The Case for a Woodwarm Water Heating Stove

For the majority of U.K. homes, the most economical and convenient form of heating is still natural gas .When burnt at the point of use, natural gas is an extremely clean and efficient fuel, with modern condensing boilers achieving efficiencies of well over 90%. However, for areas that are not on the natural gas grid and who do not want to use gas, the options available are more limited and careful consideration of a number of factors is required in order to make the most appropriate choice.

The main heating fuels available to non gas users are oil (28sec), electricity, propane (bulk tank), propane (cylinders), solid fuel, and wood .Within these fuel types, wood is  the only fuel that can be regarded as carbon neutral, and on current trends, can represent an economical and effective way of heating properties off the gas grid. In the UK, log fired and multi-fuel stoves still represent the most popular and simplest method of burning carbon neutral fuel, in addition to this, the supply chain for wood is well established and reliable and many users of this fuel are self sufficient and manage or own woodland.

Fuel cost comparisons show that hardwood logs are commercially available in small delivered loads on a national basis from locally based regional suppliers, and have a local (Devon) cost of approximately £80 per unstacked cubic metre. This has a weight of 350-500 kilo and will vary considerably due to moisture content and type of wood. An average value for the heating capability of this wood (energy density) is 4 kWh/kg. This would result in an average energy yield of 1700 kWh per cubic metre of well seasoned, stacked hardwood.

This provides an energy cost of 5.0 pence per kWh, which has no comparison with the present cost of bulk deliveries of heating oil.. Bulk buying of unprocessed wood could be cheaper with typical prices of less than 20% of the above example, this would yield an energy cost of below 1 penny per kWh but would require work to cut, split and stack the wood and an adequate timescale must be allowed for seasoning. This requires space and forward planning but is possibly the cheapest way to heat a home in the UK

Fuel Cost Comparisons

FUEL...		Per...
Mains natural gas	£0.032	1 kW/hr
Heating Oil	£0.650	litre
LPG	£0.650	litre
Anthracite	£14.00	50kg sack
Wood logs	£180.00	ton
Wood pellets	£240.00	ton
Bituminous coal	£12.00	50kg sack
Electricity	£0.123	1 kW/hr
Economy Electric	£0.050	1 kW/hr

 

FUEL...	Cost per supplied kW/hr	Appliance	Efficiency %	Cost per useful kW/hr	kg carbon emitted per useful Kw/hr
Mains natural gas	£0.0320	Condensing boiler	90%	£0.036	0.059
Mains natural gas		Standard Boiler	78%	£0.041	0.068
Mains natural gas		Gas fire	35%	£0.091	0.151
Mains natural gas		LFE fire	25%	£0.128	0.212
Heating Oil	£0.0591	Oil Boiler	78%	£0.076	0.095
Heating Oil		Oil Stove	70%	£0.084	0.106
LPG	£0.0929	Condensing boiler	90%	£0.103	0.076
LPG		Standard Boiler	78%	£0.119	0.087
LPG		Gas fire	35%	£0.265	0.194
LPG		LFE fire	25%	£0.371	0.272
Wood logs	£0.0350	Boiler	78%	£0.045	0.001
Wood logs		Stove	72%	£0.049	0.001
Wood pellets	£0.0464	Boiler	85%	£0.055	0.006
Wood pellets		Stove	83%	£0.056	0.006
Anthracite	£0.0324	GF Boiler	77%	£0.042	0.112
Anthracite		Stove	72%	£0.045	0.119
Bituminous coal	£0.0323	Open Fire	25%	£0.129	0.344
Bituminous coal		Open Fire & Boiler	62%	£0.052	0.139
Bituminous coal		Stove	50%	£0.065	0.172
50/50 Wood/Coal	£0.0337	Boiler	85%	£0.040	0.051
51/50 Wood/Coal		Stove	72%	£0.047	0.060
Electricity	£0.1230		100%	£0.123	0.113
Economy Electric	£0.0500		100%	£0.050	0.113

The above table was taken from the fuel cost calculator on www.soliftec.com on 20 June 2008

 

 

Wood-firing as a Central Heating Heat source

A wood-fired stove is an effective means of heating and offers the benefits of being a carbon neutral heat source and of isolating the user from issues of security of energy supply. The fuel can be stored in large quantities and if procured and processed efficiently is one of the lowest cost heating fuels available.

To evaluate wood-firing effectively as a method of heating, it is important to be aware of and understand a number of issues including comparative fuel costs, property type and location, available space, type of lifestyle, user convenience, and security of supply and heating system design.

Considerations when Deciding on Wood-firing as a Heat Source

Advantages

Carbon neutral fuel (if managed sustainably)

Cost effective especially if bought in bulk

Environmentally clean

No toxic ashes

Low capital cost

Low / Zero maintenance costs

Extremely reliable

Versatile and can be linked with other heat sources

Can be backed up by alternative energy sources

 

Disadvantages

Large volume of wood required to heat a typical property

Storage space required under cover

Must be correctly seasoned for best results

Manual handling and firing of fuel

Commercially available logs can be expensive

Not easy to synchronise firing with usage of heating

When considering the above factors, the suitability of the property must be included. The most suitable property will have good access and covered log storage space. In addition to the above factors, consideration of a suitable plumbing system will make a huge contribution to the user friendliness and running costs of a wood fired heating system.

 Wood fired Plumbing System Design

Effective and user friendly utilisation of wood firing for central heating depends on the layout and design of the system. The system should be robust, safe and where possible include a back up fuel / heat source that can be used if required. Critical to all Woodwarm stoves is the residual heat that the stove must be able to generate before it generates heat to water. Always refer to the specification sheets for the different sized models and their boiler possibilities

The following is an outline of two of the systems that can be used for wood fired central heating.

Gravity Hot Water and Pumped Radiator System

Suitable wood fired stoves can be directly plumbed in to the heating system & can be used with a heat store.

Without a heat store, the preferred system is with a gravity domestic hot water circuit from the stove to an indirect hot water storage cylinder, and a pumped radiator circuit. This system should be open vented with a suitable header tank at least 2 metres above the highest radiator or cylinder, and with a 22mm open vent pipe to the header tank, and an open cold water feed pipe from the header tank. The header tank must not be plastic.

No valves should be fitted in the primary gravity circuit, thereby allowing fail safe venting via the vent pipe, and re-filling of the system from the header tank. The radiator circuit will usually be pumped and will utilise the other top and bottom boiler tapping's (created by joining the flows and returns of the split saddle boilers) to feed the radiator circuit via. a suitable circulator pump

The hot water storage cylinder should be an approved insulated cylinder of minimum size 450mm x 1200mm high (160 litres capacity) with a coil suitable for gravity circulation. It is recommended that a heat leak radiator is fitted within the gravity circuit to help circulate heat from the stove when it is idling without the heating pump running, this radiator would normally be located in a bathroom adjacent to the cylinder cupboard.

An essential  safety feature for this type of system is the addition of a pipe  thermostat fitted to the gravity flow pipe from the stove This would be set to 90-95 degrees centigrade and must have a permanent live mains feed. This thermostat should switch on the central heating pump when the hot water has reached a suitable temperature and will safely disperse the excess heat around the radiator circuit. There should also be a low level pipe thermostat set at 50-55 Degrees centigrade to stop the pump and ensure that cold water is not circulated, so not causing condensation in the combustion area of the stove.

This system has two main disadvantages. The first is that firing of the stove is required whenever heat is needed from the radiators, this can be inconvenient, if for example, the radiators are required to come on first thing in the morning. . The second disadvantage is that when low heat or no heat is required from the radiators, the stove is idling (and possibly still producing more heat than the hot water cylinder can absorb), this can result in unwanted heat to disperse and results in the wood burning inefficiently at low temperatures. Low temperature smouldering of wood results in more tar formation and higher usage of fuel.

Use of a Heat store or Thermal Accumulator

A Heat store or Thermal Accumulator is a vessel designed to store a volume of heating water, the vessel should be well insulated to reduce heat loss. The size of the vessel depends on the performance required and the size of the heating load. Typically, the ideal heat store will be large enough to store sufficient heat to provide one full day of heating and hot water requirements for the property being heated.

The amount of heat that can be stored (measured in kWh) by a heat store also depends on the temperature range of the output of the heat store, this is the difference between the maximum temperature and the lowest usable temperature of the water at the outlet of the heat store. A maximum water temperature of 85 degrees is practical and a minimum usable temperature for heating radiators is 45 degrees C. This gives a temperature differential of 40 degrees C and would allow the maximum heat storage capacity of a given size of heat store to be calculated. With this temperature differential, a 1,500 litre heat store will store approximately 70 kWh of heat energy, enough to provide full heating and hot water requirements for a typical detached house. The correct size of heat store would need to be calculated using information including the size of the house, the usage of the heating required and the heat loss of the house. The calculation is based on the quantity of heat required over a given period rather than the rate of producing heat in the case of a stove only.

The benefits of using a heat store are substantial. Because the heat is stored, the stove can be fired at a convenient time when some one is available. The stove can be fired in the evening, and the heating timed to come on first thing in the morning and again late afternoon. In winter, the stove can then be relit in the evening to repeat the process. In spring and autumn, it may be possible to reduce stove firing to once every two days, and in summer, for hot water only; the stove may be fired even less frequently. Because the stove can be fired at maximum firing rate, the combustion temperatures are higher and the stove works more efficiently, with significant fuel savings, there is also a dramatic reduction in tar and creosote deposits in the stove and flue, as these compounds are burnt up in the combustion process. A Heat store can be used to link up the stove with additional back up sources of heating. This can be with solar, oil, natural gas or propane boilers, or by the use of night time tariff electricity to charge the heat store by immersion heating elements. The heat store provides an easy method of harnessing off peak electricity to heat a property with conventional central heating and has the benefit of a low initial installation cost and running costs that are lower than oil.