Replacing my heating/water controller - is Shelly the way to go (and, if so, which one)?

Hardware
1

Hi there.

I am looking to try and make my heating and water system smart and add it to my Home Assistant server.

Shelly looks perfect for the job but there are lots of models and I am struggling to work out which one to get.

My system is a Y plan system:

Ultimately, I need a 2 channel Shelly to replace my current controller (unless two single channel Shellys would be preferable).

You will note that the first channel on my controller (for hot water) uses a SPDT switch and I note that Shellys do not offer this - as such, I shall have to use a separate SPDT relay on that channel as well.

Two main questions:

  1. Which Shelly should I buy to do this?
  2. I want to use a Shelly add-on board as well as I need four probes:
  • A temperature probe at the top of my water tank and a temperature probe at the bottom of my water tank - I am going to use these to try to turn on hot water earlier/later, depending on current water temperature.
  • A temperature/humidity probe that runs into my loft and a temperature probe that runs into my livingroom - I am going to use these as failsafes oustside of Home Assistant (i.e. turn on heating if the loft temperature falls to humidity goes to high and turn off heating if the Livingroom gets too warm).

Will a single addon be able to accommodate all of this?

Thanks

2

Hi,
I've looked at this before over 20 years, and these days frankly, I'd just buy a UK-specific connected commercial thermostat with local support.

There are a load of complications such as pump overrun-timers (dumb boilers don't control pumps efficiently), limits on boiler cycling (could be a max of 6x per hour for a gas boiler), modulation (if supported, this makes your boiler much more efficient), frost protection if HA dies, and the cost-engineering of a Y-plan system means it is far from logical (although John Ward does do a good job of explaining it).

Shelly is a good choice for many automation tasks and it has replaced Sonoff+Tasmota for many of my automation tasks. Getting sensors into a Shelly is harder than a Sonoff as without an extra clip-on module, they don't have isolated GPIO for a DS18x20. They also don't have many SPCO relay output options.

The simplest automation of a Y-Plan is to replace the SPCO timer switch with an automated relay, and consider bypassing the room themostat. The issue here is what happens if HA crashes? Moving the temperature control down from HA to a microcontroller should increase reliability (less SPOFs), but personally, I'd still add a mechanical frost thermostat.

The room stat is usually distant from the AB valve and tank so comms is needed. Shelly can connect some Zigbee kit directly (not via HA), but you may prefer HA averaging sensors - just consider a HA failure.

Hot water is simpler as sensors are usually next to the tank and AB valve so all local. Be careful if you have a 3kW immersion heater as I'd not personally trust most automated switches at their current rating as the immersion can run continuously leading to heating. This is specifically called out in BS7671, and is why you can plug in a 3kW kettle, but NOT a 3kW immersion.

Shelly has a 25A module that I've used to switch 10A and the terminals look pretty solid.

I replaced my Y-plan with a Honeywell SmartFIT as this had a serial interface and Honeywell dealt with all the complications (hysteresis, cycling limits, etc). If this had died, I may have used a generic ESP8266 module with SPCO relays and DS18x20 as you suggest - but it lasted until an ASHP upgrade became possible.

How water tanks stratify - create water layers at different temperatures, so include more than one port for temp sensors. You can stop heating when just the "top" is warm if water demand is low, but remember it is recommended to heat to 55-60C to prevent Legionella bacteria which can form especially if the tank is not used for a while.

Note that the old "rats nest" choc block "wiring centre" in most UK heating systems can be easily replaced with a much neater box with dedicated Wago connectors.

Yes, you can DIY (I considered this a few times), but unless you are sure of your skills (I am an Electrical Engineer), just buy the convenience of a connected thermostat. Many boilers now include modulation for efficiency so use OpenTherm or a proprietary version that a commercial stat might support. Commercial kit also includes fault detection.

A few folks on the forum have automated Y-plan systems, so search. This DIY automation is not a simple replacement - more a rewire.

If this helps, :heart: this post!

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3

Thank you so for much for this - extremely helpful.

Loads of great advice here.

Firstly - thank you for the tip on the rats nest. I shall get mine sorted with Wago connectors as I have wires all over the place and it is really difficult working on it.

Great point about the mechanical thermostats. I currently have a dumb programmable thermostat that I use for my heating, in addition to my heating/water controller.

If I kept that and wired it in series with the heating channel, that would act as a failsafe in case the Shelly somehow got stuck with the heating on. Conversely, a second mechancial thermostat in the loft (wired in parallel) could also act as a failsafe if the Shelly failed to fire up the boiler on a very cold morning.

As the heating is so critical, this would help to add some independence, separately from the Shelly itself.

In addition to this, I have read that the Shellys have built in failsafes themselves so that they can be programmed to never be on for more than x hours at a time (this would be useful if Home Assistant crashes whilst the heating and water is on) and would hopefully act as a first line of defence before the thermostats eventually kick in.

Loft humidity is less crucial than temperature, so I might just go with a Zigbee humidity sensor for up there.

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4

Is retaining the existing AB Valve part of your upgrade strategy?

It may be limiting your choices.

Break down your automation to the basics. You are monitoring a few sensors, and based on the information gathered, switching devices. They all appear to be fairly generic and easily interchangeable with other industry standard devices.

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5

Yes. And also Shelly can be controlled with toggle_after parameter, which means that when you turn it on, it's already hardcoded to turn off after x independently from any connections.
I personally would put Shelly in parallel with actual dumb thermostat and set that thermostat to lower temp just as backup.

I didn't study your heating system, but if you need dry contact relay, you need Shelly 1, if not, you can go with 2-ch one.

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