With no mains gas available, the heating fuel options come down to:
- LPG (which is what I used for the static caravan) either in cylinders or a larger storage tank
- Oil, which has traditionally been the fuel of choice in semi-rural locations
- Biomass (in other words: wood) in the form of logs, pellets or chips
- Electricity via a Heat pump, either ground-source or air-source
A key factor is that building a Passivhaus means the space heating requirement is really quite small – a good rule of thumb being that one candle per square metre will provide all the heat that is needed – although a large Passivhaus still needs a fair bit of heat on the coldest days. For the Main House the prediction is that 6kW will be required for space heating, with additional capacity required for Domestic Hot Water (DHW) and in a well-occupied Passivhaus it’s not unusual for more energy to be required for the DHW than for space heating.
Solar power – either photo-voltaic (i.e. electricity) or solar-thermal (i.e. hot water) – is an option as a supplementary heat source but it’s not practical to rely on that to meet more than a small fraction of the heating needs in winter. Below-freezing temperatures coupled with short days and bad weather (freezing fog is particularly problematic) mean that something else is required as a primary heat source.
My initial preference was for biomass, on the basis that it can be carbon-neutral, that I have plenty of space to store logs while they’re drying (takes a couple of years) and that there would be plenty of scope to locate a boiler and accompanying thermal store in the outbuildings. If the heat demand was higher a log boiler could make sense but the logs need quite a lot of preparation and handling, which doesn’t worry me too much (for as long as I’m fit and healthy) but might not suit everyone (so possible issues when selling the house). It would also mean having some sort of “heat main” between the outbuildings and the main house, which would be expensive to insulate to the required level.
I never really seriously considered oil or LPG, since I want to steer clear of fossil fuels if I possibly can.
For various reasons I didn’t initially like the idea of a heat pump. Part of my logic was that it seems a “waste” to use electricity “simply” to power your heating (when it can be used for so many other things) so you’re better off using something that can only produce heat, like burning wood, and “preserving” electricity for the important stuff, like running computers and LED lights. That logic certainly applies to direct electric heating (something like a fan-heater) but a good heat pump produces four times as much heat as it consumes in electricity which pretty much offsets the losses involved in generating and transmitting electricity over the grid. When you also factor in the options to produce electricity from renewable sources and the sheer flexibility and controllability compared to biomass it starts to look a lot more sensible. I’ve therefore come to realise that a Ground Source Heat Pump (GSHP) is really the best option and most of the “normal” disadvantages of them don’t really apply in my case:
- They need a lot of ground and require some significant excavation to bury the ground-loop collector pipes – but I have loads of space and all of the “garden” has been dug up at least once before anyway so doing it again won’t cause any (more) disruption.
- Since the main house has such a small heating demand it only needs a small GSHP unit so the risk of needing to beef up the electricity supply connection to the site is small.
- A good quality GSHP unit should last for many years with little maintenance, so while it’s an expensive option it should represent a good long-term investment – which is in keeping with my approach to the rest of the product choices. The Renewable Heat Incentive (RHI) also acts to guarantee a pay-back of this investment.
- For a new-build it’s easy to design the heat emitters (i.e. radiators or under-floor heating) to suit the low output temperature of a heat pump and hence keep it working at maximum efficiency. For a retro-fit installation that can be more problematic and hence options like biomass which can easily produce high temperatures are often a better match, which is why biomass boilers are more common in retro-fit than in new-build.