Heat Recovery System

There are a few fundamentals to a Passive House; high levels of insulation, building design & orientation, air tightness and a good heat recovery system.

The heat recovery system is required to bring in fresh air to the house & expel stale air. The two air streams pass through a heat exchanger to raise the temperature of the incoming air. We did a bit of research and were looking for two things from our unit, Passiv House certification & high efficiency. We wanted Passiv House certification so if in the future we go down the route of certifiying the house, we get maximum benefit from the unit in the calculations. A non certified unit will automatically have a derating of its stated efficiency whenthe calculations are made.

The house is actually entered onto the passive house database based on its preliminary & design calculations:


Secondly a high efficiency was required to reduce the amount of heat lost from the building.  The unit we chose was a PAUL Novus 300. This is a German made unit with a stated efficiency of 92%. This is highly impressive to think that 92% of the heat expelled from the house is recovered back into the incoming fresh air.

The unit appears to work OK and it is a nice compact unit that is wall mounted (albeit supported from the worktop as well). The benefits of a HRV unit as well as keeping the air fresh inside are that damp air from bathrooms & utility room is removed. Drying clothes in the utility room is great and towels dry nicely in the bathrooms.

Utility Room showing PAUL HRV unit

The unit is also fitted with a frost heater (black box above right of the HRV unit) which pre-heats the incoming air if it is too cold (This prevents any danger of the heat exchanger freezing and getting damaged).

The downside of this unit is the lack of any feedback. There is no display showing temperatures or whether the summer bypass or frost heater are operational. I presume that being a German unit the principle is that it works and you don’t need any confirmation of that fact. This was highlighted recently when the unit kept cutting out in freezing conditions and it was only after several conversations with the manufacturer that I realised that the control for the frost heater was setup incorrectly by the installers. A change of setting and now the frost heater delivers just enough heat to maintain a temperature above freezing into the unit.

The second downside of the unit was the installers, they didn’t seem to have much knowledge of the unit, the duct installation & insulation was poor and the unit had to be changed as it was installed or set up incorrectly. During the sumer I will re insulate some of the ducts as they are not properly vapour sealed and we get condensation dripping off on top of the unit and sometimes seeping down the back of the unit. Also no coordination was made with the electricians or builders despite them all being on site at the same time so no provision was made for cable routing.

I would recommend the unit especially as a new version looks to be coming out with an integral frost heater, but would recommend that the builders install the ductwork and the unit. The suppliers will have to  do the commissioning. Installing temperature probes into the ducts would be recommended also in case you want verification.

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