How hot to make a hot water cylinder heated via PV?

Bendy_House

A friend was recently asking me how to use surplus leccy from their PV system, so when they informed me they had a hot water cylinder, I suggested an Immersun-type controller.

This led me to wonder just how hot it should be set to?

I understand hot cylinders should be taken to 60+oC for Legionnaires and stuff, but for most energy sources heating above this would be wasteful. However, if the leccy is free, then surely you'd want to use as much of it as possible? If a nice sunny day got your tank piping hot, even above 90oC, surely that's a good thing as it's extra stored energy?

But...there would then come the risk of scalding.

So, is this a situation anyone is familiar with? Is it worth tweaking up the 'stat?! Should a regulating thermostat be fitted to the outlet to blend it with cold in order to prevent accidents? Should every tap in the house be thermostatic?

Ta.

MX5huggy

I have a 500 litre thermal store the immersion is set to 80, and use spare PV production as you describe. I have a 2 commercial grade thermostatic mixer valves on the outlet mixing down to 45 ish (ie hot washing up water that you can just use but not hold your hand in, my children are old enough not to climb into a too hot bath). It’s a thermal store so I’m not directly heating the water I use, it’s treated water (same as in radiators) so it does not cause a build up of lime scale which is an issue at higher temperatures. 

[Deleted User]

 I have a 250L unvented cylinder which uses surplus PV solar. The immersion heater thermostat is set to 60C which is at the lower end of the recommended 60 to 65C heating band.

We have 3 showers with thermostatic controls and standard taps. If you try and heat an unvented cylinder to 90C, then you are one train stop past Barking. Fortunately, for those that do there are 2 temperature limiters built into a standard installation. Most immersion heaters are limited to 70C and there is also a top stat which will operate to expel hot water if the immersion heater stat fails.

I suggest that you take a look at videos to see the damage that can be caused if an unvented cylinder explodes.

From the Regulations:

• Thermostats – The thermostats are in place to prevent the temperature of the water in the system exceeding 99°C. These include a control thermostat which is set to maintain the temperature of the water between 60-65°C; this gives the first level of protection against the overheating of the water. A second thermostat incorporates a thermal cut-out which switches off the immersion heater and shuts off water from the boiler if the control thermostat fails and temperatures reach between 85-89°C. This thermostat has manual reset feature in place which cannot be self-resetting.
• 
  
• A temperature and pressure relief valve is fitted near the top of the cylinder. This is set to 90°C + and is designed to remove pressure from the system preventing the water temperature exceeding 99°C. It provides protection against the failure of the pressure reducing valve, failure of the expansion vessel or the loss of the internal air bubble.

Reed_Richards

I have my immersion heater thermostat set to around 75 C.  There is a second thermostat with a thermal cut-out for heating via my "boiler".  Although in principle this operates at 82.5 C it is occasionally triggered by the immersion heater-heated water and has to be manually reset or water from the boiler bypasses the cylinder.  There is a TMV on the outlet from the cylinder which takes the hot water temperature down to around 40 C.

On a sunny summers day I still generate more electricity than I can use.     

FreeBear

If you are in a hard water area, heating the water up increases the amount of scale being generated. For domestic hot water, 65°C is plenty hot enough, and it will still scald.

I run my DHW at 45-55°C which is plenty hot enough. Even with free electricity from a PV panel, I don't think I'd want to go much higher.

Bendy_House

Thanks everyone - food for thought.

FB, this is a very soft water area - an island in the Hebrides!

Bendy_House

MX5huggy said:

I have a 500 litre thermal store the immersion is set to 80, and use spare PV production as you describe. I have a 2 commercial grade thermostatic mixer valves on the outlet mixing down to 45 ish (ie hot washing up water that you can just use but not hold your hand in, my children are old enough not to climb into a too hot bath). It’s a thermal store so I’m not directly heating the water I use, it’s treated water (same as in radiators) so it does not cause a build up of lime scale which is an issue at higher temperatures. 

Thanks Huggy.

So you have the TS set high, which I understand is the whole point of them? Ie - it'll store as much energy as possible that's provided to it.

Can I ask what the main form of heating to it is? And have you any idea how much the spare PV leccy contributes? I mean, in summer, do the PVs keep it fully heated for showers and other DHW? Any way of knowing how much winter sun contributes?

The thermo mixer valve blends the DHW output (I understand it's via a heating coil immersed in the TS?) with cold?

Why two thermo mixers? Tobesure tobesure?

Bendy_House

[Deleted User] said:

 I have a 250L unvented cylinder which uses surplus PV solar. The immersion heater thermostat is set to 60C which is at the lower end of the recommended 60 to 65C heating band.

We have 3 showers with thermostatic controls and standard taps. If you try and heat an unvented cylinder to 90C, then you are one train stop past Barking. Fortunately, for those that do there are 2 temperature limiters built into a standard installation. Most immersion heaters are limited to 70C and there is also a top stat which will operate to expel hot water if the immersion heater stat fails.

I suggest that you take a look at videos to see the damage that can be caused if an unvented cylinder explodes.

From the Regulations:

• Thermostats – The thermostats are in place to prevent the temperature of the water in the system exceeding 99°C. These include a control thermostat which is set to maintain the temperature of the water between 60-65°C; this gives the first level of protection against the overheating of the water. A second thermostat incorporates a thermal cut-out which switches off the immersion heater and shuts off water from the boiler if the control thermostat fails and temperatures reach between 85-89°C. This thermostat has manual reset feature in place which cannot be self-resetting.
• 
  
• A temperature and pressure relief valve is fitted near the top of the cylinder. This is set to 90°C + and is designed to remove pressure from the system preventing the water temperature exceeding 99°C. It provides protection against the failure of the pressure reducing valve, failure of the expansion vessel or the loss of the internal air bubble.

I don't want them stopping off at Barking :-) Especially if they are launched there.

This tank will, I'm assuming, a vented type, a normal 'hot cylinder' fed by a CWS in the loft. Is there any reason why that cannot be kept at 90oC-ish? It ain't going to melt or anything :-)

Bendy_House

Reed_Richards said:

I have my immersion heater thermostat set to around 75 C.  There is a second thermostat with a thermal cut-out for heating via my "boiler".  Although in principle this operates at 82.5 C it is occasionally triggered by the immersion heater-heated water and has to be manually reset or water from the boiler bypasses the cylinder.  There is a TMV on the outlet from the cylinder which takes the hot water temperature down to around 40 C.

On a sunny summers day I still generate more electricity than I can use.     

Ah, that sounds like what I was suggesting to my friend.

Is this a standard hot cylinder, or a Thermal Store? Vented or un?

So, surplus PV leccy is used to charge it up, and you have this set to around 75oC? Why not higher?

Ah, the boiler gets this up to 82-ish, so I'm guessing this is a TS? Which will therefore be designed for such temps?

And, in summer, the TS gets charged up fully, so you have 'free' hot showers and DHW? Do you have any thoughts on just how effective it is - eg, on a sunny day, how well would your system heat up the tank from cold?! (I guess that depends on what other call is made on the PV, so a silly Q...)

I wonder how much it helps on sunny winter days?

Bendy_House

So, to modify an existing domestic setup - a (presumably) vented hot cylinder, charged via surplus leccy from a PV system - is there no benefit from setting the immersion 'stat to ~90oC? Wouldn't this allow the PV to provide as much as it possibly can to the DHW system?

Although, yes, it would need thermo-mixing for safety...

QrizB

I heat my weater with surplus PV in the summer (stopping some time in September, when the surplus stops).

My immersion heater thermostat only goes as high as 65C!