Hot water from PV

Hi all.

The I smaller I'm talking to about fitting a PV system with batteries has mentioned using excess power to provide hot water via an unvented cylinder with immersion heater.

I've read a couple of threads here that touch on this but not in detail.

My central heating is a combi boiler. Are we talking about a stand alone copper cylinder just heated by the immersion heater? Not connected to the boiler at all?

He reckons we may be looking at 8Kw worth of panels so would this make heating water viable in your view?

Any advice greatly appreciated
Mick

A PV diverter to hot water is usually the simplest way to ensure you use all that your panels genrate.

As you are having a battery system, you will be able to make better use of the stored PV running real loads at any time of day, so no need for the PV diverter.

Hi Dave. Not sure what you mean? Are you saying because of battery I shouldn't go for hot water?
I think the installer was suggesting both.
Mick

The hot water option would only make sense IF you currently have a hot water tank and a dedicated circuit for the immersion heater

Murdoch,

No, the guy is suggesting buying a cylinder to be run by the solar, I'm guessing you mean an electric circuit to run the immersion heater if it needed topping up?
It wouldn't be hard to run a feed from the Consumer unit while we are at it.

Mckwhitts said:

Murdoch,

No, the guy is suggesting buying a cylinder to be run by the solar, I'm guessing you mean an electric circuit to run the immersion heater if it needed topping up?
It wouldn't be hard to run a feed from the Consumer unit while we are at it.

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Then it would need to be plumbed in ..... a good idea in many ways, not only for the "free" hot water, but when your combi goes wrong you'll have another way to have hot water.

Probably best to get a plumber to quote for that part of the solution - the diverter is a relatively easy addition

Mckwhitts said:

Murdoch,

No, the guy is suggesting buying a cylinder to be run by the solar, I'm guessing you mean an electric circuit to run the immersion heater if it needed topping up?
It wouldn't be hard to run a feed from the Consumer unit while we are at it.

Click to expand...

With a combi boiler, that's probably complete drivel. When my Combi finally dies, I'm planning to replace it with a system boiler and hot water tank. It's not cost effective to do that before the Combi dies - NB I may be wrong about that given the current price rises in energy. Anyway, customers of mine who have a tank and an immersion diverter gadget report getting most of their hot water for about 2/3rds of the year from their panels.

As a side thought..
and I may be barking up the wrong tree here...

But many years ago I did a rewire for a property that also had a solar-water system fitted..
i.e. some panels consisting of pipes filled with an anti-freeze solution, that was pumped around a coil inside a hot water cylinder with dual coils..
One from solar, on from boiler..

Just made me think what are the advantages / disadvantages between electric and water solar for heating your domestic water supply?

Was involved in something like this on a previous job. the electrics was straight forward but the plumbers had a job!! they kept 1 hot water outlet running from the combi and then put a cylinder in for the rest. then treat the boiler like a system boiler on a Y plan. use a immersun , I boost or similar with the boiler as a back up.

Wet systems are certainly popular in more Southern climates. When we visit Cyprus it looks like nearly every property has them and they've been operating since solar PV was just a dream.
I've no idea what the efficiency figures are but it strikes me that only converting the energy once must have some advantage.

Using PV to heat water seems rather wasteful to me. PV panels are fairly inefficient. A wet system can use a much smaller panel. Save the PV for something else such as EV vharging.

binky said:

With a combi boiler, that's probably complete drivel. When my Combi finally dies, I'm planning to replace it with a system boiler and hot water tank. It's not cost effective to do that before the Combi dies - NB I may be wrong about that given the current price rises in energy. Anyway, customers of mine who have a tank and an immersion diverter gadget report getting most of their hot water for about 2/3rds of the year from their panels.

Click to expand...

Would it be possible to use the unvented, mains pressure cylinder to feed the combi boiler? The thermostat could be set so that it only heats to, perhaps, 35°. That would at least mop up some excess PV power and reduce the energy the boiler uses to heat the domestic hot water.

BorisJ said:

Using PV to heat water seems rather wasteful to me. PV panels are fairly inefficient. A wet system can use a much smaller panel. Save the PV for something else such as EV vharging.

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Not sure I agree with you Boris.

If you have a hot water tank, but no EV and add solar, then heating water from the PV excess does make a lot of sense.

Billy sheds said:

Would it be possible to use the unvented, mains pressure cylinder to feed the combi boiler? The thermostat could be set so that it only heats to, perhaps, 35°. That would at least mop up some excess PV power and reduce the energy the boiler uses to heat the domestic hot water.

Click to expand...

depends on you boiler, not many will accept preheated water.

Billy sheds said:

Would it be possible to use the unvented, mains pressure cylinder to feed the combi boiler? The thermostat could be set so that it only heats to, perhaps, 35°. That would at least mop up some excess PV power and reduce the energy the boiler uses to heat the domestic hot water.

Click to expand...

downside of solar thermal is that you can only use it for hot water, and it has maintenance costs going forward, where as PV needs no more than the occasional wash.

I had both on my old house. Solar thermal is roughly twice as efficient as PV but as already mentioned you have pipes and maintenance costs. I changed my system from open vented to mains tank and increased it to 250litres, free hot water from April to November. I've changed from water in the open vented to a sealed Glycol mix when i changed the system. My current house has a Solic 200 Pv diverter, not as efficient but panels are so cheap now if you have space its a no brainer. Any questions just ask
Stuart

There’s nothing more rewarding these days than knowing your hot water is free. Especially when you have a 210 litre tank and 2 women in the house that like hot deep baths

Having had a new PV system installed, along with batteries and a diverter to heat water with any excess, I now question the economics of the water heating part.

Heating water with gas costs me about 5.5p a kWh. So every spare kWh I get from the solar PV only saves me that much.

The fee I've managed to get for selling my spare electricity back to the grid is also 5.5p/kWh.

So I don't actually see any advantage to heating water this way. Especially as I don't get any spare power until after the batteries are full, which can be any time of the day from say lunchtime to never, depending on how much sunshine there is and how much we self use.

Currently I'm using the boiler to top up water temperature to 50degC every morning. Then if/when there is excess power, that will take it up to 60degC. On a good day, that's enough to not need the gas boost in the morning. But I still do wonder if there's any point.

Are my sums correct or have I missed something obvious?

Your gas price is low!

Yes - sounds like you've got it covered. My response above was on the basis that batteries weren't part of the set up.

Some people think that they can get batteries charged, their EV charged and get free hot water - all year round which in the UK isn't realistic with a standard 12 panel set up

My gas price is low, because by some fluke I signed up for 2 year fixed tarrifs last November.
Prices were a little higher than other companies variable tarrifs. But for once in my life a gamble appears to have paid off.