Monday, June 25, 2007

Bob Matthews on Ventilation

Good series of articles recently on home ventilation in Selfbuild & Design by Bob Matthews, the oldest surviving selfbuild writer in the business. In June, he summarised his findings. In July, he peered into the future. There simply aren’t that many writers out there who would even know where to begin in handling such an assignment. I hope his readers appreciated it.

What did Bob conclude?

• SAP 2005 presumes a ventilation rate of 4 litres/second/occupant, that’s 14.4m3/hr each. I didn’t know that: it seems quite low. For four people living in a 350m3 house, that’s one air change every four hours. Conventionaly, we’ve worked on around one air change every two hours, but I don’t think there is much evidence that this ventilation rate is actually required.

• That 4lts/sec/person requires 100watts of heat to keep at 20°C above outside temps. So for four people, that’s 400 w. And for double the amount of air changes, that would be 800w. Generally, ventilation systems are set up to move much larger amounts of air than this 4lts/sec/person would indicate, so there is a design inefficiency built into most systems.

• Part F of the building regs suggests that you have a 10mm gap undercut on every door, to facilitate air movement around the house. A lot of people won’t like that because it increases noise transfer.

• The default system of home ventilation is trickle vents for supply air extract fans in the wet rooms. Trickle vents must be 5000mm2 in dry rooms (inlets) and 2500mm2 in wet rooms (outlets).

• Passive Stack Ventilation is one of the main alternatives. Trickle vents stay but the extract fans are replaced by air ducts rising from wet rooms to ridge. Kitchen extract requires 125mm tube, utility room, bathroom – 100mm, WC – 80mm or opening window. It doesn’t work in every configuration – bungalows are not tall enough and room-in-the-roof designs have nowhere to run the ducts, and some critics suggest that it only ever works intermittently anyway.

• Assisted Passive Stack is where a fan is added to the system, to ensure it works at all times.

• Central Extract: very similar to assisted passive stack. One fan in the loft pulls air from all the wet rooms. Works with smaller trickle vents than default, and lower extract rates, but designed for continuous operation.

• Positive Input: in many ways the opposite to Central Extract. Instead of drawing air out of the house, it blows it in, putting the house as a whole under a slight positive pressure. The chief exponents of this system are Nuaire. I’ve been rude about Positive Input Ventilation before, but Bob is much more generous.

• Mechanical Ventilation with Heat Recovery (MVHR): discussed in some detail in recent post. Bob reckons that air leakage should be below 4 q50 (don’t ask – it’s a measurement of air leakage under pressure), which is much lower than we are currently achieving on non-manufactured houses. The current building reg standard is 10 q50, whilst the Passivhaus standard calls for 0.6 q50. Quite a difference. MVHR is the only system here that requires both input and output to be balanced, as they are both controlled by the fan or fans. Everything else is either entirely passive (i.e. no fans) or has fan controlled output or input (only one here). This balancing requirement is a crucial and little discussed feature of MVHR. I suspect it needs adjusting quite frequently but is rarely done.

• Individual Room Ventilators with Heat Recovery: rather than whole house solution, you have a individual room ventilators with a heat recovery capability. Designed for continuous operation.

And what did Bob speculate about in the later article?

• He cast doubt the accepted mantra of the current generation of energy wonks: Build Tight, Ventilate Right. Systems like dynamic insulation (see recent post) mean that airtightness may not be quite the holy grail that it is held up to be.

• Intelligent trickle vents, which self-adjust according to wind speed, air pressure and humidity levels.

• Intelligent controls for mechanical ventilation: at the moment, it’s off/on/boost. In the future, systems may begin to emulate the sophistication now seen with space heating controls.

• Supply Air Windows: Howarth are now making these windows which work by drawing air in between the two panes and pre-heating it on its way into the house. In effect, it’s a sort of heat recovery trickle vent, designed to work with the house under negative pressure, so usually installed with passive stack ventilation, or continuous central extract fan.

• Earth tubes: these draw supply air through underground pipes in order to pre-heat or cool the air before it gets to the house. Much experimented with over the years, but nowhere widely adopted. There are fears about contamination and mould growth within the underground tubes and this seems to hold back more widespread adoption of the technique.

• Dynamic Insulation: the capacity of walls and roofs to draw air in through their structure and to pre-heat it on its travels. A topic covered in a recent blog post here.

In fact, supply air windows, earth tubes and dynamic insulation are all ways of passively pre-heating the supply air. All have great potential but they are all also a long way from becoming mainstream.

I particularly like Bob’s concluding remarks. In the future, we will probably reach more understanding about ventilation processes, and we will be able to design systems more accurately. Then there will be less need to over-ventilate in order to be on the safe side, as at present. There is a need for greater knowledge about the complex issues involved in domestic ventilation. A lot more research is required so that we can create healthy environment in our homes while at the same time minimising the damage we do to the environment as a whole.

There are indeed lots of issues still to be resolved:
• Our understanding of house ventilation seems to be being driven by cold climate countries like Canada and Sweden, where ventilation is more critical. Does their take on it really transfer across to our lukewarm, maritime climate?
• Specifically, is airtightness quite as important as we are being led to believe?
• Are fully passive ventilation systems reliable? Or do we require fans to drive at least part of the system?
• Should we be driving towards more sophisticated controls and sensors in order to balance running costs with air quality? Or is dumb good?
• Are the health concerns about drawing supply air through ducting legitimate, or just a fear of the unknown?