Friday, 28 December 2012

Hot air about ventilation

There's something about mechanical ventilation systems that seems to go against the whole idea of an eco-house. The idea of using electricity the whole time to pump air in and out of a hermetically sealed box just seems downright un-environmental.

But of the options it's probably the best one available.

To survive comfortably and healthily, you need the house to be substantially higher or lower than outside temperature. That's assuming that human health and survival are compatible with ecology, but that's a different discussion.

So, unless you're sitting on or near a source of heat that is free, or very cheap both financially and environmentally, you're going to need insulation.

For insulation to work well, it should also be airtight. However well insulation works, you're going to lose heat if air can escape in and out.

And if you're in an airtight envelope, you need some kind of ventilation, unless you're in a few acres of thermal envelope with trees purifying the air, or you use oxygen tanks like they do in submarines.
Ventilation means air leaving as well as coming in, and the leaving air is going to contain a lot of heat. So unless you recover heat, you're going to have to produce or procure a lot more, and unless it is controlled by a fan, you are often going to be exchanging too much or too little.

Natural ventilation depends largely on external weather conditions, so if it's windy, more air will change, and if it's still, less air will change. Pressure fluctuations will also change the amount of air coming in and out, and this is likely to mean loosing too much heat or not having enough fresh air.
So this leaves two options. The simplest is probably the Passive House solution of a ventilation system with a heat exchanger. This pumps the appropriate amount of air in and out of the house and, in the winter, transfers most of the heat out of the expelled air into the incoming air to keep the house warm, and in the summer, transfers the heat from the incoming air into the expelled air to keep the house cool.

The other option, which I thought about before deciding on the Passive House approach, was to recover heat from the extract air using a heat pump, and make hot water with it. In this case, as long as the air was being extracted in suitable locations around the house, airtightness becomes less critical. In fact relatively thick, permeable walls would have a temperature gradient and may warm the air as it comes through them, although unless it was arranged carefully, most of the air would leak in through specific gaps.

This may be less efficient than the Passive House method, as the heat exchanger is passive, while the heat pump is active. It would also mean more heating in the winter, since the ventilation system is not going to contribute to the heating any more. More heating means more losses through the heating system. The heat pump would be working on air at a higher temperature—20 degrees above freezing rather than the -6 outside that it was struggling against last night—so would be more efficient.

Rather critically, there would only be a fixed amount of air leaving the house, and this may not contain enough heat to meet the needs on a cold winter day. At first sight, it would seem that there is not going to be enough heat in the expelled air, since you're going to have to heat the air coming in up to that temperature, but first of all there is solar gain, so the house is gaining heat. Secondly, if the hot water tank is over-sized, and you are storing heat a large thermal mass like our concrete slab, it would be possible to store heat for a few days. Thirdly, it's possible to get more heat out of the air, but the temperature will drop below outside temperature.

At the moment, the heat exchanger is getting heat out of night-time air well below freezing. I'm not sure how fast the fan is blowing the air over it and what kind of volumes we're talking about. The amount of heat in air depends on change in temperature but, unlike humidity, the actual temperature makes practically no difference, so if you change the temperature of some air from 30 to 29 degrees, it's going to release the same amount of heat as a similar volume dropping from minus 9 to minus 10 degrees. The difference is in the amount of energy you need to get that heat up to the temperature you want, which is going to be over 70 degrees to be sure to wipe out those legionellas.

Also, recovering heat to make hot water would need 24-hour energy use to run the heat pump since the house is being ventilated 24 hours, and so we would not be using cheap night time electricity, and the bills may be higher.

Another advantage is that you could perhaps turn the fan the other way in the summer, so you can cool the house while making hot water from incoming air.

This all makes sense in terms of design simplicity for the overall system, but in terms of economics would end up much more expensive than getting separate systems for hot water and for ventilation. Air conditioners are becoming standard in Japanese new-builds, and atmospheric heat pumps a popular way of producing hot water, but it's rare to find systems that combine these two, rather than throwing away the heat from the air conditioner.