The gas that keeps your house and car comfortably cool is contributing to global warming. So what are the best choices when it comes to space and water heating in a high-performance home? Jason Quinn dives into the numbers to emerge with some recommendations.
The Passive House community talks a lot about operational energy and embodied carbon. But there’s a third culprit for greenhouse gas emissions (GHG) that dodges its share of the limelight and that’s refrigerants. It needs our attention because it offers the potential for quick, easy wins as we strive to moderate climate change. (If you want to skip the techy bits, you can go straight to the recommendations.)
In the high-performance homes I’m involved with, I usually specify heat pumps for hot water heating and high-wall heat pumps for heating and cooling. Some people argue that the energy and carbon savings from HVAC usually outweigh the potential GHG of the refrigerants over the equipment’s lifespan. You could sigh with relief and skip on by, but the Ministry for the Environment’s report last year calculated that since 1990, hydrofluorocarbon-based refrigerants (HFC) are the fourth largest source driving the increase of CO2-equivalent emissions[1]. That’s dumb, because there are already solutions that can dramatically curb HFC emissions. Those solutions will also save money in the medium term with only small initial capital costs.
Are you old enough to remember the ozone hole and the panic about ozone depleting gases in aerosol cans? The culprit in spray cans and fridges was fluorinated gases. They’ve been replaced by hydrofluorocarbons, which are also used as refrigerants. Big win for the ozone layer but not without other environmental costs.
Four things affect the impact of HFCs: their potency, the quantity used, the lifespan of the unit they power and recoverability at end-of-life.
Potency
Some HFCs produce more emissions than others. This is helpfully represented by a unit called GWP. It’s a conversion to an equivalent amount of CO2 over a specified period of time—assume 100 years unless otherwise stated. GWP takes into account how long the HFC stays in the atmosphere before breaking down, among other things.
For example, one of the most common residential refrigerants for heat pumps and fridges is tetrafluoroethane, often referred to as R134A or HFC-134a. It has a GWP of about 1400, meaning just one kilo of R134A has the same impact as 1400kg of CO2 released into the atmosphere. Ouch.
The graph below rates a number of HFCs on GWP as well as their efficiency. As you can see, there’s a big range on both counts.
“A graph comparing refrigerants based on several properties, including flammability, required compressor displacement volume, GWP and coefficient of performance (COP). COP for a refrigeration system is the ratio of the heat removed from the cooled space to the work performed to remove that heat. Source: Optimized Thermal Systems, Inc. ” Reference
Manufacturers are already shifting to refrigerants with lower GWPs. You can find otherwise identical heat pumps with different HFCs and thus significantly different impacts on the environment. These heat pumps (photo below) look identical but the top one uses R410a (GWP 2088). The bottom one uses R32 (GWP 675!): that’s two-thirds less CO2 equivalent.
Photo: N Patrick
Quantity
Second, how much refrigerant does any heat pump contain? This bit is easy as it will be marked on the unit.
Lifespan and recovery
We’ll consider these two together: both are hard to quantify. A reasonable life expectancy for a air conditioner is 10-15 years, according to Consumer NZ. A refrigerator can last 20 years or more. Some 486 tonnes of HFCs are “lost” to the atmosphere every year, according to the Ministry for the Environment. This happens via leaks, during equipment servicing, or as a result of catastrophic failures of critical components (eg car crashes).
Rates of recovery of refrigerant gases vary a lot across the world. We manage about 7%, or 35 tonnes, which leaves lots of room for improvement.
Armed with this, I started running lots of numbers. You can see my calculations here if you want to deep dive into the data with me. Otherwise, here’s what the numbers show are the best, most responsible choices.
Recommendations
- Specify heat pumps for heating hot water
Even on very conservative estimates (a five year life span), they produce fewer emissions than an electric resistance cylinder. They remain the best choice even once our grid becomes fully renewable. And a lifespan of 20 years is not crazy, in which instance, the case for heat pumps grows even stronger.
Bonus: like heat pumps for space heating, they are extremely efficient and will also save your client money on ongoing bills.
Downside: the initial purchase price is more than an electric hot water heater.
- Choose air-conditioning units with the best HFCs
Look for CO2 refrigerant systems like the Mitsubishi EcoDan (Ecodan QUHZ 4.0kW) or the Reclaim Energy CO2 heat pump. (CO2 can be used as a refrigerant and its GWP is 1: there’s no multiplier effect.) At least make sure it’s R32.
It goes without saying that designing buildings with high thermal performance is the most important thing we can do to reduce GHG emissions from HFCs. A certified Passive House in many New Zealand climate zones may need heating or cooling only on a few days a year. Even in colder climates, energy savings of 90% can be expected. This means fewer and much smaller air-conditioning units are required.
CO2eq savings versus years for several hot water heat pump refrigerants and amounts used versus an electric resistance cylinder. The point where the line crosses zero is where the Heat Pump starts saving CO2 versus a resistance heat pump. Note R744 is CO2 and has a GWP of 1 (nearly zero) which is why it starts saving CO2 almost immediately.
Even for a fully renewable grid (much cleaner than currently projected by MOE and BRANZ) hot water heat pumps with a small charge make sense even with current refrigerants if the charge lasts five years.
[1]Since 1990 New Zealand’s gross emissions have jumped by 24%. The sources driving that increase are, in order, more cows, fuel for transport, increased fertiliser use (in part to feed all those extra cows) and then those hydrofluorocarbons used in refrigeration and air-conditioning, both by industry and households.