Heating a Passive House

In our last post, we mentioned two major design changes that we’ve made just before the point of no return. The first was the choice to go with a factory-cut wood fibreboard-clad CLT structure for the house.

The second was about heating. Now some people reading this, especially those planning a passive house themselves, might be thinking: “Heating? We don’t need no stinking heating!’ And, yes, we too had planned to have a house that essentially had no major heat source, that would be heated almost entirely by the passive solar gain and the warmth of bodies in the house, all of which would be kept inside by the air-tight envelope and incredibly efficient Energy Recovery Ventilator (ERV) – more about which in a later post.

However, something else came up: hot water.

I should mention that there are certain peculiarities with our site. Where we’re building the house, there is hardly any soil, literally inches, before you hit the limestone bedrock, and beyond, further down the slight slope where there is more soil, we have to install the septic field (there is no mains water or sewage on Wolfe Island). This means that ground source heat pumps etc. are not really a realistic option.

We were intending to use a combination of solar thermal water heating panels for three seasons of the year, supplemented in the winter by on-demand electric water heating batteries powered by a solar PV array. But the latter turns out to be very inefficient and expensive. There are systems that can supposedly heat water directly from solar PV panels (i.e. without inverting the power and storing it), but these are relatively new and we can’t find any working examples. And the other thing with all these complex PV-based systems is that they all need monitoring and the number of high-tech control panels etc. soon starts to get ridiculous. We’re far more interested in building a home for living in with appropriate technology, not a high-tech ‘smart’ home.

So, we’ve going back to basics and, as with the structure of the house, are looking to wood.

We’re going to have a wood stove. Yes. But not just any old wood stove, we’re going to install a Walltherm ‘natural down-draft hydronic wood gasification stove’ from Power Strength & Energy Solutions Ltd in Nova Scotia, (it’s actually made by Italian/ German company, Wallnofer). This not only burns wood, but also burns the gas that is generated from burning the wood, in a secondary chamber. What’s more, although it still looks pretty smart and you get the wonderful visual warmth that only a fire can provide, it is designed primarily to heat water. The stove has a water heating jacket, which in normal use takes 70% of the heat generated. For small, highly insulated and passive houses like ours, which don’t need much in the way of ambient air heating, there is also an extra insulating jacket that redirects even more of the ambient heat to the water pipes.

Walltherm Wallnofer hydronic wood stove

Even better, the suppliers of the Walltherm also sell a multi-source water tank that also takes solar thermal energy, as well as the roof panels etc to go with this (PDF). We probably won’t need to use the Walltherm itself for much more than an hour or two a day to generate all the water we need, so we’ll use relatively little wood, but this will also mean that the house will be nicely warm and not just tolerably warm, in winter. If we end up with too much hot water, we can probably send some of the water heated in a simple pipe out around the edge of the downstairs concrete floor – a simple version of underfloor heating – as well as having more baths, of course.

Full Wallnofer System

The obvious question is why not a hydronic pellet stove? Pellet stoves are highly efficient, produce very little waste and pollution and use up waste sawdust. What’s not to like? Well, we have a pellet stove as the only heating in our current house, and to be honest, we haven’t been impressed for several reasons:

1. They require electricity to operate. Both the ignition and the auger that supplies pellets to the firebox are electrically operated. With the relative frequency of power cuts here, this has been an issue, and while it won’t be in our new house with the PV-supplied battery system, it does make the system less resilient than a wood stove, which can operate without electrical power.
2. No-one delivers pellets to Wolfe Island, however it is easy to have wood delivered. What this means is that we have to drive to suppliers to collect ten bags at a time every week for about four months before winter. It’s just inconvenient and inefficient.
3. The pellets come in plastic bags, which is unnecessary waste and, whatever is said about local production, in practice most of the pellets seem to be trucked in from the USA.
4. Our pellet stove just doesn’t seem to produce a very high level of heat, whatever its nominal rating, which means we’re using a lot of bags of pellets in winter, and it just doesn’t feel as comforting as the wood stoves in friends’ houses.
5. Last, the Walltherm’s two-stage burn makes it almost as efficient as a pellet stove. In fact, we’ve decided we will probably sell on our existing wood stove and replace it with a small wood stove in our current house (which will be converted into a studio and guest space).