Shelly PM: Heat Pump + Immersion power doesn't equal full house usage

Rablet · 2024-08-15T13:03:52.247Z

I have a Daikin heat pump (installed by Octopus) along with an immersion heater.
The power of both of these are monitored using CT clamps from a Shelly PRO EM 50 in the outside CU.

During the hour of free electricity from Octopus today I thought I would set the water tank to a higher temperature and use the immersion + pump to heat it up.

Doing this I noticed something strange. My Shelly is showing ~3kW use for the Heat Pump and ~3kW use for the Immersion heater, but my smart meter is showing a total use for my entire house as 3.4kW - I would have expected this to be at least 6kW.

What am I missing?

Smart Meter Electricity: Power = Whole house from my smart meter
Heat Pump Power = Heat Pump from Shelly Pro
Immersion Heater Power = Immersion heater from Shelly Pro

image992×822 27.2 KB

Edit: I am a fool, this was a clamp issue. The HP clamp was on the main incoming wire to the CU rather than the outgoing wire to the HP.

Robert.Wall · 2024-08-15T15:27:06.546Z

Caveat: I know very little about heat pumps.

Are you reading that display correctly? It looks to me as if the heat pump itself is idle, the fan and heat pump electronics are consuming 18.8 W, (± measurement errors) which seems on the verge of plausible. I.E, the immersion power is included in the heat pump total, and the difference is the fan etc. Even if this is right, it’s still nearly 8% lower than your smart meter.

[Edit]
The true solution is in post no.5: The display was being fed the wrong information.

Rablet · 2024-08-15T16:07:37.354Z

Thank you for your response!

The strange thing is: I have one fuse for the Immersion and one for the Heat Pump. Each one has a CT clamp around them so I can monitor their power independently. I would expect if both of them are pulling 3kW I would see at least 6kW total use on my smart meter but either I am missing something, or something is wrong with my Shelly setup.

Robert.Wall · 2024-08-15T20:54:25.537Z

In the absence of someone who knows the Shelley, what I’d do is experiment.

What happens when you unclip each c.t. in turn? Does only one reading (Heat Pump Power, Immersion Heater Power) fall to zero? Are the c.t’s polarity sensitive? How does reversing each on the cable affect what you see on the display?

Rablet · 2024-08-16T06:32:32.796Z

It turns out you were actually spot on in your original assessment.

The sparky I had in to install the Shelly had put the HP clamp on the incoming cable coming in to the CU rather than the HP cable so that meter was measuring the entire CU rather than just the HP.

I must not have looked at the HP usage before when the immersion was running (during the legionella cycle) and therefore not noticed it.

After switching the clamp to the right wire it appears to be working as expected.

Thank you for pointing me in the right direction!