"Mike Clarke" wrote in message
...
In article , Christian
McArdle wrote:

Is retrofitting a flow switch a practical option without modifying the
internal boiler wiring?

Correct. You may find it works with an additional external pump set to
minimum, if the boiler doesn't mind the extra help! The flow switch
output
would connect to the thermostat input on the combi.

Whether this would work or not depends on the internal design on the
combi.

That's an interesting possibility but I'm not sure if it would be
possible with our boiler. After living in a house with ducted warm air
heating for the last 35 years I'm seriously lacking in experience with
radiator systems so any comments and advice will be welcome.

To summarize my original post, we've just bought a house where there is
no room stat and only the bedrooms have TRVs. The "standard" approach of
a room stat in the living room and TRVs everywhere else doesn't appeal
since there will be times when we'll want to heat parts of the house
without heating the living room. Separate zones would obviously be the
best solution but I don't fancy the task of ripping up floors and
modifying the pipework layout to achieve this so I was considering
fitting TRVs to all the radiators and relying on the boiler's internal
bypass.

I was back at the "new" house last week and picked up the manual for the
boiler. It's a Worcester Bosch "350 Combi" (HC350.FSN), probably about 4
years old. The boiler has a single heat exchanger in the combustion
chamber, this normally feeds the CH circuit unless there's demand for
DHW when the CH pump stops and a separate pump starts up to divert the
flow to a water to water heat exchanger to provide DHW. In case my
description isn't clear I've put a copy of the boiler water flow diagram
on http://milibyte.co.uk/boiler.gif. As I said I'm a newbie to
radiator systems so I don't know if this is an unusual design or not.

It looks to me that an extra CH pump would have a detrimental impact on
the DHW mode by stealing some of the flow intended for the heat
exchanger unless it was connected across the flow and return lines with
a motorized valve (or a couple of non return valves) and configured to
cut in only when the flow valve sensed that all the TRVs had closed.
Since the extra pump would always be pumping into a "blocked" line I
suppose it would also need it's own bypass valve. I suspect I'd end up
with an overly complex setup just to avoid a bit of boiler cycling.

The boiler manual claims that "the electronic controls prevent rapid
cycling of the appliance in the central heating mode" but fails to go
into any detail. I suspect this is achieved by an imposed minimum "off"
cycle of 3 minutes which is mentioned elsewhere in the manual.

I suppose it all boils down to whether the cost of the fuel wasted in
cycling the boiler is sufficient to justify the cost of installing a
more complex control system.

Your gif is too fuzzy for me to see, so I can't see if there is an internal
by-pass. If not, then install one between the flow and return. On the
outlet side of the by-pass, bwteen the two tees, fit a flow switch which
switches on-off the boiler. When the by-pass opens there will be flow, when
it opens further the flowswitch cuts in and cuts out the boiler.