14 Nov 2011

Does Passiv mean Massive?

Ken Neal makes some interesting points on my last but one post.

• I do prefer to design passive houses rather than a PassivHaus, which are really active houses with all the kit and controls required. I do use and agree with all the standards, especially the airtightness but prefer to use passive stack ventilation. The additional heating load is about 1kW on a reasonably sized house which can be met, in rural areas where most of my designs are, by using a small wood stove.

Is he right? I don't think so. The Passivhaus only really requires one bit of kit - the mechanical ventilation system (MVHR). Sure, this kit requires controls, but they are not overly complex. Think Off — On — Boost. That's pretty much it, although there may be some element of timing involved as well. But nothing more complicated than a conventional heating system. And if you do choose to use natural ventilation instead of MVHR, you have the problem of not being able to circulate the warm air around the house, so that your wood stove wouldn't be heating the other rooms in the house. Natural ventilation requires significant input on the air intake side and that can realistically only mean trickle vents, which immediately blows a hole in your airtightness strategy. You can see why Passivhaus and natural ventilation don't really go hand in hand.

• I also prefer thermally massive houses, where you can get several days carry over of heat, to typical PassivHaus lightweight structures.

Passivhaus is agnostic on this point. There is no presumption in favour of lightweight or heavyweight structures. You can have whatever you want, so long as the thermal sums add up.

• I like a house which just sits there and does its thing on its own with very little control or active input.

Well, that is pretty much what a Passivhaus is aiming to do. But to get there, you need MVHR. Natural ventilation just doesn't cut it.

• Regarding embodied energy in materials, this only becomes significant if you are designing for a short life, say sixty years. In our energy constrained future we won't be able to build quantities of new houses, or alter significantly existing ones, because the energy to do so will be in short supply or too expensive or both. If you look at the age of some of our present housing there is no reason to suppose that a well built passive house won't last for three or four hundred years. So over that lifespan embodies energy is a very small proportion of the energy used in a building.

Basically, I agree with Ken on this point. But it does rather contradict his first point about the additional 1kW heating load placed upon a house without MVHR. Because that 1kW load will mount up over the lifetime of a house. After about twenty years, it will equate to the embodied energy of a lightweight house. After 40 or 50 years it will equal that of a heavy, masonry house. Those kilowatts, they all add up.

Mike Jones also makes some interesting comments on the same post.

• I plan to self-build to Passivhaus standard but I'd like a simple efficient MHRV system that I can service myself so I do not have expensive maintenance costs. Are the Passivhaus recommended MHRV systems easy to maintain by the householder or is maintenance a factor to be added to the building running costs? Anyone know?

I don't know enough about MVHR to answer that for sure. One of the issues to be resolved is that whilst there are dozens of MVHR systems on the market, very few are "Passivhaus certified." And whether these need professional servicing, I have no idea.

1 Nov 2011

Half FIT

One morning two weeks ago, I was awoken by some banging coming from a neighbour's roof. Draw back curtains and, lo and behold, across the street, some guys are up on the roof, seating some PV panels. Now this is a street running North-South, which means the panels are facing due east, so the amount of power they will create will be well down on their designed output.

To me, this was a sure fire sign that the Feed-in-Tariffs (FITS) had gone too far. Harvesting sunlight to make electricity in this manner is never really going to make much sense in a northerly latitude like ours, and doing it inefficiently like this really offends me. No way would anyone ever conceive of erecting east-facing PV if it wasn't for the promise of a big fat subsidy cheque. I've never liked these subsidies (having bleated about them on this blog often enough) and I don't have a huge amount of sympathy for these businesses which have apparently been "caught out" by this week's announcement that the subsidy is to be approximately halved forthwith (actually from December 12th).

Building a business plan on the whims of government renewable subsidies has never been a clever idea, especially in the UK which has a lamentable history in this area. We had Clear Skies, launched in 2003: they cocked that up. We had the Low Carbon Buildings Programme, launched in 2006: they cocked that up. Now the FITs, launched way back in April 2010, show every sign of being cocked-up too. Roll on the Renewable Heat Incentive!