live "call for heat" feed to an optimizing controller is essential.

A timed live can confuse the optimizer. The optimizer will notice that when it calls for heat and the timed live is off the result is no rise in ambient temperature. It will thus incorrectly learn to turn the heating on unnecessarily early.

[RufusA]

John, Thanks for the response, your suggestion of a permanent live is a very neat solution to the problem that didn't occur to me!

Off to hassle my local plumbing supply shop now!

Rufus.

[John Phillips]

But please check - after some thought I am not now sure it works with a gravity hot water system.

[Roger Mills]

Whereas swapping a manual room stat for a programmable stat and leaving the main CH timer on 'constant' works well for systems which also control the HW temperature, it will *not* work with an un-modified gravity HW system because - as you rightly identify - this will keep the boiler running 24x7, and the HW will get up to boiler temperature which will be far too hot if you run the CH at (say) 80 degC.

The best solution is to convert to a fully pumped system - either S-Plan [1] or Y-Plan - which automatically provides a boiler interlock, and thus only the runs the boiler and pump and either or both CH and HW are calling for heat.

Depending on the layout of your pipework, that may be difficult to achieve without a lot of upheaval. An alternative solution which is *almost* as good is to convert to C-Plan [1] - under which you keep the gravity HW system but insert a zone valve and tank stat which, together, shut down the boiler when it's not needed.

[1] For details of the various control 'plans' see