Sunday, 24 March 2013

How much ventilation do you need?

We need fresh air in our homes, and that means exchanging nice warm air for cold air coming in. Too much ventilation wastes heat and too little is unhealthy. It isn't easy to get the balance right and that applies both for new build and for draught proofed older house. When we extensively retrofitted our victorian house we were given the choice of trickle vents in the windows or, as our architect advised, MVHR - mechanical ventilation with heat recovery. The idea is you have extractors in kitchen and bathroom which connect to a heat exchanger; the extracted air warms up fresh air coming in that is pumped into the other rooms. It works very well, but it is a bit clunky. It uses electricity to pump the air around and it only has three speed settings. Also it has filters which we have to change regularly at an extortionate price. At Ecobuild this year I discovered a company called Aereco which has an ingenious systen for of demand controlled ventilation. They claim you can get almost as good heat savings without a heat exchanger provided you only ventilate when you need to.

Ventilation is a difficult subject which involves almost as much psychology as engineering. The most important aspect of air quality is CO2 levels but we don't notice them unless they are very high. When we feel a room is stuffy it is usually because of musty odours or high humidity which don't actually do us any harm directly. Still humidity is a good indicator of poor ventilation because when we generate CO2 - by breathing or by burning gas or other fuels - we generate water vapour too. Also, high humidity can easily lead to condensation and mould which certainly is bad for both us and the building.

Most ventilation systems are designed to change the air often enough for the expected level of occupancy. Hence windows come with trickle vents that are fixed open and MVHR systems are more or less fixed in settings. This means you get more ventilation than you need most of the time and not enough some of the time.

Why not just open the windows when you need to, I hear you say? Well that solution is fine if you take care to do it properly but very often we don't. Imagine you've just had a shower and the bathroom is steamy. When you leave you open the window and you shut the door to keep the cold out of the rest of the house - so where is the air to come from to replace the steamy atmosphere you are trying to get rid of? Even if you have an extractor it will struggle to achieve much against a closed door and an airtight house. You need to make sure you let air in as well as out. The German government recommends shock ventilation, where you open all the windows and internal doors for just a few minutes, several times a day. ( See 'Energy saving achievements and shock ventilation'.) Most people in the UK have never even heard of this and our building regulations assume that we can't be trusted to open windows hence the rules requiring trickle vents. That means we have continual air flow whether we need it or not.

Demand controlled ventilation requires a mechanism to measure the air quality and adapt the ventilation accordingly. Some systems sense CO2 but the Aereco ventilation is triggered by humidity only and it does this using a purely mechanical effect - as a strip of acetate material gets moist it changes length. The air inlets and outlets open automatically when the air is humid with no electrical power needed. I really like that. You can use them with either mechanical or passive ventilation system. A passive system requires no power as it relies on warm air rising and chimneys generating a good draught. Even if you use a mechanical system you don't need as much power as in an MVHR system because it takes a bit more oomph to drive air through a twisty heat exchanger. Also Aereco recommends a grille in the bathroom door to facilite air flow in to replace the steamy air being extracted.

I have an instinctive mistrust of complicated technological solutions to pretty much anything but the Aereco system is elegantly simple and they have an impressive set of field test reports to show that it works. On their downloads page you can find a study by the Fraunhofer institute showing clear energy savings almost as good as an MVHR system - and it is considerably cheaper both to install and to run (less power needed and no filters). I don't want to sound like an advert but I do wish I'd known about this system 3 years ago when we were doing our house up.

1 comment:

  1. Thanks for this, Nicola. It helps me understand the humidity reading in my room (see "Humidity2" on and compare with temp1). I was thinking of installing temperature sensors in all rooms (DS1820 to raspberry pi), but now I think it would be more useful to put in temp/humidity sensors (DHT22).