Dear UK-DIY,

I am in the process of replacing my very old gravity fed CH and HW
system. To simplify the job and because of the size of the property, I
have opted to convert to a simple vented fully pumped "Y" plan system
for CH and HW.

I have a semi detached property with just seven radiators and one
bathroom (although adding an on suite shower room at a later date is
not out of the question). The existing setup has more than enough head
to power the one shower without the need for a pump.

I have taken a segmented approach, replacing the radiators last year
and the rest of the system (hot water tank, pipe work conversion to
fully pumped system and new boiler) this year. So far I have replaced
the old copper cylinder with a new Part-L compliant one and am running
all hot water from the immersion heater until the indirect and boiler
installation is completed.

I have installed all the pipe work for the indirect system, including
new pump and three port mid position valve; however I now face a
challenge. In plumbing terms, the house is split into two parts, the
one hundred year old main building, and the newer extension to the
rear. The old part has four radiators connected to a 22mm single pipe
system; the extension has three radiators which are plumbed using a
15mm two pipe flow and return system.

I am planning on having plumber install a Worcester Greenstar 15Ri to
this new setup, but from what I understand condensing boilers rely on
a significant temperature differential between flow and return in
order to modulate properly and that single pipe systems often do not
provide enough of a temperature drop between flow and return.

Because of the age of the house, converting the four radiators in the
two bed rooms and two living areas on the existing single pipe system
to a two pipe flow and return version would be a major challenge
requiring a significant amount of additional work.

I appreciate that the problem with single pipe systems is their lack
of efficiency, however since replacing all the radiators, I used the
heating with just new radiators during the winter and I found that the
radiators on the single pipe section got hot at the same rate and
achieved the same operating temperature as those on the two pipe
system - hence my reluctance to make any changes!

I guess my question is, In order to meet the any new regs and get best
efficiency from my new boiler, am I going to be forced to add another
pipe to the single pipe system to convert the whole thing to a two
pipe system?

Many thanks in advance!

James