Heat Pump related electrical question - should I have a contactor controlling my immersion with the safety interlocks controlling the contactor?

[MikeSharp01]

I am finalising the wiring for the Heat Pump install and have got to the immersion. When I originally drew up the wiring I had a 24V contactor in the circuit and all the safety / interlocking done on the 24V side to give the BMS full control. Now I am wondering if:

 

a. Am I allowed to have the immersion controlled by a contactor with all the thermal cutout and setpoint control stats in the 24V circuits managing the contactor. (See Image below)

b. Should I consider a solid state contactor - if so which?

c. What provision should I make for a possible PV diverter, ie terminals below the contactor or what!

 

[JohnMo]

32 minutes ago, MikeSharp01 said:

I am finalising the wiring for the Heat Pump install and have got to the immersion. When I originally drew up the wiring I had a 24V contactor in the circuit and all the safety / interlocking done on the 24V side to give the BMS full control. Now I am wondering if:

 

a. Am I allowed to have the immersion controlled by a contactor with all the thermal cutout and setpoint control stats in the 24V circuits managing the contactor. (See Image below)

b. Should I consider a solid state contactor - if so which?

c. What provision should I make for a possible PV diverter, ie terminals below the contactor or what!

 

You sound like you are making it difficult.

 

If you are using a 3rd party cylinder, the immersion will already have can in built safety cutout as part of its thermostat.

 

If you want a diverter as well, I would simply move all the immersion stuff away from the heat pump controller, and do it all stand alone. Then use a standard immersion controller and diverter in parallel. So either diverter or normal timer override can power the immersion. Take power direct from consumer unit.

[ProDave]

Does your heat pump expect to connect to the immersion heater?   If so that dictates a lot of the controls and you have to work or adapt around that.

 

In my case the ASHP does connect to the immersion heater (though I have disabled all use of that in the settings)  and it does so with a control box supplied with the heat pump, which contains a contactor and mcb's.

 

To integrate that with my PV dump controller, I modified the supplied box to single pole switching, so only switching immersion L via the contactor, and then connected a solid state relay in parallel with the (now single pole) immersion contactor switched from my dump controller.  The use of a remote SSR for the dump controller was a big motivation for making my own.

 

The immersion heater will have it's own normal thermostat and secondary over heat protection should that one fail.

 

The cylinder mounted thermostat with a probe in a cylinder pocket is to protect the cylinder from over heating when fed from an external heat source, so should be wired to close the motorised valve on the cylinder input coil if the cylinder is overheating.

[dpmiller]

whilst/ as all the stats and cutouts look after the safety side of things, I'd personally be thinking more about the possibility of (say) a stuck contactor going unnnoticed and the immersion taking over tank heating duties unknown. I certainly wouldn't presume that an SSR was totally trustworthy...

/but then I come from a place where accidentally leaving the Willis on was punishable by death

[JohnMo]

2 minutes ago, dpmiller said:

whilst/ as all the stats and cutouts look after the safety side of things, I'd personally be thinking more about the possibility of (say) a stuck contactor going unnnoticed and the immersion taking over tank heating duties unknown. I certainly wouldn't presume that an SSR was totally trustworthy...

/but then I come from a place where accidentally leaving the Willis on was punishable by death

I suppose you get to the question of why with r290 heat pump do you need the immersion connection at all. The heat pump itself can do a legionella cycle, if you have risk assessed and deem required.

 

If you haven't switched the output off as mentioned by @ProDaveyour immersion can take over, because the heat pump control thinks it's taking too long to do a DHW cycle.You may decide to use immersion exclusively in winter, does the heat pump control allow that?

[ProDave]

2 hours ago, dpmiller said:

whilst/ as all the stats and cutouts look after the safety side of things, I'd personally be thinking more about the possibility of (say) a stuck contactor going unnnoticed and the immersion taking over tank heating duties unknown. I certainly wouldn't presume that an SSR was totally trustworthy...

/but then I come from a place where accidentally leaving the Willis on was punishable by death

I used to build industrial control systems and there is a defined risk assessment that considers likelihood of a failure and consequence of a failure to determine what sort of control system was required.  For most of what we did (machines that could kill if the safety systems failed) that was 2 contactors in series, with a safety relay monitoring the state of the contactors and would not allow the machine to start if it detected one of them had a stuck contact.

 

Now given a boiling unvented cylinder could explode and kill someone if the safety systems failed, I would not want to be the one standing up in court explaining  how I designed my own 24V contactor driven "safety" controls rather than using industry practice as described in the installation manual.

 

I did encounter an ASHP installed to it's manufacturers instructions (I forget which make / model) and that connected to the immersion heater in the UVC WITHOUT a thermostat on the immersion heater itself.  I was most uncomfortable with that, but I neither installed it or altered it, just observed what was fitted.