Using Mini Split Air Conditioner for Space and Hot Water Heating

Mini split air conditioners are great little air-source heat pumps that can work in the reverse and provide heating to the building, too. For highly insulated houses built according to the Passive House standard they technically provide enough power to support both space and hot water heating.

Passive houses require a heating load of only 10W per square meter or 15kWh of total energy per square meter per year to condition the inside air. That means that technically a 100m2 house requires only a 1kW heater which can be easily served with the smallest mini split units on the market that have a total heating/cooling power of 3kW.

Wouldn’t it be great if the remaining 2kW could be used for heating the hot water? However, such units are pretty much non-existent or they are way over-sized and expensive for the needs of a highly efficient house.

So I’m starting a project to add an additional 3kW plate heat exchanger to the indoor gas/fluid line of the cheapest mini-split air conditioner with the heating functionality, to transfer the heat to another fluid loop passing through a 1m2 heat exchanger of a 100 liter hot water tank:

Mini Split air conditioner for space and hot water heating
Mini-split air conditioner heating another fluid loop through a plate heat exchanger (with bold outline) that is connected to a hot water tank.

Design Questions

  • Does it need a bypass for the plate heat exchanger to decouple the water tank from the air-con while the unit is in the cooling mode? Or is it sufficient to just turn off the circulation pump for the hot water tank heat exchanger?
  • What are the flow rates for the hot water tank heat exchanger loop with a 1m2 tank heat exchanger surface?

Proposed Components

1. Mini Split with Heating Support

Midea Blanc, AlpicAir Premium Pro, Gree Lomo Nordic or similar costing around EUR 500. Requires knowledge of the communications protocol with the outside unit to control the device without relying on the inside unit.

2. Hot Water Tank with Heat Exchanger

Kospel SWK100 hot water tank (around EUR 400) with an internal heat exchanger loop area of 1m2 or more.

3. Plate Heat Exchanger

Eko Air NB-238 brazed plate heat exchanger (cost unknown) with support for 3MPa (30bar) pressure and 3/8″ SAE flare 45deg connectors for the gas line and G3/4″ connector for the water loop. The particular unit has a maxium float rate of 5 m3/h.

4. Circulation Pump for the Hot Water Heating

Wilo-Yonos PICO circulation pump (around EUR 130) with support for variable flow rate. Ideally with electronically adjustable flow rate.

5. Pipes, Expansion Tanks, Valves and Connectors

TODO: List them all.

Next Steps

Order the main components and tools to make it work.

Last updated: October 2021.

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