On 12/06/2021 22:41, Fredxx wrote:
On 12/06/2021 22:24, Roger Mills wrote:
On 12/06/2021 19:36, Fredxx wrote:
On 12/06/2021 19:14, Roger Mills wrote:
On 12/06/2021 18:15, jkn wrote:
Hi All
Â*Â*Â*Â* I am musing about finally replacing our anciente back boiler
(Baxi Bermuda, part of a Plan C system dating back ... quite a bit)

One possibility I am considering is having a system boiler fitted
rather than a combi. On question - would it be necessary/advisable
to replace the cylinder at the same time, or could this be kept?

Thanks & Regards
J^n


I would say that it wouldn't be strictly necessary, but it would be
highly advisable.

The cylinder will almost certainly be the indirect type with a
separate coil inside through which the primary water from the boiler
flows. Whilst would work with a system boiler, it may not work very
well. If is was installed in Baxi Bermuda days it is pretty old and
will likely be scaled up unless you are in a soft water area. Also,
it's unlikely to have a fast recovery coil inside so the domestic
hot water won't be heated anything like as fast as it would be with
a new cylinder. Finally, all recent cylinders are covered with foam
insulation which is likely to be better than fitting a loose jacket
round it.

I'm assuming that you plan to alter the pipework and controls in
order to make it a fully pumped system? Modern low volume boilers
just won't work with gravity circulation HW systems like the old
cast iron lumps did. Unless the pump maintains the flow until the
boiler has cooled a bit after each firing cycle, it will overheat
and trip out due to the residual heat.

The OP could consider S-plan. And if an issue to minimise alteration
of pipework, the OP could have separate pumps for CH and DWH with
non-return valves.


That would make it pretty difficult for pump over-run conditions,
where the boiler needs to control the pump. Not impossible, but lots
of relay logic!

Some heat only boilers don't have a pump over-run facility. The Baxi
EcoBlue is an example.




Fair enough if it can cope with the flow stopping as soon as the flame
stops. But, in any case, the OP was talking about using a system boiler,
which included a pump inside the casing.
--
Cheers,
Roger