February 5th, 2021 by Barry A.F.
One of the ways to slow the advance of climate change is to reduce your personal carbon usage. While we can’t efficiency our way to climate neutrality, we can buy ourselves time by slowing the rate of carbon emissions and conservation, as Negawatts are often the cheapest form of clean energy available (and the least polluting). Also when you have less energy to replace, it’s cheaper to do so (i.e. if you cut your energy use in half, then only half the renewables are needed to make it sustainable).
Our homes can seem like a monolithic entity — they need heat and or cooling, they use water and heated hot water, they consume electricity, and need lighting and plumbing. But the structure plus our actions can alter how much carbon is produced by several orders of magnitude. Two equivalent homes standing side by side could have 5 to 20 times the difference in carbon pollution produced in daily operation. A 100+ year old leaky home with inefficient appliances and high electricity use creating dozens of tons of CO2 a year can stand next to a Passivhaus or Net Zero home, which has very low or even no carbon emissions whatsoever. And there is a huge continuum in between these extremes. Many existing homes that are inefficient can be upgraded to various degrees to reduce their carbon footprints.
This will be a four part series:
Series One: Insulation And Air Sealing
Series Two: Heating/Cooling And (Hot) Water
Series Three: Plug Loads
Series Four: Building For Net Zero Or Better
The standard disclaimers apply, all advice is for informational purposes only, CleanTechnica is not responsible for any damages caused by inaccurate information or following any information provided, consult professional expertise before making any modifications to your home, all information is subject to change as our knowledge evolves, and the coffee may be hot.
This article series is focused on detached and semi-detached homes, but many of the concepts are applicable to all building types.
Almost all cooling is done with electricity. Evaporative systems exist but are uncommon, especially in high humidity areas where evaporation cooling performs poorly. Hence most cooling is done mechanically by vapor compression heat pumps (devices that move heat from one side of a barrier to another like your fridge/freezer). In many hot climate countries, air conditioning is required to survive, in some places it is actually law that air conditioning is provided to tenants (much like many cold climate countries have laws that heat must be available and minimum temperatures). The efficiency of air conditioners varies greatly, with newer high efficiency systems having a huge edge in BTUs moved per watt. There are three factors in energy used for cooling: the cooling load of your home, the efficiency of your equipment, and the duty cycle (how many hours of cooling per year), also known as the annual cooling load. This is location-dependent, with the equator having many more heatwave days vs the North or South Poles.
As a starting point you should figure out your current cooling load. The energy audit, if done from Series One, should already have this information for you, otherwise you will have to calculate it, with a meter if you have a plug-in unit, or by other means. This can get tricky because your testing will likely not be over an entire year. If you can determine the number of watt-hours per day and the peak temperature, you can approximate what your cooling load is using Cooling Degree Days (but it gets tricky as the hours of sunlight and temperature change daily). Hence a cooling load calculation from an energy audit is probably the best benchmark available. Reducing air leakage and adding insulation can reduce your cooling load, and hence kWh consumed.
The typical cooling options are window air conditioners, portable air conditioners, central air/heat pumps, and geothermal.
Window air conditioners are often the more efficient solution because they only cool part of a home, versus central air that may be more electrically efficient (BTU/W) but has ~3-20x the volume to cool. However, if you can’t do with only a room or few air conditioners then central air may be required. In many countries efficiencies are stated on the package when you’re buying the unit, or you can compare the BTU divided by the wattage to get the efficiency. In the US and Canada, Energy Star provides ratings for all new units on the market. Many other nations also have efficiency ratings for products or national efficiency programs.
Portable air conditioner units are typically less energy efficient than equivalent output window units (so comparison shop) but not all homes have windows in convenient places or of a convenient size. In addition, the portable units also often use indoor air to vent the hot air though the exhaust pipe, meaning conditioned air is lost and hot/humid outdoor air has to come from gaps in the house to replace it. So look for dual pipe units.
Central Air units cool your entire home. If your home is efficient or you did the upgrades from Series One, then you have likely also reduced your cooling load. These units can be very inefficient if they are old, and you should be able to determine its efficiency by determining its BTU rating divided by its wattage. Be sure to include the furnace blower motor usage. You can contact manufacturers to determine these numbers if they are not provided on tags, or use a clamp energy meter or a whole house energy monitor with everything else in the home turned off.
Geothermal cooling has the same considerations as it does for heating, spoken about in Part Three.
You can save cooling energy by letting the house warm up a bit at the end of the day and opening windows at night (if you’re expecting a cold night and the humidity is low, this is a big consideration, as high humidity adds latent load which is extra water vapor that requires extra energy to get rid of, so on high humidity nights it’s best to keep the windows closed even if it is cooler outside), insulating your home/low SHGC windows (see Series One) or by temperature setbacks and allowing the indoor temperature to rise while you’re not home or sleeping. Setbacks will save a few percent of power used as a smaller temperature difference equalizes more slowly due to thermodynamics.
Older air conditioners are often less efficient then newer devices, but as mentioned you can determine its efficiency by dividing the BTU rating by the wattage used. Both should be stated on a placard which is often on the side or bottom of the unit. If the difference is small you can just wait until your current unit wears out and then replace it with the most efficient unit you can find. If the difference is big, then replacement may be more cost effective than using your current unit. Do the math to determine payback period, not forgetting to include the electricity used by the blower motor.
If your unit is old enough that it is still using R12 (Freon), then replace it now so that it does not leak and further damage the ozone layer. Many units will also state the refrigerant on the placard. If it does not, contact the manufacturer. There are many decades-old units still in use so don’t be surprised if you have one, especially if it came with your home and you did not purchase it yourself.
Stay tuned until next week for Part Five – HVAC Efficiency considerations
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