|In a fully vented system (with gravity
cold water feed to hot water cylinder and radiator circuit venting into a
small cistern - often sited in the loft) water is typically heated by a boiler
then circulated via (zone valves) two 2 port valve or a single 3 port valve
(depending on whether you have a 'Y' or 'S' plan configuration) to either:
- The hot water cylinder's boiler coil (heat
exchanger) to generate hot water, and/or
- To the radiators for heating.
Control of what your boiler heats is by
virtue of a programmer and the zone valves.
The term 'fully vented' comes from the fact
that no part of this type of system is pressurised. There are usually two
water tanks situated in the loft (one smaller than the other). The larger
cold water cistern (or tank) supplies water to the hot water cylinder and
does so purely by the force of gravity. The domestic hot water system (DHW) and cylinder
vent back into this cistern. The smaller 'feed and expansion' tank (F&E)
that vents the radiator circuit (and also keeps it topped up) also operates
courtesy of gravity. Both cisterns will usually be fitted with a float valve
so that a level may be maintained within the cistern and fed from a mains
A gravity system is inherently safe.
Because it should be impossible to have a dangerous pressure build-up in any
part of the system there is no risk of something bursting under pressure -
or at worst ... an explosion! That's the good news. There are however draw
backs (trade-offs) as there are with each and any system.
The main draw back with a fully vented
gravity system is the pressure you can expect from your domestic hot water.
As mentioned, the cold water feed to your hot water cylinder is under the
force of gravity. It is therefore going to be relatively low pressure.
|To explain this in a
more tangible way you should consider the following:
As you elevate a vessel of water it
gains potential energy. Keeping the outlet point lets say at
ground level and raising the vessel of water, you increase the
pressure at the outlet the higher you raise the vessel. For the
purpose of this example when you raise the water vessel by approx 33
feet (10 metres) you create 1 bar of pressure out the outlet.
The pressure throughout a gravity
system will differ between outlets as the height differential
between the cistern and the outlet changes. For example, the height
difference between a cistern in a loft and an outlet on the ground
floor of a two a storey house will be greater than the height
difference of an outlet on the first floor of the same house when
fed from the same cistern. The pressure at the outlet will therefore
be greater on the ground floor than on the first floor by virtue of
the height differential.
This is why the performance of a
gravity fed shower on the first floor of a house is generally pretty
poor - especially if compared to a shower on the ground floor when
fed on the same system. The height differential between the loft
cistern and the first floor outlet (in this case the shower handset)
can be extremely small - especially if you raise the handset as high
as it will go. The nearer you go to the ceiling with it the nearer
you get to the under side of the cold water cistern.
It is important to realise that it
is the height differential between the cistern and the outlet that
makes the difference - not the size of the cistern or the amount of
water in it. The position of the hot water cylinder will
for intents and purposes not affect the pressure at the outlet by
virtue of its relative position in the system.
Providing the cold water cistern is higher
than the outlet point you have what is known as a 'positive head'. If you
attempt to create an outlet higher than the cistern you will have a
'negative head' scenario. In this situation you will not get water to flow
from the outlet under normal gravity conditions - for the simple reason that
water will not voluntarily travel up hill.
Gravity systems (open vented systems) as
mentioned are simple and safe. They may be low pressure, but they do have
the advantage of being to ONLY type of system to which you may add a booster
pump. You can even get 'negative head pumps' to cope with the problems of
outlets above cistern level.
By comparison, Water Bye Laws prohibit the
use of pumps on mains pressure systems to increase flow or pressure. Combi
systems are ains fed and will already be working flat out so attempting to
increase flow here is folly. So, if you have a low pressure gravity system
and are fed up with the poor performance of your shower you can add a
booster pump - that's the good news.
Positives: Inherently safe. Suitable for addition of
booster and power shower pump to provide exceptional flow rates.
Negatives: Require loft tanks and a
hot water cylinder, so provision must be made for the space these will
require. An unpumped system will offer low pressure at taps and outlets.
Economy: Water must be heated and
stored whether or not it's used. Important to make sure your system and
storage is insulated well to minimise standing heat losses. No less
economical to produce hot water than any other system.
Typical System Requirements: Boiler,
Hot Water Cylinder, Loft Tank/s, Rads.