An interesting letter in last week’s Building from Tim Gough who has a CV as wide as a Passivhaus wall.
In it, he berates the Zero Carbon Hub for being no such thing. They are courting compromise in asking for the mandatory use of mechanical ventilation with heat recovery (MVHR) to be ditched. They say here that is a step too far for a temperate climate like the UK. Problem is that without MVHR, you can’t really build to PassivHaus standards because it’s so airtight that you would keel over every time someone farted.
Passivhaus standard uses just 15kWh/m2 for annual space heating requirements. Zero Carbon Hub are suggesting that this is relaxed to somewhere between 30 and 45kWh/m2 – i.e. two to three times as much, which is about as low as you can go safely without MVHR. And is really not much lower than where Part L will pitch us next year.
Maybe this debate is a bit obscure, but it’s nevertheless interesting. I rather tend to side with the Zero Carbon Hub on this. I think mandatory MVHR is just possibly a step too far in the UK. If you look closely at Tim’s text, you can begin to make out just what the problems are.
This is not a question of diminishing returns – to halve or quarter heating/cooling costs by means largely of a MVHR system costing a few thousand pounds can hardly be said to be that writes Tim.
Except that it ignores the cost of running the fans which drive MVHR system. OK, they are low. We now have fans operating at under 100w, but multiply that by the 8760 hours in a year and you have consumed enough power to heat and cool 58m2 of Passivhaus. The actual energy saving achieved by MVHR is therefore pretty minimal. Some commentators have even suggested that it’s not a net energy saver at all. They certainly aren’t if residents are so bold as to open their windows.
MVHR systems do not, contrary to what is claimed, adversely affect indoor air quality – even if the filters are not cleaned.
That is true. The reason Passivhaus insists on them is because of air quality, not energy saving. The problem is that they don’t always work as planned; they break down; people turn them off (ironically sometimes to save energy). That is where the problems may lie.
Housebuilders worry about reliability, but this is a proven technology that is no more complex than an extract fan.
And extract fans never break down?
What, I wonder, is the real agenda of the promotion of these poor standards?
In a word, caution. What ought to happen is that several hundred new homes should be built to PassivHaus standard, complete with MVHR systems installed, and then lived in by ordinary folk for something like five years. That would be a sensible test of whether the system is workable. If the MVHR systems proved to be reliable and popular and people learned to live in the houses the Passiv way (i.e. not opening windows), then MVHR could be safely rolled out as a mandatory building regulation requirement in new airtight homes. But until that’s done and dusted, making MVHR mandatory would be pretty rash. Stupid even.