Plumbing – Connect 2 heating controllers to one boiler

As long as the boiler is capable of providing enough hot water through the system to heat the whole house if necessary you shouldn't have a problem.

What you are proposing is a "coarser" version of thermostatic valves on each radiator and that is perfectly safe for a combination boiler.

I don't think you need return valves.

Make sure that you leave one radiator "open" (usually the bathroom one) so that there is some flow around the circuit.

By the way the fact that it's a combination boiler is a red herring. The "combi" bit refers to the hot water heater, the central heating aspect is pretty much the same as a "standard" boiler with integral pump.

Update

If you want to control the times each room is heated independently you can get combined timer and thermostatic radiator valves - My Greener Home has some for example.

When the boiler is running (especially in heating rather than hot water mode) watch the pressure gauge: does it rise almost all the way to maximum? And do you then get a sudden discharge of hot water through the pressure relief discharge (PRD) valve and pipe to the outside of the building? (If the boiler is mounted on an external wall the PRD pipe usually goes directly through the wall behind the boiler.)

If so this is where you're losing water (and hence pressure). It indicates that the expansion vessel (probably built-into the boiler) is faulty. It may just have lost pressure (there's a Schraeder(sp?) car-tyre-style valve you can use to top it up) but could have failed entirely (e.g. if water comes out of the air valve!). For expansion vessels built into boilers it's often more trouble and expense than it's worth to replace the fitted one and quite acceptable to fit a vessel externally.

The uk.d-i-y usenet newsgroup is generally a good place to discuss this sort of problem. There is also a wiki/faq at http://wiki.diyfaq.org.uk/