How they can be utilised for the safe provision of
central heating and pressurised hot water


By Richard Hanson-Graville, MA.Mech.Eng FIOP, Managing Director DPS Ltd.         Download PDF Version

  
 

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.  

With the growing demand we are also seeing a rise in the number of systems that have been installed dangerously, and without a though for the safety measures that must be followed when using a fuel that cannot be turned off with the flick of a switch. In this guide we wish to clarify the basic restrictions that must be considered when designing a wood burning system, and to cover the methods we employ to design a fully integrated and safe system, while still following the cardinal rules...

  • A wood burner must be part of  a vented water system, with an uninterrupted path to atmosphere.

  • 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.

Click here to see Case Study


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:

  1. 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.

  2. Basic Room and Water Heaters (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). 

  3. Thermostatic 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. 

  4. 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:

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

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

  3. 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...

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

  2. Peak hot water demand.

  3. 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. 

 

Multi-Fuel Systems...

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...

Wood burners
Gas or oil boilers (vented or pressurised system boilers)
Solar panels
Heat pumps
Electric elements (powered from mains supply, or from wind turbines)
Central boiler plants on a communal system

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.
  

Connection to a Vented Gas or Oil Boiler
GX Method: Patented DPS Technology

Connection to a Sealed Gas or Oil Boiler
Via Plate Heat Exchanger (also used for central boiler applications)

Connection to a Solar Panel
Standard Sealed system on Coil

Connection to a large Solar Panel Array
Standard Sealed system on Plate Heat Exchanger

Connection to a Heat Pump
Direct Connection

Electric Heating
From 3kW to 15kW


 
Complete Multifuel System using Wood Burner, Gas Boiler, Solar Panels, & Electric Backup
   Driving Mains Pressure Hot Water, Underfloor Heating on Ground Floor, Radiators Upstairs.
 

 

 

 

 

3D Models of a complete system can also be found on our web site at www.heatweb.com/3d

These are in 3D PDF format and can be viewed, rotated, and zoomed in on using the free Adobe reader.

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. 

GX Recovery System, Solar Coil, Solid Fuel Connections (28mm), CB18-40 Plate Heat Exchanger (up to 55lpm), Themostatic Mixer: Heatguard 28mm, 28mm GX Primary Valve, Twin Cylinder Thermostats (Buffer), Economy Mode Cylinder Thermostat, Boiler Pump:Standard 6m, 22mm Htg Circuit (Upper Store), Standard 6m Head Pump (Upper Store), 28mm Htg Circuit (Lower Store), Underfloor Heating Temperature Control, 28mm, Standard 6m Head Pump, Two Channel Programmer (Ht+Htg), TP7000 Programmable Room Thermostat, 3kW Immersion Heater (Half Store), Overheat to Heating, Overheat to Drain via Finned Coil, Temperature Gauge.

1 Plate Heat Exchanger, CB18-30 (145kW)
2 Flow switch
3 Heat Exchanger Pump
4 Thermostatic mixing valve, 28mm
5 Drain off cock
7 White plastic coated steel casing
8 Vent
9 Cold Feed
11 Wiring Centre
13 Boost Immersion Heater
17 Cylinder Thermostat, Immersed [70°C]
28 Danfoss TS715 Programmer (Hot Water)
30 Danfoss TP7000 Programmable Thermostat
32 Flow to Solar Coil
33 Return from Solar Coil
34 Solar Sensor Pocket
35 Overheat Relief Regulating Valve
36 Overheat Relief Discharge Valve
40 Primary Return Valve, 28mm
42 Heating Pump 5m
44 Heating Pump 6m
48 Temperature Gauge
53 Y-Pattern Strainer
55 Solid Fuel / Gravity 1'' Connection
74 Economy Mode Cylinder Thermostat
75 Finned Coil (Overheat)

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.


Connecting a Wood Burner to a Sealed Heating System...

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.

Smaller central heating loads can be drive through a coil, rather than a plate heat exchanger.  


The Heat Bank Thermal Store and The Zero-Carbon House...

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.   

"It is all very well having all this zero carbon heat if it then has nowhere to go. For many years this has held back the harvest possible from solar thermal and biomass and hence payback. Forming a partnership with DPS we found a like-minded manufacturer able to design heat stores to make the most of the renewable technologies we have sourced."   ZedFabric Innovations

Read about The Beddington Zero Energy Development, or BedZED, the UK’s largest eco-village, by Zedfactory.  

 

 


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.

    Weight per
m3 in kg
Gross heat value
kWh/kg (btu/lb)
% Moisture
when green
Seasoning time
in summers

Hardwoods
(fully air dried)

Ash 674 4.1 (6,350) 35 1
Beech 690 4.3 (6,700) 45 1-2
  Birch 662 4.1 (6,350) 45 1
  Elm 540 3.6 (5,600) 60 2-3
  Oak 770 4.5 (7,000) 50 2-3
  Poplar 465 2.6 (4,100) 65 1

Softwoods

Pine/Fir 410 2.6 (4,100) 60 1

 
As an example, if the internal dimensions of the wood burner are 500mm x 250mm x 250mm then the volume of the space is calculated as follows...

Volume (m3)  =  Length (m) x Width (m) x Height (m)   =  0.5 x 0.25 x 0.25   =  0.03125 m3

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...

Max Weight (Kg)  =  Volume (m3) x Density (Kg/m3)   =  0.03125 x 674  =  21 Kg

As logs cannot be perfectly packed (as some briquettes can) multiply by 60/100 (for 60% by volume)...

Max Weight of Logs (Kg)   =   (60 / 100)  x  21   =   12.6 Kg

Next, to work out the energy given off, multiply this figure by the value in Column 2 (Gross heat value) for your wood type...

Gross Output  (kWh)  =  Weight (Kg)  x  Gross Heat Value (kWh/Kg)  =  12.6  x  4.1  =   51.8 kWh

To work out the Net heat output, multiply this further by the efficiency figure quoted for your wood burner... lets assume 80% efficiency...

Net heat value (kWh)  =  Gross output (Kw) x  Efficiency  =  51.8 kWh x (80 / 100)  =  41.45 kWh

Next, divide this by the percentage of heat that goes to water (rather than to room)...
lets assume 60% ...

Net heat output to water from a full load   =   41.45  x  (60 / 100)   =   24.9 kWh

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...

Volume of storage required (litres)   =   Output (kWh)  x 860 / 40   =   24.9 x 21.5   =   535 litres

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...

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


Appendix 2.  Overheat Relief Valve, Honeywell TS130.


Application

The TS130 Temperature Relief Valve for heating systems to
DIN 4751, Sheet 2 is a self-acting valve which is activated by
the flow temperature of the heat generator. It opens and discharges
water from the heat generator or condensing coil at a
flow temperature of 95°C (203°F) and thereby prevents a
significant temperature rise in the heat generator.

Features
• Construction tested to DIN 3440
• Immersion pocket with double heat sensors
• With test facility
• Capillary tube protected against kinking by steel sheath
• Immersion pocket with external thread

 

Function
The temperature relief valve is actuated by the flow temperature
of the heat generator. It comprises a spring-loaded
valve and a bellows operated temperature sensor. When a
boiler flow temperature of 95ºC is reached the force exerted
by the bellows system becomes greater than the force of the
spring and the valve opens. Heated potable water then flows
out and this is replaced by cold water from the supply network.
This absorbs excess heat from the heat generator and
prevents overheating.

Specifications
Heating system capacity max. 93 kW (80 000 kcal/h)
Operating temperature 95°C (203°F)
Flow capacity 2000 kg /h water at a minimum
flow pressure of 1,0 bar  (14,5 P.S.I.)
Connection size Rp 3/4" (DIN 2999)

Design
The Temperature Relief Valve comprises:
• Housing with internal thread
• Bonnet
• Valve piston with seal disc
• Spring
• Immersion pocket
• Remote double temperature sensor with capillary tube
• Immersion sensor G 1/2” (ISO 228)

Materials
• Brass housing, bonnet and immersion pocket
• Copper temperature sensor
• Copper capillary tube
• Brass valve piston
• Hot-water-resistant elastomer seals

 


Appendix 3.  Approximate Running Costs of Various Fuels.

The following table has been composed on 14 November 2008 and compares the costs of various fuels when used for heating. We have approximated boiler efficiencies and used readily available suppliers for our comparisons.

Cost
  pence/kWh  
Wood  2.7 From Surrey Logs (www.andrewheard.co.uk)
Gas 4.0 From Energy Comparison Web Site
Oil 4.7 From Southern Counties Fuel 42.97p/ltr
Cheap Rate Electricity 5.2 From Energy Comparison Web Site
Peak Rate Electricity 10.9 From Energy Comparison Web Site

 


Appendix 4.  Wood Suppliers.


Article on how Croydon Council is making locally sources renewable heating via wood a reality for communities...  http://www.grownupgreen.org.uk/library/?id=880

The following links can be found at  http://www.carbonneutralfuel.com/logs.html

UK Mainland RWS Bio Fuels Wood Pellets, Sawdust logs and Bio Bricks
North England Bingley Logs

Logs delivered in Cheshire, Cumbria, Derbyshire, Lancashire, Merseyside, North East, Yorkshire

North West England Grange Wood Fuels Seasoned hardwood logs in nets, bags or bulk loads
Lincolnshire, Market Rasen Lincolnshire Tree Services Seasoned hardwood logs delivered
Nottinghamshire & Lincolnshire logs2buy.com Bagged hardwood logs with a free friendly delivery service Newark, Nottingham and Lincoln areas.
Surrey, Shalford Surrey Logs Logs and kindling - collect or delivered
Sussex, Worthing Sussex Logs A mix of ash, oak and beech logs in full or half loads
West Lothian, Linlithgow Champfleurie Estate Bags of seasoned logs delivered

 


Appendix 5.  Thermo-Siphon Calculations.

The following data may help in calculating the forces generated through thermo-siphon (gravity) action.

 
Temp
( °C )
Density
tap water
( g/cm3 )
4
0.99999
20
0.99823
40
0.99225
60
0.98389
80
0.97487

 

Pressure of Water = height x 9.81 x density

With a temperature rise from 60°C to 80°...
Difference in Pressure between hot and cold  =  height x 9.81 x ( 983.89 -  974.87 )
=   height  (m) x  88.4862  (i N/m2)
=   height (m) x  0.009023061  (metres head of water)

This force needs to balance out with the pressure losses in the pipework under circulation.