Passive House in 17° weather stays at 65° without heater
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So far autumn has been fairly mild and quite sunny. As far as I know our (floor) heating has not had to kick in, or if it did it has been limited. I was still walking around in T-shirt + shorts until well into October. So yeah, I can confirm passive houses work, and I'm glad they do with the current energy prices.
When we built our house such passive houses were rare. Nowadays basically every newly built house should be (more or less) passive due to regulations that were tightened every couple of years. But there are still many many old and very energy inefficient houses that are in need of a good renovation...
Comfort wise I think floor heating is the best. It's radiated heat instead of heat from hot air circulating around, and since it's coming from below it tends to give you a more evenly feeling of warmth across the whole room/house.
Comparing cost is difficult... you have to look at the complete picture of all equipment you need in the house: how do you produce the heat? Is the same appliance also used for heating your tap water? How do you ventilate your house? Do you want/need some way to cool the house during summer?
We've chosen a geothermal heatpump that handles both heating the house and the tap water. Since we don't need much heating capacity the additional cost compared to the commonly used (at that time) gas heater was manageable. Additional benefits:
- Gas/oil heaters require a mandatory (bi-)yearly maintenance/checkup while a heatpump doesn't. - Using only electricity as energy source means I only have to pay one set of installation cost + yearly fixed subscription cost. - Electricity I can generate locally on my roof. - The heatpump can also be used for cooling via the floor heating system. It's not the same as real A/C but on the other hand it costs way less energy (basically just running the circulation pumps).
Both airflows pass the heat exchanger so the outgoing air can heat up the incoming air to minimize heat losses. These airflows are balanced, i.e. the various inlets/outlets in the rooms are calibrated so the total inflow matches the total outflow without the fans in the device having to run at different speeds.
An alternative system that is frequently used here is mechanical extraction while fresh air is pulled in 'naturally' via slits at the top of the window frames. This is obviously cheaper but has some disadvantages:
- The incoming air is cold and thus your heating system needs to work harder to keep the temperature constant. - You can feel this cold air flowing in, typically around your feet and this can be uncomfortable. - Less control over how much air flows in: a bit of wind outside can mean more (cold) air is blown into your house. There are sliders on these slits but they are manual so people either keep them open (resulting in more incoming air than strictly needed) or they close them (resulting in too little ventilation).
Don't get me wrong, this walls and roof with lots of insulation and windows orientated for the sun makes so much sense for passive heating, and in general insulation seems like the most obvious way to save energy.
At night it has been getting to 17° outside, with it getting in the mid to high 30s during the day. But we have been hovering from 65-69° inside despite the much larger outdoor temperature swings.
To only swing a few degrees means a huge thermal mass that charges during the day from the sun and discharges at night with little temperature change and huge insulation along with say body heat and cooking etc. Not that it's not possible but only varying 4 degrees F with no heating system other than the sun during the day seems like a stretch, will be happy to be proven wrong.
I'm in the process of building a house - it has windows and is nearly fully insulated, but there is nothing done inside yet (the floor is sand). In September we had a cold patch of 10C maximum for a few days, and on the last day I went to see how warm it was inside the house. The masonry on the south wall was 17C, and on the north 15C - and that was with some windows open upstairs. Masonry takes an awfully long time to respond to temperature changes.
My house has a lot of thermal mass so the sun will shine all day on the block walls and heat it up and it takes a while to cool back down at night. It does smooth out the temp swings, but bottom line my air conditioner must remove the BTU's that make it into the house, and it's a lot.
Obviously having much more insulation helps, and ideally you would want the thermal mass inside the insulation, so insulation on the outside of blocks. Then in a cold climate you need the irradiance from the sun to go through windows and heat the interior having the interior thermal mass absorb the heat while minimizing temperature rise.
It got warm in the summer. The main floor got to about 80 while the pitched roof in the upstairs got warmer. The house really wants to retain heat. But it's been incredible in the winter so far with it barely getting above freezing.
Note: it sucks in the summer but because of our latitude we can quite effectively build an awning that will keep the windows in shadow during the summer and let all that light in during the winter. That is this year’s project for the spring :)