How do you heat water with a wood burning stove that has a back boiler?

Functional and aesthetically pleasing, a wood burning stove, also known as a wood burner, provides an attractive focal point in any living area.

It is also one of the most efficient ways to heat a property as a back boiler wood burner can heat an entire home, while helping to keep your energy bills low.

Suitable for heating a wide variety of different living spaces, wood burning stoves and back boiler stoves (also known as wood burners) are incredibly popular. Not only do they provide an efficient and carbon neutral form of heating, they also convey an enormous sense of wellbeing, a phenomenon which was explored by American scientist Professor Edward O Wilson in his 1984 publication of the biophilia hypothesis which suggested that human beings have deep intrinsic connections with the natural world, including fire.

What are the different types of back boiler stove or wood log burner or wood burning stoves with a back boiler for heating water?

A wood boiler stove (or back-boiler wood burner stove) is an aesthetically pleasing wood burning stove with a glass in the door for seeing and feeling the heat from the flames. It looks much like an ordinary wood burner except that the heat from the flames is also used to heat a volume of water (the boiler) within the stove. As a general rule, the ratio of heat transferred into the room versus that transferred into the boiler is roughly 50; 50.

There are four main design types of wood burners or wood burning stoves that can heat water, depending on the fuel used (whether it be wood logs or wood pellets) and the design of the heat-exchanger used for transferring energy from the firebox (where the wood fuel is burning) into the water, whether it be a “wet base” or a “dry base” boiler design.

WET-BASE versus DRY-BASE design of back boiler wood burner stoves.

A wood burning boiler stove with a “Wet Base” boiler design is one where the steel case of the firebox is surrounded with water. It is a much simpler and cheaper design of boiler compared with the alternative Dry Base boiler. Most log wood burning back boilers available for sale in the UK are based on a Wet Base design although many of these products have recently been removed from the UK market as they are unable to meet the recent ECODESIGN emissions limits. This is due to the cooling effect of the water jacket on the firebox which results in lower temperature combustion and poor emissions.

A “Dry Base” boiler design on the other hand is one where there is no water surrounding the firebox. All the water is contained in a separate section of the boiler away from the flames. With a Dry Base boiler, the firebox is surrounded by ceramic insulation (also known as refractory) which allows the temperature in the firebox to get much hotter than in a Wet Base boiler, resulting in cleaner combustion and higher efficiency.

Dry Base boilers often use a “shell and tube” heat-exchanger design whereby the products of combustion pass through the boiler tubes which in turn then transfer heat to the surrounding water. They are also sometimes known as “firetube” or “smoke tube” boilers.

A “shell-and-tube” heat exchanger is the most common type of heat exchanger used in large commercial and industrial boilers and is suited for higher-pressure applications. As its name implies, this type of boiler design consists of a shell (a large pressure vessel) with a bundle of tubes inside it. The products of combustion run through the tubes, and water flows over the tubes (through the shell) in the opposite direction to the products of combustion to transfer heat from the products of combustion to the water. The set of tubes is called a tube bundle and may be composed of several types of tubes. Cross-baffles can also be used to force the water to flow perpendicularly across the tubes to develop a more turbulent flow, increasing the boiler efficiency.

The Lundy 8 pellet boiler-stove is an example of a Dry Base boiler design that uses a “shell and tube” heat-exchanger.

How does the choice of fuel affect the design of a wood burner back boiler stove system used for heating water with burning wood?

Unless you own your own forest or are happy to pay premium prices for buying a single bag of wood fuel at a time, then you will need to allocate some storage space so that you can buy wood-fuel in bulk which is the most cost-effective way. For the same energy content then you will need approximately twice as much room for storing logs compared with pellets. For a typical 5 kW stove then you will probably use up to a tonne of wood pellets (or 2 tonnes of logs) over a winter. This much wood-pellet fuel will take up storage space for a “euro-pallet’s” worth of wood pellets, typically delivered in 10 kg or 15 kg bags and stacked on a pallet with a footprint of 800 mm x 1200mm. An out-building is the ideal space for storing this. The storage space required for logs depends largely on how neatly stacked they are!

Should you use LOGS or PELLETS used as fuel for wood burning back boilers?

A wood burning back-boiler stove sits at the heart of the home, and will provide the perfect focal point for your guests to gather round and keep warm as the evening draws in.

The choice of fuel, however, will have a big effect on the design of the plumbing circuit connected to the boiler.

A back-boiler stove that uses Logs as fuel is considered to be a slow-acting device because if the water temperature from the boiler gets too hot there is nothing that can be done to reduce the energy transferred from the firebox to the water, you can’t remove burning logs from the boiler stove!

For this reason, most back boiler stoves that use logs as a fuel should not use an electric pump for maintaining flow through the boiler, but instead should use large diameter pipes and the “thermosiphon” effect to create natural convection currents to maintain a flow rate through the boiler. They will also often have emergency cooling devices fitted to them to prevent the water in the boiler from overheating.

A back-boiler wood burner with a controlled feed of wood pellets on the other hand, is a fast-acting device because the water temperature from the boiler can be quickly reduced by slowing or stopping the rate at which pellets are fed into the firebox. A back-boiler pellet-fed burner stove such as the Lundy 8 back-boiler stove

will therefore generally use a pump to create flow through the boiler. It will also have an overheat thermostat as a safety device to shut it down in case the water temperature gets too high in the case of a power outage.

Other things to consider when selecting a wood burning back boiler stove or wood boiler for heating water with a wood burning stove.

You can make a wood burner boiler-stove easier to integrate with other heat sources (such as gas boilers, solar panels or heat pumps) by using a thermal store tank (sometimes called a buffer tank or an accumulator tank).

Put simply, a thermal store tank is a large, well-insulated hot water tank and forms the heart of your home heating and hot water system, they are typically sized around 100 – 200 Litres.

A typical combination is a wood burning boiler stove (for winter heating and hot water) with solar panels (for summer hot water) and with a conventional fossil fuel gas or oil boiler (to provide a backup).

As an example of a heating system design for a passive house using a pellet fuelled boiler stove with a thermal store and a heat pump.

Other sources can also include a ground or air source heat pump or an electrical heating element.

Because the heat from the wood burning back boiler stove is being stored it means that the  stove only has to be lit once a day and it can be run efficiently, rather than being kept smouldering all day. The classic cycle is to light the wood boiler stove in the evening to heat the house and enjoy the fire with the heated water stored in the thermal store. After the wood burning boiler stove has been shutdown then the heat in the thermal store tank is used to warm the house in the morning and provide the Domestic Hot Water (DHW) for bathing and washing.

In the afternoon the central heating controls draws the rest of the heat from the thermal store to warm the house before you return from work and light the boiler stove again.    

A good thermal store takes in the source energy efficiently and transfers it back out on demand quickly, maintains good stratification (hottest water at the top, coolest water at the bottom) and, most important of all, minimises heat loss.  Some of the considerations that need to be made when choosing a thermal store are listed below; –

You need to know what you want to do with a thermal store to know what coils you will need.

• If you are planning to store heat from a back boiler stove for central heating or to supply an independent hot water tank, then no coils are needed.
  • If you are planning to supply mains pressure Domestic Hot Water (DHW) from the thermal store for bathing and washing then it will need a DHW coil. Some tanks have a pre-heat coil in the lower part of the tank and a top up coil near the top which avoids taking all the heat from the top and so helps maintain the stratification.Storing heat from a solar panel will require a solar coil.
• What are the coils made of?
  • The better tanks have copper coils, which transmit the heat more quickly and effectively.
• Are the coils finned?
  • Finned coils have a much greater surface area and so transmit the heat much better.
• How long are the coils?
  • The longer the coil then the quicker and more effectively it conducts the heat, thus giving a higher flow rate for Domestic Hot Water for bathing and washing.
• Are the tappings recessed or flush mounted?
  • Flush mounted tappings make it easier for the installer to do the tank connections and, most importantly, make it easier to achieve continuous insulation from tank to pipe.

• How good is the insulation?
  Learning what the insulation is made of, and how thick it is, will determine the quality of the insulation around the thermal store. The best insulation for a thermal store is sprayed-on polyurethane (PU) and it should be around 100 mm thick. Cheaper tanks use foam rubber. To give you an idea of the difference in performance, a 100 mm thickness of polyurethane PU insulation is equivalent to a 600 mm thickness of foam rubber. A thermal store with 100 mm thickness of polyurethane PU insulation which has been properly plumbed in and properly insulated shouldn’t be losing more than 1-3 degrees Celsius of temperature in 24 hours. A foam rubber insulated tank will lose around six times as much heat as one insulated with polyurethane.  Some tanks use a synthetic fibre in a flexible plastic jacket which has a very limited insulation value.

• A thermal store also helps to make the heating system more “future proof” as it allows multiple energy generation sources to be easily integrated into a single heat distribution system. Heating systems which combine a wood burning boiler stove, solar thermal, solar PV (electric) , an air source heat pump and a gas or oil boiler can be easily controlled and sequenced by using a thermal store. The thermal store can also come solar-enabled so that you can link in solar panels to provide hot water when it is sunny. Many other heat sources can also be connected to the thermal store, including a conventional oil or gas boiler as backup.
• Modern houses tend to have extractor fans which will create a negative pressure in the house which can cause problems for a wood burning stove fitted with a conventional flue as there is a risk of “downdraught” whereby the exhaust gases created during combustion can linger in your chimney. A wood burning boiler stove fitted with a balanced flue will solve these problems as it is an entirely self-contained system not affected by the air pressure in the room. A balanced flue wood burner such as the Ramsey pellet stove.

or the SKYE pellet stove, a mighty small pellet stove which does not require a chimney, making them a good option for a wood burning stove  installation. They are completely sealed, with a glass door, from the room into which they are installed. This means there are no draughts, and heating efficiency is increased, with a concentric balanced flue pipe venting directly to an outside wall.

This also means no additional ventilation is required.

• Choose a stove with features such as a wide glass window to enjoy the flickering flames such as the Lundy 8 wood pellet burning boiler stove.

With careful planning and preparation, installing a boiler stove in your home can provide an effective way to enjoy cosy warmth in this social space. Talk to us to ensure it is done safely and legally.

How to get further help for choosing a wood burning boiler stove for heating.

Island Pellet Stoves Ltd has been manufacturing and installing wood boiler stoves in Cardiff, Wales, UK since 2015.

Our website has a range of resources and frequently asked questions to help you plan your project and to get the best from your wood burning boiler stove or back-boiler wood burner. Follow the link below for more information or give us a call on – 0330 111 4747