Thermostat issue in the Immersion heater - how to check this?


Postby juewra » Fri Jul 25, 2014 3:10 pm

Hi there,

I have a 1 kw immersion heater installed in my home that runs on electricity. Also installed is a 5 kw solar PV system on our rooftop as well as 5 kw single phase inverter. Recently we got a solar pv load controller switch installed in our home to optimize most of the power that the summer's shine gave us. We got it configured to divert excess to the immersion heater and an additional inverter. First the immersion tank heats up and then any extra goes to the inverter. The diverting happens only when the thermostat in the immersion tank cuts off. Lately there was no power going to the inverter and we checked the immersion tank. We called the controller switch company and got it checked. The device is doing fine but the inverter is not receiving power. We checked the wiring, it is fine. What could be the reason? I have a doubt of the thermostat not working properly. How do I check this?
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Postby ericmark » Fri Jul 25, 2014 6:22 pm

Not sure I understand the problem here.
Either the excess goes into water or to inverter from what you say so if the thermostat on the immersion switches off too soon you get cold water and if too late the water boils. (In fact likely a temperature fuse which would rupture first).

Your not complaining of water too cold or too hot so it would seem working as designed.

With a 5 kW solar panel I would have expected a series of immersion heaters to match the load to what is being produced rather than also having an inverter which I assume it charging batteries but you don't say.

Clearly one could use an inverter to power the immersion heater and vary the voltage so it matches the spare power produced. But since these devices will stop working so well once smart meter role out it would seem rather pointless to go to extreme lengths for such a short life.

I did look at them but gas is same price as exported electric so just not an economic proposition when one has gas.

I would have thought if you have a 5 kW panel then 1 kW immersion is rather small I would have hoped at times you have 3 kW to spare so it would seem I am missing something?
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Postby Freacn » Mon Jul 28, 2014 11:38 am

Hi there,

Regarding the thermostat, check the voltage at the incoming power leading to the immersion itself.

If there is voltage, then the possibility is that the element itself or the thermostat is damaged.

Also, check if there is a reset option for the thermostat. If it is, then reset the thermostat to try.

By the way, what is the controller device you are using?

Being all through this could be dangerous, hence call in an electrician.
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Postby ericmark » Wed Jul 30, 2014 9:32 am

Since no reply I assume what ever was the problem is now solved?

My best tool for fault finding is the clamp-on ammeter measuring current without one is a real pain.

I have re-read your post and it still seems a bit odd.

My understanding is normally one would use an inverter connected to the solar panels which both converts to synchronised AC (Grid Tie) and adjusts the DC voltage to extract the maximum wattage.

To connect direct to the DC although it would work it would cause problems in adjusting the output voltage to give maximum wattage.

What I can't understand is what is the additional inverter used for? One could charge batteries and technically the device which drops and controls the voltage to charge a battery is an inverter but we would normally call it a battery charger.

We normally call a device which turns DC to AC an inverter there are many types the rotary one was used a lot at one time but now most are static. Of the static inverters there are those which give a AC output and there are also specials which feed that AC output into the grid called grid tie.

There are some special rules with grid tie to ensure it does not give an output when something has gone wrong. This is a multi-system as it has to still work when there is a street full of solar panels so if the cable supplying the street is damaged all the grid tie inverters will shut down.

So it looks at the voltage either over or under it will shut down, it looks at frequency again too high or too low it shuts down, and once it shuts down it has to remain shut down for a minimum time to allow all other grid tie inverters in the street to also shut down.

The supply authority has to supply 230 vac + 10% to - 6% so between 216.2 volt to 253 volt RMS. There problem is the step down transformer and cables all have a resistance so before solar panels they would set the tapping to give 253 volt with no load to ensure at the end of the cable run the voltage never drops below 216.2. If they find it does drop below that they may even exceed 253 volt no load as in real terms there is never no load.

However with solar panels they also have to run by same rules and as voltage reaches 253 volts they shut down. If one complains often the supply company can drop the supply voltage to ensure you can export but only if by doing so they will not drop below the 216.2 limit at other times of the day.

Economy 7 is a real problem as you get a whole row of houses taking a really heavy load in the night when solar panels are not running so with a street full of solar panels and night storage heaters keeping the supply between the 216.2 and 253 volts is a real problem.

The Economy 10 helps a little with the boost but still in the winter the power is needed but no sun and in summer loads of power available just at the time when your panels want to export power.

In hot countries with air conditioning solar panels work well but not in our climate.

In the USA it would seem they have rolled out smart meters well before us and their tariff system varies through the day to encourage people to use washing machines etc as a set time of day which is causing the consumer a real headache with timers one most of their domestic appliances and forcing devices like "Hive" to be used to control what their house uses from the phone.

As smart meters roll out here we may follow and the tariff will vary through the day. However we have a legacy where the government offered silly money for power supplied into the grid from domestic solar power. They tried to drop the rate but European court stopped it so they are left with one option which is common with all energy suppliers just don't take the power being offered.

It would seem firms who rent roof space and put solar panels on house roofs for free and in return get the power put into the grid have been having a real problem ensuring they can in fact get this power into the grid. If they raise their cut off point just 0.1 volt above the level of other solar panels in the street then theirs will be the last to shut off. Since one has to be registered to install and all the controls as to cut out voltage are not accessible to the user these firms can get away with just raising their voltage just that little bit to ensure theirs are the last to lock down.

The problem for private user is when installed there may only be a few panels in the street so for the warranty time all works A1 only as more and more houses in the street get them fitted does the problem with voltage raise it's head.

There is talk about changing the rules to +/- 10% the extra 4% would clearly help but at 207 volts add to that local volt drop of 3% on lighting so 200.79 volt many florescent fitting will fail to work. For our sockets with volt drop of 5% that means 196.65 volt RMS and although switch mode stuff may work any old radio or Hi-Fi will have mains hum and would be useless. So really 56.35 volt variation is just not acceptable.

So only option is not to accept power offered by owners of solar panels.
ericmark
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