Whereas some low performing MVHR systems have post heaters in order to prevent an influx of cold air, Paul’s high performing systems provide warm supply air without active heating; e.g. even at frost temperatures outside, the supply air is 18 degrees Celsius (with extract air temperature at 20 degrees).
Post heaters are necessary whenever a building is partly or fully heated solely by air without radiators or underfloor heating. Such heaters are operated either electrically or with warm water; they are positioned in the supply duct. Usually only one post heater is situated close to the HRV unit, but in order to differentiate temperature levels, two or more heaters can be installed at different places of the supply ducting system.
For buildings following the German Passive House design criteria it is quite common to use a post heater in the HRV supply duct system. The most common approach is to have one post heater for all supply rooms. These are either operated electrically or with hot water. Alternatively a mixed mode is possible, where two post heaters are used for e.g. for the ground floor and first floor separately – or one floor has got underfloor heating, etc.
The heat output of water based heaters heaters heaters depend on the overall air flow volume and the water temperature. E.g. for a 3 bedroom dwelling for 5 people, either 1.3kW or 2.2kW (50 or 70 degrees Celsius hot water) is the output.
Things to be considered in general:
1) Please be aware, that heating solely via air is only possible for Passive Houses/ Passivhaus buildings, and then only if detailed themal calculations with the PHPP (Passive House Planning Package) verify this possibility. The certification of such building as Approved Passive House (Passivhaus Institut, Dr. Feist) is recommended. As only limited heat can be transferred by air, the heat load of the house has to be very low, if air heating is the only space heating system.
2) Sometimes a small additional heat source, such as a wood burning stove can be added for space heating, when the PHPP does not allow for sole air heating.
3) All supply ducting should be insulated
4) The heating via air is slow – this is to be considered when dwellings are unused for longer periods, e.g. holiday homes.
5) There will be a temperature differentiation from room to room, depending on the level of thermal losses and the amount of supply air per room.
6) There should be ample capacity of the HRV unit, as higher temperature demands can only be met with increased volume flows.
7) In the European climate with tempertures down to minus 25-30 degrees Celsius and low outdoor humidity levels, this set up sometimes leads to very low levels of indoor humidity. For this reason humidity recovering heat exchangers are sometimes used. However these have a reduced efficiency. This problem is not so much known in the UK climate.
The file below shows details about our post heaters: