DPS MultiFuel Heat Bank Thermal Store

DPS MultiFuel Heat Bank Thermal Store

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 systems.

The system must ensure all heat generated to water is removed from the heater (and system) without reliance upon electrical power, the operation of pumps or human intervention. i.e. the system water must not reach

100°C under any circumstances.

The Basic Principles of the Heat Bank Thermal Store.

With a modern Heat Bank Thermal Store from DPS the following are all possible.

A wood burner can be used to drive central heating including radiators, underfloor heating, with both vented and pressurised heating circuits.

A wood burner can generate mains pressure hot water taps and showers.

A wood burner can be combined with a gas or oil boiler, electric elements, heat pump, solar panels and other biomass boiler or wood burner to form a true multi-fuel heating system.

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:

Room Heaters. where the burner is not connected to a water system, and the heat given off is used to heat the room it stands in. At DPS we are not interested in pure room heaters — they are not connected to a water system or a thermal store and as such there is no further design needed other than ensuring the flue in installed correctly.

Basic Room and Water Heater s ( boiler models). where a percentage of the heat generated goes into the room, and the rest goes into heating water for driving central heating or generating hot water for taps. The burn rate of these systems is manually controlled by opening or closing the air intake to increase or decrease the intensity of the fire. These are the ‘most demanding’ to the designer as one must be able to ensure that the maximum amount of heat that can be given off from the burner is carried away from the stove to prevent water in it boiling, and can be stored or dumped somewhere safely (even during a power cut).

Th ermostatic Room and Water Heaters. are the same as above but also has the facility to shut down the air intake rate automatically when the temperatures get too high. This takes a little pressure off the designer as it limits the amount of heat that must be stored or dumped somewhere safely to prevent water in the system from boiling.

Pellet Boilers. which are electrically controlled and burn pellets (as opposed to logs). These devices are highly controllable and can turn themselves off safely when no further heat is required, and as such are easy to design for as there is no need for additional overheat protections.

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:

What is the maximum output to water (in kW) from the heater when fully loaded ?

What is the maximum output to the room (in kW) from the heater when fully loaded ?

Does the heater automatically shut down its rate of burn at higher temperatures. If yes then what is the output (in kW) to water that needs to be dumped at this lower burn rate?

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 rooms.

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.

Total amount of heat (in kWh) that the wood burner generates in a full load.

  • Peak hot water demand.

  • Size of gas or oil boiler (in kW) connected to store as a backup.

  • 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.

    Volume of storage required (litres) = Length (cm) x Width (cm) x Height (cm) x 0.017

    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.

    One must not forget that the size of thermal store also depends on the total volume of hot water required between store reheats. A cylinder that is required to deliver five full baths of hot water within a half hour period will need to be much larger than one that is only required to deliver one shower. Again, to take the donkey work out of the calculations we have provided our WaterLoad Calculator which allows accurate sizing based on your selection of hot water loads, as well as providing adjustment for an additional boiler input.

    We use these tools daily in the designing and sizing of Heat Bank Thermal Store systems, and please feel free to make use of these and all the other tools on our Flash Tools page .

    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 second uses a coil fitted inside the Heat Bank Thermal Store, through which we pass cold mains water that then heats up (and cools the store as it does so) before being discharged to drain. A mechanical valve (approved for this purpose) initiates the flow of mains water when the store reaches 95°C, and requires no power to be present. This form of overheat protection is capable of removing over 30kW of heat from the store, and it suitable for the largest domestic wood burners. More information on the discharge valve can be found in appendix 2.

    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.

    Overheat Protection using a Plate Heat Exchanger.

    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.

    DPS can supply a pre-fabricated overheat assembly for pumped wood burner systems that incorporates pump, temperature control, plate heat exchanger and overheat discharge valve, as well as a backflow preventer that allows the swing from pumped to gravity.

    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.


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