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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.
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.
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.
With a sufficiently large accumulator tank that is connected in the correct way, the boiler can work at full output, at high temperatures and with 70–90 percent efficiency.
Good control equipment allows the boiler to get up to full working temperature, approximately 80–90°C. Then the boiler heats the house in the first instance, while the surplus energy is stored in the accumulator tank. After 3–4 hour’s firing the tank is fully charged. How long, and how often do you fire the boiler for at present?
The minimal smoke keeps both the neighbours and the chimney sweep happy, while saving you both wood and work.
Depending on the size of the house and the tank, the stored heat will keep the house warm for 10–20 hours before you have to fire the boiler again.
To exploit your tank’s accumulation capacity optimally it is important that the water in the tank is layered, both when charging and when discharging (Laddomat MR40/MR50). Good layering extends the time between charges and increases hot water and heating comfort.
Layering means that hot water, introduced slowly to the top of the tank, lies on top of the cold water and creates a hot zone at the top of the tank. The water in the hot zone maintains the same temperature, for example 80°C. The cold water maintains, for example, 30°C. The layer border between the hot and cold water is 10–20 cm high and the temperature here rises from 30°C to 80°C.
Without Laddomat there is no layering, neither when charging nor when discharging (Laddomat MR40/MR50). This means that the charging must stop before the whole tank is hot. In practice, this means that you only get approx 20 kWh energy from a 500 litre tank when the temperature in the tank falls from 70°C to 35°C. The hot water that you then get is not even enough for a shower!
With Laddomat the whole tank can be charged to at least 80°C and therefore the temperature falls from 80°C to 30°C across the whole volume, thanks to layering being retained. You get approx 30 kWh, that is 50 percent more energy, from the same tank.
If the water from the tank is not preheated the risk of corrosion in the boiler is increased. Corrosion significantly shortens the service life of the boiler; some boilers require replacement after only 3–5 years. Laddomat ensures that the boiler gets to operating temperature quickly by pre-heating the cold water in the tank before it is routed into the boiler.