Over the last fifteen years we have seen solar panels and wind turbines slowly make their way into the mainstream, and more recently heat pumps have had growing interest, however the bottom line has always been that fossil fuel technologies are cheaper to install than renewables, and far more capable of satisfying a properties demand for energy, and when compared to renewable fuels such as wood or biomass are also cheaper to run. That is until recently, with oil, gas, and electricity prices now soaring, the pressure to find alternative cheaper sources of heat is driving a huge increase in demand for wood burning systems.
At DPS we have been designing thermal storage systems to run using multiple
fuel sources for over fifteen years, and in the last year we have seen the
popularity of wood systems go through the roof, with most customers are now
asking that their hot water systems are supplied with suitable connections for
wood burning stoves or cookers, even if only for 'future-proofing' their
With a modern Heat Bank Thermal Store from DPS the following are all possible...
This is all possible because of the unique way that a Heat Bank Thermal Store works, far different to a traditional or unvented hot water cylinder. The key difference is that the stored water is not the same water that comes out of taps. In fact the stored water is the same water that fills the wood burner, and central heating system. The domestic hot water that feeds taps is heated up as it is drawn, using the heat stored in the Heat Bank Thermal Store as a battery, and using a Plate Heat Exchanger to transfer the heat as it is needed.
The diagram below shows a simple system with a wood burner heating the store, which in turn heats radiators, under-floor heating and mains hot water to taps. The system if fully vented, with a feed and expansion tank used to fill the system, keep it topped up, and to take up expansion of the water in the system.
The system shown is however incomplete as we have not shown a
form of overheat protection or dump to get rid of excess heat. Before we
cover this part of the design it is best to go over the basics of the wood
burners and the level of protection required.
Types of Wood Burner...
There are hundreds o models of Wood burners on the market, however they can generally be sorted into one of four categories:
To ensure that you are fully aware of what type of wood burner you have and what precautions must be taken, the following questions should be put to the manufacturer of the heater you intend to use:
When selecting a wood burner it is also important to ensure that the room
output is matched to the heating requirements of the room in which it is placed,
and not oversized. This is important in boiler models as the room can get
uncomfortably hot while insufficient heat is provided to water for use in
heating the rest of the property. On thermostatic models one may find that if
they are oversized for the room they are in that the heater will shut down the
burn rate and again provide insufficient heat to water for the rest of the
property. To overcome these problems make sure that the output to water is
relatively high when compared to the room output... one can always have a
radiator in the same room as the wood burner to top up the heat in that room if
required. Another solution is to install ducting for circulation of warm
air around the property, and to move heat from the primary room into adjacent
Sizing a Heat Bank Thermal Store...
As the Heat Bank Thermal Store is used to store and distribute heat energy, the size of store used is dependent upon how much heat you want to store. There are a few factors that influence this...
Working out the total amount of heat generated depends of types of fuel, how densely it is packed, the efficiency of the burner, and how much of the energy goes to water rather than to the room If one follows typical assumptions regarding wood type, 60% loading of the wood burner by volume, and 80% efficiency, then the following rule of thumb can be used to estimate the volume of thermal storage that is needed to take a full burn (for the full calculation and assumptions used, please see appendix 1) from a known combustion chamber size filled with logs.
It is not always required to store a full burn as it is fairly common for the central heating to be running at the same time as the burner is alight. If this is the case then the store size can be reduced.
To help with other common calculations regarding the size of store required to take a charge, or how long a certain size of store will run central heating for before it goes cold, we have created our EnergyStorage Calculator, which can be accessed or downloaded free of charge from the Flash Tools page on the DPS website, www.heatweb.com
There is also a useful Cylinder Size Calculator to work out the dimensions of stores for various storage capacities and store diameters that are available from DPS.
Circulation and Pipework...
Making sure there is suitable circulation of water to prevent it from boiling in a wood burning system is probably the greatest challenge to the system designer. Armed with a figure for how much energy is generated to water by your chosen heater, one can then proceed to plan out the pipework layout that will be the key to making things work, and the first principle that needs to be understood is that of gravity circulation, or thermo-siphon. Quite simply this is the way that heat rises relative to cold, and is the only way to ensure circulation of hot water without the use of pumps.
The forces generated by thermo-siphon are small, nothing like the power of a pump, so it is important to size pipework larger than one would for a typical boiler installation in order to reduce the resistance to flow. 28mm pipework is fairly standard, however larger sizes may be necessary for higher output heaters. Pipework should also rise (and fall) continuously, with air locks to be avoided at all costs, and horizontal runs kept to a minimum as they will reduce the output that can be transferred.
If it is required to connect to a thermal store on the same level as the wood burner then it will generally be necessary to connect a gravity circuit to radiators upstairs, as well as a pumped circuit to move the heat to loads on the same floor. There is no problem installing pumps on a wood burner system provided there is also a gravity circuit that can take away the full output of the burner in the case of a pump failure or power cut. The diagram below shows one possible configuration.
Overheat Protection on Heat Bank Thermal Stores...
Sometimes it is simply not possible to install heat dump radiators that can remove all the generated heat using gravity circulation. In these circumstances DPS can offer alternative forms of overheat protection on Heat Bank Thermal Stores to guard against boiling of water in the system. It is still a requirement to transfer the heat to the thermal store via gravity, but the store can then provide the dumping facility for excess heat when the store starts getting too hot.
There are two levels of protection that we use on a Heat Bank Thermal Store to remove heat. The first requires power to be present and works by turning on the central heating pump to get rid of heat when a thermostat on the store reaches a preset temperature, typically 90°C.
The two forms of protection are typically used together so that the central heating is used as an automatic dump under normal circumstances, but in the event of a power cut of pump failure the discharge to drain comes into play.
It is possible to build overheat protection into a pumped wood burner system by using a plate heat exchanger in place of dump radiators. They do the same job as a radiator - removing heat from the system - except the heat is transferred to cold mains water, rather than to air. The plate heat exchanger has one big advantage in been able to dump huge amounts of energy - far more than radiators are capable of.
The following diagram shows a suitable circuit which allows both pumped operation to a thermal store, and also has a gravity circuit that comes into play when the pump is not running. During overheat conditions the TS130 valve opens up to allow the cold mains to flow through the heat exchanger, cooling the circuit and driving gravity circulation. This form of overheat works without power, although power will required to pump water from the store to provide central heating.
The circuit shows the use of two other components that are extremely useful in designing circuits for wood burners, the Acaso Termovar and Termobac valves. The Termovar is a temperature control valve that mixes two water inputs (hot/cold) to achieve a target temperature. It is most useful for ensuring that the temperature of the water in the wood burner circuit is brought up to a minimum temperature to improve combustion and prevent 'smoking' flues. The Termobac is a non-return valve with a swing gate that provides a route for gravity circulation. It is used to connect to gravity circuits that should only come into play when required.
One of the main advantages of a Heat Bank Thermal Store is the ability to combine numerous heat sources in a system, allowing the end-user to choose what fuel to use. Typical heat sources that are connected are...
The following diagrams show how connection of various heat
sources is possible. All these schematics have been generated using
our online Schematic
Designer, available on our web site for anyone to use.
Multifuel Heat Bank Thermal Store Design CXC-290-DBADD-CIMZH+OHCOIL+STAT...
This is an example of the final DPS Heat Bank Thermal Store, embodying the aforementioned multi-fuel heating system in a fully pre-fabricated, wired and tested product.
This design can be generated using our PANEX Heat Bank Thermal Store Designer, available to use on our web site, as well as almost any combination of the systems talked about. The tool includes a wizard to help automatically select a design. A design code is generated and this makes it easy for us to bring up any designs our customers show an interest in, check the designs, provide further technical consultancy, and generate quotations.
Something that is often asked is how do you connect a vented wood burner to drive a pressurised central heating circuit ? This is easily achievable with a Heat Bank Thermal Store using a plate heat exchanger to transfer heat from the vented store into the pressurised heating circuit. The diagram below left shows this, and also shows how a system boiler can be brought into the equation. Running a pressurised heating circuit from the store also allows radiators to be sited higher than the feed and expansion tank, and this is sometimes necessary if the feed and expansion tank cannot be sited at the highest point in the system.
At Ecobuild 2008 we were invited to participate as part of the Zero Carbon House by Zedfactory and Bill Dunster Architects, a full-size installation, stripped back to reveal the anatomy of the building, the design features, products and solutions which combine to make zero carbon housing a reality now. Visitors were able to tour the entire structure and learn about the materials used, its systems and energy performance as well as the pre-fabricated timber frame panels that made it possible to complete the entire structure in just three days ready for the exhibition!
DPS has worked closely with ZedFactory to provide the ZedStore... not just a hot water cylinder, but rather the centre of the house's energy system. The ZedStore has been designed to work in conjunction with a centralised wood chip boiler, feeding typically six properties on a communal circuit, as well as wood burners and solar panels fitted in each of the properties, and demonstrates how wood burning technologies can be combined with other renewable heat sources to form a basis for zero-carbon design.
Appendix 1. Calculation of storage required for full load burn.
Using the following information (from the Solid Fuel Association) it is possible to calculate the total amount of heat generated from a wood burner, provided you know the internal dimensions of the burner for loading wood. This is important if the plan is to store up the heat given off by a full load of wood, possibly on an overnight burn for running central heating in the morning before the wood burner is reloaded.
To get the maximum load weight, multiply this figure by the density values in column 1 (Weight per m3) for your wood type ... we will assume Ash...
As logs cannot be perfectly packed (as some briquettes can) multiply by 60/100 (for 60% by volume)...
Next, to work out the energy given off, multiply this figure by the value in Column 2 (Gross heat value) for your wood type...
To work out the Net heat output, multiply this further by the efficiency figure quoted for your wood burner... lets assume 80% efficiency...
Next, divide this by the percentage of heat that goes to
water (rather than to room)...
To convert this figure into a suitable volume of stored water, assumed to be heated through a 40°C rise for a typical thermal store, multiply the net heat output by 860 and divide by the temperature rise to give...
If one follows similar assumptions regarding wood type, 60% loading by volume, 80% efficiency then this can be simplified to the following rule of thumb...
Appendix 3. Approximate Running Costs of Various
Appendix 4. Wood Suppliers.
The following links can be found at http://www.carbonneutralfuel.com/logs.html
Appendix 5. Thermo-Siphon Calculations.
The following data may help in calculating the forces generated through thermo-siphon (gravity) action.