Phil Addison wrote:
non-condensing 5-ish years old Saunier Duval SD30e, which is apparently
a Vaillant in disguise?) - it just says restrict the flow using a valve
across flow and return.

As I said in the reply to your other post, I don't follow that. Maybe
someone else with that manual can clarify it.



It means using the pump valves or similar to restrict the flow rather
like you would use the valves on a radiator. I'd be surprised if the
pump wasn't adjustable but if you set it low then it means it takes
longer for the house to heat up.

I've balanced mine by feel and it seems ok. No idea what the temp
drops are.

For the "bypass" you need enough water volume so that it doesn't
overheat and boil. If the boiler gets very hot then you need more
water than it it doesn't. If you don't have enough water in the
regular pipework then a radiator will add some volume. If the bypass
loop is too open you'll get boiler cycling. if it's too closed you
might get kettling. Auto bypass valves supposedly give you the best of
both worlds - all the heat to the rads when they need it and then flow
through the bypass when they don't.

To avoid having a bypass radiator you need enough water volume in the
pipes to avoid kettling etc but it often depends on the boiler type.