In article , Christian
McArdle wrote:

Remember that such a system would not comply with building regulations,

Agreed, but it will be a big improvement on the existing situation where
almost half of the radiators have no thermostatic control at all. Since
the system was installed about 4 years ago I assume it doesn't have to
comply with the latest regs though I do want to make it reasonably
economical to run, providing the cost of any improvements can be
recouped from the energy savings.

though. If you really do want to continue with such a system, it is
essential that the entire primary circuit is insulated to reduce
losses.

The pipework does seem to be quite well insulated but I don't suppose
this is relevant to the situation where all TRVs are shut and the only
flow is through the internal bypass in the boiler.

--
Mike Clarke