"John Aston" wrote in message
.. .
Thanks for your help to date. I've distilled the advice from various
threads
in this newsgroup to come up with a possible heating design for my house.

The drawing HD01 at http://tinyurl.com/3zv2g shows the proposed hydraulic
design for a large domestic heating system. The boiler and hot water
cylinder is on the left hand side of the drawing, space heating is on the
right. (You might need to rotate the view so that the drawing is in
landscape orientation in your browser)

Some explanatory notes on the design are appended below. Any comments
would
be gratefully received but my principal questions a

(1) The internal space heating requirement when it's -3°C outside is 31kW.
In addition, there is a 250L cylinder serving three showers and one bath
for
a family of five. Is a 38kW boiler sufficient?

(2) I'm specifying 28mm pipe through the water softener and up to the
cylinder, 22mm for the boiler flow and return, 22mm to potable water cold
taps and 15mm everywhere else. Is that reasonable or over the top?

(3) A 22mm pipe is teed off the secondary side of the low loss header.
From
this 22mm pipe, I propose to tee off 15mm pipe to each heating zone. What
is
the maximum distance between these 15mm tees, and what's the maximum
permissible distance from the furthest 15mm tee to the header? (Keston
told
me that there is NO restriction)

(4) What's the best way of incorporating two towel rails (bottom right)
into
the circuit so that the towel rails come on all year round when there is
either a call for heat or a call for hot water?

Design:

The mains cold water supply is treated by a water softener (bottom left).
The softened water is fed to a 250L cylinder under pressure from an
accumulator which keeps the cold water at about 2.5 bar and provides a
high
flow rate to all taps.

The water in the cylinder is heated indirectly by the primary of a 38kW
fully-modulating condensing boiler in the room below the cylinder. Hot
water
is circulated to the taps by means of a secondary pump. Because the system
is unvented, the hot water is also at a pressure of about 2.5 bar.

A diverter valve allows the boiler to heat the central heating system when
there is no demand for heating from the cylinder. The boiler flow is
diverted into a low loss header from which the distribution circuits
(heating zones) are pumped. The header buffers the boiler primary control
against sudden changes of flow in the distribution circuits.

The central heating system comprises both underfloor heating and radiators
in all rooms (only a few rooms A-E are shown, for clarity). There are two
heating zones: The kitchen and reception rooms on the ground floor are
zone
1, and the upstairs bedrooms and bathrooms are zone 2. Since each zone
comprises one underfloor heating system and one set of radiators, there
are
a total of 2 x 2 = 4 heating distribution circuits.

The underfloor heating circuits are connected to a manifold and
temperature-controlled by room thermostats which operate the circuit
valves.
The water through the underfloor heating system is limited to 55°C maximum
by a thermostatic mixing valve at the manifold entry. The underfloor
heating
has a maximum output of 17kW and is only capable of maintaining a internal
temperature of 9°C or 10°C above the outside temperature.

The purpose of the radiators is to supplement the underfloor heating with
up
to 14kW additional heat when the external temperatures are cold. The
temperature of the water pumped to the radiators is (almost) equal to the
temperature in the low loss header. The boiler's primary flow temperature
is
a function of the outside temperature and typically falls from 70°C to
40°C
when the outside temperature rises from 0°C to 15°C. The radiators are
switched on and off by thermostatic valves.

Every underfloor heating circuit has its own room thermostat and the
circuits are terminated at a manifold which has its own local controller
that sends a signal to the master controller when there is a call for
underfloor heat. The master controller has provision for switching the hot
water and heating off independently at user-adjustable times.

The system has a distribution of drain cocks and isolation valves to
simply
maintenance.

Sources of information:
This newsgroup and, amongst others,
Viessmann http://tinyurl.com/6cqtt (Page 41)
Geminox http://tinyurl.com/5glxs (Bottom of page 7)
MAN Heiztechnik http://tinyurl.com/3vzwz (Page 27)

I'll get back tomorrow . Have been out a lot today.