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Is it worth it?

After a lot of searching I have found that there is a Heat recovery unit for hot water from refrigeration systems which transfers heat from refrigerant into water.

What astonishes me, I have found only this one manufacturer making it, available for home use. Is it really not that effective, or just people are not aware of this opportunity to save some energy/money?

Maybe the commercial out-of-box solution is not (yet) available.

I have written to the company and got no response about availability in my country (Slovakia). On the regular Condenser unit, there is no option to access coolant in hot state (between compressor and condenser, as far as I know).

My theoretical DIY ideas

Would it be safe to ask a professional to open the device and extend the coolant tube from compressor towards condenser, to have it go through Heat Recovery Unit? I would do it with an old appliance, so voiding the warranty is not an issue.

HotSpot installation diagram

Simplification

Assuming this scenario is possible, Would it be much less effective to drive hot cooland through pre-heat water tank directly (without heat recovery unit)? Of course, using water tank with tubes designed for coolant. This could greatly simplify the system, as there would be no need for water circulation pump.

Pre-heat tank with heat exchanger tube

Of course, in all scenarios with water tank I assume the water inside is cold (used frequently), and that coolant tubes would be split and connected by a technician who would in the end refill the coolant medium to a proper amount. Maybe the question could be: Can regular AC devices installing technician do this?

More extreme DIY

Going even more crazy in brainstorming, would it be safe to build the condenser of an external unit into the pre-heat water tank? It is weather resistant, and is getting wet anyways every time it goes in reverse mode, while heating. I would have concern only about the pressure made by water onto the condenser.

AC Condenser built into boiler

Simplification, and maybe something more practical

Comparing to above scenario, would it be effective using regular, factory made boiler, with coolant heat exchanger tube (from previous example), with coolant compressor?

Does it exist? Coolant compressor out of the box, without condenser, ready for installation (with internal consumer-grade AC unit)?

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  • Why not just stopping using air conditioning at all? It's not used that much in Europe, despite it getting quite hot in summer.
    – Ottavio
    Jul 16, 2022 at 15:32
  • The cost of the equipment and maintenance is much greater than the value of low temperature heat. Jul 16, 2022 at 21:04

3 Answers 3

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The problem of heating water from waste heat from air conditioning is that the water needs to be heated to at least 60 degrees Celsius to prevent Legionella bacterium from growing. On the other hand, the cold side needs to be at about 10 degrees Celsius to effectively cool a room to 20 degrees Celsius.

In regular air-to-air air conditioning units, you are moving heat from 10 degrees Celsius evaporator coil (cold side) to about 40 degrees Celsius condenser coil that is situated in outdoors where air temperature is 30 degrees Celsius. The condenser coil needs to be above 30 degrees Celsius by a safe margin, let's say 10 degrees Celsius, to effectively remove heat without needing massive airspeeds.

So using the waste heat for water heating would require discarding heat at 60 degrees Celsius, not at 40 degrees Celsius. Increasing the heat discarding temperature will reduce the efficiency of the air conditioning unit.

Carnot efficiency of 10 C -> 40 C is 9.44. Carnot efficiency of 10 C -> 60 C is 5.66. So in an ideal Carnot air conditioning unit, your energy usage increases by 1.67x if you increase the hot side temperature to temperatures enough to heat water. I assume that in real units (less efficient than Carnot), you would still multiply energy usage by 1.67x.

Also the state of art water heating system (ground source heat pumps) have real coefficient of performance of 4. This means that if you for example need 7000 kWh/a of water heat, that's only 1750 kWh of electricity.

It would be better to use discarded air conditioning heat only if the air conditioning usage is less than 2611.9 kWh/a (because 67% of 2611.9 is 1750). Otherwise the air conditioning electricity bill would increase more than the savings of not having to use that ground source heat pump.

Some other problems in the plan:

  • If you use air conditioning heat to heat hot water, eventually if little water is used, the water could become so hot that temperatures rise to such high levels that the system becomes very inefficient and a secondary way of discarding that heat needs to be present. So you essentially need the ability to switch between two condenser coils situated in different areas. Would make quite an expensive system.
  • Refrigerant pipes would need to be installed between the indoor unit and the water heater. These pipes would be long and installation would be costly.
  • You still need an alternate heat pump or resistor heater to supply heat to water in cooler days. If it's resistor, it's energy-inefficient and costs lots of money to operate. If it's heat pump, then you need to pay a lot of money for that heat pump, so you can't expect any savings from reduced water heating infrastructure, you need to pay for that infrastructure anyway.
  • Air conditioning usage is heavily seasonal, whereas hot water usage happens in all seasons. Thus if for example there are 5 air conditioning months in a year, you wouldn't save 1750 kWh of electricity but rather 729 kWh of electricity (so in that case air conditioning to heat hot water would only make sense if air conditioning electricity usage would be below 1088 kWh per year)
  • Even during summer, there are hot days and cooler days. In hot days, the temperature of the heater would become so ridiculously high that the alternative heat discarding pathway is needed. So even if your air conditioning usage is below 1088 kWh per year, that wouldn't necessarily translate to any savings because different days during summer have different air conditoning needs. Part of that heat would be wasted. So I'd say it's realistic to expect any savings if you need less than 600-700 kWh per year energy for air conditioning.
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  • *I did not know about Legionella bacterium, thank you. Plan is that water will go to washing machine, dish washer, or boiler, to be heated above 60 degrees. Is it a problem when there many killed bacteria? *Saving comes from shorter AC on-time, not from heating the water. It is functioning only when I am at home, so cold water should flow, and I would excect the Carnot efficiency to be better than 10. How can I calculate how quickly the water heats? *Refrigerant pipes installed between indoor unit and tank? My pipes at indoor unit are cold. I address that in second paragraph of question.
    – Jakub M.
    Jul 19, 2022 at 13:36
  • Legionella are a problem only for drinking water (including shower etc - as opposed to warm water heating systems), and even there lower temperatures are safe iff the warm pipe volume is sufficiently small and the water doesn't sit extended times at the low temperature. A big tank whose water is used directly for drinking or shower and is sitting around at lukewarm temps for long time is very much of a concern. At 30 - 45 °C, they double every 1 - 3 days in clean water (in nutirent-rich environments doubling time can be as low as 3 h). Our home has a heat exchanger off a larger warm... Jul 29, 2022 at 8:08
  • water tank that also supplies the general heating system. With the heat exchanger to warm up drinking water as needed (no big boiler), we can even set the thermostat to lukewarm temp e.g. for just showering. Every once in a while, we nevertheless give it hot temp (70 °C is the max setting) when the tank is hot and the hot water is needed. Jul 30, 2022 at 15:05
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I think it likely does exist, though I cannot point you to any specific system. What you'd want to look for is

  • a ground source heat pump system
  • which works also in reverse mode, and
  • on the house side an AC/air heating system (rather than hot water central heating)

Some thoughts in addition to what @juhist already said:

  • AC will be the more energy efficient the colder the cold pool/heat sink to cool against is. @juhist has outlined a scenario (10 °C) that will allow for efficient transfer of heat on its own.
    Nevertheless, any heat sink that is cooler than the ambient air you are presumably cooling against will help. From that perspective, a water tank with less than ambient air temperature would help making your AC more efficient, regardless of what additional use you can put that then warmer water to.
    Even more efficient is evaporation (except in very humid conditions). That is what industrial cooling towers use (which btw. have been sources of important legionella infections in the past)

  • Whenever the weather is such that you'd want to run AC, getting really hot water via solar thermal collector, and the proposed tank would maybe better be employed as heat buffer for an STE system.

  • Ground source heat pump systems use either soil or e.g. a pond as heat source that keeps higher temp in winter than ambient air. That basically would also give an opportunity to use the thermal buffer as heat sink in summer.

    (I don't know whether it is acutally used this way - I'm in Germany where for residential buildings we still do fine without AC because masonry walls supply sufficient heat capacity that with thoroughly airing during the night and closing window lids during the day we get along without AC (unless you are living in a flat right under the roof). And heating installation over here is anyways water-based rather than hot air. These systems don't lend themselves well to cooling since you'd easily get into trouble with moisture condensing on the cool surfaces)

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I built something like that, out of repurposed air dryer.. It can be efficient, but there's a catch. I'm using it to preheat the water in the lower part of my water heater tank. The tank itself is 300l tank with 3kW electric heater in the halfway. There is a preheat copper coil on the bottom and i use the 'heat pump' to cool down the house ventilation air going to roof. I have condenser replaced with small plate heat exchanger and water/glycol transporting the heat to the tank and getting the bottom temp up from average 15 celsius up to around 40.. I'm consuming about 180w electrical power (+circulation pump and control circuit abt.25w) and getting about 500w+ heating power.. You can't really use much higher power compressor with house exhaust air because you'd need to either turn up air ventilation so high that it's impossible or to interrupt compressor often due to completely iced and blocked evaporator.. Also the same but looked from another side is the water temp.. you can't go much higher, since the pressures in the system would grow and cop turn to 1 or even negative.. (theoretically one could achieve high temperatures by trying to utilize the superheat portion but that type of system beats me.. :D) As a refrigerant i used r290..not much though, only about 50-60g.

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