Do we need MVHR systems for our new generation of super-insulated homes?

Thought 02

writes Chris Twinn

My proposition is that on balance there are benefits in designing out the need for MVHR systems. The driver is to value engineer solutions to achieve a less complex way of achieving CSH6* at a cost premium nearer to £5k per home than cost current suggestions.

Prior to doing the Kingspan Lighthouse and the Barratt Greenhouse we did a set of parallel calculations, not least because we believe SAP is somewhat simplistic when it comes to solar gain, ventilations, domestic hot water demand, etc.

Among the things we found was that in a super-insulated home with very modest space heating demands, the fan power carbon emissions can exceed the heat recovery carbon savings. This was also the case if we used the new generation super-low energy fan units (SFP<1W/l/s) from Europe. This was particularly the case if it has room exposed thermal mass.

Note this is carbon emissions not energy. This is where the grade of energy used (& its carbon content) becomes so important instead of simply the number of kWhs. The carbon measure also makes a reasonable approximation for relative operating cost and of the relative capital cost of renewable electricity verses renewable heat.

That said we did not pursue the issue further at that stage for these projects because BRE would not accept alternative compliance routes for CSH6*, SAP does not model thermal mass energy influences, and Kingspan wanted to market a MVHR unit without a high thermal mass option! The client is always right……!

Further separate analysis illustrated the variations on this. For example, if the fans are only run when heat recovery is of benefit, then the carbon equation moves more towards MVHR. However, using useful heat recovery as the criteria for deciding when to run the fans falls down when considering indoor air quality. Unfortunately human senses mean occupants tend to open windows on room over-temperature, not on indoor air quality terms. Consequently, we would need to add back the trickle ventilators to provide the continuous background fresh air needs for the rest of the year. It is probably for this reason that Building Regulations require mechanical ventilation running continuously or installed trickle ventilators.

My particular interest in this is broad ranging, but particularly with regard to reducing the overall implementation costs for achieving CSH6*. I am seeking ways of making use of our milder UK climate compared with Passivhaus’ mid European background (UK surrounded by sea that does not go below 4degC verses Lower Germany with temperatures down to -7degC in winter), looking for ways of reducing the cost of achieving super-standard homes, and trying to avoid the normal engineering/supplier approach of throwing more kit at an issue.

I am also particularly aware of the occupant interface and trying to understand how people react to their built environment. There are clear different national characteristics, with the UK having a fundamentally different approach to building and home maintenance compared with say Germany or Scandinavia. Not only do UK home occupiers hardly maintain anything, but where there is a landlord responsibility they can’t get easy access into individual homes anyway. In practice there is a UK perception that the basic long life building should be ‘dumb’. Occupants appear happy to have ever more complex entertainment systems, based on no-maintenance requirements and a built in obsolesces of little more than 2 years, but this is hardly the basis for adding relatively complex plant into millions of homes!

So I put forward a proposition that the UK (at least the south) does not need the additional capital cost, complexity and operational cost and bother of MVHR if a home has very high levels of passive super-insulation! Instead I suggest the direction might be to achieve a level of heat recovery using room exposed thermal mass (redistributes internal heat gains across the 24 hours) and trickle ventilation brought in between the triple glazing panes to recover perhaps half the glazing heat loss (similar to Dwell-vent but not needing the complex vents) coupled to passive stack extract ventilation (using direct-acting moisture opening grilles in showers, etc).

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