Post-heating with electricity or water?
The need for energy of the post-heating radiators of ventilation units has become smaller
The task of heat recovery in the ventilation unit is to heat the air coming in with the heat of extract air. This not only saves energy but also enables a comfortably warm supply air also in winter. If the temperature of supply air blown to the dwelling remains too low in cold weather, the temperature can be raised with an electric, or in some unit models with a liquid-circulating, post-heating radiator.
New, highly efficient cross-counterflow cells are capable of heating supply air to more than + 17°C with the heat energy of extract air alone. Additional heating of supply air is then usually not needed. In ducts installed in a warm space – in the ceiling of a bathroom for instance – supply air usually gets warmer.
The small Vallox ventilation units with a high efficiency are always equipped with electric post-heating. This is because the higher purchase and installation costs of a water radiator cannot be amortised over a sensible time period. Also, an electric radiator is more dependable: a power failure or the breaking down of the circulation pump does not cause damage to the electric resistor.
As an example, the annual energy consumption of the post-heating radiator of the Vallox 096 ventilation unit, with the air flow of 37 l/s in a middle-size dwelling in a terraced house or a block of flats and with the supply air temperature of 17 degrees, is less than 100 kWh in South Finland and less than 200 kWh in Central Finland. The cost of the post-heating of supply amounts to about 10 € in South Finland and less than 20 € in Central Finland per year. The higher purchase and installation costs of a water-circulating radiator cannot be covered with the energy price difference. The raising of supply air temperature naturally increases the energy consumption of the post-heating radiator.
Electric post-heating and the primary energy indicator of the building
The new energy regulations “punish” electric heating with an energy factor of 1.7. However, the consumption of energy for post-heating and therefore its impact on the primary energy indicator is very small if the correct starting data and a sensible supply air temperature are used. An electric radiator is still the only sensible option for small-dwelling ventilation units as it is not the purpose to heat the dwelling.
Use post-heating correctly
The correct temperature of supply air blown into the dwelling is a very personal matter. It pays to keep supply air temperature as low as possible without there being a sense of draught. Then the heat adjustment of the dwelling operates correctly even in those instances where the sun or the fireplace brings in overheat. It is never advisable to raise supply air temperature higher than that of indoor air as it is not sensible to try to heat the dwelling with the ventilation unit.
If the air coming from the valve feels uncomfortably cool (it is true, though, that air with a temperature of less than 37 degrees always feels cool on the skin close to the valve), it pays to measure the temperatures of air going out from the unit to the duct and that of air coming from the valve. If air cools down in the duct by several degrees, duct insulation is inadequate. Other factors that may have on impact on the sense of draught include the direction of air flow in the valves, windows or the heating system.
In poorly insulated ducts installed in the attic both supply and extract air get cooler, which deteriorates the functioning of heat recovery and increases the consumption of energy for post-heating. When extract air cools down on the route from the extract valve to the ventilation unit by one degree on an average, the annual efficiency of the ventilation unit goes down by nearly 4 percentage units. When the ducts have been installed on the warm side of the vapour barrier, heat recovery operates efficiently and the temperature of supply air from the valve is even higher than when leaving the unit.
Here you can download a calculator for calculating the energy consumption of your ventilation unit.
Actual energy consumption of post-heating radiators
The adjacent table shows the consumption of energy for post-heating in different ventilation units with different air flows and supply air temperatures. The calculation concerning Vallox 90 corresponds quite accurately with other smaller units with cross-counterflow cells, such as TSK Multi 50 and 80, 096, 121, 110 and 145. Correspondingly, the calculation for Digit2 can be used for estimating energy consumption for the models Vallox 75, 95 and 132E. Similarly, the calculation for Vallox 150 can be used for larger units, such as Vallox 245.