View Full Version : Using an old r22 split unit to heat water

02-11-2012, 10:48 PM

The story...
I am tinkering about with an old Fuji Electric r22 split system, making it into a water heater for my small inflatable pool, about 6000L. At the trial stages at the moment, I have made a heat exchanger and have timed ho long it takes to heat about 110L of water. The water went from 14*C to 36*C in 32min with a flow rate of 90L/min. I cant quite remember what the unit capacity was as a split, I think it was about 6kw. It has worked out to be about 5.3kw without calculating heat losses (haven't insulated anything at this stage). What I need to find out is if the unit has too much refrigerant in it. The pipe-run is about 1.5m and the exchanger has about 7m of 3/8" pipe in it. With water at 36*C the pressure on gas side is 310psi which seems way high to me. I remember when I took the unit out of my mates place he said that someone had charged it up about a year beforehand and at that stage I thought it was high.

The questions...
If I take the gas out and charge in the correct amount (1600g) is this still going to be too much? How do you do a superheat calculation if the indoor coil is immersed in water? I have also been told that maybe I should put some sort of pressure reduction in so the compressor wont fail. It already has capillaries as the txv but was he talking about on the suction side near the compressor and would this just create a pressure build-up somewhere else that is equally serious?

03-11-2012, 07:35 AM
Hi beatnik
You can measure the superheat at the suction pipe going to comp pressure & temp. The more gas (to a limit) the more heat will be put out so make sure you are within current rating of compressor and you have sufficient cooling to comp. I worked on a pool heat pump and I was supprised at how high the HP was but this system had run for years on that pressure. Chlorine corrosion could be a problem & the unit at that rate would take 27 hours to heat 6000L. The HP I feel is not too high for the application & R22 gives the best performance for a heat pump. The important thing to do would be take measurements under various ambient temps and record them including current, superheat and discharge temp 6" from the comp. For the charge you need just enough to maintain the liquid seal at the metering device which you might see (capillary frost line) whilst charging. You can have CPR or EPR to limit the work done on compressor.
Mad is a bit of an expert on this type of application & can advise better than me..

mad fridgie
03-11-2012, 08:49 AM
Not enough pipe in the heat exchanger
It is sub cooling you need to measure.
Do not over worry about the suction pressure.

cheers Tesla

04-11-2012, 12:05 AM
Hey thanks for the replies. Tesla - great info and yes the chlorine corrosion might be a problem though I have read that if I keep ph level in check then there souldnt be too much risk - but I have been slack on that part before. Mad - how much pipe should I have? The reason I used 7m was I had an old 10kw ducted indoor unit and measured how much it had, not very scientific though.

mad fridgie
04-11-2012, 01:31 AM
Draw pic of how you built your heat exchanger.

Have another look at your 10Kw indoor unit, I think you will find there is a lot more than 7m.

04-11-2012, 03:37 AM
Crude Shell and Tube heat exchanger but seems to work. Made out of plumbing waste pipe 100mm with screw on cap at top and 100-50mm strap boss for water inlet and outlet. I use Hansen screw type fitting to reduce to any size water pipe and at the moment I am using 25mm pressure pipe. This is Heat Exchanger v2 (v1 had sealed caps at both ends and leaked water, also out both ends) and as for the length of 3/8" tube, 10kw is no more but I have a 7kw and it has about 20m of pipe. What the hell was I thinking! though I vaguely remember being told it doesn't have to be as long as that because the water will suck the heat out quicker. So am I able to use 40m (2 lines of 20m) of 1/4" instead, solely for ease of coiling and maintaining the size of the exchanger?


22-01-2013, 01:03 PM
A couple of years ago, I made a pair of coils out of 1/2" for a glycol chiller - about 14m in each side. Seems to work fairly well at 5kw evap duty.

It doesn't have the contact from the flow that you will have in your pipe arrangement, so you may be able to use considerably less pipe.

Have you seen the heat exchangers attached to the HWHP/Aquahort machines? Similar arrangement to what you are describing, except in titanium.

If you are going to use copper, probably the most important thing would be not to have any welds in contact with pool water, as the welds will be the first to corrode.