In article ,
Andy Hall wrote:

Thanks for the quick & detailed reply.

On Tue, 6 Jan 2004 00:00:50 +0000 (UTC), wrote:
Does this sound feasible? Effectively it creates a combination of single
pipe loop and micro-bore piping, but other than locating and hooking into
the split of the hot feed into the manifolds & the corresponding return is
there anything else I can do?

It's a bad idea to try to combine a one pipe and a two pipe system and
to be honest a one pipe system is not great to begin with.

Fair enough. However if the CH is split in two (as described below) surely
a single pipe system of just two radiators is not such a bad thing? It's
not long enough to end up with the last radiator being too cool.

Really, you should plan to add the new pieces in two pipe. This
doesn't need to be done in microbore or into a manfold. You need to
tap into the main (probably 22mm) run after the 3 port valve as you
say but then find the return.

How would I go about finding the return? There is no access to any of the
pipework except in the airing closet where the hot comes in. Presumably
where the coil exits the hot water cylinder is a return - would that join
the CH return and would it therefore do to connect the loft radiator
return to the hot water return?

Would it work to tap into the main with a simple T (and same again at the
return)? Ie would enough water flow up 2.5m in this situation, or would it
require a motorised valve to shut off the "easy" route of the existing CH
piping?

Since the pipe runs are quite long, it is important to size the pipes
adequately - don't guess because you will be disappointed if you make
them too small.

There is a note on how to do this at

http://www.cda.org.uk/megab2/build/Pub150%20UKCB.pdf

Excellent. Looks a bit complicated but following step-by-step will
probably do it.

You need to size the radiators to meet the heat loss and then the flow
rate required and pipe size are determined from this. Again, do do
the proper heat loss calculations using one of the radiator company's
programs - don't guess or you may have another disappointment like the
towel rail.

Agreed.

Alternatively I could install a new 2-port valve on the Ch side of the
3-port valve, to split the CH system into two separate circuits - one for
the existing house and another just for the loft.

You could do that, but it might need more work on the rest of the
system to convert it to use entirely two port valves. If you look on
Honeywell's web site it is called an S-plan plus system then. You
could just zone it off with a two port valve, but then the loft
heating is at the mercy of the rest of the house if you see what I
mean.

What about a second 3-port valve installed on the CH side of the existing
3-port valve? This could split the CH water into two circuits. If
controlled by a thermostat in the loft, then this could switch the CH hot
water between the circuits, favouring the loft first. This could then be
independent of the rest of the system. The loft would heat before the
downstairs but once warm enough the downstairs would heat up. As long as
the loft thermostat had a range (eg heat to 20 degrees, but don't
reactivate until temp drops to 16 degrees) there wouldn't be excessive
switching between the two.

Don't work on gut feel. This is not a large boiler so again you may
be disappointed. On the other hand, it is probably due for
replacement and you could simply try it out and replace the boiler if
it proves to be inadequate.

I'm hoping not to have to do this for a while, but might be forced into
it.

Thanks,
Zane.