Do you consider Zigbee devices reliable enough to use for hot water control?

Hi there.

I recently updated my bathroom extractor fan with a Sonoff ZBMini and it has been brilliant.

I will get around to adding an humidity sensor but, at the moment, I tell Alexa to turn on the extractor and it automatically runs for 45 minutes - it has, so far, never failed to work.

This has got me thinking whether I could/should use the same device to control hot water. The current timer is very basic and I would like the flexibility to have the hot water come on a little bit earlier on colder days or have the hot water turn on remotely.

My question is whether I should trust the device with something as import as the hot water tank.

Although the extractor fan device has never failed (in many months) if it somehow managed to get stuck on, it would not be the end of the world.

However, if the hot water were stuck on permanently all day, that might be more problematic.

Do you think I am being overly concerned?

The only way I can imagine this happening is that Home Assistant tells the hot water to come on and then HA somehow completely crashes (which does very occasionally happen).

Is there perhaps something clever, that I hadn’t thought about, to avoid this scenario?

The other option might be to use a Shelley relay, which (I think) has a built in timer and can be set to never run for longer than a set period (say, 3 hours).

Any advice would be much appreciated.

I would control your heating/water with a smart device (I use Hive, but it could be Nest, Honeywell or any of the others) and then control that via Homeassistant.

This means if HA fails for any reason, you still have full control via your smart solution.

I schedule my hot water and heating purely via HA automations (using presence, temperature, windows opened/closed, time of day, am I awake/asleep etc as influences) and my Hive is completely in ‘manual’.

HA has never failed in the ~5 years I have been using HA.
HA has never failed in the 2 years I have been running my heating/hot water this way (via Z2M) and the 2 years I was running it before that (via the Hive cloud integration).

For what it’s worth, the Hive thermostat talks to the Hive controller over Zigbee (hence it integrates into HA via Z2M). Bringing my Hive into my (single) Zigbee network means there aren’t 2 networks competing over the same frequency - I would say that makes it a more reliable connection.


Thanks for that, jchh.

If I explain exactly what I am after, perhaps you might be able to help me.

I have a dumb hot water controller that tells my boiler to come on and off and heat my water tank. The timer is really basic and, to make any adjustments is a real pain (i.e. just for one day).

The other problem I have is that the timer has no awareness of how hot the water is in the tank already. This is not a problem if there is already a lot as it just heats the additional water until the thermostat turns the boiler off.

However, if someone has used lots of hot water the night before (or is particular cold), we sometimes find that there is not enough hot water for baths etc. in the morning.

What I could do with is a smart relay that also has GPIO pins on it - I could then add a Dallas senor to it (attached to the hot water tank).

I am then hoping that I can set the hot water to come on at, sat 6am, but even earlier if the sensor senses that the tank temperature is particularly low.

My initial thoughts was to go with these as a solution:

Shelly 1PM Mini Gen3 | Shelly Store UK | Smart Home Automation

Shelly Plus Add-on | Shelly Store UK | Smart Home Automation

DS18B20 Sensor | Shelly Store UK | Smart Home Automation

The main benefits being that:

  • The ESP has its own internal settings, so can by timed to turn off if is every left on for more than a set amount (as a safety feature).

  • It can work off mains voltage, so can just be placed exactly where the current timer is.

  • That particular relay has temperature and voltage control built in to it, which is important, given that it is going to be placed in the airing cupboard, next to my gas supply.

However, my main issue is that is Wifi and I would really prefer Zigbee control.

Do you think it would be possible to do what I am after with a ESP32-C6, instead?

So far, I have not found any off-the-shelf Zigbee dry relays that will let me add additional sensors to them.

Agh, man - I don’t have a clue about the hardware side of it - sorry.

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Thanks mate.

I have done a lot of extra research on this and I think it would be safer to copy exactly what you have for a number of reasons:

  1. I am thinking of moving house within the next couple of years - having an upgraded smart system in place might be a bit of a selling point.

  2. My family tolerate my constantly tinkering with our house but if I mess up the hot water, it might be a stop too far. Hive seems a nice non-DIY method that will avoid me completely messing things up.

So just to recap - am I correct in thinking that all I need is Hive for separate hotwater:

Hive Active V3 Wireless Heating & Hot Water Smart Thermostat White / Grey - Screwfix

Do I need to plug in the hub at all or can I just ignore that?

You will not need the Hive hub if you use Z2M, but then you need to know how to do that, also install Mosquitto (MQTT*) and also have a Z2M stick.

Do a bit of reading on that and them come back with questions.

The stick I use (and live is) the Sonoff Dongle-P

[edit: Typos*]

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This sounds perfect - I already have Z2M already set up, so it sounds like I do not need the hub at all. I might buy one, however, as I assume that the next owner of the house would probably use it.

Just to clarify - is the Hive Controller connected directly to the Hive Controller as well as both being independently connected to HA?

Correct. If you have Z2M already running, you’re laughing.

I am happy to share my yaml to control the Hive once you have added it to Z2M. Here are my notes on that (from Z2M docs and what I learned from this forum):

For information on pairing the thermostat controllers see the pairing instructions for the Hive SLT3B. Note that the thermostat’s Central heating light will remain amber until a controller is paired with the thermostat, however the thermostat will still function correctly.

SLR2c (receiver)
  1. Remove the batteries from the thermostat (SLT) to turn it off.
  2. Turn boiler off at the mains in order to turn off the hive receiver (SLR).
  3. Turn the boiler on and receiver.
  4. Hold down the central heating button for 10 seconds on the Hive receiver until top LED turns flashing-pink, then release.
  5. Hold down the central heating button again until the light turns double-flashing-amber.
  6. Pair with Zigbee2mqtt using “Permit Join (all)”.
  7. The double-flashing-amber double flash may change to a single-flashing-amber.
  8. The receiver will be added to zigbee2mqtt. Rename it to something like Hive.
SLT3c (thermostat)
  1. Make sure the thermostat is close to the receiver to pair properly.
  2. Replace the battery in the thermostat and allow to boot.
  3. Press and hold the menu and back buttons to factory reset the device. Allow the countdown to finish and release when you see ‘welcome’ - after selecting - a language, it will enter pairing mode.
  4. In Zigbee2Mqtt use the dropdown arrow next to Permit Join to select the Hive receiver device you added earlier.
  5. Start pairing with the newly labelled Permit join (Hive Receiver) button.
  6. Wait around 30 seconds.
  7. The thermostat should now pair to the boiler receiver. The flashing-amber light should turn green, and the thermostat will go through the getting started wizard.

note: it can take 24h before the battery status is shown

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Thank you so much for this - really do appreciate it.

I am currently looking at Ebay and Screwfix/Amazon to get my hands on a system.

There are some deal to be had on Ebay but the model numbers seems to vary slightly - do you happen to know what the difference is between receivers SLR2, SLR2b and SLR2c and thermostats SLT3, SLT3b, LST3c and SLT3d?

No worries. I got mine from Screwfix.

  • SLR1x means 1 channel (water only)
  • SLR2x means 2 channels (water and heating)

I think a,b,c is the model - so ‘c’ is newer than ‘a’.
…I think. Worth checking on the Hive website.