In an earlier contribution to this discussion,
tvmo wrote:

I'm moving into a new home that has an old boiler, the label on it
says "Thorn EMI Heating Limited M56/76 & M80/100 gas boilers". There
is a cylinder above it in the bedroom and in the attic there is a
cold- water cistern and a feed-and-expansion cistern. I'm after some
general advice (be gentle on my I am totally ignorant re central
heating/water systems!):

1. The timer on the system is not working - could I fit a modern
thermostat and timer to the boiler?

Probably, but we'd need to know a lot more about the pipework layout before
being able to recommend the best solution. How many water pipes connect to
the boiler, and what is their diameter? Are there any motorised valves? If
so, how many and what type (2-port or 3 port)?

2. Alternatively, if the boiler is replaced would I need to replace
all the radiators as well? Is there a difference between modern
pipework and older (possibly over 20 years old) pipework diameter?

Not unless the current radiators are shot - which they *may* be if the
system has been run for years without inhibitor, so that they have rusted
inside. In this case, fitting a new boiler - which will almost certainly use
a non-vented (pressurised) system - the extra pressure may just be the last
straw which may cause them to start leaking. On the other hand, they may be
perfectly ok.

Another consideration is that a new condensing boiler needs to run at a
lower flow temperature than your existing boiler in order to maximise its
efficiency. So if your existing radiators are only just adequate in terms of
heat output, you may need bigger ones (or probably the same physical size
but with fins on the back) to get the same heat output at a lower water
temperature. On the other hand, the standard of insulation of the walls,
windows and attic may well have been improved since the system was
installed, making the existing rads more than adequate.

Don't worry about pipe sizes. The original system may use imperial pipe
sizes - 1", 3/4", 1/2" (though probably not if it's only 20 years old) - but
it's easy enough to join up to the equivalent metric sizes of 28mm, 22mm and
15mm. [1]

3. Can the cylinder be moved into the attic? What affect would this
have on the water cisterns already in the attic?

It *could* but you'd have to make sure that you had an adequate structure to
support the weight. You'd also need to raise the cold header hank to be
higher than the cylinder - for which you may or may not not have enough
headroom.

4. If I replace the boiler, what should I replace it with? I think a
combi would be out of the question as the house is a large 5-
bedroomed, with a couple of bathrooms.


There are plenty of non-combi boilers around which will suit your purpose,
and give you central heating plus lashings of stored hot water. To satisfy
current building regs it will have to be a high efficiency (which
effectively means condensing) boiler, and you'll have to update the controls
and room and cylinder thermostats to provide boiler interlock. In other
words, the boiler must shut down completely and not keep keep firing to keep
itself warm once both the CH and HW demands are satisfied. It also makes
sense (not sure if it's mandatory?) to fit TRVs to most of the radiators.

[1] These sizes may not *look* equivalent (based on 1" = 25.4mm) but they
are actually much closer than that because imperial pipes are measured on
the *inside* and metric on the *outside*.
--
Cheers,
Roger
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