WiFi control of hot water cylinder: feasible?

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kiwigander

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#201371 15-Aug-2016 22:57

I've got a small cottage that's usually used only one or two days a week. We control the heat pump remotely via the Pebble Air controller and app. When we're away the heat pump is off. Returning on a winter's day after several days' absence, we can start the heat pump a couple of hours before we arrive and enter a warm cottage.

The hot water cylinder is electric, and it seems a waste of energy and money to keep it operating when we are routinely away for several days at a time. But the HWC is hard-wired to a 30A (if I remember correctly) breaker, and I haven't seen any device (30 A rated WeMo or the like) that would allow us to control it as we do the heat pump.

What devices exist to allow remote control of a HWC? (disregarding the power utility's ripple controller)

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#1612252 15-Aug-2016 23:03

Have a sparky wire in a large contactor to control it, and the power that off a wemo or one of the other many widely available much better value wifi controlled switches like the broadlink or wewo ones.

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timmmay

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#1612277 16-Aug-2016 04:58

Modern hot water cylinders are really quite efficient in terms of insulation. Ours is a few years old and is up in the ceiling cavity. We turned it off and went away for a week, when we got back it was warm enough to have a shower - just. You might save a little doing this, but a contactor is around $100, an electrican could be the same or more, plus the rest of the electronics you could easily spend $400 to $500. I think the payback time will be years.

Jase2985

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#1612280 16-Aug-2016 05:31

it may cost you more heating the water from scratch than maintain the temperature anyways.

i would be making sure its as insulated as it could be so its not heating much at all when not in use

timmmay

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#1612282 16-Aug-2016 05:40

Question: when you go back, before the hot water is switched on, is the water cold, lukewarm, warm, or hot? That will tell us how well insulated the cylinder is.

Jase2985:

it may cost you more heating the water from scratch than maintain the temperature anyways.

i would be making sure its as insulated as it could be so its not heating much at all when not in use

I doubt it will cost more. Heat loss is relative to the difference between the hot water and the internal temperature. So the hotter the water, the more the loss.

frankv

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#1612286 16-Aug-2016 07:05

timmmay:

... a contactor is around $100, an electrican could be the same or more, plus the rest of the electronics you could easily spend $400 to $500. I think the payback time will be years.

Google "30 amp 230v ac relay" -- they're about NZ$10. http://www.ebay.com/itm/Relay-5V-Control-voltage-230V-30A-Power-relay-30V-20A-Amps-Volts-L90A-5W-/181790369127?hash=item2a538ce567:g:nYMAAOSwxH1T6LvG

Add in a 5V power supply (about $5) and an ESP8266 device (e.g. Wemos D1 mini) (about $4) and a transistor and a resistor, and a $5 click-clack box to hold it all... Say $25 for parts, some time to program it and connect it all up. You will need an electrician to sign it off.

timmmay

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#1612288 16-Aug-2016 07:16

frankv:

timmmay:

... a contactor is around $100, an electrican could be the same or more, plus the rest of the electronics you could easily spend $400 to $500. I think the payback time will be years.

Google "30 amp 230v ac relay" -- they're about NZ$10. http://www.ebay.com/itm/Relay-5V-Control-voltage-230V-30A-Power-relay-30V-20A-Amps-Volts-L90A-5W-/181790369127?hash=item2a538ce567:g:nYMAAOSwxH1T6LvG

Add in a 5V power supply (about $5) and an ESP8266 device (e.g. Wemos D1 mini) (about $4) and a transistor and a resistor, and a $5 click-clack box to hold it all... Say $25 for parts, some time to program it and connect it all up. You will need an electrician to sign it off.

A quality contactor, bought in NZ, is around $100 according to my electrician. It's illegal for a homeowner to connect anything to a switchboard: "The placing and connection of any cable into the switchboard must be carried out by a licensed electrical worker."

I wouldn't use cheap ebay parts for important electrical parts. Anyone know if it's legal? I suspect things have to be certified for use in NZ. It's probably not a good idea even if it's legal.

kiwigander

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#1612313 16-Aug-2016 08:44

Thanks to all for your replies.

The cottage is <2 years old so I have to assume the cylinder is insulated to modern standards. We have not yet tried switching off the breaker when leaving and back on immediately upon returning. I imagine that might work well for an absence of three days, less well over a fortnight.

I realise too that any built solution is not likely to be cost-effective. But if it can be done by a local sparkie using off-the-NZ-shelf gear, then I'd like to consider it and get a quote or two. The sparkie who wired the place is not interested.

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frankv

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#1612318 16-Aug-2016 08:55

timmmay:

It's illegal for a homeowner to connect anything to a switchboard: "The placing and connection of any cable into the switchboard must be carried out by a licensed electrical worker."

I don't see the relevance of this... I wasn't suggesting placing or connecting anything to the switchboard.

And you're probably right that the device would need to be NZ certified.

timmmay

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#1612321 16-Aug-2016 09:05

Where else would you put a controller for the hot water cylinder? Most things are controlled from a switchboard. Mine has a couple of timers and such in it, that's where I'd put anything like that.

Jaxson

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#1612326 16-Aug-2016 09:19

Warm water is a nice breading ground for bacteria. Personally I'd be quite focussed on ensuring the hot water cylinder gets fully up to temp for well over an hour or so before I used the water. Not saying not to proceed with a timer, but would suggest timescheduling the cylinder back on several hours ahead of what you initially had in mind.

Bung

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#1612327 16-Aug-2016 09:20

kiwigander: But the HWC is hard-wired to a 30A (if I remember correctly) breaker, and I haven't seen any device (30 A rated WeMo or the like) that would allow us to control it as we do the heat pump.

Next time you at at the cottage check whether it is 30A. Even if it is it is likely that the HWC only has a 2000W element. You might not need to plan around controlling a pottery kiln.

We have a Bach and also turn the HWC off. Mainly because if someone steals the copper vent pipe heated water will be running down the roof until someone notices. The normal load doesn't hurt as much as the daily line charge.

timmmay

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#1612333 16-Aug-2016 09:33

Legionnaires is an issue. You should keep the cylinder above 60 degrees at all times if possible, but I think the regulation is it has to hit 60 degrees either daily or weekly - I forget which. So if you turn it off you really need to let it heat up fully before you use it.

Jaxson

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#1612357 16-Aug-2016 10:27

The recommendation is that water temp reaches 60°C in the tank at least once per day.

The issue is that the in a water tank, the top can be warmer than the bottom, so the real objective is that the whole tank be raised to this temp.

http://www.stuff.co.nz/national/health/70285141/legionnaires-disease-from-hot-water-cylinder-results-in-amputations

It's a funny situation though, in that with tempering valves the water in the lines from the HWC are never at this high temp.

Fred99

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#1612397 16-Aug-2016 11:56

Jaxson:

The recommendation is that water temp reaches 60°C in the tank at least once per day.

The issue is that the in a water tank, the top can be warmer than the bottom, so the real objective is that the whole tank be raised to this temp.

http://www.stuff.co.nz/national/health/70285141/legionnaires-disease-from-hot-water-cylinder-results-in-amputations

It's a funny situation though, in that with tempering valves the water in the lines from the HWC are never at this high temp.

The legionella species can't multiply at over about 47 deg C, the number of viable cells slowly decrease. About 55 deg C, the cells die in about 8 hours vs 1/2 hour at 60 deg C.

60 Deg C cylinder setting is considered optimum setting taking into account variation in temp in the cylinder, risk of scalding where there's no tempering, energy use/waste.

Relevant to the OPs situation perhaps is that poor rural water supply quality may be an issue, for presence of the bacteria but also because the water may contain high(er) levels of nutrients than treated town supply water - allowing the bacteria to proliferate. I'm on a bit of a guessing mission here, but if the cylinder was to be powered on/off at those intervals, water supply quality was a bit dodgy, and when powered off it was going to take a long time to cool down to low temps - possibly meaning that it was sitting at the ideal temperature range for rapid growth as it cooled down over many days, I'd set the thermostat higher than 60 deg C - to be more confident that the bacteria were killed very quickly when the cylinder was turned on.

If I had a small bach/cottage used only occasionally, I'd probably not have a HWC at all, but use a gas califont and 9kg cylinders (to avoid fixed rental costs for 45kg cylinders etc). But I'd also probably want it off-grid and use a log burner (or gas) for heating when needed, gas fridge, and small PV system for lighting and low power appliances.

frankv

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#1612400 16-Aug-2016 11:59

timmmay:

Where else would you put a controller for the hot water cylinder?

At the water cylinder.

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