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Three tips if you want to double efficiency, halve your wood consumption and remain friendly with the neighbours.

1. Install an accumulator tank

A normal wood fired boiler for a house often produces 25–50 kW. When it is at maximum output the efficiency is 70–90 percent. This should be compared with the average output requirement of an average house during the winter - 3-4 kW.

A boiler that is fired without an accumulator tank and that is choked to 4 kW, will have an efficiency level below 30 percent and at the same time will function as a tar boiler. The result will be that a lot of wood is burnt and that you will need to sweep the chimney frequently; the risk of a chimney fire will still be large. You can also reckon on the neighbours complaining about the smoke and the soot particles polluting the neighbourhood.

With a correctly sized and installed accumulator tank all these problems disappear. The tank should be sized to at least 10 l/m² heated living space.

2. Install by-pass control

Install by-pass control

During the winter your house requires 3-4 kW for heating. At certain times of the day you get free heat through the sun, people, lamps, TV, stove or dishwashing machine etc.

The Thermomatic automatic by-pass valve measures the indoor temperature quickly and accurately. If, for example, you set it to 21°C, the automatic control ensures that the water to the radiators is exactly the temperature needed to maintain the correct temperature in the house.

The indoor sensor continuously monitors to ensure that no more heat than absolutely necessary is fed out to the radiators. The return temperature is the lowest possible and the maximum stored energy of the tank is exploited. According to the measurements reported by SP, the Technical Research Institute of Sweden, in Råd&Rön no. 2/01, a house with Thermomatic by-pass control uses 22 percent less energy than a house without automatic controls. By-pass control with an outdoor sensor also reduces energy consumption, but requires more accurate setting and adaptation to achieve the same result.


Non adjusted system
Adjusted system

3. Adjust the distribution of the water flow between the radiators.

It is very common for the flow to the radiators in a house not to be adapted to the size of the radiators. The result of this is that the smallest radiators do not have time to take advantage of the heat in the water – the return is as hot as the supply. The larger radiators, on the other hand, never get hot, which is usually compensated by a larger pump and by the automatic by-pass control increasing the supply temperature. The result is a higher return temperature, which leads to greater energy consumption and poor layering in the accumulator tank. And through less energy being stored, comfort is reduced.

This problem is usually easy to rectify. Many houses already have adjustable radiator valves. Otherwise, plenty of good, cheap and easily adjustable radiator valves are available on the market.

If you have a normal valve like a Danfoss 10/15, it takes a maximum of an hour to set the choke to 2 for small radiators, to 4 on medium size and 7 on large radiators. If you have a two pipe system this adjustment is usually enough. If you have a single pipe system the radiators are connected in two or more loops. If you reduce the flow by lowering the pump speed, you should also adjust the loop valves so that all the loops maintain the same return temperature.