Knowledge/Refrigerants/R744 CO2 is a different world: 63 bar at 25 degrees
R744 CO2 is a different world: 63 bar at 25 degrees
R744 (carbon dioxide) sits at 63.33 bar gauge at 25°C. Its critical point, the temperature above which it won't condense, is only 31.0°C. So transcritical running is normal for a CO2 heat pump water heater here, not an exception. Gauges, hoses, relief devices and recovery gear all have to be CO2 rated. Ordinary HVAC gear isn't rated anywhere near this.
- At 25°C
- 63.33 bargagainst R410A's 15.56
- Critical point
- 31.0°CR410A is 71.3°C
- Above 31°C
- Transcriticalany day over 31°C
- Your normal gauges
- Not ratednowhere near it
How does it compare with the gases you know?
| Refrigerant | Critical point | 0°C | 25°C | 45°C |
|---|---|---|---|---|
| R744 (CO2) | 31.0°C | 33.84 | 63.33 | Transcritical |
| R32 | 78.1°C | 7.12 | 15.88 | 26.93 |
| R410A | 71.3°C | 6.99 | 15.56 | 26.32 |
| R454B | not computed | 6.66 | 14.83 | 25.04 |
| R290 | 96.7°C | 3.73 | 8.51 | 14.33 |
| R134a | 101.1°C | 1.91 | 5.64 | 10.59 |
R744 at 25°C runs about four times the pressure of R410A at the same temperature. That's not a margin you work around with care. It's a different class of equipment.
What does transcritical mean for you?
Above 31°C, CO2 can't turn back into a liquid on the high side, so there's no condensing. The condenser becomes a gas cooler, and the pressure on your gauge no longer tells you the temperature the way you're used to.
With a critical point at 31.0°C, that's no edge case in Australia. A 32 degree day puts the system there. So transcritical running is normal for a CO2 heat pump water heater here, not an exception.
In practice, you can't read a saturation temperature off the high-side gauge. The trick you use on every split system doesn't work here.
What don't we know yet?
We haven't got an Australian CO2 gas cooler operating pressure figure from a manufacturer, and that's an open gap. If you work on Reclaim, Sanden or Panasonic CO2 units, their own documentation is the authority, and it's worth having before you touch one.
We also couldn't confirm an Australian cylinder band colour for R744. That fits the general rule anyway. Identify the gas by the label, not the colour.
If you haven't worked on CO2 before, here's the safe version. Don't put your normal gauge set on it. The pressures are about four times what your gear is used to, and the diagnostic logic you rely on doesn't apply above 31°C.
Working out what the transition means for you?
There's a lot of noise about what's banned and when, and most of it is either early or wrong. What matters is what you buy and stock this year.
Kevin will give you the straight version.
Who this is for: small and medium HVAC contractors in Victoria. If you're a homeowner after a rebate or a repair, you need a licensed contractor, not us, and yours can point you the right way.
Related
- R410A: what's actually banned, and what isn'tThe phase-down limits bulk gas, not equipment. The equipment ban is narrow and ducted is exempt, the opposite of what a lot of contractors have been told.
- R290 and R454B: where they actually are in the marketR290 air conditioning here is almost all portables, not splits, and the reason is electrical law, not refrigerant law. R454B has only just started arriving.
- Which refrigerant GWP figures count in AustraliaAustralian law uses the IPCC AR4 2007 GWP figures. New Zealand uses AR5. A GWP that doesn't say which set it's from can't back a claim here.
- A1, A2L, A2 and A3: what the classes actually meanHow each safety class is tested, how flammable each one is, and why propane charges are capped in grams while R32 can run to kilos.
Review date: 1 April 2027. Figures in this area change without much notice and this page carries some that will go stale. Every number above is dated and sourced so you can check it yourself. If you find something out of date here, tell us and we will fix it.