Why the HEATBANK Xcel beat all the competition
Xcel uses a Plate Heat Exchanger for Hot Water rather than a Coil
This gives a huge performance benefit to the Xcel. The following graph shows the output of a Heat Bank compared to a a well known make of coil type thermal store. The graph details the temperature of water provided over time, with taps running at 18 litres per minute (a good bath tap).
The graph demonstrated two key differences between plate heat exchangers and coils.
The first is how the output temperature from a coil type thermal store drops with time as the thermal store cools down. From the graph, within 5 minutes draw off the temperature of water produced becomes too cold to drive hot taps. Given that the draw off rate is 18 litres per minute, the coil delivers approximately 90 litres before performance drops off.
By comparison, the Heat Bank draws water from the top of the store, returning it cold to the bottom, and hot water temperatures are maintained until nearly all the stored heat is used up. The result is 200 litres of hot water - over twice as much - from the same size 180 litre store.
The performance of hot water coils is usually quoted for the initial draw off, when the store is at full temperature, and it is important to know that this is not the whole picture.
The situation gets even worst when the store is only partially heated. For example, if only the top half of the store is heated, then with a coil type store only some of the coil is sitting is hot water and performance will be much lower than quoted. The Heat Bank however will give full performance with only the very top of the store heated, and as such allows it to operate in an economy mode with only smaller volumes heated up. This is very noticeable when starting a system from cold. The Heat Bank will generate hot water to taps in only a few minutes from cold start, where a coil type store will take some time while the majority of the store is heated up.
Coils also have another disadvantage - they scale up. We pioneered coil type thermal stores some 30 years ago, and have a great deal of experience with them. One thing we found was that in hard water areas the coils scaled up rapidly, becoming unusable within a year in some instances. This is a result of the fact that the coil is sitting in hot water and the mains water inside the coil heats up to store temperatures. At these temperature limescale rapidly deposits and if not serviced in time will result in the coil becoming unusable and the cylinder may need changing (there are a few sites where coil type stores have failed this way and the entire site has had units changes to Heat Banks).
On a Heat Bank the heat exchanger sits externally to the store and is not submerged in hot water. It is still possible for a plate heat exchanger to scale up, but it is far more gradual and easy to service. There have only ever been a handful of instances where plate heat exchangers have been left to the point where limescale became a real issue. Most were solved with a powerflush, and in the couple of occasions where this did not work the heat exchanger was replaced.
Another advantage of plate heat exchangers over coils is the power of them to generate hot water. The standard heat exchanger on the Xcel is capable of generating over 100kW of mains hot water, or over 40 litres per minute, and at pressures up to 10 bar. In order to get anywhere near the performance of a plate heat exchanger, a coil size needs to be so large that the cylinder itself usually needs reinforcing to carry the weight of the coil. There are also options for over 300kW plate heat exchangers, achieving levels of performance beyond the reach of any coil.
This leads onto one more disadvantages of coils - decent performance coils are sometimes easily damaged in transit due to their sheer weight,
In summary therefore:
- Plate heat exchangers can produce approximately twice as much hot water from the same size store fitted with a hot water coil.
- Plate heat exchangers can generate higher flow rates at higher pressures,
- Plate heat exchangers function to full performance with only small volumes of the store heated, allowing the use of an economy mode, and providing hot water within minutes of a cold startup.
- Coil type stores suffer badly from limescale depositing within the coil.
Patented Top-Down Reheating of Stored Water
When using a boiler to reheat stored water, Xcel Heat Banks offer advantages that NO other thermal store provides. We spend a number of years perfecting our patented top-down reheat system that uses a return temperature control to ensure that the water fed to the boiler for heating is always at the perfect temperature to ensure optimum efficiency, and in turn guarantee that the water fed into the top of the Heat Bank is at a temperature that can be used immediately.
Other makes of thermal store use a simple flow and return pipework to boiler, where water is pumped from the bottom of the thermal store to the boiler and back. The problem with this system is that the temperature of water in the bottom of the thermal store varies greatly, from 25°C up to 70°C, and a boiler will only lift water temperature 20°C in a single pass, typically 15°C. If the water in the store is at 55°C, then the boiler will heat it to 70°C - fine. The problems start happening when the bottom of the store drops to say 35°C. At this point the boiler will heat the water up to 50°C, and this is too cold for achieve decent performance. This 50°C water is fed back into the thermal store, and can adversely affect the performance of hot water outlets currently running, as upper store temperature start dropping.
To look at the problem with other stores from another angle, look at a thermal store as it is heated from cold (25°). On the first pass the boiler raises the store temperature from 25 to 40°C. It has to heat up the whole store so this takes some time (15 minutes for a 300 litre store running of a 20kW boiler). The boiler then has to heat the store from 40 to 55°C, and again this takes another 15 minutes. Once the store is at 55°C it should be possible to achieve low flow rates to taps, although to achieve stated performance you still need to wait even longer as the boiler heats the store from 55 to 70°C.
With the Xcel, as soon as the system is started water is recirculated around the boiler until it reaches 70°C. This usually takes less than a minute, and then the system feeds the 70°C into the very top of the Heat Bank where it can be used immediately for hot water. Most customers have great pleasure in pointing out how they manage to obtain showers within minutes of starting the system up - a big difference from their previous systems where they had to wait over half an hour for any hot water.
The benefits of this type of reheating are multiple. If, for example, the aim of reheating is to run a shower or wash up some dishes, then there is no need to reheat anything more than the very top of the thermal store. As just described, when using other thermal stores one will need to heat up the entire store just to obtain a few litres of hot water. This is very inefficient, especially where there are renewable heat sources such as solar in the picture. The Xcel top-down reheating allows the user to only heat up the water they need, and no more. This saves time and money, and also ensures that there is enough cold water in the store for the solar panels or wood burner to work with.
These benefits make it possible to fit quite a large store into a property to allow for a 'full house' without loosing efficiency when there is only one or two people living there. The amount of water heated can be adjusted to match the occupancy level of the property, and this is a benefit unique to the Xcel Heat Bank.
- Rapid reheating using the full output of the boiler.
- The ability to easily set how much stored water is heated, and how much water is left unheated for use with renewables such as solar.
- Lower standing heat losses as minimal volumes of water are heated.
- Maintains condensing mode on condensing boilers, while stopping boiler rot in oil and pellet boilers.
- Maintains temperature differential across boiler.
- Automated control of economy/full store modes using programmer and two or more cylinder thermostats.
- Patented technology.
Patented Overheat Protection
Another unique aspect of the Xcel Heat Bank (again, patented by us so unavailable anywhere else) is its ability to protect from overheating, even during a power cut.
This system was designed primarily for systems connected to a wood burner. It is needed because wood burners provide a large amount of energy, and they don't turn off, even when there is a power cut. As such they easily have the ability to boil the water in a thermal store if there is no other form of protection.
The standard approach is to use a heat leak radiator - located higher than the wood burner so that heat generated in the wood burner circulates naturally (thermo-syphon) to the radiator where heat is dissipated and the cooled water drops back into the wood burner. This is fine providing:
- The wood burner is thermostatic, and slows its burn rate down when it gets hot.
- The heat leak radiator is large enough to dump the output of the wood burner.
- A location can be found above the wood burner for a dedicated heat leak radiator.
The problems start occurring when not all of these criteria can be met, and given that most wood burners are not thermostatic and have an output of approx 9kW to water, the chances of dedicating 9kW of radiators (three large double convectors) to this function becomes impossible.
The protection system on the Xcel works on the basis that the heat can circulate from the wood burner to the Heat Bank naturally using thermo-syphon (gravity) circulation. The Xcel is fitted with a heat exchanger in the top, and if the temperature of the water in the store ever goes over 95°C then a mechanical valve opens up to allow cold mains water to pass through the heat exchanger and out to drain, cooling down the stored water as it does so. The heat exchanger can extract over 12kW of heat and as such is suitable for all types of wood burner, and in many instances is the only way that the latest building regulations (now covering vented systems) can me met when using a wood burner.
When someone is designing a system for you to run with a wood burner, ask them to guarantee that the system will not boil if there is a power cut and the wood burner is kept going at full output. Some installers may state that it is ok to get away with a fairly small heat leak radiators, but they are often not taking into account changes in building regulations that now cover vented systems.
There is a reason why the Xcel Heat Bank is the only system that all well known wood burner manufacturers approve is that we know how to ensure systems are safe. Again this is a patented system we have developed ourselves and is available no where else.
It is also worth pointing out that there are two other forms of overheat protection built into Xcel Heat Bank systems. The first is an overheat thermostat that turns on the central heating when the store reaches 90°C. This protection is the normal day-to-day protection and requires power supplies to be on, and pumps to be functional. The other form of protection is the system is open vented. Whatever happens and if ever the system does boil, you have peace of mind that the vent pipe will protect the system from catastrophic failure.
- Protects against system boiling even during a power cut.
- Often the only way to comply to latest building regulations.
- Approved by most wood burner manufacturers.
- Patented technology.