FEATURE
DOMESTIC BOILERS
& RENEWABLES
“Simply chatting to customers about their
condensate will help to ensure that the pipework
is much less likely to freeze come wintertime, and
ultimately reduce those easily preventable call-outs”
Turning up the pressure on PRVs
Martyn Bridges, director of technical communication and product management at Worcester
Bosch, outlines the common causes of Pressure Relief Valve (PRV) failure and how to stop it.
Every combi boiler on a sealed
primary heating system has a
Pressure Relief Valve (PRV).
Without a PRV, pressure in
the heating system could become too
high (generally around three bar).
A PRV, therefore, is an essential
boiler component and means that
excess pressure can be released in the
form of water, or sometimes steam, and
travel through the discharge pipe to be
safely removed outside.
It’s not unusual for PRVs to last the
lifetime of a boiler if used correctly, as a
PRV isn’t a wearable part. If a PRV is used
incorrectly though, it can quickly become
irreparably damaged.
So what are the common causes of PRV
failure and can it be stopped?
1. A DRAIN ON RESOURCES
Unfortunately, PRVs are frequently used
erroneously to drain heating systems down.
This isn’t what it was designed for and can
cause complications, however, due to PRVs
being situated at the bottom of a boiler, in
addition to being attached to a permanent
pipe, it can be tempting to drain a system
this way.
When using a PRV to drain a system, any
contaminants in the heating system are
drained through the valve itself and end
up on the seating of the valve where the
valve should re-seal and shut off again.
Contamination from the heating system,
other than through the PRV,
causing potential damage.
In general, expansion vessels are able to
cope with the demand of most domestic
heating systems, but there are occasions
where the heating system is too large.
For example, if a system has a large
number of radiators or large radiators
and pipework, then the water volume will
be greater than the expansion vessel can
accommodate. This, once again, will cause
the PRV to open.
4. THE MISSING LINK
Occasionally, heating systems need repressurising
or “topping-up” to maintain
optimum performance, and the easiest way
to do this is with the help of a fi lling link or
loop to fi ll up the system.
However, if this piece of equipment is
left connected and turned on slightly, too
much water can be added to the sealed
heating system causing the PRV to open.
PRVs are designed to continuously
control the pressure of a heating system
and should only open when the system
pressure is too high.
Avoiding the above misuses will ensure
that a PRV protects the boiler as it should
and won’t need replacing for the boiler’s
operational life.
ENQUIRY NUMBER 119
such as magnetite and other
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Heating & Plumbing Monthly | OCTOBER 2018 | www.hpmmag.com
foreign bodies, doesn’t allow the valve
to re-seal correctly. A noticeable sign of
this is found when the heating system is
put back into operation, and water can be
seen periodically dripping from the valve
and the discharge pipe outside.
For the homeowner, this means that they
have to re-pressurise the heating system
from time-to-time, but this is a quick fi x
and eventually the PRV will need to be
replaced.
Simply put, the system should contain
drain-off valves at all low points in the
system, and all drops to radiators from
above. These are the only places that a
system should really be drained from.
2. PUTTING THE PRESSURE ON
Generally, it is recommended that you cold
fi ll a heating system to around one bar, but
it isn’t uncommon for homeowners and
installers to mistakenly charge the heating
system to too high a pressure with cold
water, which in turn can make the PRV
operate when it shouldn’t need to.
For example, if a heating system is
charged to two bar with cold water, when
the system heats up it could easily reach the
three bar limit, causing the valve to open.
3. LIKE A BROKEN VESSEL
The expansion vessel’s role in a boiler is
to absorb the expansion of water which
occurs when water is heated up. If the
expansion vessel is faulty, has no air charge
in it, or if the membrane is leaking, there
is nowhere for the expanding water to go
It’s not unusual for PRVs
to last the lifetime of a
boiler if used correctly
PRVs are frequently used erroneously to
drain heating systems down
PRVs are designed to continuously
control the pressure of a heating system
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