Too Hot?

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 (if the extract air temperature is 20 degrees C.).

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 MVHR 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 depends 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.

We now offer ceiling and wall valves with an electric heating element, which offers individual control of the heat per room. The heating element is a 250W PTC (pulse controlled) element with overheating protection.

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.

Feasibility of heating via air

Heat Recovery Ventilation Systems are an efficient way of providing ventilation with hardly any heat losses for relatively airtight buildings. Whereas they are primarily not heating systems, in some cases they can be used for space heating or back up heating.

In order to give you a better guideline when MVHR systems are recommended and what needs to be taken into account if they should be used for space heating, we have produced the following chart:

A – Feasibility for commercial buildings

B – Feasibility for dwellings

C – Choice of MVHR system and heating method for dwellings

D – Choice of Post Heater

De-central electrical post heaters are a simple and effective way to get additional heat into individual rooms. Compared to central post heaters, these allow the zoning of heating, thus room which are not being used don’t need to be heated.

Our de-central heaters come either as ceiling or wall outlet for air supply rooms. They have an efficient and safe PTC heating element with a maximum output of 400W. The connections are 2x 75mm ports for 63/75mm semi-flexible ducting. With connectors these can be used for 1 or 2x 78/90mm semi-flexible ducts, too.

The post heaters need to be controlled by individual room thermostats or a in intelligent house control system.

Although our high-performance MVHR systems cut out almost all ventilation losses, the system will in itself not level out the temperatures between heated and un-heated rooms. Temperature differences of 4-10 degrees between rooms can still be achieved, depending on the ventilation volumes and internal heat transfer.

The same counts for the distribution of heat from wood stoves around the house: The MVHR system will help to move heat around, but will not sufficiently heat unheated rooms from a different room with a wood stove.

However, we can offer experimental ways of heat distribution. Please talk to us in the early design stage if you intend to use the MVHR for heat distribution.