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[Solved] Feasibility of Converting Inverter AC to Air/Water Heat Pump with Water Exchanger

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Can an inverter air conditioner be converted into an air-to-water heat pump by placing the indoor unit's coil inside a sealed water boiler without modifying the refrigerant circuit?

Not really as a simple plug-in conversion: the thread consensus is that enclosing the indoor coil in water may work only as a custom build, but it is not very feasible to do safely or efficiently without a proper heat exchanger and water circulation [#15851715][#15869705] The split unit refrigerant side is usually R410a at about 20 bar, so sealing, corrosion, and pressure compliance are major issues, and the indoor coil/aluminum parts may not last submerged in water [#15851715][#15851402] Several replies recommend using a real plate or pipe-in-pipe exchanger, or a dedicated hydrobox, rather than trying to repurpose the stock indoor unit directly; the exchanger itself can cost around PLN 1500–2000 [#15851715][#15868214] One user reports a working DIY air-to-water setup, but it used a custom copper coil in a boiler with sensor changes, full refrigerant handling, and a circulation system—so it was a redesigned heat pump, not just a sealed indoor coil dropped into water [#17472247][#17472466][#17473723]
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  • #31 17535984
    kikis1
    Level 11  
    Posts: 30
    Help: 2
    Rate: 7
    For my thumb, the air conditioner was working, but it had too little power then the heater was turned on; this was generally the case in March; in summer I added radiators and now 30 degrees on the output is enough; what will be in the frost I do not know;
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  • #32 17766888
    andrzejkm
    Level 17  
    Posts: 374
    Rate: 84
    Kikis1 what power is this air conditioner with you? I plan to do something similar on the basis of an air conditioner. I have a 400-liter container and an 18 cm flange in it. I have an air conditioner with a heating capacity of about 3.5 kW. And now I wonder if a condenser made of coiled tubes that would enter through these 18 cm will be sufficient for my heating power. I would like to add that my goal is not to heat the tank in this way, but to cool the attic of my house, and heat the lower part of the tank with the heat recovered in this way, so that the main heat pump that heats the house and domestic hot water is switched on less frequently in the summer. Two birds with one stone.
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  • #33 17771254
    kikis1
    Level 11  
    Posts: 30
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    I don't know what the author means; what does the 400 l hot water tank have to cool the attic? in my case, in a 300-liter tank there is a coil 10 m of pipe 15 cu, 10 m cu fi 12 and 20 m cu fi 8; this creates a condenser and heats the wash water to 45 degrees; temperature measured at the bottom of the tank; the air-conditioning system has a power of 3.5 kW, it can also cope with frost; my water heating air conditioner is constantly turned on for heating; it is switched on by a 4-contact relay where the switching between two thermistors takes place;

    Added after 8 [minutes]:

    placing the outdoor unit in the attic may prove to be harmful; the unit will not work in summer due to too high temperature in the attic, and in winter it will flood the attic with water during defrosting;
  • #34 17771376
    andrzejkm
    Level 17  
    Posts: 374
    Rate: 84
    kikis1 wrote:
    What does the 400 l hot water tank have to cool the attic?
    I want to do it like this: The condenser is more or less like yours, i.e. coiled copper pipes in the tray. The compressor, pressure switches, filter, sight glass and all electrics would be on the ground floor, i.e. in the boiler room, somewhere near the storage tank. In the attic, I am going to install an evaporator + fan through which air would flow sucked in with one diffuser from the rooms and ejected (already cooled) with the other diffuser to the same rooms in the attic. I would also have a place to drain the condensate from the evaporator from the attic, so it would not be a problem.
    Regarding the condenser made of pipes, this is how I counted, that it takes about 40 meters for the power you give - I would have a similar power. Of course strongly dependent on the evaporation temperature.
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  • #35 17771431
    kikis1
    Level 11  
    Posts: 30
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    I am skeptical about the flow of air to this evaporator of yours through the anemostat; but it's your project; read the air flow through the evaporator; air at high speeds makes noise; these 40 m of pipes is rather a calculation of the exchange area and optimization of the distance of units inside and outside
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  • #36 17771472
    andrzejkm
    Level 17  
    Posts: 374
    Rate: 84
    I know. I have already calculated the surface of the condenser and for this power you need approximately 0.7 m2. And this is as much as I remember correctly 42m of fi12 pipe, although exaggerating upwards will not hurt. Regarding noise: The unit I am going to convert is a portable air conditioner which somehow is tolerated as an indoor unit. It is known that it is not as comfortable as the wall air conditioning, but .. we'll see.
  • #37 17771497
    kikis1
    Level 11  
    Posts: 30
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    20 m of fi 12 pipe is 0.7536 m2; good luck in your experiences;
  • #38 17780935
    Arbiter
    Level 15  
    Posts: 407
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    The outdoor unit cannot be in the attic because it will not have where to get heat. He'll freeze the entire attic, and that's it.
    You will not bring any heat anemostats from the room to the external unit, because these are not the rows of flows. You can take 100 m3 out of the house with one anemostat and the unit needs 2000 m3 / h

    As Kikis1 noticed in the external language, it freezes in winter, even at high flow, because it collects moisture from the environment and liters of water pour out of it in the defrost cycle
  • #39 17780948
    andrzejkm
    Level 17  
    Posts: 374
    Rate: 84
    Where did I write that the outdoor unit should be in the attic?
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Topic summary

✨ The discussion revolves around the feasibility of converting an inverter air conditioning unit into an air/water heat pump by utilizing a water exchanger from the existing indoor unit. Participants express skepticism about the effectiveness of using freon for this purpose, with concerns about heat transfer efficiency and the durability of aluminum exchangers submerged in water. Suggestions include using plate or counter-flow heat exchangers and considerations regarding the pressure of refrigerants like R410a and R134a. The conversation also touches on the importance of thermodynamics, plumbing, and the potential for heat loss in the system. Some users share their experiences with DIY heat pump conversions, emphasizing the need for proper sealing and the challenges of maintaining efficiency in various temperature conditions.
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FAQ

TL;DR: Converting a 3.5 kW inverter AC into an air-to-water heat pump can deliver ~32 °C floor-heating water at COP ≈3.0, but “aluminum exchanger will corrode” [Elektroda, scalak4, post #15851402]; field data show COP falls 40 % below −5 °C [IEA, 2022].

Why it matters: A cheap split AC can offset boiler fuel, yet only if you manage pressure, materials and control logic.

Quick Facts

• R410A discharge pressure: 20–30 bar at 35–45 °C [Elektroda, Arbiter, #15851715; ASHRAE, 2021] • Copper coil surface area needed ≈0.7 m² per 3.5 kW unit [Elektroda, andrzejkm, post #17771376] • DIY plate heat-exchanger price PLN 1500–2000 (€320–430) [Elektroda, Arbiter, post #15851715] • Floor-heating supply 30–35 °C gives COP >2.5 [CIBSE, 2020] • One user logged 7 kWh/day for DHW+heating with 3.5 kW system [Elektroda, kikis1, post #17473609]

Can I swap the indoor coil for a water tank without opening the refrigerant loop?

Yes. You may submerge the factory coil inside an insulated tank and seal all pipe penetrations. The compressor still “sees” the same evaporator/condenser, so no refrigerant work is required [Elektroda, Arbiter, post #15754190]

Why is material choice critical?

Immersed aluminum fins corrode quickly in warm water. A pin-hole can vent 20–30 bar R410A into the tank and flood your heating loop [Elektroda, scalak4, post #15851402] Use copper or stainless coils wherever possible.

What pressures must the replacement heat exchanger survive?

With R410A at 40 °C, saturation pressure is about 26 bar. Design for 30 bar working and 45 bar test pressure [ASHRAE, 2021; Elektroda, Arbiter, #15851715].

Is a 3.5 kW split powerful enough for whole-house floor heating?

Only for small or well-insulated homes. One user heats 70 m² at 23 °C using a 3.5 kW unit, drawing 7 kWh/day in mild weather [Elektroda, kikis1, post #17473609] Larger houses need 5–8 kW units or multiple systems.

How do I size a copper-coil condenser inside a buffer tank?

Rule of thumb: 0.2 m² tube area per kW. For 3.5 kW you need ~0.7 m², equal to 20 m of 12 mm OD tube [Elektroda, andrzejkm, post #17771497]

Will I need water circulation inside the tank?

Yes. Without flow, stratification raises condensing pressure and cuts COP. A small 15–25 W pump keeps ΔT below 2 K [Elektroda, ja9, post #15869705]

What control strategy works for combined heating and backup heater?

  1. Thermostat enables AC when buffer < 30 °C. 2. If outdoor air < –5 °C or buffer < 25 °C, electric heater stages in. 3. Both stop when buffer hits setpoint [Elektroda, kikis1, post #17473609]

Can I use an automotive radiator blown by the indoor fan?

Possible, but car radiators are rated for R134a at ≈14 bar. They may rupture under R410A pressure [Elektroda, Arbiter, post #15851715]

Is mounting the outdoor unit in an attic acceptable?

No. The unit needs thousands of m³/h fresh air. In an attic it will starve, ice up and dump defrost water indoors [Elektroda, Arbiter, post #17780935]

What are common failure modes in DIY conversions?

  1. Coil corrosion causing refrigerant loss. 2. Undersized heat exchanger raising high-pressure trips. 3. Poor insulation lowering COP. Edge case: stagnant water can freeze if pump stops, bursting the coil.

How much does a factory hydrobox cost versus DIY?

Commercial hydroboxes with pump, plate HX and controls start at €600–800, roughly double the parts cost but save build time [Mitsubishi Ecodan Catalogue, 2021].

Three-step How-To: building a copper coil condenser

  1. Cut 15–25 m soft-copper tube; braze ends to flare fittings.
  2. Spiral it around a PVC form, slide into tank, connect to AC lines.
  3. Pressure-test with nitrogen at 30 bar, then vacuum and recharge [Elektroda, kikis1, post #17472466]
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