Es Tresidder from Highland Passive explains what an air-to-air mini split heat pump is, why he believes it’s a good choice for heating a high performance home and the various pros and cons of going down this route. He also reflects on one of the toughest stages of his retrofit when they took the roof off the existing dwelling and then faced torrential rain.
Interview with Es Tresidder
Es Tresidder is a certified Passivhaus Designer with over a decade of experience in low energy buildings. His consultancy Highland Passive helps self-builders and retrofitters create better homes, that are healthy, comfortable and energy efficient. He’s also been working on various projects for John Gilbert Architects over the years, and he’s a regular at House Planning Help as part of our ‘ask the expert‘ team in The Hub.
Es has been on the podcast before, explaining how he upskilled in DIY during his retrofit project. In this session he’s reflecting on the whole process, including a key challenge he faced with water damage, and he’s going to go into detail about how he heats his home with an air-to-air mini split heat pump.
Es is a regular in The Hub where he helps answer questions on building physics and retrofit
Vehicle-to-load allows you to use an electric car’s battery to power external devices
This podcast was recorded during a power cut and was only possible because Es has an electric car capable of vehicle-to-load, which means that you can plug an adapter in to where you normally plug it in to charge, and then plug a three pin, normal domestic plug into that, and run that into your house. He explains that you can’t power your whole house, but you can take three kilowatts from it, which he was using with three or four extension cables strewn around the house, to power the fridge freezer, the dishwasher, the Wi-Fi, to charge computers and even to make a loaf of bread overnight!
“…logically, you know that electricity is very, very useful in life, but life just becomes much harder as soon as you don’t have electricity.”
When Es put the services into the house, he created a service void, which is effectively a pipe that goes through the external wall with an airtight membrane on one side and a windtight membrane on the other. This means he can add new cables into the house and make them airtight easily. He did this when adding solar PV to the roof and now he’s done it again, putting in a permanent extension cable that can connect to the car, ready for the next power cut.
Home battery systems with ‘island mode’ help you operate independently from the grid during a power cut
Es explains that domestic inverters shut down when the grid fails, preventing the house from drawing power from solar PV panels during an outage. However, if you have a home battery system you can pay extra so that you can island from the electricity grid, which would happen automatically during a power cut. However, domestic batteries are only around seven to 12 kilowatt hours, so if it was January or February, with little solar power generation, you’d quite quickly go through your battery’s power, even in a Passivhaus. A car battery is much bigger, around 77 kilowatt hours, which would last several days during mid-winter, running heating as well as everything else. Another advantage is that if the power cut lasted much longer, he could just drive into town to a public rapid charger and top the car back up.
Getting the MVHR back on was a priority because it immediately improves the health and comfort of the home
Es says the MVHR has a really small energy requirement and is beneficial in lots of ways, so it was the first thing they plugged in. Until that point they had been keeping windows open for ventilation and, with an outdoor temperature of 12-13 degrees, the temperature inside had dropped down to around 17 degrees.
The existing dwelling before any alterations
The completed project
Photography by Alex Trowski, commissioned for a case study in Passive House Plus Issue 50
The retrofit, extension and remodel has made the whole house work better as a home
Es’s house was a 1975 timber-frame build of one and a half storeys, where the first floor rooms felt quite pokey. So the strategy was to take the whole first floor off and rebuild it as a one and three-quarter storey house. This may not sound like a big increase, but Es says it gave a lot of extra floor space on the top floor, with enough for an extra bedroom, and an increase in size for the other bedrooms. The floor plan just worked much better.
The upper floor is a new build Passivhaus, with an I joist timber frame sitting on top of the existing structure, which was quite complicated to do. Downstairs is a retrofit of a 100mm timber frame house, where the plasterboard was taken off and insulation put between the studs. He then added another 40 mm of wood fibre insulation board, an airtightness membrane, and then outside of that is white rendered blockwork. There’s a cavity between the two. As a Passivhaus Designer, Es did all the energy side of things and a lot of the retrofit work himself. The new build was carried out by a local contractor.
The project involved removing the pitched roof and extending upwards
Keeping a flat roof dry is incredibly difficult
This project required the first floor and roof to be removed, which is slightly unusual. When they were looking at the options, one way to keep everything dry was to put up a massive tent over the whole structure, but this would set them back £30,000. The builders, however, were confident that they could keep the water out because they had done this many times before. There was also the fact they would be doing this in May which is statistically the driest time of the year in Fort William. Unfortunately the weather did not play ball and it was an unusually wet month. Es also points out that although the builders take off lots of roofs they probably don’t remove the whole structure leaving effectively a flat roof to keep dry.
So it just became impossible to protect the structure from the torrential rain. Es had already insulated the ground floor and achieved good airtightness but the rain soaked the floorboards and insulation, forcing him to strip them out and replace everything.
“You don’t want to install any insulation until the building is watertight.”
A lot of insulation got waterlogged and had to be replaced
Sequencing the work differently would have avoided damaging the insulation
If doing the project again Es would not necessarily pay for the giant waterproof tent but he would sequence things differently. The work downstairs had been done first to ensure structural stability for the new first floor and roof. However once rain had compromised the work, they had to go ahead without the extra racking boards on the walls anyway or the racking strength from the floor. He would therefore consult a structural engineer to look at any structural elements that could be done without involving the insulation.
The unit inside the house
An air‑to‑air mini split heat pump is a cost effective way to heat a low energy home
For Es, a heat pump is an obvious choice over direct electric resistance heating. However, instead of a conventional air‑to‑water heat pump system, Es chose an air‑to‑air mini‑split system. He explains that this is like an air conditioner, which usually pumps heat from inside to outside a building, but can also be used to pump heat from outside to inside a building. They have a similar efficiency to an air-to-water heat pump, but are much cheaper to install, because there’s no need for radiators or a wet system. The mini split has an outdoor unit and an indoor unit, with refrigerant lines between the two. The unit and installation cost was around £1,500 compared to £12 000 for the air‑to‑water system. Es is clear that this system is great for a Passivhaus but wouldn’t work for a normal new build house as it only delivers heat into one room and relies on heat spreading to other rooms.
The unit outside the house
Reversible operation provides useful cooling in heatwaves
Mini‑splits can run in reverse to deliver modest cooling, which has proved handy for Es during a heatwave which coincided with the midge season (when they don’t really want to open any windows) and a large group of friends staying over and thus raising the indoor temperature.
Sizing the mini split above peak load ensures comfort in cold snaps
The house’s peak heating demand is around 1.3 kW and by selecting a 2.7 kW mini‑split Es has provided a comfortable margin, which allows the system to cope with occasional freezing cold days without needing supplementary electric heaters. In cold weather he will run the system overnight when electricity is cheaper, so increasing the temperature in the main parts of the house, whilst the bedrooms stay cooler. Once the doors are open the heat can spread through the house. He admits that having his office in his bedroom means occasionally it can be cooler than he would like and so a small electric heater would probably be a good addition.
Photography by Alex Trowski, commissioned for a case study in Passive House Plus Issue 50
Locate an air-to-air mini split system in a large open area
Es explains that the mini split system needs to be in a space big enough to deliver a lot of heat into it, before warming that space to above the set point temperature. A small room would reach that temperature quickly then the system would shut off and so not provide enough heat for the rest of the house. The indoor fan also produces a low hum comparable to a refrigerator, louder than the house’s MVHR, so you need to place it where the noise won’t matter too much. Es stresses that whilst the MVHR supplies fresh air, and recovers heat it only moves a small amount of warm air between rooms. Keeping interior doors open lets the warm air from the mini‑split circulate more effectively.
Grants favour air-to-water systems, but air-to-air systems may still be cheaper
Even though Scottish renewable‑heat grants are available for air‑to‑water heat pumps and not for air‑to‑air units, Es feels the air‑to‑air solution may still be cheaper. The grant system also involves a lot of paperwork and restricts you to using MCS certified contractors, who are not necessarily the best. For a Passivhaus going down the air-to-air route also means you keep things simple.
A standalone hot water pump works alongside the air-to-air system
Whilst air-to-water heat pumps can also be used to supply hot water, an air-to-air mini split system is just warming the air in the house. So Es has installed a heat pump water heater, which is a 200 litre insulated hot water tank with a small heat pump on top. They are normally ducted to the outside to bring fresh air into the heat pump, the heat pump takes some heat out of that outdoor air, and then it ducts the exhaust air back out. Es has set his up to take heat from the house, which gives some free cooling in the summer and a much faster reheat time in the winter. It’s also easier from an airtightness point of view as it is not ducted to the outside at all. In the winter, the water heat pump is slightly cooling the house, so the mini split has to work a bit harder, but overall, it is still very efficient.
Living in the house has been amazing
The house before Es started had quite serious mould and damp problems. They were always a bit chilly in winter and couldn’t dry things because there was no reliable ventilation, and the house wasn’t warm enough. Now things are completely different.
“It’s hard to remember how it was before, because it’s just so nice now.”
The house is warm and comfortable all the time in every room. Es believes it’s almost more impressive in hot weather than it is in cold weather, when you expect it to be warm. Es says that the first summer they moved in, they had two separate weeks of more than 28 degrees as peak temperatures in the afternoon. And in those conditions, the house was at 23 degrees, and it felt like walking into an air-conditioned space. It has really improved the quality of life. The air is clean and just walking outside the house, Es immediately notices if the neighbours oil boiler is running or there’s a coal or wood fire on locally. He thinks his sense of smell has improved, because he’s used to clean air and the house smells fresh all the time. The best thing isn’t the bills, but it’s the most quantifiable thing. He’s reduced his heating bill by at least 80% while also massively increasing the average temperature of the house.
“After one full year of monitoring we have spent £175 on heating and hot water for a family of five, which is remarkable.”
Find out more
Transcript
Download a transcript of this episode.
Please connect with us
Subscribe, rate and review us on Apple Podcasts
