analysis high-performance building
The houses are super-insulated: basements are
within the thermal envelope, with r- 20 walls and
sub-slab insulation. above-grade walls are r- 31,
roofs are r- 50.
The single point source of heat — the heat pump — should be able to provide all the required heating,
as long as the doors to the bedrooms are left open to allow heating by natural convection.
figure 1, two households achieved zero-annual
net energy, and an additional two households
had energy consumption that was within 20 percent of the Pv production.
Figure 2 shows the breakdown of total annual energy use
by end use. The meter that logs
heat pump energy usage is covering usage for both heating and
cooling. In the data analysis, I
made the simplifying assumption that June, July, August and
September usage was for cooling, and all other usage was for
heating. The combined usage
for lights and appliances is calculated for each household by
subtracting the usage of the
heat pump, the electric radiant
panels and the domestic water
heater from the total energy
usage. occupant choice matters
hugely, as the key insights gathered from figure 2 highlight:
energy used for cooling is
small at this northern location,
yet it varies by a factor of 26 to
1. It’s clearly discretionary energy here — some
folks use it, others hardly at all.
With the exception of one house, heat pump
energy is relatively even. The electric radiant
panel energy, however, varies by a factor of 14
to 1. Since the panels are much less efficient than
the heat pump, it is in the occupants’ financial
interest to minimize panel energy. It may not be
in their comfort interest to do so, though.
Heating energy varies by a factor of 2 to 1. If
the highest and lowest usages are discarded (as in
olympic judging) the variation is much tighter —
the next highest usage is 35 percent higher than
the next lowest, and only 38 percent higher than
the lowest. The highest usage is 52 percent higher
than the next highest usage. do folks at this house
keep the house warmer than others do? In this
house, the panel energy is more than twice the
amount used by panels in the next highest house
— yet the heat pump energy used is second to
lowest. I don’t think that the heat pump is faulty,
but perhaps it is worth checking.
dHW energy varies by a factor of 2. 4 to 1.
The data reveal that this type of energy usage is
much more linked to number of occupants than
heating and cooling is.
In all but one household, dHW energy exceeds
energy used for heating. That is what happens in
low-energy-use houses using heat
pumps and electric resistance for
d WH. It is clear to me that the
next available investment in ener-
gy savings in these homes would
be either solar thermal or a heat
pump water heater!