9
Unvented Systems
Definition
An unvented hot water system differs from conventional
open vented systems in that no vent pipe is provided
and it is connected to the mains water supply, eliminating
the need for the provision of cold water storage tanks in
the roof space. The functions, therefore, of the open
vents and cold feeds have to be performed in other
ways. There are two basic types of unvented system:
(1) Direct off the mains,
(2) Boosted water supply.
Direct off the mains
Where mains pressure is sufficient the system can be
connected straight off the mains. However, between
the cold water supply and the DHW generator (i.e. the
CT-plus calorifier) there must be an unvented system
kit fitted.
Warning: Some pressure booster
sets have an accumulator (expan-
sion vessel). This is to reduce the
amount of pump starts and should
not be confused with the system
expansion vessel.
This kit should contain a minimum of the following
items:-
•An expansion vessel should be fitted between the
non-return valve and the calorifier to take up expanded
water during heat-up.
•An expansion relief valve should be fitted in the cold
feed between the expansion vessel and the calorifier.
No means of isolating the calorifier from this valve
should be installed.
•A temperature and pressure relief valve should be
fitted directly in the calorifier as a thermal protection
to prevent the DHW reaching 100 deg C.
•A hand reset limit stat should be fitted in the calorifier
to prevent the DHW overheating.
Balanced Unvented Systems
Some systems may be fitted with blending valves or
mixers that require a balanced hot and cold water
supply. When sizing the strainer, the PRV, and check
valve care must be taken to ensure they can pass the
flow rates at the required pressure.
Hovals’ standard unvented system kits are sized to
reduced incoming pressure to 3 bar at a flow rate that
meets the peak output of the calorifier only.
As shown in the above diagram, an unvented system kit
comprises of the following components:-
•Strainer (this is optional, but good practice, to prevent
debris passing through the valves)
•Pressure regulating valve (PRV), this allows the
pressure to be reduced and controlled at a preset
pressure. A pressure gauge is supplied to indicate
system pressure.
•Double non return valve, this prevents the expanded
water back-feeding into the mains supply and
subsequently seals the system.
•An expansion vessel should be fitted between the non
return valve and the calorifier to take up expanded
water during heat-up.
•An expansion relief valve should be fitted in the cold
feed between the expansion vessel and the calorifier.
No means of isolating the calorifier from this valve
should be installed.
•A temperature and pressure relief valve should be
fitted directly in the calorifier as a thermal protection
to prevent the DHW reaching 100 deg C.
•A hand reset limit stat should be fitted in the calorifier
to prevent the DHW overheating.
Boosted water supply
Where mains water pressure is insufficient a cold
water booster set can be fitted. Normally these sets
have integral pressure controls, in the form of either
pressure transducers, pressure switches, and or
pressure regulating valves. They also have a non return
valve which prevents back-feeding. Systems where a
booster set is installed should not have a vent and as
such are unvented.
Boosted systems should have an unvented system kit
fitted down-stream from the booster set.
T & P Relief Valve.
Control & hand
reset limit stats.
Expansion relief
valve c/w tundish.
Expansion Vessel.
Check valve.
PRV.
