How it works


The Function, Construction And Location Of The Accumulator Tank

An accumulator tank acts as a buffer and intermediate store for water that is pumped around the system. The boiler heats the water in the tank, and the hot water is then supplied to the consumers. When the water in the tank gets so cold that it no longer can meet its requirements, the boiler should fire up and reheat the water in the tank.

This means that accumulator tanks are especially suitable for wood or pellets-fired boilers as there are long pauses between the times the boiler is fired. In addition, the boiler can work at full power and optimum efficiency until the tank is once full of hot water.


A tankless system (direct system) must be started several times per day to maintain heat. Each time the system starts, losses occur, which means that systems containing accumulator tanks are much more efficient that tankless systems. Those who change from a direct system to a system with an accumulator tank experience significant cost reductions! Most accumulator tanks that are sold today are made from metal or a composite material, are cylindrical and come in various heights and diameters. There are also square tanks that are e.g. customised for door apertures’ standard widths. If you want to locate your tank in a space with special dimensions, there are also manufacturers who can build a tank to measure.

In order for heat losses not to compromise economies, it is important that the accumulator tank, pipes and connectors are well-insulated.

A 2,500 litre tank, insulated with 1 cm thick polyurethane and heated to 75 °C, has nearly ten times as many losses as a tank in which the insulation is 10 cm thick.

A range of options

A flexible accumulator tank is a shortcut to good heating economy, both today and tomorrow. But the tank must be able to also accept and store energy from other energy sources, e.g. solar panels and heat pumps. The future proof principle is easy – the more separate connection options the better!

The tank can also be fitted with an electric immersion heater that can be used when it is uneconomical or inconvenient to use the boiler, e.g. during the summer or when you are away from home.

Where should the tank be put?

Your hands are pretty free about where to locate your accumulator tank. The only requirement is that the space must be frost-free, which in practice means indoors. Of course, you could build an extension that is just intended to house a good-sized tank. An interesting alternative, if you have space problems, is to locate both the tank and the boiler in another building. You then connect the buildings with a culvert and, perhaps, complement the system with a small tank indoors.


Different Hot Water Systems

If you do not have a separate hot water heater, your heating system must also ensure that the house has access to sufficient volumes of hot water. The hot water heater can be integrated in the boiler, but a smarter alternative is also to use the accumulator tank to heat the hot water. The hot water supply can be constructed in four different ways:



1. Hot water heater in the boiler or tank; often a large tank 2. Separate hot water heater, often electrically heated and independent of boiler/tank. Can also be connected so that it is heated by the boiler.



3. Hot water loop/loops in the boiler or tank. If you have double loops, the cold water that enters the tank low down can be pre-heated, and then leave the tank and be sent back into the top of the tank where it is heated to its full temperature. Double loops are, therefore, also very good for separation.4. Tapped hot water automatics, that means that hot water from the top of the tank or boiler is sent to a separate unit where it heats the incoming cold water. The cooled water is then returned to the bottom of the tank/boiler.

Separation—the Key To Efficiency

The function of an accumulator tank is simple, and is built around the physical principle that hot water is lighter than cold water. Hot water from the boiler is pumped into the top of the tank, at the same time as cold water is retrieved from the bottom of the tank.

If this process is to function as intended, the boundary between hot and cold water must be sharp. If hot water and cold water are continually mixed, the water temperature will be lower. This means that capacity is reduced – in a worst case scenario, there will not be enough energy to heat the water for a single shower.

The secret behind perfect separation is to control the intake of the tank so the separation is not disturbed. The fitting that accomplishes this is called a charging unit, and the market leading charging unit is the Laddomat.