What is the difference between the heat exchange methods in mechanical ventilation heat recovery systems?
When considering the measures required to build an efficient home we will find that we are faced with the challenge of air-tightness: Build it to a good air tightness level and you will need to include some form of mechanical ventilation. In order to mitigate heat loss we will then need to recover the heat from the exhausted air.
There are various different heat exchange methods used in heat recovery ventilation and they each have their own advantages and disadvantages. One of the most common heat exchangers is the flat plate system, where the opposite air flows are completely separated by very thin plates. This has an advantage in the fact that it has no working parts that may require future maintenance though the fan power may need to be increased to overcome resistance in the system. The air flows are separate so no air pollution takes place but its efficiency can suffer slightly against other methods.
Thermal wheels have a matrix of thin tubes that are placed half across the inlet and half across the outlet ducting. As the wheel rotates the outgoing stale air warms it up and it then rotates into the flow of fresh cold air which in turn absorbs the heat and becomes warm. A thermal wheel is very efficient at recovery but it does require a motor to turn it so this energy needs to be taken into consideration when considering it’s overall efficiency. These units should also not be used in areas where slight cross pollution would be a problem.
Run around coils are used where inlet and outlet ducts are not situated next to each other. A finned coil is situated in the flow of air in each duct and a heat transfer fluid mixture is pumped around this micro system. Naturally the electrical consumption of the pump and the losses acquired due to the transfer, need to be calculated when considering the overall efficiency of the method.
The difference being that a refrigerant gas is included in the tubes and a pressure reducing valve and compressor are added to facilitate the higher efficiencies and the heat gain process. This system can also be used to facilitate cooling in the summer as the heat can be extracted from the incoming air and transferred to either the exhaust air or domestic hot water. If you combine this system with a Photovoltaic (PV) panel array you will be maximising the return on investment for the Photovoltaic panel as well as delivering free ventilation and hot water for quite a large percentage of the year without the need for costly controllers.
Naturally there is a big capital difference between these systems and it is therefore absolutely essential to get the systems correctly specified and appropriately designed to be compatible with the building envelope as well as meet your lifestyle expectations. This is where certification can make the difference for an efficient installation.