Pete’s Blog

by pete

What questions should I ask when buying an eco/green/low energy home?

Posted by on May 9, 2019 in Pete's Blog | Comments Off on What questions should I ask when buying an eco/green/low energy home?

From many years of painful conversations with people who have “bought a pup” I bring you this simple list which I hope will empower you to get what you think you are getting. I have to declare my bias: I moved to Passivhaus after many years designing the thermal envelope and building services for low energy buildings. In the early years there was a great deal of “greenwash” and we came up with some simple ways round it, but were wowed by the Passivhaus standard and the detail that went into the calculations, and more importantly, the monitored evidence. So don’t be surprised if I’m pushing Passivhaus as the complete package – I know it works. That said, there are other people working away to give you a comfortable home without costing the earth. But there are still some people who, how shall we say, aren’t quite on the ball as regards the energy performance of their offering. How do you tell who is OK? Simple, 3 questions: 1. Has it got a Mechanical Ventilation System with heat recovery (MVHR)? Any low energy home will have this, so if it hasn’t, walk away. There are some other ways of dealing with good ventilation with low energy use, but I haven’t yet seen anything that is as robust as MVHR units. a. Does the system use flexible or plastic rectangular ductwork? Walk –they leak like sieves. Spiral wound or semi rigid ductwork is OK b. Can you see the Design and Commissioning documents? We have seen so many badly designed MVHR units I want to scream. Commissioning documents show the air flow through each terminal, as set up, and form the basis of performance. No documents, walk! 2. What is the airtightness of the building. If it hasn’t been tested, walk away, as they obviously haven’t got it. Current building regulations have many get-outs so contractors can declare that they “tested one the same 3 years ago”. If you do have a test result you can grade it as follows: a. Over 10 m3/m2/h at 50 Pa test pressure – impossible as this is the Bregs limit if its tested, so if it is over 10, someone will have pretended to have never tested it and they will have moved onto another house/flat on the same site. b. Between 10 and 5 m3/m2/h @50 test pressure. About the mean for UK new Housing. Walk. c. Between 5 and 3 m3/m2/h. Bad but better than the average. d. Between 3 and 1 m3/m2/h. Not bad. Could do better. e. Below 1 m3/m2/h, hey, near Passivhaus territory. Congratulations! 3. Well if they have done the above, they have cracked Passivhaus, so they might as well get it certified- have they? Beware of contractors claiming “Passivhaus principles” which might be fine, but without the evidence in the first two questions I wouldn’t go near them, life is too short. If it is a certified Passivhaus, it will answer all the above. (just to be clear, I am a certifier) Notice I haven’t even looked at insulation yet, the above are usually all that is needed. Good luck. Below a simple pictorial of the above points.   Buying an Eco-House? (128.7 KiB, 1,007...

Read More

Airtightness, Airtightness, Airtightness

Posted by on Mar 18, 2019 in Pete's Blog | Comments Off on Airtightness, Airtightness, Airtightness

What makes a building low energy? As most readers know, Passivhaus is one of the leading ways to realistically reduce the need for heating or cooling energy in buildings, both new build and retrofit.  I say “one of the” but I don’t personally know of any other that works. Minergie in Switzerland is OK but it’s really a Passivhaus copy. So what are the fundamental propositions of Passivhaus?  I would say there are three: The building must be airtight.  Wow – living in a plastic bag! How horrible!  Well actually its quite comfortable. No draughts, constant temperature, maybe a bit weirdly uniform. In fact, after the initial exposure, most people don’t even notice the environment internally, which is a definite success!  After all, thermal comfort = lack of thermal stress.  But how can we ensure that the air is fresh and smells and excess humidity are removed? This is the second point: The building must have a controlled ventilation system to ensure indoor air quality is maintained at all times.  So far, the only systems we have used are mechanical, since all the natural ventilation systems we have found are unable to provide a consistent ventilation rate,  instead vary dramatically with the weather.  People generally will open windows only when the internal air quality conditions are quite bad. And the third is insulation.  Lost of insulation, all joined up to itself like a good polar suit all round, no leaks anywhere. And that’s where airtightness comes in again.  If air can leak through to the other side of our insulation, then its not doing anything. Bob Lowe reported this as “convective bypass”, but it’s basically crummy workmanship (or to be kind, installation without any understanding.) So why have I titled this Airtightness, Airtightness, Airtightness? Well if we look at the three options above, they all end up requiring airtightness to make sense. The heat recovery aspect of ventilation is a waste of time if you home has a leakage of much above 3 achr at 50Pa. And if the insulation has air washing around both sides due to a lack of airtightness, its not going to do anything.   So:  low energy building = airtightness...

Read More

What Kit do I need to commission MVHR systems?

Posted by on Feb 26, 2019 in Pete's Blog | Comments Off on What Kit do I need to commission MVHR systems?

We get asked this frequently, and the answer depends a lot on how much work you expect to do and what level.  You need a capture hood to get any sort of accuracy at all, but there are three main types:: The basic  is the Testo or Airflow anemometers, Testo better built.  What you have to recognize with these hoods is that they are seriously affected by turbulence, so if you put them on a supply terminal designed to give a jet your readings will be seriously out – BSRIA have done a report on this. Extract ok.  The other main issue with them is that they cannot measure much above 60 m3/h (~20 l/sec), which means they cannot be used for measuring total flows on outside terminals, which in turn means you cannot check for duct leakage (sum of internal flows should equal the external flow).  This limit is simply due to the small size of the vane: 100mm diameter, which at higher flows puts an extra resistance on the mvhr system and so gives daft readings.  Cost a  few hundred £ depending on how many hoods. Very light ( 1kg?) The next step up is the large area hot wire anemometers with a much larger throat (200mm sq) so can measure larger volumes and doesn’t seem to have the turbulence issues above.  £2k? Swemma main brand. Fairly light ( 2 kg?) And finally we have the balometers, which have a powered fan which runs to balance the pressure inside the measuring chamber to outside, so there is little change on the whole system balance.  Most accurate but a little heavier (3kg?) and cost (£3k?). Which you get depends on what you are doing.  The anemometers are fine but only if the system has no leaks (tricky to know) and you can’t really do a Passivhaus without a separate duct leakage test. The hot wire are fairly light and accurate, and for all day use are the best choice. If you get mostly called out for fault finding, as we are, then the balometer is best for accuracy, so we put up with the extra weight. Remember that you will have to calibrate the unit once a year, which costs a couple of hundred quid.  It worth also thinking about hiring: BSRIA has reasonable rates, and each unit is calibrated before it is sent out.   Its also a good way to find out what each unit is like to use. postscript: It has been pointed out that the CIBSE commissioning code says the only absolute measurement recognized is a pitot tube, with measurements taken at intervals across the duct.  This is fine for commercial pipe ducts, but hopeless for residential, where the small ducts and lack of straight runs before and after measurement points  means that it is wholly impractical. Don’t try...

Read More