adder1969 wrote:
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.

(snipped)

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.

My boiler has a bypass pipe in it, from one side of the heat exch unit
to the other, from what I can make out. It's about the same thickness
as a bit of microbore (yuck/spit/hiss) so the flow rate through it is
going to be pretty poor, and the volume of water also practically
nonexistant (we're talking maybe a foot, foot and a half length at
absolute best).

I've never adjusted a CH pump in my life so not really sure what I
should be looking at on it to twiddle. If someone could give me a few
clues as to what to look for (bearing in mind it is part of the boiler,
I know where the pump is on it - rather than being external to the
boiler type). The manual is a bit crap generally for the boiler, so
it's not completely infeasible that the pump *is* actually adjustable,
and they've just not actually said that anywhere (though the bit about
using a flow valve across flow/return does rather make one wonder).

I definitely don't have the water volume in the boilers own bypass in
order to prevent kettling, as I discovered later that night when I shut
all the trvs down fully. Even with the bathroom rad half-open on the
LSV there was insufficient flow to prevent kettling, and even when the
CH boiler stat was at minimum too.