X3 Immersion heaters on a single radial

[TerryE]

On 12/01/2026 at 19:12, Nickfromwales said:

 

@TerryE used solid state SSR’s to power up his heating and hot water gubbings, so the attraction is there if you want silent operation vs the clunk of a contractor.

 

I moved away from using SSRs just under 2 years ago to an ESP32 module using 24 VDC to drive contactors.  See p3 and 4 on this topic.  

 

My issue with the SSRs was that they had about a ½-1% heat loss in the SSR when on -- that's 15-30 W per relay.  This caused real overheating issues with the DIN mounted SSRs.  The contactor coil losses are a few W, so absolutely no overheating issues.  I prefer using 24 VDC for relay control as this minimised safety issues in the microcontrolled modules.  You just need to include flyback diodes to prevent the 24V relay contact arcing / wear.  You really need to add snubbers for the 240VAC switching relays for the same reason.

[Mike]

6 hours ago, TerryE said:

15-30 W per relay

That is a lot! Double checked my contactors and they seem to be 0.8 to 1.8 W.

 

6 hours ago, TerryE said:

You really need to add snubbers for the 240VAC switching relays

Mine claim to be good for 100,000 to 150,000 switching cycles so until your post I wasn't too worried about that - but maybe I should be? The only snubber at the moment is for the circulation pump.

 

[TerryE]

@Mike, The power loss of SSRs is appalling IMO.  The main issue is that the DIN rail form-factor ones get hot and this can cause oxidation on the power leads from the resistive heaters.  Even with crimp covers the oxidation can cause probs.  I haven't had any issues with the contactors.   No you don't need snubbers on the resistive loads and I don't bother on the AC pump either as this only toggles ½doz times a day.

 

No this issue is with your  Shelly Pro 4PM  which outputs 4 × wet 240 VAC signals for contactor coil switching. The inductance of the contactor coils is quite high so you can get field collapse arcing when they switch.  I don't know if Shelley has internal snubbers, but this is something you need to check IMO

[Mike]

2 hours ago, TerryE said:

The inductance of the contactor coils is quite high so you can get field collapse arcing when they switch.  I don't know if Shelley has internal snubbers

It doesn't, so I'll get myself some more; thanks for the heads-up!

 

2 hours ago, TerryE said:

The main issue is that the DIN rail form-factor ones get hot and this can cause oxidation on the power leads from the resistive heaters.

Yes, there is a recommendation to use no more than 2 adjacent contactors unless there's a gap between them for cooling. I'm populating the row with alternating breakers & contactors, which should help.

 

Edited 8 hours ago by Mike

[TerryE]

I use ½ blanks between my contactors, but a couple of W in a steel CU isn't really an issue.  I had probs with my DIN mounted SSRs especially as my sparky had used an abs box.  Luckily it was in a large closed cupboard so I could leave the cover off.  I prefer my current setup. The RH unit was my sparky's work. The LH contains: 24VDC power,

<gap>, contactor 1,  <½gap>, contactor 2, <flyback diodes>, contactor 3, <½gap>, contactor 4

 

The 24V Tasmota 4×relay module is below. The red /black switch loom is all 24V.

 

[Nickfromwales]

I do like this forum.

 

Some very useful information , and real life feedback here.

 

Thanks for the additional detail  @TerryE

 

Tres bien. 👌.