Running a ULV immersion heater avoids the losses incurred by the inverter.

Whether those losses are significant will depend on whether the inverter has to stay 'live' in any case, due to the boiler.
If the battery and DHW tank are in close proximity, then there would be little voltage drop across the cable supplying the immersion.
You could run a 500W heater element from a 12v battery using 6mm² cable.

The high currents I was concerned about were those between the inverter and the battery.
That's where you need the thicker cabling.

I've checked the Installation & Technical documents for the Worcester Bosch Ri boilers:

But that range of boilers does appear to require 240v AC to operate.

Thanks,

I've read this whole post again from the start, as I was wondering exactly how solar thermal is usually integrated with a gas-fired direct water heater (HW and space heating via a combi maybe, but not necessarily), as per the OP. @johnmo seems to have done it by diverting the water from the solar thermal-fed cylinder when it is at the set temperature, but there is a bit of topic drift since then.

The 'modulating boiler' option seems different, as my understaning is that in this scenario the boiler feeds the cylinder and 'modulates' in response to the water temperature in the cylinder by heating the cylinder water.

I am not a hot water engineer, and I wonder if anyone could elaborate please on how exactly a solar thermal/gas-fired direct HW hybrid system could be set up so that the temperature is managed (the gas firing is modulated). Are the above described ways indeed different and viable, and if so what components are needed please, to modulate the gas firing and/or divert the water from the cylinder?

Thanks,

Andrew

Posted by: @anr

↑

johnmo seems to have done it by diverting the water from the solar thermal-fed cylinder

When I had the combi connected as per a combi (now system conversion, ASHP hybrid), it was run as preheat of the cold water prior to entering the combi. In winter you can heat the preheat cylinder via central heating and get same benefits even when the sun doesn't come to play. The combi has way less work to do so consumption comes down, but importantly for me, flow rate as a consequence increases, by a big margin. Could feed 3 showers at the same time from a combi.

In summer you need a diverter to stop water entering the combi, then you get zero gas usage DHW.

But if you have room do an unvented cylinder, convert the combi to be either S or Y plan or more ideally PDHW. Do a normal thermal insulation.

Thanks @johnmo  

I'm sorry, but there are some acronyms that I am not familiar with. Could you expand ASHP and PDHW please?

Also, when you say that you used/need 'a diverter' for summer to stop the gas from firing, would you tell me what one of these is exactly please? Is it just a manual valve, or is it some electronic control that monitors water temperature?

Thanks once again for responding.

Posted by: @anr

↑

I am not a hot water engineer

Nor me...

... at best I'm an engineer who gets into hot water.

Here's diagrams to show how solar thermal can be integrated into both a DHW cylinder or a thermal store.

For the purposes of understanding the theory, it matters not whether the main heat source is a Heat Pump or a Boiler.

But the solar-thermal coil is usually the bottom one when you have a multi-coil cylinder.

I have a 300-litre thermal store,
which has appreciable thermal layering of water within the cylinder, assisted by internal baffles.

I can draw water from the bottom section for UFH,
and get 46°C from the DHW coil at the top.

Posted by: @anr

↑

ASHP and PDHW please

ASHP - air source heat pump 

PDHW - priory demand hot water. So heating the hot water cylinder on priority over heating, but more importantly allows different flow temperature for central heating and cylinder heating. 

Not what I used but a simple diverter like this, looks ideal

https://atmos-i.co.uk/boiler-spares/15mm-solar-diverter-valve/

Thanks @johnmo  

That water diverter is the component that I wanted to know about.

It looks solid state. I wonder how it works.

Andrew

@johnmo I hadn't previously heard of such a diverter. 🙂 

@anr  before you fit one, have a look at Building Regulations; Approved Document G, and particularly Section 3.64 onwards.
Those paragraphs contain guidance on excessively hot water and prevention of scalding.

It is still the case that if you wish to install DHW in an existing dwelling which can scald the inhabitants, then you are permitted to do so. 🤨 

This is the part I actually used, it has a mixer valve as part of the assembly 

https://www.bes.co.uk/thermostatic-solar-valve-kit-with-bypass-21082/

I feel much happier now that you've suggested that part from BES @johnmo

And I note the footnote on their page that the mixer-valve portion can be adjusted 30°C - 65°C

My own house has an overall mixer set at 45°C
and the showers can go lower still if you wish.

May I also point out that the water temperature in the manifold of a solar-thermal array can go far higher than 100°C if it's only being drawn off 'on demand' from a pressurised system.

I don't think there are mandatory safety regulations about this, but common sense suggests:

• use only pipes made of metal (copper or stainless steel)
• don't use push-fit connectors
• ensure that the pressure/temperature relief valve directs its outlet somewhere safe

Thank you @johnmo and @transparent .

I prefer the look of the Atmos version, and the price!. I see that this one from BES has a separate cold water feed to always limit the solar thermal water temperature, and the definitions of 'cold water' and 'hot water' (for the temperature operating ranges) confuse me since there are multiple different hot water and cold water lines. But I can phone the company to query this.

On a different subject, does a direct water heater merely 'kick in' when it senses a pressure change caused by water draw-off? Or is it more complicated.

We too can help with which pipes go where and the definition of 'cold water'.

That unit from BES is bound to be more expensive because it's based on a 1-inch connection.
That's the same for almost all plumbing fittings.
The devices to fit 15mm and 22mm pipes will be much cheaper than that for 1-inch or 28mm.

When using a thermostatic valve to mix hot & cold water, the main point is to ensure that the hot and cold inputs are approximately the same pressure.
The valve will struggle to output water at the set-point if the cold feed is running at twice the pressure of the hot!

Here's a mixer valve at the top of my thermal store:

Both the hot-input and the cold-input are derived from the same source, and are therefore at the same pressure.

The only difference is that the hot pipe has picked up heat from the body of water within the thermal store.