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.

[MikeSharp01]

6 hours ago, ProDave said:

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.

I cannot imagine that the heat pump can overheat the cylinder so I have discounted that issue. My motivation for wiring as it is was just protection for the immersion but also so I could add the PV control later. I looked at the details of your system and thought I might give it a try at some point. There are several other probes in the tank pockets - one for the heat meter, one for the heat pump and one for the BMS monitoring.

 

3 hours ago, ProDave said:

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.

There are so many safety systems in this system my failure mode analysis (FMA) figured all would be OK. Here are the bare facts:

1. The immersion has a stat of its own that limits the water temperature it can achieve.
2. The immersion has a over temperature cutout should it get that far.
3. The tank has a temperature and pressure safety device.
4. The BMS probe has an upper limit setting that can raise an alarm at a chosen set point (85^oC variable)
5. The 24V system has the over temp cutout and the temperature control cutout I could put them in line with the immersion - one or both, but doing so would limit the ability to use all the PV up to 80^oC when you wanted only 50^oC from the standard cycle. 
6. I did consider taking a feed from the immersion live connection, at the immersion and using the BMS logic to tell me if it is powered up when it should be off but have discounted the need for this as the protections above mean it is not really needed.

6 hours ago, JohnMo said:

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.

 I did figure the Heat Pump would do the whole job but having an immersion as a standby makes sense to me.

 

[Gus Potter]

33 minutes ago, MikeSharp01 said:

The tank has a temperature and pressure safety device.

This is the key safety control. That needs to be coupled with a safety device that stops the cold feed exceeding the safe working load of the cylinder and an expansion vessel if need be. Ideally the flow pressure regulator should be set to say 2/3 of the safe working load top to prevent over stress in the rest of the system. 

8 hours ago, MikeSharp01 said:

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)

In my view you can play with your electronic controls as much as you like, as they are not safety critical. As always,I mention that at some point you may not be able to maintain this, say you go senile! At some point you may wish to sell the house and if you want to maintain the asset value you'll have to make a case for that. 

 

Remember to earth everything electronically. 

Edited 5 hours ago by Gus Potter

[JohnMo]

As I said, you have made a simple system very complex. I think you are trying to reinvent the wheel. For systems that have already been designed tested and approved, overlaying a 24v system, just seems OTT. 

[Gus Potter]

1 minute ago, JohnMo said:

overlaying a 24v system, just seems OTT. 

It does to the lay person.. but to the electronically and folk minded to treat their heating system as a hobby it's not.