What's probably wrong with this circuit - water heater not doing heating.

Thread Starter

anunusedusername

Joined Nov 12, 2020
7
Bit of background: I have PV arrays on the NE and NW elevations of my roof (live in the southern hemisphere). The NE also has a solar thermal HW system. Feed in tariffs are punitive, so I try to shift as much consumption to periods of high solar yield.

I wrote and fine-tuned some python code which establishes at midday each day how long it feels it should run the immersion heater based primarily upon the insolation of the NE PV array until that point, but also considering season, ambient temperature and forecast (these values are all scraped from the web rather than measured locally.

The way in which this controls the immersion heater is by way of an SSR, which remarkably (to me) was able to energise from just the 3.3v logic level signal from the RPi GPIO. It has worked well for ~8yrs, but yesterday we had cold water.

Testing the system from bottom to top, the RPi is up and responsive. It's giving up 3.3v on it's GPIO on demand, and this is readable at the SSR in the roof. Switching the GPIO in software gives rise to 240vac reaching the thermostat, and that in turn gives up 240vac to the element terminals when water temp is below the setting of the thermostat, typically about 60 celcius.

The thing is, the way in which I have established if the element is in operation in the past is to monitor the frequency of the flashing from the consumer meter, which increases with rate of consumption, and this is not happening.

As I pondered this with my multimeter connected to the element terminals, I saw the voltage drop to zero for a second, and then return to 240. This repeats every 4-10seconds. I observed the same on the input side of the thermostat after I'd noticed this.

This is where I'm out of my depth. The circuit isn't tripping the RCD, so it seems unlikely the element has a short circuit. I can hear a faint click and see an LED illuminate on the SSR when the RPi sends 3.3v to the GPIO. I don't hear or see it opening/closing in line with these voltage drops, but haven't yet probed the voltage before and after as I don't have a neutral handy there, it just switches the positive of one of the three 240v phases in the house. I don't know if anything else is on the same phase, it is entirely possible that there is not.

Is it possible that there's some kind of failure mode of the element that could cause the conditions observed? The thermostat appears to be working, I feel like the SSR is doing what it always has - would appreciate any suggestions.

Cheers,
Alex
 

jpanhalt

Joined Jan 18, 2008
11,088
I would check the heating element.

I have no experience with electric water heaters, but when the element in my oven failed last year, there were a few sparks and then it failed open. Never did cause the circuit breaker to open. The on/off cycling could be some feature to check for current when on. If that fails, it then cycles. That is common for gas heaters (flame proven, fail, cycle). I don't see why that would be done for an electric heater. Maybe in case the circuit breaker blows, its keeps checking for a reset.
 
Last edited:

s14rs4

Joined Sep 15, 2016
73
It looks like your immersion heater element has failed open circuit. Isolate your supply and measure the resistance of the heater. I don't know your mains voltage is but the resistance should be in the region of tens of ohms. The thermostat you are referring to is this an external thermostat you have added or the inbuilt thermostat of the heating element this could fail open circuit.

P.S, I don't have solar panels, I live in Scotland, it's 8am here and still dark.
 

jpanhalt

Joined Jan 18, 2008
11,088
Is the thermostat line powered or battery powered? I have seen systems cycle when the battery in a battery powered thermostat went dead.
 

Thread Starter

anunusedusername

Joined Nov 12, 2020
7
Thanks both. Thermostat is original equipment, but is separate to the element. It's a bimetallic strip arrangement, and is operational, effectively a DPST switch. The setting screw just varies the distance between the contacts. It takes 240VAC live and neutral inputs and the closed circuit (below target temp) connects both to their respective outputs to the element.

Isolated element measures 25 Ω.
 

s14rs4

Joined Sep 15, 2016
73
Try disconnecting your control circuit and connecting the heater via the thermostat directly to the mains and see if the water heats.
 

Alec_t

Joined Sep 17, 2013
12,201
Could be the contacts failing on the thermostat. Try temporarily bypassing the thermostat terminals with a suitable guage insulated wire.
 

Thread Starter

anunusedusername

Joined Nov 12, 2020
7
It appears the issue may well have been grid related, insofar as a neighbour had problems with one of his phases, and that the unchanged setup for heating the water has now returned to adequately performing it's solitary job. How odd. Thanks for all suggestions.
 
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