Builders not wanting to zone house.

[chris watkins]

Ric2013 I don't think modern boilers have much of a requirement there is so little residual heat in the condensing exchangers.
The manufacturers may ask for a minimum flow rate to allow the electronics to control the boiler /pump outputs but this is better done by using hydraulic separation via a low loss head or close spaced (coupled) tees rather than a auto by-pass.

 

[Ric2013]

I was thinking of old cast iron boilers when I mentioned minimum heat dissipation, though I note the MAIN HE 24 heat-only boiler has a requirement of a 4m run of 22mm pipe to provide a suitable by-pass. That said, point taken: the MIs do not say anything about not being able to insulate the bypass circuit (as long as there is no plastic pipework used in the system).

 

[johnduffell]

Agree, In fact the building regs require the bypass pipe work to be insulated, a heat dump would be frowned upon even more than a pilot light nowadays!