2.
Domestic hot water (DHW) using a heat pump
The water you see appearing from
your hot water taps is referred to as Domestic Hot Water and
abbreviated to DHW. It differs from say the hot water
circulating around your radiator system or through your boiler's
heat exchanger which is often contaminated with inhibitor and
waterborne debris and sometime referred to as 'primary hot
water'. DHW can be produced by various methods.
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Combination (Combi) Boiler.
It is necessary to acknowledge this method of heating
although Combi systems have little or no interaction with
heat pumps. In a Combi system hot water is produced
instantaneously by heating on demand and delivered directly
to whatever outlet might be calling. Whilst this method has
the advantage of space saving (because there is little or no
storage required) the hot water delivery rate is often
frustratingly slow.
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Vented or unvented
'Indirect' hot water cylinder. By this method a volume
of water contained within a cylinder is heated by a remote
heat source using a heat exchanger (coil) through which hot
water from the heat source is passed. (An 'Indirect'
cylinder may be fitted with immersion heater/s, but a
cylinder heated exclusively by electrical immersion heater/s
is deemed to be 'Direct'). Indirect and Direct hot water
cylinders may be vented or unvented by design. A vented
cylinder is, as the description suggests, vented to
atmosphere and only ever subjected to the pressure gravity
from the water feeding into it from a header tank. Vented
hot water cylinders typically only supply a low pressure
'gravity' feed of hot water. An unvented cylinder is a
pressure vessel fed with water directly from a mains supply.
Generally regarded as the high performance option because of
higher hot water output pressure, they are only as good as
the supply pressure to them and bring with them a raft of
conditions and regulations as they are potentially dangerous
if incorrectly installed or not serviced. A cylinder
constructed for use with a gravity supply must never be
connected directly to a mains pressure supply as the risk of
explosion exists. (Gledhill
Unvented Heat Pump Cylinders) (Newark
Vented & Unvented Heat Pump Cylinders)
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Thermal Store cylinder.
Using a simple bit of reverse logic a cylinder with multiple
capabilities was born. Instead of heating a volume of clean
water contained within a vented cylinder by a remote heat
source via a heat exchanger, a Thermal Store uses the water
contained within a cylinder as a medium in which thermal
energy may be stored (hence the title). (See
Heat Pump Thermal Stores)
Thermal stores can produce
mains pressure hot water and a reservoir from which
space heating (UFH or Rads) may draw their hot water.
By immersing a heat
exchanger within the cylinder through which mains water
is passed a rapid heat transfer takes place whereby the
incoming cold mains water becomes hot water at mains
pressure which is then delivered to taps and outlets.
A circuit of water for
space heating can be drawn directly from the cylinder -
in effect stealing the preheated water.
One of the beauties of a
Thermal Store is the fact that the large volume of hot
water in the store (that features as a concern with an
unvented cylinder) remains at atmospheric pressure, only
the significantly small amount contained within the
mains water coil is at pressure at any given moment. For
this reason a Thermal Store is regarded as the 'safe'
way to achieve mains pressure hot water as it utilises
an inherently safe vented store of water.
Additional
considerations for the type of cylinder to use.
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Heat Sources
There are certain heat
sources that are incompatible with certain types of
cylinder. The cylinder type that gives the cause for
most concern is the 'unvented' type.
More about unvented hot
water cylinders
An unvented hot water
cylinder is essentially a pressure vessel in which water
is heated, then by virtue of pressure maintained at the
inlet, contents are forced out at mains pressure
whenever an outlet calls for hot water. The problem with
unvented cylinders is the risk of explosion if an 'over
temperature or over pressure' scenario were to go
unchecked. For this reason pressure relief equipment
must be fitted to an unvented cylinder. Other notable
issues are G3 Building Regulations for these devices and
the fact that ONLY controllable heat sources may be
connected. This precludes the use of such things as wood
burning stoves, AGA's , Rayburns, etc as they cannot
generally regulate their output.
Solar systems, heat pumps,
gas and oil boilers are normally controllable and
therefore safe to use with unvented hot water cylinders
and may be used in combination. (see
Multi coil
unvented cylinders)
Using 'uncontrollable'
heat sources
If however you wish to use
a heat source deemed 'uncontrollable' such as a wood
burner you may only do so with a vented cylinder (being
vented there is zero risk of explosion). As referred to
earlier in this section, there are essentially two
families of vented cylinder -
1. An open vented gravity
hot water cylinder (for gravity hot water), and
2. A thermal store (for
mains pressure hot water from vented cylinder).
Thermal stores are
considered attractive because of their ability to
deliver mains pressure hot water while remaining open
vented, being able to accept heat from multiple heat
sources simultaneously (including those deemed
uncontrollable) and being able to supply stored energy
directly to a wet heating system (rads or UFH). In
short, any and all heat sources contribute to any and
all outputs. Quite unique! (See
Heat Pump Thermal Stores and
Thermal Stores /
Solar Thermal Stores)
A vented gravity cylinder
by contrast can be configured to accept different heat
sources by the inclusion of several heat exchanger coils
(multi coil cylinders) but can only ever deliver gravity
pressure hot water. Extraction for heating purposes is
not an option. (See
multi coil cylinders ,
Stainless Lite Vented and
Heat Pump Cylinders)
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