How do you heat a passive house?

Some people may think the answer is "you don't," but Passive houses do need some heating. 

The real answer is that you don't need very much, so it's not so important how you heat it. 

A comprehensive discussion can be found here from Zehnder Passive House, listing the pros and cons of each approach.

My personal favourites at the moment are underfloor heating and air-source heat pumps, athough I have not always thought so, and cannot guarantee that as a final answer. 

I wrote about underfloor heating before, highlighting the advantages: saving space, evenly distributing heat and increasing thermal mass; and also warning of the problems that can come from incorrect design and installation. One disadvantage that I forgot to mention is that it's very difficult to get to the underfloor pipes if something goes wrong with them, although they are just pipes and it's very unlikely that anything will go wrong.

Air conditioning units are becoming standard installations in Japanese houses, and their COP is getting better all the time. As well as cooling, they can reverse the circuit to heat air. In a regular house using these for heating can be uncomfortable since they are only heating the air, while the building itself stays cold and the temperature is not balanced. In addition, the hot air can rise giving you cold feet and a hot head when you stand up. This may also be an expensive way of heating your house, and it may even be both uncomfortable and expensive. 

Since a passive house has such low heating demands, modest air conditioning units can easily provide the heating needs of a building. The peak heating load of a Passive house should be around around 10 Watts per square metre, so for a 100-square-metre house, you need 1 kW. Units are typically rated at several times this, so one air conditioner can produce all the heating or cooling you will need for a whole house, although you may have to think carefully about where to put it. It can go on a wall or ceiling so, similar to underfloor heating, it won't take up any floor space. 

An additional advantage of an air conditioning unit is that it can also cool the house, and may also have a dehumidifier. The units are stand-alone, rather than incorporated into the ventilation system, so they may be less complicated. And depending on where you are, you may get something that comes with a guarantee, and can be easily be maintained or replaced.  

And by the way, if you're in Japan trying to work out how to read the symbols on the remote control so you can use the air conditioner this post from Surviving Japan may be useful.