On 2007-12-25 14:56:38 +0000, said:

On 24 Dec, 23:33, Owain wrote:
wrote:
Another question:
After being recommended to us a S plan system with 2 portvalves, I'm
thinking why 2valvesand why not one 3 port valve?

Because 2 portvalvesare simpler and more reliable than one 3-port,
probably.

What happens if you were to use 1 2 port valve?

You don't get separate control of heating and hot water.

Owain

The main thing is that I want to be able to have HW only or CH only or
both at the same time if a S plan system can do this then thats what
is right for me

You can do that, although often controllers work in a hot water priority mode.

In old systems which used "gravity" for heating the HW cylinder,
convection was used in the primary circuit to it to transfer the heat
from boiler to cylinder. This ran pretty much continuously with the
boiler thermostat effectively controlling the temperature. The
central heating was controlled by a thermostat effectively operating
the pump. This gave simultaneous hot water and central heating, but
the cylinder would take quite a time to reheat because the transfer
rate is poor. However, at least the house didn't get too cold while
it was doing it.

With 3 port valve systems, the hot water takes priority. This means
that if there is a demand from the cylinder, then the valve opens in
that direction, and the boiler and pump run. This gives much faster
reheating but even then because the coil in most cylinders is of
limited surface area, it can still take quite a time to reheat and now
the boiler output is directed away from the CH. Of course that will
be worse if the cylinder is large and the transfer rate poor. In bad
scenarios, the boiler will even cycle.

However, you can use a fast recovery cylinder. These have a coil of
much larger surface area than traditional ones by having more turns or
multiple small tubes and can take all or most of quite a large boiler's
output. Reheat on these can be very short indeed and thus the boiler
is returned to CH operation much sooner, and thus there is little drop
in room temperature and probably nothing noticable if the system is
designed well.

Another factor is that the boiler will run more efficiently in this
way. During HW operation, the return temperature from the cylinder
coil will be quite low and a lot of heat will be transferred. The
boiler will thus modulate up to full power or close to it and
condensing models will be well down the curve into efficient operation.
When the boiler is returned to CH only operation, it will modulate
down and run at lower power and temperature, again putting it into an
efficient part of its working curve.

Certainly you can arrange the controls such that there are separate
timed periods for CH and HW. That simply means that one or the other
or both *can* operate. However, running the system such that both
CH and HW demands are met simultaneously may well not achieve a
desirable result of fastest results and best efficiency.