When I was building my house, I had to figure what to use for heating/cooling plus a hot domestic water solution. I came to think that the best for my case is to use a heat pump. The advantages are obvious – a heat pump moves heat/cold using somewhere up to ¼ of the electricity needing to produce that amount of energy with resistive electric heaters. So I ended up looking at Daikin Altherma as a solution. Well, until I found out about the price and a shortcoming that is quite annoying.. The Altherma can’t heat the house and the hot water tank at the same time.. so that means I have to wait for the hot water tank to warm up to the setpoint before the house can be heated. Taking a bathtub will leave me without heating for at least an hour. No good for me. So I came across a local forum topic where some guys are discussing the option to hack a split type air conditioner into air-to-water type system that also takes care of domestic hot water. To understand how it works, lets take a look at a typical split air source air conditioner’s scheme:
That one is modified as per the following diagram:
I ordered a 120L non-corrosive steel hot water tank with two heat exchangers in it – one will be used for the refrigerant and the other is a spare one, I plan to have a fireplace connected to the water system at some point in time and will hook that to the water tank as well. The heat exchangers have been carefully designed with the help of local forum gurus. The indoor heat exchanger of the air conditioner is replaced with a brazed plate steel heat exchanger. The electronics from the unit is extracted and used to control the system. The air temperature thermistor is trimmed with extra resistor, to accommodate for the expected lower temperature of the fluid compared to the original design with air and placed on the returning cold water outlet of the brazed plate heat exchanger.The original indoor unit tube thermistor is placed unmodified in the middle of the brazed plate heat exchanger.
That setup provides me a 120L of domestic hot water regardless of the operating mode, heating or cooling. I have invested extra efforts to make sure the house is overly well insulated to accommodate for the low-temp heat pump setup I have in place.
The heat pump I used is a Daikin FTX71GV / RX71GV. Needless to say that all warranties are voided, but worth the risk.
To monitor the heat pump and provide some controls have created a special Arduino-based system that I will be describing in a post to follow.
I use the system for a second year now and am extremely pleased with the results. My electricity bill is ridiculously low for what I get.
Here are some pictures