Thermal Stores... if you haven't used one then why not?
The range of hot water systems on the market today is quite staggering, and it is
important to sit back for a while and look closely at the advantages and disadvantages of
each. At DPS we have been central to the advance of heating technology for a number
of years, holding the original designs and patents on a number of ground breaking systems.
To date we have designed over 250 separate hot water cylinders, ranging from
standard systems to advanced solar thermal stores, including custom made systems for a
number of local authorities and building groups.
We have always been thorough in investigating old and new hot water technologies, and adapting them to produce more effective systems. I hope here to impart some of our knowledge to the reader, and to clarify any confusion regarding which type of systems are best suited to which applications. Firstly I would like to list the basic methods of providing hot water currently available on the market:
Methods of providing domestic hot water using Hot & Cold water storage:
Methods of providing Pressurised domestic hot water using only Hot water storage:
Methods of providing Pressurised domestic hot water without the use of any water storage:
We have dealt first hand with the installation and use of every one of these systems, and have come to understand the benefits and pitfalls of each. The first important fact is that there is a big difference between flats and houses. The main difference is space. A house usually has a loft, as well as a full size airing cupboard. This makes it easy to install any choice of system. When space is not a problem, the choice comes down to price and performance. When space is a problem, such as in a small flat or bedsit, then performance is limited by the space available.
Combination of a cold tank and hot water cylinder:
Where loft space is available, a cold water storage tank can usually be installed with
relative ease, assuming one is not already fitted. The basic combination of a cold
tank and hot water cylinder is often the easiest and cheapest choice. Where cold
tanks are already installed, the re-fitting of a British Standard cylinder is very
economical at less than £100 for the cylinder. The disadvantages of this system
become apparent if there is a desire for a quality shower, especially in upstairs
bathrooms.
The performance of each hot water outlet depends upon its distance below the water tank. Lower outlets, such as a downstairs kitchen tap, have a higher supply pressure, and can achieve relatively high flow rates. Higher outlets, such as a shower head high on the wall of an upstairs bathroom experience pressures as low as 2 metres head, and will provide poor flow rates. This effect is amplified when both an upstairs and downstairs outlet are open simultaneously, as the downstairs outlet will draw more water and tend to starve the upstairs outlet, which will rapidly go cold. Not nice if your in the shower. When the downstairs outlet is closed, the upstairs outlet will increase rapidly in temperature, and may again prove unpleasant to the person in the shower if they have turned down the cold to compensate for the previous lack of hot water.
Combination of a cold tank, hot water cylinder and booster pump:
A good way to overcome the disadvantages of the basic system is to fit a booster
pump. These vary from single shower units through to whole house
high pressure booster pumps, with prices ranging from £100 to £600, and
performance can be extremely good when using a quality pump. The disadvantage is
noise and vibration, which can often be easily heard throughout the premises.
When fitting pumps, measures should always be taken to reduce the transfer of
vibrations from the pump.
Combination Boilers:
Where space is limited to the extent that their is only room for the boiler,
then combination boilers are the obvious choice. They also work out quite
cheap when considering the savings from no separate hot water system. However,
the flow rates of hot water they deliver to taps is rarely complementary, and
are affected by opening two outlets at once in just the same way as a basic
gravity system. The effect is usually more apparent however, due to the
lack of stored heat. Without any hot water storage the hot water system
can only deliver water to taps at a rate equal to the maximum boiler output.
This is usually equivalent to 8 litres per minute. Considering that a baths and
luxury showers can easily draw 25 litres per minute, it becomes clear that
combination boilers are not the best choice when flow rate is important.
If the limiting factor is space, and not so much cost, then it is always worth considering fitting a store under the kitchen worktop. This is rarely considered, but often provides an excellent solution. Another alternative is to do away with the boiler and fit an electrically heated store, the disadvantage being that electricity is still more expensive to run than gas, and is not so quick to recover.
Plate Heat Exchanger running off a Central Boiler:
This is by far the most compact and cost effective hot water system
when it comes to domestic installation. The system also performs extremely well.
However, a large central boiler supply, such as provided by district heating
schemes, is required, and hence this is not often an option.
Unvented Cylinders:
The range of unvented units on the market is quite extensive, and
allows the easy installation of a high performance hot water system.
Unvented cylinders run directly of the mains water supply, avoiding the use of a
cold water tank. Mains water pressure in the UK can often reach 7 bar (70
metres head) pressure, and to prevent the cylinder from exploding, a pressure
reducing valve is fitted onto the incoming mains supply. The standards
regarding unvented cylinders are strict, requiring the use of at least two other
safety devices for protection, as well as a qualified installer.
Thermal Storage:
In a basic hot water system, hot water from the boiler is pumped to the hot water cylinder, and heat is transferred to the stored domestic water through the primary coil. When hot water is needed the cylinder thermostat switches the boiler and boiler pump on.
The way that a thermal store heats hot water is very similar. Hot water is pumped from the store to a plate heat exchanger, which transfers heat to the domestic water flowing to outlets. When hot water is needed a flow switch switches on the pump. In the same way that a vented primary system can indirectly heat a pressurised (unvented) cylinder, a vented thermal store can indirectly heat domestic water under high pressure.
In addition, just like the primary water in a boiler circuit, a thermal store is filled with a set volume of water that never changes. The water never mixes with the domestic water and the system can be protected by the addition of corrosion inhibitors and anti-freeze. Fresh scale is not constantly introduced into the system, as is the case with a domestic copper cylinder, and the life expectancy of the store is greatly extended. Immersion heaters used to heat the store are also immune to the build up of scale, the most common cause of failure. Thermal stores are the only hot water storage systems that allow this level of system protection.
Once it is understood that the thermal store can be looked upon a primary system, containing primary water, it follows that the water in the thermal store can be pumped directly to a boiler for heating, providing the boiler system is vented. This provides a huge benefit with regard to recovery rates as the full output from the boiler can be pumped directly into the store. Heat transfer from the boiler to the stored water is no longer limited by the size of the coil, as there is no coil. Furthermore, boiler cycling is reduced, and fuel efficiency increased.
The GXV Heat Bank Thermal Store, designed by DPS for local authority use, effectively demonstrates how direct heating of the store can produce some quite spectacular results with regard to efficiency, recovery and performance. Not only will the store prevent boiler cycling during recovery, it also keeps condensing boilers operating in condensing mode for a longer period. It allows the full output of both the boiler and thermal store to be directed to hot water outlets, providing over 150 litres of hot water from a system employing an 80 litre store which fits under a kitchen worktop. Hot water can be drawn from taps within three minutes of a cold start, with flow rates in excess of 30 litres per minute, and mains supply pressures up to 6 bar. This kind of specification is simply not achievable by any other type of domestic system, and may explain why the GXV is the only hot water cylinder to ever reach the finals of the Institute of Plumbing Challenge, which has quickly been recognised as a true mark of excellence for innovation in the plumbing industry.
One might expect to pay a premium for a system offering the performance of a Heat Bank Thermal Store, however this is not the case. A typical domestic Heat Bank Thermal Store, factory wired and fitted with a pump, flow switch and heat exchanger, sells to the trade for more than £100 less than an equivalent unvented cylinder. The larger the capacity of the unit, the larger the saving. Unvented cylinders providing 500 litres of storage work out to more than double the price of an equivalent Heat Bank Thermal Store. The simple reason for this is the fact that thermal stores are vented and can be made to the British Standards for Grade 3 Copper Cylinders, whereas unvented cylinders need to made typically from steel to hold the high pressures, and require strict safety testing and approval.
Thermal Stores offer one more, quite significant benefit when compared to
unvented storage. Thermal stores can not only me made as cylinders of
virtually any height and diameter, they can be made as tanks, designed to fit
into specified clearances. As they are not sealed systems and do not
need to be submitted for safety approvals regarding pressure, thermal
stores can be custom made to order without the costs and delays associated with
obtaining these approvals. This flexibility makes thermal stores ideal for
both flats and housed, as well as new build and re-fit installations.
Trade Price List - Heat Bank Thermal Stores
Comparison of hot water systems
System D Heat Bank Thermal Store