2021 Webinar Series - 12 Commercial Hot Water Applications with High Temperature CO2 Refrigerant Heat Pumps

In case you haven’t heard, CO2 refrigerant heat pumps have some very unique properties, including being one of the most environmentally friendly refrigerants available. Also, CO2 heat pumps can efficiently increase (i.e., lift) the temperature of water over 100F in one pass of the heat exchanger – which is well suited to produce domestic hot water. In this session we will hear from three CO2 refrigerant heat pump manufacturers talking about their unique products and applications – largely focused on commercial and multi-family projects. 



  • Troy Davis / Mayekawa USA
  • Kyle Korman / Mitsubishi Electric Trane
  • Andrew Macaluso / Watts Water Technologies








  1. What tonnage range does the aww heat pump come in?
    1. Mayekawa - We have one capacity at this time, 100kW heating capacity at standard condition, but this also depends on heat source temp and inlet water temp, this affects the performance.
  2. Do we have any special ventilation requirements for CO2 leak in mechanical room
    1. It depends on the total CO2 charge in the room, we do install a CO2 detector, sometimes multiple detectors and these can sound an alarm. Sometimes we also need ventilation to activate when an alarm occurs. But an A-1 refrigerant is nothing special.
  3. Troy - simple question - why is CO2 not commonly used as a refrigerant for residential heating & cooling?
    1. Mayekawa - It is used for residential DHW, Sanden makes a small unit around 2-5kW I believe. CO2 cooling is not as efficient as hot water heating due to cycle inefficiencies, many are working on this issue now to improve the cooling only performance. It is also a cost to scale issue. In Japan there are millions of the EcoCute CO2 heat pumps in residential use, primarily driven by Utility incentives.
  4. Would a High Pressure CO2 leak be similar to that of a CO2 release from a CO2 Fire Extinguisher?
    1. Not really, it depends on if the high side or low side leak or pipe burst. The leak can dry ice and freeze for a moment, then release again. It is not a major rupture event.
  5. One of the biggest hurdles in the GSHP industry is first cost. What is the rough price comparison per Ton for CO2 WSHP's vs. conventional WSHP's?
    1. Mitsubishi - Great question - I'm not totally sure we have all of that information available yet.  Our QAHV units are going to market late summer/early fall and they will be at a fixed price in targeted markets (NYS, Boston, Northwest) initially.  We haven't found out yet what that price will be but it will be something to track once rolled out.
    2. Watts - It is important when considering first costs for CO2 heat pumps to look at the entire system cost, not just the heat pump engine.  While CO2 heat pumps may be more expensive per BTU over conventional refrigerant heat pumps, being able to install less (or even zero) auxiliary support equipment will often pull the first costs down equal or lower than other technologies.
  6. Question about the CO2 systems, Do you offer factory training for technicians planning on installing and maintaining the equipment? How much training is required to work on the units versus a conventional or Geothermal system?
    1. Mitsubishi - Yes, absolutely.  This is a specialized product that is not being included in the product portfolio of City Multi VRF. It's release is being targeted in key areas with a limited rollout so our teams can keep a close eye on the installations. Contractors will need factory training and there will be multiple teams assigned to a project.
  7. Kyle: Do you plan on introducing a water source unit as well?
    1. Mitsubishi - At the moment we do not.  The properties of CO2 are very conducive to being air-source and will provide very high temp/high pressure refrigerant to the gas cooler for high delta-T heat transfer to the water loop.  I think the main focus right now is how to most effectively control the unit(s).
  8. You are now making the spiral twisted copper pipe heat exchanger? I used to buy them in the late ’80’s but they ceased making them?
    1. Mitsubishi - Indeed we are having that direct pipe-on-pipe heat exchange.  Each QAHV will have (6) gas cooler heat exchangers piped in parallel. Coldest water comes into contact with warmer refrigerant, and warm water comes into contact with warmer refrigerant.  The twisted design allows for greater surface area heat transfer and a turbulent effect for higher velocity and uniform heat transfer.
  9. What policies would be useful in increasing the use of CO2 heat pumps for water heating in New York State?  Also, why don't more of New York City's one million buildings use CO2 heat pumps for water heating? What is the most important reason more don't use it?
    1. Mitsubishi - One of the target markets for the QAHV units is NYC.  This is a much newer technology so there hasn't been the opportunity to satisfy the demand.  I would imagine that we will be saying many installations in this market.
  10. Kyle: Did you say that the QAHV is being intro'd/marketed under a new name in the US market? If yes, what will be the name used in marketing?
    1. Mitsubishi - Yes, QAHV is a pretty dull way to introduce it to the general market unless you're talking to Mitsubishi employees but Heat2O is much more engaging for the general audience. It plays on the H2O molecule in that water is being heated.
  11. Do any of the presenters have experience with using waste water, sewer water, as a thermal source for their systems? If so, how did it go? Why isn't that done more?
    1. Mayekawa - Yes we have done a few installations of both waste water from a process or building and sewer water at waste treatment plants. They do work well as the heat source temperature is usually in the 55-75F inlet range which is good for a CO2 water source heat pump. One issue is the heat exchanger design, for sewer water direct we use special spiral low fouling heat exchangers and they can be costly, so usually a larger system could have a nice ROI, whereas a smaller capacity system that still requires the same components etc might not have a decent ROI to make sense. So these are more special applications that you need to look into all the details to see if it makes a good ROI.
  12. How extensive is training for an HVAC contractor that would like to get involved using CO2 technology.
    1. Mayekawa – there are now many options for CO2 refrigeration training in the US which will give the Technician a good background on the use and handling of CO2. There are schools like Garden City Ammonia/CO2 School in Kansas and several other options as well as many sources/videos online from many different mfgs. While these are more geared towards CO2 transcritical for refrigeration duty, they give a good overview to start with. The smaller packaged heat pumps that we mfg do not require any long term special training, we usually do it onsite with maintenance personnel and local contractors to maintain the heat pump after startup.
  13. Several presenters spoke about capacity in terms of kW or kWh rather than "tons." Does this indicate that the USA's industry is moving towards the international practice of using metric units rather than the old "tons" metrics? (Note: I think that would be a good thing....)
    1. Mitsubishi - kW is easier for Mitsubishi to use since they do a lot of work in Europe and are Japanese based
    2. Mayekawa – yes I use kW for heating capacities so not confused with Boiler HP, Boiler Tons of Refrigeration Tons which can be slightly different values depending on where you are in the Globe, so with kW there is no confusion for heating capacity.
  14. What hot water temperatures can you generate for space heating at an acceptable COP (say over 2.2) at low ambient (say -10 F) before you go with backup elec resistance or aux fuel heat.
    1. Watts -I think this was mostly answered in discussion, and there are a few differences between manufacturers, but generally speaking we should be able to produce heating temperatures between 140-185°F.  The important thing to remember is that CO2 heat pumps often have low return water requirements that must be met.  See also the answer to #17
  15. Troy - are the EcoCutes in Japan being used for all 3 functions - space cooling, space warming and domestic hot water
    1.  Mayekawa – so the EcoCute product in Japan is actually the term we use for the air source CO2 heat pump, this name came from the saying “Shizen Reibai Hīto Ponpu Kyūtō-ki” in Japanese when they were first introduced which loosely translates to Natural Refrigerant Heat Pump Water Heater. The word Eco is basically for Ecology and a near homonym of Kyūtō as Cute, meaning supply hot water. Little history lesson. For our water source CO2 heat pumps, yes we have done the Trifecta of simultaneous chilled water cooling, hydronic heating and DHW production from the same CO2 water source heat pumps. The trick as mentioned in the presentation is the inlet water temp from the hydronic heating return, this can greatly influence the performance of the system.      
  16. With CO2, wouldn’t backup be required incase of compressor lockout? This might be provided by all-electric backup or unit redundancy.
    1. Mayekawa – any compressor lockout with any refrigerant heat pump will cause a loss of hot water production, so redundancy might be needed for more critical applications like hospitals or processes that need the hot water and another approach is to split the capacity between two or more units so you have something to operate versus a single heat pump in a critical hot water application. HW Tank storage capacity can also help with this issue depending on how long the shut down of the heat pump occurs.
  17. Can high temperature heat pumps be feasible for heat only site applications for remote sites that have minimal to no need for cooling?
    1. Mayekawa – They can be depending on the HW load and use at the site. We would use an air source type model if no water cooling is required. Remember another advantage of the CO2 heat pumps is the efficient generation of high temperature water to prevent Legionella and other water borne contaminants during storage whereas HFC heat pumps are not very efficient above 140-145F outlet, this might be an advantage to remote sites is used for DHW to prevent illness to folks using the water. Also remember the CO2 heat pumps can have the fastest or one of the fastest 1 HR recovery times of heat pumps, so that is also an advantage when sizing storage.
  18. With the technology moving forward in the commercial and industrial market, do you see a place in standard residential or single family homes for the technology?
    1. Mayekawa – there is already a small air source CO2 heat pump available here in North America and offered by Sanden Company. They have had some good success with their product in Residential homes here in the US. In Japan, there are millions of EcoCute air source units for Residential use as they are subsidized by the Gov’t and Electric Utilities. Before they started using CO2 heat pumps before about 2006-2007, almost 30% of Japan’s energy use went to Domestic Hot Water heating, and they like their hot water much hotter than other Countries, so they wanted a very efficient technology to address this huge energy consumption, and CO2 heat pumps were a big part in their overall reduction of energy use and emissions, since they started almost 15 years ago with this technology.