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