In article ,
Mike Barnard writes:
Hi all.

Any CH design guru's aid me in my small dilemma please?

I have a 1960's mid terrace Wimpy (spit) three bed house. It had no
CH at all untill a couple of years ago when I put a new combi boiler
in [1]. I also put a couple of rads in. It was not a complete system,
DHW was what I needed mostly. Now the time has finally come to finish
the job.

The dilemma is whether to zone off the rear two rooms upstairs. The
bathroom and bedroom 2 are north facing and in winter they can be much
colder than the rest of the house. The thremostat and timing controls
are in the open plan lounge; see description below. If the lounge is
nice and toasty, then the thermostat will not be telling the boiler to
fire up. Therefore the water in the pipes will not be heated and
under circulation. No matter how far the TRV's in these rooms open
they will recieve no heat.

Well that's resolvable by matching radiator sizes to heatloss
in each room, which should have been done anyway. The room
with the thermostat could be relatively slightly undersized
to ensure other rooms are normally all up to temperature before
the thermostat switches off.

However, to go back to your main point, I installed central
heating in an end-terrace house 5 years ago. I zoned it and
I don't regret that one bit. I split it into 3 zones;
downstairs, upstairs, and bathroom. The bathroom sticks out
the back of the house and was prone to be cold and takes a
long time to warm up. The bathroom zone is simply the logical
OR of the upstairs and downstairs zones, i.e. the bathroom
heating is on if either upstairs or downstairs heating is on.

Some things to bear in mind...

A small zone is likely to have insufficient heat output to
absorb all the power of a modulating boiler even at lowest
power output, so you will get boiler cycling. Some modulating
boilers, particularly condensing with a system designed for
efficient low flow temperature, are not all that good when
cycling at low temperature. Even a medium sized zone will
have this issue when only low heat output is required, and
you will end up with periods where multiple medium sized
zones end up taking turns with heating demand resulting in
a long period of cyclic boiler running where fewer/single
zone would avoid this. My own designed heating controller
attempts to avoid this, but I don't know if there are any
multi-zone commercial heating controllers which do that
(and multiple single zone ontrols could not do that).

If you are intending to be able to operate the system with
some zones switched off (or frost setting), remember to allow
for the heatloss to a cold room from a warm room on a different
zone, bearing in mind that rooms are often not well thermally
insulated from each other. This is particularly important for
upstairs verses downstairs zones. Often much of the upstairs
heating comes from downstairs through the floor, but if you
make that normal assumption in a heatloss calculator, you will
find it difficult to heat the upstairs alone if downstairs is
cold and not contributing.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]