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 Monday at 01:25 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. 👌. 

[ProDave]

The only SSR I use is for my solar dump controller.  I guess I went into it knowing at the heart of an SSR is some kind of semiconductor that will have a voltage drop when on, and thus dissipate heat.  I did not use DNI rail as they have no particular way to get rid of that heat.  Instead I used one that has a metal back plate that bolts to a heatsink with some heatsink compound.  Even at full power it is barely warm to the touch.

[RedSpottedSev]

Thank you guys

 

This really is a great forum. I have to say a fair bit of this is over my head, I do however want to pin down my final plan.

 

To recap I want to have tight and remote control of x3 3kw immersion heaters. 50Amp RCBO at the main CU. 8m straight 10mm 3 core to loft. Aux CU.

Option a) x3 of: Shelly Pro 1pm (16amp) , then 16amp RCBO then contactor then to immersion. RC snubber but.not really clear if this goes across the shelly pro? Does it have to be physically very close and how does it sit in a DIN CU? 

 

Option b) Shelly pro 4pm (40amp), then 16A RCBO, then contactor... same question re RC snub

 

Option c) forget Shelly, 16A rcbo in aux CU, then immersion heater, and use this for remote control https://www.tsmart.co.uk/

 

I couldn't get my head round the 24vdc setup yet.

 

Thanks again

Edited 6 hours ago by RedSpottedSev

[JohnMo]

On 11/01/2026 at 13:54, RedSpottedSev said:

It is a custom build. Main hot water cylinder has x2 (top and bottom), another in a buffer tank.

Still think you are making things way more complex than you need. 

 

Main water cylinder top immersion, if you are a low water user just use that set the thermostat and a charge window.

 

If you are a heavy user ignore the top immersion and set thermostat on the bottom element and use a simple immersion time switch. Hot water sorted mice on with your life. Change from £30.

 

Buffer immersion - depends what you empty the buffer into, UFH or radiators. But just do simple.

 

You don't need any of this "smart stuff"

 

20 minutes ago, RedSpottedSev said:

want to have tight and remote control

I doubt very much you really do and doubt it will save a penny over a sensible simple to run system.

[Mike]

As @JohnMo says, if you can use a time switch that's the easy and cheap option.

 

For the Shelly solutions, the main practical difference between the Pro 1PM and the 4PM is that you need 1 x 1PM for each device (heater, in your case) that you want to control. The 4PM can control 4 devices (and has manual controls in addition to smart controls) and would work out cheaper.

 

Here's a diagram (modified from my installation to cut out some bits) that shows the wiring; your electrician may prefer to use RCBOs instead of the breakers & RCCB shown here. The latest version of the 4PM has a single live supply terminal, not 4.

 

I'll be wiring the snubbers directly across the contactor terminals, as shown. The one Shelly sells has bare leads, which would need covering in heat shrink sleeving for safety.

 

I don't know the T-Smart, but if it only works over WiFi and by using their proprietary app, then it wouldn't be my choice - that's all their brochure mentions.

 

Edited 5 hours ago by Mike

[JohnMo]

I just use a Sangamo PSS2 time switch. Paid £17 for it. Connected direct to CU dedicated fused outlet, then 2.5mm² cable, to an isolator switch near the cylinder, then direct to immersion - all standard stuff, any electrician can fix, any day of the week, with bits from local electrical factors.

[RedSpottedSev]

Thank you,

I do appreciate I am making this more complex than it needs to be.

 

Part of the need for 'smart' control is that the setup is in the loft and I can't run any more cables. I would like to easily adjust away/holiday settings. 

 

@JohnMo sorry what does this mean? "Hot water sorted mice on with your life. Change from £30"

 

I am now leaning towards simple timer switch for hotwater tank. Our water usage is 2 showers morning, 1 shower and bath(3 young kids) evening.

 

Buffer supplies ufh only and in recent cold spell the flow temp of 37drgrees was sufficient (and still some more insulation and airtight work to do, loft hatches to close etc...) trying to run it as low as possible. 

I would definitely want on a smartish system for the buffer, Minimum remote control, love Shelly's power monitoring.

 

@Mike thanks for that diagram, very helpful

 

 

[Mike]

17 minutes ago, RedSpottedSev said:

love Shelly's power monitoring

It's good in principle, however by using a contactor you bypass that function - it will only tell how how much power the contactor itself is using, not how much power is passing through the contactor...

[TerryE]

You can get a range of contactors typically with 240VAC, 12VDC or 24VDC coil voltage.  They all draw ballpark 1W to keep the contactor closed to the load on the control relay is ~40mA in the case of 24DVC, doubled for 12VDC, and a tenth of that for mains.  I prefer using DC simply because all of the control stuff is low voltage which makes it easier to get signed off.