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Topic: Heat Pump for Pool (Read 4233 times) previous topic - next topic


Dear all,

I have an air-to-water heat-pump for my pool in the garden. The device worked fine until lately where the control board gave up and no spare parts are available (at leats not for a fair price). So I'd like to finally tackle what i have long since in my mind: replace the control with an Arduino and finally get the heat pump integrated to the rest of the home-automation via Ethernet also.
As I could not find any info of relevance (neither in this forum nor in the rest of the I-Net).

My question: does anybody have experiences with the control-strategy for a heat pump? Basically I have two questions:
- The heating strategy: is the compressor always on or is there a strategy like "on for 2 minutes, off for 2 minutes"?
- the defrost strategy: the heat pump doesn not have a reverse valve nor any electrical heating device. How to do defrosting?

The heat pump has the following sensors & actuators:
- 230V electrical motor for the compressor pump (activated by a relay)
- 230V electrical motor for the fan (activated from a relay)
- NTC for water inlet temperature
- NTC for water outlet temperature
- NTC for ambient temperature
- NTC for heat spiral to detect when it's time for defrosting
- two pressure switches for water pressure (on-off only). Low pressure and high pressure switch
- flow sensor to detect if pool water is flowing

Basically, the ambient temperature and the flow sensor would not be needed in my control strategy, as both are already known to my home automation and can be send to the Arduino via the network.

So basically no rocket science except for the control strategy.

Please no answers like "230V is dangerous. Leave that to the Pros". I am a pro and I'm running a bunch of Arduinos in my house already to control various other stuff successfully.


The way some of these defrost is to run the fan across the coils until the coil temperature sensor goes to whatever the ambient temperature is. This means you have to pay attention to what the actual temperature is and shut the whole thing down if it's too cold to do any good.  The tiny number of them I've seen don't allow operation under 40F, so you might use that as a starting point.   The only strategy I've seen on compressor operation is to run it at least 10 minutes, and let it set idle for 10 minutes after running.  The idea is to keep those short cycles at a minimum to reduce the wear of start-stop cycling.

They're actually pretty simple devices so you should be able to get it working to your satisfaction.  One thing I do recommend is to make your new controller as self sufficient as you can.  Sure, it's great to incorporate it into the house controls, and you should do that, but allow it to operate on its own if necessary.  My experience has been less than stellar using centralized control with ethernet and devices that depend on a datum from somewhere else often don't get it.  I try to make every new device capable of working to some degree without the need for a solid connection.

While you're at it, take a look at replacing the contactors for the motor and fan with solid state relays.  The biggest problem with these devices is those darn big relays that wear out and get contact burns.  I'm replacing all of mine over time; basically when they fail, they get replaced with an SSR.  Haven't had an SSR fail or get dirty contacts.



thanks !

So you basically say I turn the compressor (and fan) on for a while and monitor the temperature for the defrosting signal. If it drops e.g. below 0C (32F), than I go to defrost. And defrost means I shut off the compressor and leave the fan on (to have warm air come over the piping).

The current control in the device will not allow the heat pump to be working at any ambient temperature below 10C (50F); I assume that is a kind of marging to allow defrosting. So I'll implement that also.

Self sufficient: that is for sure. All my other Arduinos in the house work on a standalone basis and just get set values from the master computer (a rasperry). So in the heat pump case, the set values would be "turn on/off" and "desired water temp is 28C". Anything else, the device needs to handle on its own. And all the necessary sensor are in there, so should be no major issue.

For mission critical stuff (e.g. my water valves for the lawn watering), I have a applied a kind of double dead-man switch: the RPi sends the ON command to the Arduino. The ARD opens the valve and starts a timer. If the timer reaches 0, it closes the valve by itself. So the master RPi needs to send the ON command on a periodic basis to restart the timer. In turn, if the RPi doesn't receive an ACK for the ON command in say 30 Seks, it cuts the power supply for the valves (hence they close).

SSR: you're right ! Although I was thinking of purchasing some of those relay cards, that are ready to use. Not sure if this is SSR


I use a few of the ready made relay cards; relays work great in lots of applications.  It's the high current applications that I have been replacing the contactors with SSRs.  They're noisy and the arcing on the contacts causes them to fail too often in my climate.  So, water heater, A/C units, outdoor motors, that kind of thing.  I reserve relays for small items.


I have no experience with SSRs so far so therefore was drilling into the subject the last couple of days. That souns promising, but I have one thing, that gives me headaches:

both motors in the heat pump are pretty powerful animals (2,2 kW in total) and both of them use motor-capacitors. These are basically used to create two phases from one 230V phase. Besides needing to use cooling devices for the SSR, I fear three effects:
- when switching on, the capacitors need to be charged. That might mean, they might draw pretty significant current well above the current drawn while running. The comprssor motor capacitor has 50µF
- when running, the voltage at the two SSR pins might be higher than 230V
- when shutting down, the typical voltage might see a high peak. That could be elminitaed by a free wheeling diode of course

Does anybody have recomendations on that?


My peak draw on startup of two A/C units is fairly high and SSRs stand up to it just fine, but it would be good if you find a better source than me on this.  I recommend that you find one of the hardware engineering forums and ask about this exact thing.  Some of those folk have designed the devices and can give you definitive answers.

Then, of course, come back here and post your results .  :)


Jul 23, 2014, 05:56 pm Last Edit: Jul 23, 2014, 07:17 pm by Boardburner2 Reason: 1
What is the technology of the old control board ?
Is it micro controlled ?

Ones i played with were relativley old tech but that was 20 yrs ago.

Failures were normally the motors, or as mentioned the contactors, have you tested that the motors run when power is applied.


The capacitors are purely for motor starting, these are a fairly common cause of failure, normally easy to replace.

Winding failure is more serious.


Jul 23, 2014, 09:34 pm Last Edit: Jul 23, 2014, 09:38 pm by bugster_de Reason: 1
the old control board uses mechanical relays to apply power. If I apply 230V to the motors, they just run fine (and that is what I'm actually doing to get the heat pump running :-)
As Murphy Law also applies at my place, the SSRs will fail when
a.) I'm away from home for some days
b.) my lady wants to go in the pool but feels the water is too cold

So I will go with mechanical relays and if they are shot (which might be in some years from now) simply replace them. The major goal of this 'hack' is to have a heat pump that is working and that is integrated into the home automation system.

I had exchanged the capacitors a while ago (one was shot so I replaced both to be sure)


Jul 24, 2014, 03:34 pm Last Edit: Jul 24, 2014, 03:44 pm by Boardburner2 Reason: 1
Ok , sound that you know what you are doing.

20 yrs back they were fairly uncommon in the uk.

Every time the " experts " turned up they spent a lot of time on the phone.

First thing they invariably tried was the most expensive one, replace the control board.

On one occasion one of the motors had a visibly melted winding which i pointed out, they still tried to replace the controller though.

One thing i never understood though.

For household heating i believe some degree of cycling is to be expected.
A swimming pool however has large mass high flow and low temp difference, this should be ideal for a heat pump.

Should run continuously until up to temp then shut off.
So not much cycling.

I have experience of industrial motors and never understood why the contactors failed so often on the heat pump..

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