In hot climates (like in Australia where I grew up), heating systems aren’t something you need to worry about much, but in colder climates even the best insulated house will need a heating system of some sort to keep the space warm on a cold winter night. And every house will need hot water heating of some form.
Gas boilers are the most common heating source in Western countries but the transition to a net zero carbon economy will change that. In Britain, regulations have already been implemented to ensure that no new-build home is connected to mains gas from 2024/25. So what are the alternatives?
Heat pumps are fairly new to the market but the technology will be familiar to us all – they are essentially just a refrigerator working backwards, transferring heat from where it’s not needed to where it is. And just like the coils on the back of a fridge, a heat pump needs a source to exchange the heat with.
The most common form is an Air Source Heat Pump (ASHP) where the heat is extracted from the outside air through a fan coil unit (similar to an air conditioning unit). Ground Source Heat Pumps (GSHP) extract heat from the earth via buried pipes or a borehole, making them more efficient and so cheaper to run than ASHP but also more expensive to install. There are also Water Source Heat Pumps (WSHP) which work with any large body of water or underground aquifers. Heat pumps can be installed as small standalone units (about the same size as a gas boiler) and connected to standard radiator systems and hot water cylinders, or as attractive integrated units about the size of a refrigerator.
Heat pumps are technically not renewable energy as they rely on electricity to operate, but they transform the electrical energy into useable heat energy by a factor of between 2 and 6 times, referred to as their Coefficient of Performance (COP).
Some heat pumps can operate in reverse, cooling the building interior. This also lets GSHP’s “store” heat in the ground during warmer weather, making winter operation more efficient.
There are lots of variations on this theme but the technology is ancient – burn wood, get heat.
Traditional wood stoves can be a practical heat source in rural locations but they are relatively inefficient, and they contribute to air pollution so are not generally appropriate in urban areas.
Modern biomass boilers are high-tech items of equipment designed to burn specific fuel types such as wood chips versus whole logs, but the simplest one for a single house is a pellet boiler. Biomass pellets are made from compressed sawdust or other plant fibres. They are small, thin nodules (think rabbit or chicken feed, or the cat litter that’s made from recycled paper) and come in bags which are easy to manage. The burning process is fully automated and can be controlled in the same way as gas boilers to operate at specific times or temperatures.
Using solar thermal systems for hot water will work in any climate, but especially in hotter climates where they can provide close to 100% of hot water demand. They are usually mounted on a house roof and there are two types of collectors, flat plate and evacuated tube, with the latter being more efficient.
There are also solar thermal hot air collectors, which preheat fresh air before it is drawn into the house. In climates with sunny winters these can make a significant contribution to space heating demand.
Mechanical Heat Recovery Ventilation
Whilst something of a mouthful, Mechanical Heat Recovery Ventilation (MHRV) is an important component of sustainable heating systems. It works via a series of ducts which extract “stale” air from rooms like bathrooms, kitchens and laundries, capturing the heat from this air rather than allowing it to be “wasted” to the outside, and transferring it to fresh incoming air which is supplied through more ducts to living rooms and bedrooms. Apart from being energy efficient, MHRV ensures excellent air circulation through the whole house, and does away with the need for individual extract fans.
MHRV systems can be fitted with high quality filters to improve indoor air quality, which is important for areas of high pollution or where residents have respiratory health problems. They are also often specified in locations where acoustic insulation is important as they allow windows to be kept closed.
For Passivhaus certification, MHRV is considered a mandatory requirement.
Waste Water Heat Recovery
On the basis that every little bit helps, Waste Water Heat Recovery (WWHR) is also a good option. These capture the heat from hot water going down the drain from showers, transferring it to the incoming cold water supply, offsetting some of the demand for hot water.
As strange as it may sound, sometimes electric heating systems are the most sustainable heat source for an ecohouse. With high levels of insulation space heating demand will be minimal, and if the bulk of hot water demand is provided by something like a solar thermal system then the need for a top-up supply is minimal. Electric heaters are relatively simple and inexpensive items, and so have a much smaller resource footprint than more complex plumbed systems. And as the electricity grid decarbonises in the transition to net zero, the energy they use will by default become more sustainable.
Underfloor heating has become fashionable to the point that it is marketed in real estate sales brochures, and there are many good reasons for installing it in an ecohouse, especially with heat pumps. It also leaves walls clear of obstructions for furniture, which is great for interior design. However, it is more complicated and expensive to install, and less flexible to alter, than a conventional radiator system, and as space heating demand reduces with better insulation the case for underfloor heating is not cut and dry. A practical compromise is to use underfloor heating in living areas where it will be of most benefit, but not in bedrooms.
Many cold climate countries have had district heating systems for generations. Hot water from a central heat source is pumped through insulated pipes to every house, where the heat is captured through a Plate Heat Exchanger (PHE) and transferred to the domestic radiator and hot water system. District heat systems work best for higher density housing units, such as terraces or apartments, as this reduces the size of the pipe network required.
The eco-benefits of a district heating system will depend on its energy source. Because of the larger scale, more efficient versions of biomass boilers and ground source heat pumps are likely to be feasible. There are Waste Water Heat Recovery systems which tap into the public sewers, capturing the wasted heat from all our showers and washing machine cycles and industrial processes. Some district heating systems are based on Energy From Waste (EFW) facilities that burn municipal waste, which is also considered as low carbon. If a new house is located where district heating is available, it generally makes sense to connect.