Heating water to 65 °C with a refrigeration system

​Hello everyone.

Have a little project to make, a water heater for keeping cast aluminum moulds at 60-65 °C.

I made a test unit from old parts laying around.

I used r134

Small 1/20th hp danfoss condensing unit flushed oil, and replaced with RL32 (ex snowfreeze machine to keep the premix cold)

Used the air cooled condensor as a evap, made a heat exchanger from a bit of 1/2" and 1/4" tube, and a old waterpump out of a ice machine, just to see what i could figure out.

I de superheated the cap tube by wrapping it around the suction line, and found the unit worked quite well.

I have only tested it to 55°C so far, will try for more once i replace cap tube..

With the water temp around 55°C i had a discharge of around 260psi and suction of 40 ish psi (+8°C) Suction pipe temp was around 10°C before the heat exchange of the cap tube.

Ambient temp was 18°C. I only had 2.5mt of capillary tube to use, i would have liked to made it slightly longer to get the evap temp down - i may buy some and give it a try.

It worked ok, and i was quite surprized. i was only heating up a bucket of water and, it was fun to just throw it together and see what happened.

Now im looking at building a proper prototype and would like to learn how to work out the best way of doing this, and have some questions.

Have searched on this subject and found a lot of useful information, but i still have some questions as how to design a system properly. (system component choice and calculations.)

The current heaters are 2kw, 6kw and 10kw each. The existing heaters are just portable insulated boxes with a element, thermostat and a circulation pump. They are portable (on wheels) and hold around 50lts of water.


The first one i will be building will be around 2kw heat output.


Ambient temps of anywhere from about 0°C to +30 °C, But is usually around 18°C. I am going to data log the ambient to get a better idea.

Has anyone ever designed a system similar to this?
What information should i be considering when designing a system like this?
How much water should i be looking at holding onboard?
How well does R134a work at high pressure (guessing around 330psi to get 65 °C water?)
De superheating and system calculations?
Capillary vs TXV, And how high discharge pressures will effect each?
Compressor selection?
Any other components needed CPR etc?


I have looked into other refrigerants, and have concluded r134a will be my best bet due to lower pressures, and availability. Is there anything else i should consider?

Any information, tools or advice i can get on this would be greatly appreciated.

Sean