In article ,
Martin Pentreath writes:
On 9 Mar, 12:11, "Geoff" wrote:
I currently have a mixer shower attached to a Puma combi boiler. *We've long
since had a problem as we appear to get cycling of the boiler....

What appreas to happen is that the flow rate is insufficient to keep the
boiler running all the time so it shuts down, the shower goes cold, boiler cuts in gets hot again, cuts out, goes cold etc. *This is not so bad during
the winter months as the water entering the house is colder therefore
requres the boiler to be on more to heat it etc. *However, it gets worse in
th summer months. *I've tried everything over the years to correct this, changing boiler settings, pressure balancing valves, changing shower head
and hose to increase flow rate etc.

I'm now about the change the shower (we want to change it anyway). *I was
wondering about the best one to pick to remove this problem.

You need a shower which is designed for combi/multipoint operation.
They have much faster acting elements (often some sort of spiral
bi-metal rather than a wax pellet), and they are designed to cope
with a large and rapidly varying pressure differential between cold
and hot inlets. They are also quite a lot more expensive than the
cheap wax pellet thermostatic showers.

I've been looking at the mira excel which says it is designed for this.
Does anyone have any experiences of this and can point me to the best one to
achieve max flow rate to get over this....?
Hi Geoff,
Are you sure it's not caused by the secondary heat exchanger being
gummed up? The symptoms you describe match. The problem usually occurs
when a build up of iron oxide on the primary side (the water that also

or hard water scale on the secondary side.

goes round the rad circuit) makes the exchanger unable to conduct heat
through to the secondary side efficiently (ie to the water coming out
of the taps). In other words the shower water is not taking heat away
from the boiler as quickly as the boiler is supplying it. After a
while the water going round the primary circuit gets hot enough for
the boiler to cut off on its thermostat. It will cycle like this in
the way you describe.
I would have thought for the problem to be caused by the shower not
using up water fast enough (as you're guessing) would need a shower of
particularly pathetic performance.
If it is the heat exchanger it should be possible to get it out and
clean it up.
Cheers!
Martin

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Andrew Gabriel
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