Living with thermal mass
You might think thermal mass is a bit boring, but bear with me. I think a good analogy for comparing heating a passive solar home and the standard Aussie house is a sailing boat versus a speed boat. Sailing boats take advantage of renewable energy to sedately travel from A to B, sometimes using a small motor when the wind isn’t blowing. A passive solar home relies on sunlight and thermal mass for warmth, with the occasional boost from an efficient heater on cloudy days. Speed boats are totally reliant on their motors to travel. Without it, they’re expensive buoys. A standard home would be a cold, uncomfortable place if it lacked a heating source.
With that analogy in mind, what lessons have we learned after moving from a standard house to a sustainable home? As David said recently, “How are you going with trimming the sails on your ‘boat’?”
Sunlight warms the interior.
Passive solar homes are oriented with large north facing windows to bring sunlight inside during Winter. On a sunny day, solar energy by itself is enough to warm the interior to a comfortable temperature.
The Sun is low in the sky so it shines directly onto the north wall. The eave is angled up and away to avoid any shading at this time of the year.
My temperature sensors clearly show the Sun’s power. On a bright, clear day, the sensor on the north facing wall peaks at 35+C.
Around the other side of the house, it’s a completely different story. The sensor on the south wall (light blue line) is shaded from direct sunlight and reads an ambient temperature of 10 to 15C. Bringing sunlight into the kitchen clearly has the desired effect of raising the temperature (green line) to a comfortable 20C or so.
Standing in the beams of light, we instantly feel warmer.
Clerestory windows in the rooms at the back ensure sunlight helps to keep these areas warm as well.
Sun’s energy stored in thermal mass.
Warming the rooms when the Sun is shining is handy, but what happens at night? During the day, our house captures solar energy in a thermal mass. Think of the thick concrete slab and Timbercrete bricks in the wall as a heat “battery”.
As night approaches, we rely on good insulation in the ceiling, walls and underfloor to keep the warmth inside. Any heat lost to the outside is replaced by energy released from the thermal mass. The mass does a great job of limiting temperature changes inside the house as the surrounding environment cools overnight. Next day, the cycle starts again with sunlight recharging our “heat battery”. Under these conditions, we don’t need the reverse cycle air conditioner (heat pump) to maintain a comfortable temperature.
Protect the thermal mass on cold, cloudy days.
Lately, we’ve been experimenting with how to respond when our sustainable home sailing boat enters the equivalent of the doldrums. Day after day of cold, cloudy weather is the equivalent of no wind. What are we doing to make the most of the home’s passive design features during these times?
First thing to note is there’s no embarrassment with using our Panasonic reverse cycle air conditioner. The house is rated at NaTHERS 8.2 Stars which means it will require some mechanical heating and cooling to stay comfortable. Only 10 Star homes are designed to eliminate the requirement for additional heating and cooling.
We use the air con to keep our thermal mass within a reasonable temperature range. Overnight, I reduce the setpoint to 16C and then increase it to 20C in the morning. In this way, the air con is just topping up the thermal mass during the morning. If we allow the temperature in the slab (or Timbercrete walls) to fall too low at night, the air con has to work hard to warm them up again.
A hard-working air con can lead to uncomfortable draughts as it circulates large amounts of air to reach the desired temperature. Hot air blowing strongly out of the air con at head height leads to a stream of cooler air going in the opposite direction at floor level. Limiting the nighttime temperature drop avoids this problem as the heater gently nudges the temperature up to the higher setpoint in the morning.
What do the numbers show?
My Wireless Tag temperature sensors show how this process works. On cloudy days, limited sunlight hits the north wall (dark blue line) so the north and south wall (light blue line) temperatures are similar. Any breaks in the cloud cover immediately show up as temperature spikes which reduce the load on the air con and help to warm the thermal mass.
Overnight, the kitchen temperature (green) remains stable while the air con maintains the minimum 16C setpoint. During the day, the temperature increases with a combination of air con heating and whatever solar energy is coming through the north-facing windows.
Minimise electricity demand for mechanical heating.
Choosing a reverse cycle air conditioner as our back-up heating strategy minimises the amount of electricity required to do the job.
Our Panasonic 3.5 kW air con is a heat pump with a coefficient of performance (COP) of 4.56. The higher the COP, the more efficient the unit. Flat out, this system consumes 0.9 kW of electricity, but pumps 4.3 kW of heat extracted from the outside air into our home. That’s an impressive feat, especially when compared to a ducted gas heater. Out of every 33 MJ of gas burned, only 10 MJ ends up as useful heat energy inside the home.
The electricity consumed by the heat pump comes from renewable sources. If the weather is sunny, our solar panels easily generate enough electricity. At night, the battery keeps things ticking over. When cloudy days limit how much “home grown” electricity we make, I buy GreenPower from Powershop.
Others are optimising their heating.
Renew issue 144 has a series of articles from other homeowners who have learned to make the most of their heating systems.
A good example is a family with a similar arrangement of one reverse cycle air conditioner in the home. It heats the front lounge room, two bedrooms, hallway and a bathroom. A timer switches the unit on first thing in the morning to warm the house and then again in the evening.
Thermal mass transition from summer to autumn.
Paying close attention to what happens with the thermal mass highlighted one interesting change earlier this year.
The eave is designed to shade the north facing windows in Summer. Going into Autumn, the Sun travels lower in the sky so more and more floor is exposed to sunlight. This is a good thing as the thermal mass transitions from being a source of coolth on hot days to a source of heat on cold days.
Unfortunately, Autumn days are staying warmer for longer as climate change takes its toll. Sunlight started warming the slab before the cold weather arrived. As a result, the inside temperature rose a bit higher than we’d like. In most cases the outside temperature was lower than inside so trimming the sails meant venting warm air. If not, I used the heat pump to limit the thermal mass temperature increase.
Adding an adjustable shading area to the eave is one way to address this issue. On warmer days, extending the eave prevents sunlight hitting the slab. Something to think about for future designs.
Our education goes on.
Who knew that living in a house could be such a fascinating educational experience?
As we sail onward through Winter, I’m sure there will be more lessons to learn about how to stay comfortably warm using the minimum amount of energy.