MORE ABOUT WOOD STOVES 1 JULY 2009.

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1 MORE ABOUT WOOD STOVES The efficiency muddle Last month I mentioned a wood stove, the Xeoos Twinfire, for which an amazing efficiency of 90% is claimed. But the official HETAS Guide states that the gross efficiency of the Xeoos is 76%. That s a remarkable difference, yet a similar anomaly is to be found for most stoves. For example, Aga claim an efficiency of 81% for their Little Wenlock Classic stove, yet the efficiency given in the HETAS Guide is only 74%. What s going on? The discrepancies arise because the efficiency of stoves has been measured in two different ways. HETAS use the gross efficiency, which is used in an old British Standard. In 2003, this was replaced by a European Standard which uses net efficiency. The names given to the figures seem to be badly chosen. In normal parlance, a net figure is a gross figure with some amount taken away, and so the net figure must be less than the gross figure. That is not the case with the efficiency of a stove. The figure for the net efficiency is always greater than the gross figure. Moreover, the figure for the net efficiency could theoretically be more than 100%. Normally, 100% efficiency is the maximum, of course, so something strange is going on here with the figures or at least, with the names that have been given to them. By the way, a similar scenario is to be found with gas boilers, and some condensing boilers do have net efficiencies greater than 100%. Their exhaust gases are cooled so that steam, which is produced as a by-product of combustion, is condensed into water. Latent heat is released and this takes the net efficiencies to more than 100%. Condensing wood boilers do not exist, but they are a theoretical possibility and their net efficiency could conceivably be more than 100%, too. The figures given for efficiency in the publicity material for wood stoves are invariably for net efficiency. This is in accord with the European standard and the higher figure looks better, too! But these figures don t match up with the gross efficiency figures on the HETAS listing. And these efficiencies are the ones that matter for building regulations. I m indebted to James Verlaque of Gastec for explaining the origins of the nomenclature. The crux of the matter depends upon how the energy embodied in the steam within the flue gases is dealt with. For a fuller explanation, see his to me in the Footnote. Caveat emptor let the buyer beware is a warning in the legal world. There is another way in which the unwary might be misled. Some stoves are dual fuel and can burn wood or fossil fuel (or a mix of the two). For some of these stoves, the efficiency with smokeless (fossil) fuel is higher than that with wood. The higher efficiency figure is quoted, leaving some naїve purchasers of the stove believing that it applies to wood burning, too. The Charnwood flue boiler One of the leading manufacturers of wood stoves in the UK is Charnwood, run by the many members of the Wells family on the Isle of Wight. Since I live on the Isle of Wight, I went to visit them and found a factory that is working flat out. They kindly allowed me to see their copy of the British Standard for stoves (BS EN 13240) to help in the writing of this article. (I had been unwilling to pay 180 to buy a copy of the standard from BSI.) Whilst there, I discovered a recent innovation of Charnwood s, MORE ABOUT WOOD STOVES 1 JULY 2009.

2 the flue boiler. This is an optional extra for some of their stoves and it replaces a section of the flue immediately above the stove. I said above that there are no condensing wood boilers, but the flue boiler does take a step in that direction. It extracts heat from the flue gases, heat which would otherwise go to waste. As the name suggests, the boiler produces hot water without any significant loss of performance in the stove itself. There is more to this device than simply wrapping pipework around a flue. A number of chains hang down inside, and they help to clean up the smoke. (Tar condenses on the chains. It trickles down the chains, and then drips onto a hot plate at the top of the stove. There the tar burns away.) Zenex GasSaver As an aside, I ll mention a somewhat analogous heat-saving device for gas boilers the Zenex GasSaver. This recent invention by a British plumber pre-warms a small store of water by capturing some of the heat in the flue gases coming from a condensing boiler. When required, the pre-warmed water is fed into the boiler where its temperature is raised further to create domestic hot water. Zenex claim that the GasSaver can reduce the annual cost of domestic hot water by a third. If you are installing a condensing boiler perhaps you are building a house down to the energy standards of the present building regulations then fitting a GasSaver may well be worthwhile. (The GasSaver is most effective in wintertime, so it complements solar water heating.) The high price of standards As another aside, I d like to express my regret about the excessive prices being charged by BSI British Standards for their standards. In these days of the free or cheap download of documents from the web, the prices of their standards seem exorbitant. They are just too costly for interested individuals. BSI British Standards, which is the successor to the famous British Standards Institute, is run too much as a (monopoly) business, and it funds itself from sales of the standards. There was a time when paper copies of the standards could be viewed at public libraries. Nowadays there is an online service offered to libraries, but very few libraries can pay the high price required by BSI. Your chances of viewing a standard at any library near you are very slim. The government likes to say that we have a knowledge economy. They should takes steps to make the knowledge encapsulated in standards more widely available to the general public. More about boiler stoves The performance of a boiler stove tends to be a little inferior to that of a dry stove. This is because the boiler tank cannot be as hot as the rest of the firebox, and a hot fire is needed for complete combustion. Knock-on effects are that the boiler tank tends to corrode and the door window becomes sootier. (Even for a dry stove, the publicity material usually promises a cleaner window, not a clean window!) The slight loss of performance of a boiler stove is more marked when cold water is in the boiler tank rather than hot water. With some sophisticated pipework at the back of the boiler it is possible to minimise the amount of time that the boiler contains cold MORE ABOUT WOOD STOVES 2 JULY 2009.

3 water. (Rather than allowing cold water to flow straight into the boiler, cold water is allowed to enter only gradually, and so it is quickly warmed up.) The control pipework could be made up in situ by a knowledgeable installer (eg, a HETAS authorised installer). More easily, a proprietary device such as a Laddomat or an H3 Panel can be installed. (See Further Info.) Log boilers As I said last month, a boiler stove complements solar panels to give carbon-neutral hot water all through the year. Larger boilers can give full central heating, too. But if that is what you want, it may be better to install a log boiler in the utility/boiler room. These boilers are designed to produce plenty of hot water, for both central heating and domestic hot water. They give out little direct space heating, and they have a utilitarian appearance. Most of them can accept fairly long logs (eg, 40 cm, or even 50 cm). With a large accumulator tank to act as a buffer, a log boiler may only have to be lit two or three times a week. (For sizing the buffer, Baxi recommend 50 litres for each kw output. Their smallest log boiler, the Solo Innova 20, has an output of 20kW. So a 1 m 3 accumulator tank would be required.) Connecting two boilers Sometimes a wood boiler and a gas (or oil) boiler are to be incorporated into one central heating system. This gives the householder a choice of which boiler to use or even to use both at the same time. But incorporating two boilers into the same system is not straightforward. The method to use depends upon whether a vented or a sealed heating system is being used. A reminder for some readers When water is heated it expands, and there are two ways to accommodate this expansion: Vented systems traditional in the UK The pipework is connected to a small tank in the loft, and the water level in the tank can rise a little to accommodate any expansion of water in the system. Unvented systems more modern, but not necessarily better The system is sealed, and expansion of water is accommodated in an expansion vessel. This contains a flexible diaphragm with water on one side and air on the other. The water can expand by pushing the diaphragm, and this in turn compresses the air. Link-up for a vented system There are a couple of devices on the market which allow two boilers to be simply incorporated into a vented circuit the Dunsley Neutralizer and the Esse Centraliser. (Actually, more than two heat sources can be incorporated, say two boilers and a solar panel.) The Neutralizer (or Centraliser) is connected directly to the header tank in the loft, and pipework for each boiler is connected both into and out of the Neutralizer. The water pressure in the Neutralizer stays more or less constant, at a neutral pressure determined by the head of water. In particular, the pressure in the Neutralizer is MORE ABOUT WOOD STOVES 3 JULY 2009.

4 unaffected by any pumps that may be in use. Hence water can be pumped through one boiler without any water flowing through the other one. The H2 Panel by Heating Innovations is an alternative method. And another method is to connect the two boilers to a vented thermal store, with the thermal store acting as the neutraliser. (Some GX Heat Banks from DPS are suitable see Further Info.) Accumulator for a sealed system On the Continent, sealed systems are invariably used not vented ones, and the continental practice of using large hot water stores, called accumulators or thermal stores, is becoming more common in the UK. Each boiler is connected by its own separate circuit to a heat exchanger within the accumulator, and thereby the water in the accumulator is heated. This hot water, typically at 80ºC, heats the domestic hot water and the central heating. Next month: Pellet stoves, and more. FOOTNOTE: from James Verlaque of Gastec to Robert Matthews The terms net and gross actually apply to the Calorific Value of the fuel (also referred to as heating value). If you consider the intrinsic energy in a (hydrocarbon) fuel, the heat released can be measured in two equally valid fashions, simplistically represented by:- NET HEAT release Hydrocarbon + Oxygen Carbon Dioxide + Water (Gas, liquid or solid Gas Gas Steam) GROSS HEAT release Hydrocarbon + Oxygen Carbon Dioxide + Water (Gas, liquid or solid Gas Gas Liquid water) The difference is that SUBSTANTIALLY more heat is released when the products of combustion are condensed to water (equivalent to the latent heat of condensation of water). By way of illustration, the Calorific Values (CV) of some fuels are: Gross CV MJ/m 3 Net CV MJ/m 3 % Diff N Sea Gas Kerosene Bituminous Coal Wood (Note: 1 MJ = kwh.) So as you rightly suggest in your (to me), the net CV figure is lower than the gross CV figure, and hence they used to be referred to as lower and higher heating values respectively. MORE ABOUT WOOD STOVES 4 JULY 2009.

5 This has an impact when considering efficiency (which can simplistically be considered as the ratio of useful heat energy produced to the energy available in the fuel). You can see that if the net CV is used, then the efficiency on a net basis is actually higher than the efficiency on a gross basis. (You will be dividing the same heat-energy produced by a smaller number.) As you quite rightly point out, the use of net CVs to calculate efficiencies on a net basis can thus result in efficiencies greater than 100% (particularly where flue gas condensation occurs). In the EU the use of net efficiency is more common for all fuels and product types, and the test standards consider net values hence manufacturers tend to use the net value (which of course is the higher value). The UK traditionally has dealt in terms of gross CV and hence Building Regulations, as referred to by HETAS, tend to cite efficiencies on a gross basis. In terms of which is more appropriate, there are arguments for both, and both are valid concepts. Personally I would suggest that the gross basis is preferable as this is the more absolute. FURTHER INFO: BSI British Standards Gastec Test heating appliances. Also train and do consultancy. Aga Charnwood Manufacturer of the flue boiler and stoves. Zenex Cool flue technology for your gas boiler. Ravenheat Their Energycatcher is similar to the Zenex GasSaver. It is included with their CombiPlus gas boiler. Laddomat Made in Sweden but widely available in UK. For a wood boiler in a sealed system. Heating Innovations H3 panel. For a wood boiler in a vented system. H2 panel. For linking a wood boiler with an automatic boiler, eg, an oil boiler. H4 panel. For linking two (non-automatic) solid fuel boilers. MORE ABOUT WOOD STOVES 5 JULY 2009.

6 Dunsley Neutralizer (or Link-Up ) and stoves. Esse Centraliser and stoves. Dedicated Pressure Systems Ltd DPS produce many variants of the (vented) GX Heat Bank. Words: Copyright article by Robert Matthews in SelfBuild & Design magazine, July, MORE ABOUT WOOD STOVES 6 JULY 2009.