Putting your back into it

Biomass stoves generally provide local space heating only, and cookers heat for cooking only, but those equipped with back boilers are designed to also produce hot water and/or space heating.

In the previous issue we covered the basic principles of biomass combustion; quality and moisture content of fuel; combustion air supply; the burning cycle; fuel types; storage and sizing. Most of those principles also apply to stoves and cookers with back boilers with the exception of some fuel sources and sizing techniques. These are important to understand so you should familiarise yourself with that article before proceeding.

Because the appliances covered in this article are usually designed to be located inside the living space of a building, fuels are normally restricted to wood pellets and logs. Wood chip appliances are normally placed in a garage or shed due to the high volume of fuel required.

As some appliances can supply a variety of heating loads (local space, cooking, hot water, general space), sizing must take account of each of these elements.

The health and safety risks associated with biomass appliances in the home are often misunderstood. There is a popular misconception that biomass fuel, being an un-reactive solid, is less dangerous than oil or gas. This is not the case!

Gas appliances have integrated safety systems that cannot be fitted to biomass stoves and boilers. Flue gas carbon monoxide concentrations from natural gas boilers are low compared to those of biomass burners. Oil burners are sophisticated devices carrying little risk of uncontrolled combustion whereas uncontrolled combustion is always a possibility with biomass burners.

An important point to understand is that all biomass burners require an adequate supply of air to enable combustion. In today’s era of airtight building design, this means that in most modern buildings and often in refurbished ones as well, a dedicated air supply must be provided.

Without an adequate air supply dangerous levels of carbon monoxide can build up and, ultimately, cause death.

Many modern buildings include a Mechanical Ventilation with Heat Recovery (MVHR) system. Where these are present, it is essential that negative pressure cannot be created in the same room as the appliance.

In these circumstances, it would be wise to ensure that any biomass appliance is fitted with a dedicated, sealed, combustion air supply routed directly from the appliance to the exterior of the building.

Regular maintenance is required to ensure combustion air supply and exhaust gas pathways are kept clear. This contributes to safe combustion, reducing carbon dioxode or monoxide risk. You should employ a registered professional to carry out the work1.

Bearing in mind the principles discussed in the previous article on the combustion cycle (the necessity for the biomass burner to dissipate heat from initial ignition through to the extinguishing of the fire), it is obvious that water must be able to pass through the boiler at all times to remove the heat from the fire.

Should the water stagnate it will ultimately turn to steam, whose volume is approximately 1,600 times that of water (depending on pressure). Thus in a sealed system if the water turns to steam and the pressure cannot be alleviated, a potential bomb has been created.

In a system open to the atmosphere, the water between the steam and the exit point will be evacuated. Therefore, it is essential that a safe system is designed where the situation described cannot develop. If you are in any doubt about the dangers, simply type in your search engine “wood stove boiler explosion”.

There are a myriad of regulations in force designed to ensure health and safety in biomass installations including Building Regulations, Safety, Health & Welfare Regulations and Legislation and Construction (Design & Management) Regulations.

The Regulations are many and complex so it is advisable to employ a HETAS2 registered installer to ensure safe installation, in both NI and ROI. Also make sure the equipment and final installation is CE marked.

Logs vs pellets 
Fuel types, volumes, moisture content and automation were covered in some detail in the previous article; other points to note specifically about the difference between log and pellet systems with back boilers are as follows.

Log fed systems rely on manual lighting and feeding, which takes time, not forgetting also the manual labour involved before the appliance reaches useful temperature. Multi fuel stoves are available for burning either logs or coal but, by definition, cannot be optimised for either fuel. Note that burning logs with coal at the same time in multi fuel stoves can produce sulphurous acids that can corrode metal surfaces and reduce appliance lifetime.

Pellets have the advantage of enabling various levels of automation; the lighting, combustion and extinguishing sequence can be timed to occur automatically, providing sufficient fuel is available. Some burner designs deliver more reliable automation than others.

Log burners are unsuitable for installations where the flue/chimney doesn’t provide adequate draught or is susceptible to down draught. Pellet stoves, which have a combustion air fan to control the burn rate, can often be used in these instances due to the forced draught from the combustion air fan.

Pellet stoves often include fan assisted room heat output for rapid space heating and there are models designed to duct blown warm air to other rooms (some log burners can do this too but they’re not the common model you’ll find on the market). Pellet fed appliances with back boilers normally require some daily cleaning and a more thorough weekly clean. Cleaning is a messy process!

The principles of system sizing were covered in some detail in the previous article and most of those principles apply here, and also to fossil fuel systems.

However, systems with back boilers generally provide at least two functions. In the case of a stove with a back boiler, the functions will be local space heating and hot water and/or wider space heating. In the case of a cooker with a back boiler, it will be cooking, providing some local space heating by default and hot water, and/or wider space heating. Therefore, each of the loads must be fully understood and the system designed to address them.

Any design of stove/cooker will heat the room in which it is located and, if the heat output to the room is too high, the room will become unusable when the appliance is in operation.

Therefore, the start point is to calculate the heat losses from that room and ensure that the heat output from the appliance to the room (normally quoted in kW by the supplier) does not exceed the calculated heat losses3.

It is important to understand that, in a modern, well-insulated house, the design heat losses may well be so low that you cannot source an appliance with a back boiler with a sufficiently low room output. If you choose to go with an oversized boiler the room will be uncomfortably hot.

Once the room output has been calculated, consider the other uses. In any event, the system must be able to dissipate the full boiler output at all times.

Thus, if the boiler is to heat a hot water cylinder, a system must be designed to ensure that, once the hot water cylinder is hot, the heat is diverted to another load.

Similarly, if, for example, the heat is then dissipated to a space heating zone, either another load must be available to accept the heat when that zone reaches temperature or the zone must be left open at all times to allow the boiler to dump heat.

Also bear in mind that during the year, the amount of heat required to keep a building warm varies according to the outside temperature. So if the boiler output is designed to meet the maximum space heating load, for much of the year the appliance will be unusable as it will need to dump too much heat.

Again, the end user must understand that, in a modern, well-insulated house, the space heating load may be low and that, even though the rooms are hot, the back boiler may need to dump heat.

The previous article discussed thermal stores, and it may be possible to include a thermal store4 in the design. However, the addition of a thermal store will increase capital costs substantially.

Many modern buildings include MVHR systems. There is an expectation that, where present, these systems will deliver recovered heat from the appliance to the rest of the building. While this will undoubtedly take place, the air-flow rates are very low on MVHR systems and the end user should understand that only a small amount of heat will be moved around the house by such a system5. The great benefit of installing such a system is that you’re not allowing cold air to be driven through the house in winter.

There is no infallible rule of thumb for sizing, but the end user’s requirements, their expectations and the loads must be fully understood to optimise it.

Traditionally, back boilers were plumbed into an open vented, gravity fed system supplying a second hot water coil in the hot water cylinder.

In this design, the hot water cylinder is placed directly above the boiler and a ‘heat leak’ radiator is fitted above the cylinder and below the expansion tank to allow excess heat to dissipate.

Large bore copper pipe work is required along with a metal expansion tank and a brass float on the ball cock in the expansion tank to ensure that high temperature water can escape and that it cannot melt the tank or float. A thermostatic blender is required on the hot water from the cylinder to ensure it does not exceed safe temperatures.

This system is in widespread use6 and often extended to deliver hot water to the central heating system with the addition of at least a pump and a minimum return thermostat (that controls the pump based on the temperature of the water returning to the back boiler). There are specific connection requirements too which need to be expertly installed.

Sizing of components is essential, but the system has the advantage of remaining open to atmosphere. If there is a power cut or the pump fails, the heat can dissipate around the heat leak radiator and the robust metal fittings ensure that a supply of cold water is always available at the expansion tank.

There are many variations on this theme, but ensuring that a good, gravity fed flow is available is the key to success. The author has visited a site where a bucket of sand was located beside the stove to put the fire out in the event of a power cut as the stove creaked alarmingly if the pump failed to operate!

More recently, systems have evolved that are pressurised and protected by both temperature and pressure control valves with automatic cold water wash through. Some of these7 have been accredited by the UK MCS scheme (see Financial Incentives below). Installation requirements are detailed and stringent for both products8 and installers9.

If you would like to know more about installing biomass burners with back boilers there are some good websites with a variety of useful diagrams10.

Financial incentives 
As far as we are aware, there are no financial incentives for biomass back boilers in ROI.

At the time of writing, the Domestic RHI11 in NI gives an initial support payment of £2,500 and an ongoing support payment of 5.6 p/kWh. The ongoing support payment is based on a calculated annual heat demand if all reasonable energy efficiency measures had been carried out at the property12.

In addition, it is capped at £2,500 per annum. The support lasts for seven years and the tariff will be reviewed annually. Both the equipment and the supplier must be accredited under the Micro-generation Certification Scheme (MCS) for projects <45kW to avail of financial incentives. ν

Additional Information
Xavier Dubuisson, XD Consulting, Clonakilty,
Co Cork, mobile 086 0476124,

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Tony Traill

About Tony Traill

Tony is a director of Element Consultants; a small, multi-disciplinary consultancy specialising in energy and resource efficiency at all scales. (em: info@elementconsultants.co.uk) www.elementconsultants.co.uk

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