Again a very long post with some pictures to try and get as much detail
as possible out there first time.

Some of you may remember my commitment to help a friend overhaul the
heating system on an inherited house, which although used regularly and
kept inhibited at least until the last 3 or 4 years or so has had no
work done other than pump replacement, and inhibitor replenishment since
about 1974.

Indeed this group helped to diagnose non spinning CH pumps last winter
and give me guidance as to how I might replace valves like this
https://www.dropbox.com/s/t5613768cj1hezn/Rad1.jpg?dl=0
with modern TRVs.

(The original thread can still be found on Google Groups
https://groups.google.com/forum/#!searchin/uk.d-i-y/ch$20system$20design%7Csort:date/uk.d-i-y/apuzmBtbWvk/JOQHT5hjAwAJ)

Anyway time has moved on and over the winter the system operated
adequately at maintaining a frost free environment but the response to
demands for increase in temperature could at best be described as
glacial. (Temp increase of 1C or at the most 2C per hour). This has
not bothered the previous owner as in recent years the heating has just
been on for 24 hrs over the winter months.

Now although the pump outputs have been hot, none of the radiators has
got more than lukewarm to the touch, despite the fact that one of the
feed legs to most radiators was hot, (the other pretty much cold).

Suspecting sludge all radiators have been removed, taken outside and
hosed through (there was indeed quite a bit of sludge).

But now we come to the rebuild. When I first asked last autumn from
initial inspection of the system I thought it was S plan with 3 pumps
(DHW, Downstairs Rads and upstairs rads). However from more detailed
inspection, the previous owners notes and a google search what now I
think I have is a *******ised "single pipe system" with three pumps.

I have drawn it out here.
https://www.dropbox.com/s/yn7ac7wk4w...ayout.jpg?dl=0
In the master bedroom and the lounge the original radiators have been
replaced (in 1974) with MUCH larger double panel radiators as shown by
the revised pipe runs.

The downstairs supply pipes are embedded in a solid concrete floor like
this.
https://www.dropbox.com/s/mod6if6hzi...upply.jpg?dl=0
and any thought of changing the pipe runs is probably out of budget and
certainly well beyond my abilities.

The return from downstairs goes up to join the upstairs return before a
common return to the boiler like this.
https://www.dropbox.com/s/k36vmmjljj...eturn.jpg?dl=0

And all of the unmodified original radiators are variations of this
https://www.dropbox.com/s/wl04fyffxc...ginal.jpg?dl=0

Note no lock-shield valve and 3/4 inch valve on one end.

I have flushed out all the radiators and they are all now clean with no
end fittings, there is no sign of serious corrosion on any of them, and
I am ready for the rebuild.

Now the questions start.
I have been googling single pipe systems (completely new to me) and it
seems that although the systems are pumped heat transfer in the rads is
down to gravity (indeed all of the pics I can find show the rads
diagonally connected with the input at the top and output at the bottom).

Although the feed pipe is 1/2 inch the original installer has fitted 3/4
valves and outlet connections.

Do I need to retain 3/4 inch connections? will a 1/2 valve cause too
much of a restriction?

One (quite old) forum I found said that Single pipe systems also seem to
need special (full flow) TRVs (like Danfoss Randall RA-G) which appear
to be about twice the cost of Drayton TRV4 (presumably because of
scarcity) and that a TRV4 had been fitted in a single pipe system and
didn't work. One responder said that he had fitted a Danfoss RAS-C2
successfully in a single pipe system.

Any comments or experiences?

I note that the original installer never fitted any lock-shield valves.
Is this to minimise flow restriction? Would I need to fit lock-shield
valves with TRV?

There is a chance that in the short or medium term the new owner might
want to fit a new boiler. Are current boilers compatible with a single
pipe system?

My plan was to get the radiator side all up and working satisfactorily
and only then consider a boiler replacement,
The minimum work now is to put it all back together as it was (with
either 1/2inch or 3/4inch manual valves - which will actually turn and
not leak :-) ).

I would like to upgrade to TRVs but don't want to spend 5 or 6 hundred
pounds on 10 or 11 special new single pipe TRVs if
a) they are not likely to work or
b) they will all need to be ripped out and thrown away if/when the
boiler is replaced.


Any comments/Thoughts/suggestions would be most welcome at this point.


--
Chris B (News)