Renewables in a Low Energy House

When we first started looking at our house, warts and all, we drew up a wishlist of all the changes we would make if we could. Almost top of the list was getting rid of the LPGas tank and heating the house with something more efficient, lower in energy requirement, and lower in annual cost. I had been keeping records of how much gas we were using and how much we were spending each year. Our LPG tank holds 2,000 litres of propane; we had a contract with the supplier for regular top ups throughout the year, so it was easy to see that we were spending over £3,000 per annum on heating our house. Only a small percentage of the gas was used for cooking, most of it went through our twenty-year-old Potterton gas boiler, and most of the heat that produced then went straight out through the roof and uninsulated walls. We were in effect paying through the nose to heat West Yorkshire!


We are lucky that we have a field next to our house, albeit a very boggy field. The bogginess, however, is a positive boon if you are planning to lay ground source heat piping, as the ground never freezes and the temperature of the ground four feet below the surface remains pretty constant. A ground source heat pump seemed to be the best bet for us; it is more efficient than air source, and we have the land to lay the pipes. These days the units are compact, only the size of a refrigerator, and fit neatly into the design for our laundry room on the ground floor.

The digging took place in February, not the best time of year in our neck of the woods, as the land is pretty saturated by the winter rains, but Green Source Heat of Glossop found a way, by dint of using large sheets of plywood, to get the mini digger onto the boggiest parts of the garden. Every trench they dug immediately filled with water, and they uncovered several springs whilst digging the two enormous loops that traversed the field from one side to the other. They managed to avoid all of our trees, apples willow and alder, and unearthed a mountain of stone in the process, as well as creating streams to carry the spring water away to the dike beyond our land.


The Ground Source Heat Pump model we have chosen is a Kensa Compact 6KW. It is manufactured in the UK and comes highly recommended. It is 570mm by 500mm by 900mm high, with a separate buffer tank which is strong enough to take the weight of the pump unit, giving it a small overall footprint. It fits neatly into the corner of the laundry room (which is not within the house envelope), with pipework through the wall into a manifold installed in a cupboard in the guest shower room.

The GSHP output is 40 degrees Celsius (for heating) or 60 degrees Celsius (for hot water); it supplements the solar thermal for hot water, but will supply all of our heating requirements.

Underfloor Heating
The ground floor, excluding the laundry room, was completely excavated, to a depth of 450mm (about 18 inches). A concrete slab 75mm (3 inches) thick was pumped into the ground floor, and a damp proof membrane laid over it. This was topped with 200mm of Xtratherm and a layer of Intello, and the underfloor heating pipes fixed to this. The whole lot was then covered by a screed, bringing the floor up to its original level.


We were advised that slate or ceramic tiles are best over underfloor heating, as they retain the heat longer than wood. We had intended to have Welsh slate to cover the floor in the kitchen, guest bedroom and hallways on the ground floor, but the cost in recent years has shot up,  making it economically unviable for us, even if it is relatively local. In searching for a reasonable alternative, we have chosen a quartz slate, which is paler than Welsh slate and has a nice rustic feel to it. This slate has probably been transported halfway across Europe, rather than the 120 miles from North Wales, but it still came in at a little over half the cost of Welsh slate; it is sad that a beautiful local product should be so prohibitively expensive.

It would have been nice to have had underfloor heating on all floors, but to do that would have meant raising all the floors by several inches and raising the roof as well. This was not feasible, so we have settled for having radiators on the first and second floors. Although each of the radiators is deeper than the usual thin panels, there are fewer of them overall, and they take up less wall space than our old radiators.

The heating requirements for the house have been carefully calculated; with the heat pump we shall be able to heat the whole house to 21 degrees Celsius when the outside temperature is minus 10 degrees Celsius, using only 4.4KW. Since our solar panels output a shade under 4KW, this means, in effect, that our heating costs have dropped from over £3,000 per annum to virtually zero!

We have settled for two heating controllers; one will allow us to program the underfloor heating and the hot water, the other will have a mobile thermostat so that we can choose optimum placement. We know that it will take a few months for us to get used to the different methods of heating the house, and this arrangement gives us the best option for getting the necessary adjustment right.


Woodburning Stove
We loved our Morso Squirrel woodburning stove. It was only small but output a phenomenal amount of heat. Every year we stocked up on seasoned wood and burnt the whole tonne over the winter, supplementing the inadequate LPGas. But such a stove is not compatible with an airtight house, because it takes its air from the room and exhausts it up a chimney, breaking the airtightness seal. It is a common dilemma for people wanting to build a low carbon low energy house in a timber-rich environment, judging by the questions asked on various internet forums. After a lot of research, Green Building Company came up with a solution: a stove which has been specifically designed for Passivhaus application. The Milan 4 Passive stove from Chesneys is built in the UK and has a direct air intake through the wall of the house. It uses a dedicated external air supply and automatically creates a curtain of air to seal the stove chamber when the door is opened for refuelling. Their documentation states that “it is the only stove independently tested that can safely be installed in an airtight environment”. Its output is 4KW the same as the Morso Squirrel, which is the heating requirement for the whole house at minus 10 degrees. Green Building Company say that we will never have to light it! It is a bit of an expensive item to have if its only purpose is as a focal point for the living room, but a fireplace or stove is one of those comforting things that humans seem to gravitate towards, so we don’t regret insisting on having it, even if it does end up as a decorative wood store.


Solar, both PV and thermal, was another item at the top of our must-have list of house improvements, but the only southfacing roof is on our garage, and this is shadowed by the house until about 11am on a summer’s day, losing seven hours of sunshine during which modern PV panels would not work efficiently. In the winter, the exposure to sunlight is even worse. Some years ago we had the house surveyed for an Energy Performance Certificate, and it failed miserably because of the lack of insulation and the usage of LPGas as a means of heating. We would not have received the feed in tariff if the energy performance could not be improved.

One morning, I was listening to the news on BBC Radio 4 and heard an item about solar energy in Germany, and the fact that so many southfacing roofs had solar PV that the grid was overloading in the middle of the day, and the government was encouraging more buildings to have east and west facing panels to even the load. A lightbulb went on over my head. OK we couldn’t have the most efficient southfacing panels, but could we have east and west facing panels that would give us similar coverage? After a bit of calculating, and advice from Eco Heat of Hebden Bridge, we decided east-west was our best option; in the summer, we would get heat and electricity from about 4am through to after midday on the eastern side, and from 10am or 11am through to sunset on the western side; in the winter, sunrise is about 8:30am, but because of our elevation (about 53 degrees north) the sun stays low in the sky all day. Even this small amount will give us roughly 4 hours of sun from the east and 4 hours from the west. I intend to monitor this carefully for the first year or two of operation to ensure that my calculations and assumptions were correct.


After careful measurement of the available roof space, and advice from Andy of Eco Heat, we settled on BenQ high performance photovoltaic panels, which generate 330watts each. With 6 panels on the east-facing roof and 6 on the west-facing we have the best coverage possible in our situation. We also discussed at length having the panels inset into the roof and this was the initial plan. The panels would lie flush with the tiles and look very neat. However, we have Velux windows in our roof, and even having wide but shallow Veluxes still did not allow enough room for the inline panels and the necessary flashing around them. We therefore had the panels installed on top of the roof tiles. The solar PV panels have been generating electricity since the middle of August. Initially, approximately 12KWh per day (figure based on the first week of operation during the last week of August 2015 when there was a lot of cloud and significant amounts of rain), although as the autumn progressed this dropped to 10KWh  per day on average. We shall be monitoring the performance closely.

The solar thermal panels are also installed on top of the tiles, two on the east and two on the west. The panels selected are Orkli Solar Keymark. The deemed input to the hot water cylinder is 2,328KWh per annum.

Hot Water
The hot water cylinder is heated by the solar thermal panels and the ground source heat pump, with an electric immersion heater as belt-and-braces backup.

Belts and braces
As you can see, we have gone a little overboard in our belt and braces approach to heating and hot water. Our caution is borne of long experience living here. We tend to suffer from breaks in electricity supply; most are brief, short glitches in power that cause everything to turn off and then back on again. But occasionally, there is a longer outage. Earlier this year, a van hit a wooden pylon a mile away and left us with no power for 5 or 6 hours. A few years ago, on New Years Eve, heavy snowfall brought down a power line a few hundred metres from us, and we were without power for 24 hours, during one of the coldest spells we have encountered. We wanted to minimise the very real disruption such outages cause, so we have tried to double up on most things. In the event of a lengthy power outage, we have decided that the pragmatic approach is to have a small standby diesel generator which will at least keep us in hot water and refrigeration. This may not seem very green of us, and normally I would agree,  but the alternative was to have a back boiler on the woodburning stove with very long flow and return pipes to the hot water cyclinder. No such model of stove exists for an airtight house, and the pipework would have been very difficult to retrofit into the fabric. We have therefore gone with the pragmatic solution. We shall probably only have to use it once every couple of years, if that.

See other posts in this series:

Our House Reborn

Airtightness and Ventilation 

Water, Water Everywhere



Filed under Our House

2 responses to “Renewables in a Low Energy House

  1. Pingback: If in Doubt, Insulate It | Things my grandma taught me

  2. Pingback: Home Sweet Home; or There and Back Again | Things my grandma taught me

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