Will it 'work'? ... probably.

Will it be effective/efficient?  ... probably not.

This comes down to the fact that your gas boiler will inevitably be a condensing type. Unless it was, then it couldn't attain the SEDBUK rating to be approved for installation in the UK.

Let's ignore the 'combi' bit because that simply means it supplies domestic hot water (DHW) on demand, without using a hot water cylinder.

The central heating (CH) system is what accounts for most of your gas bill.
Unlike DHW, the radiator circuit is a loop. It returns water back to the boiler to be re-heated.

When the thermostat first tells the boiler to fire, the return water will be somewhere around 30degC.
The exhaust gas going out of the flue has so much of the heat extracted from it that the water-vapour condenses and drips out of the condensate pipe.
In 'condensation mode' the boiler achieves at least 90% energy efficiency.

After a few minutes the radiators heat up and the return water temperature rises.
There's now less temperature differential across the heat-exchanger within the boiler.
Flue-gasses no longer produce condensate, so the boiler efficiency plummets to around 70%.

If you use a solar-thermal input to pre-heat the water before it returns to the boiler, you are simply shortening the interval before it becomes less efficient!

That's the opposite of what you want. 🤔 

The idea should be to prolong the time during which the boiler is running in condensation mode.

You achieve that by greatly increasing the volume of water which it's required to heat.
That's why my gas boiler delivers its water to a 300-litre thermal store.
The return water comes from the 'cool zone' at the bottom of the cylinder.

For similar reasons, I use underfloor heating (UFH) pipes rather than radiators.
They can heat the house using water temperatures at 43degC and below.
Compare that with a typical radiator at 60degC and more!

My gas boiler never drops out of condensing mode.

If there's sufficient solar-thermal input to the same thermal store, then my boiler won't actually fire at all. 🙂 

Posted by: @murali442002

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want to install a solar thermal at the cold water inlet to the Combi Boiler. I had a chat with the customer support of British Gas and Worchester and was informed that the Combi boiler can take a hot water inlet.

There is no issue doing that, I have a similar setup. To get best efficiency you need to divert water past the combi when it's hot enough to go direct to the tap. You can get solar water diverters and mixers. The idea is, when the water coming out of the cylinder is above 43 deg the water is directed to the tap via a mixing valve. If it is hotter than say 55 degs it is mixed with cold water. When the output is below 43 the water is directed to the combi. If you don't have the mixer and diverter, the boiler is stop starting every time someone opens the tap. And you save very little.

For the sake of clarity @johnmo can you confirm that the system you're describing is to deliver Domestic Hot Water (DHW)?

My preceding answer didn't address that scenario because @murali442002 had written:
How will the central heating system behave...?

Both of us have referred to the need for a cylinder and water temperatures in the mid-40degC.

It is unfortunately the case that houses with combi-boilers usually don't have appropriate space for installation of a storage cylinder.

The larger housing developers are members of the NHBC. They use that mechanism to pre-register new housing estates for Building Regulations purposes, often before they've even bought the land.

It is cheap/easy for them to install a gas combi-boiler as the sole source of DHW and C/H.
Thus they will continue to build houses with no energy-storage for at least a decade, even if the law were to change this year. 😥 

It is for DHW. I am using a 160l thermal store with a DHW coil at the moment it is heated with excess PV, and also used as a buffer for CH, so is always warm to hot, summer and winter.   Will be installing ST soon, as I have all the bits in the garage ready to go.

Biggest plus is the uplift in combi DHW performance as there is a low delta T required to raise the water to a usable temperature. I can run 2 showers comfortably and 3 showers if needed from my Atag combi.

Alpha boilers do a kit for their combi boilers, you can use as small as 50L and it still works. Various kits Flowsmart, Solarsmart etc

Posted by: @johnmo

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It is for DHW. I am using a 160l thermal store with a DHW coil at the moment it is heated with excess PV, and also used as a buffer for CH, so is always warm to hot, summer and winter.   Will be installing ST soon, as I have all the bits in the garage ready to go.

 

Biggest plus is the uplift in combi DHW performance as there is a low delta T required to raise the water to a usable temperature. I can run 2 showers comfortably and 3 showers if needed from my Atag combi.

 

Alpha boilers do a kit for their combi boilers, you can use as small as 50L and it still works. Various kits Flowsmart, Solarsmart etc

I am very pleased to see that more people are capturing, and better utilising, any available solar energy. Well done. 😀 

@derek-m @transparent your thoughts please

I currently have solar thermal which heats a large (oversized for needs) pressurised tank, in addition to a standard WB boiler

I think the whole set up is about 15 yrs old, we are planning some renovations so the boiler will be replaced

I prefer the idea of solar PV (mainly for use with a electric car) and potentially batteries as well

I don't really want PV and Thermal on the roof and i think it's probably time to dispose/sell due to age - well at least whilst we are doing the renovations it would be an opportunity to

So, outwith old boiler, tank, and solar PV system

Questions are .. what to replace with. PV (yes), combi (maybe), tank with diverter (maybe)

I was not planning on a replacement tank but we have the space - existing tank is in integral garage and previously was in an airing cupboard - so could take up residence there again (tank i assume was moved when the solar PV was fitted)

I like the idea though of using the PV via an immersion to heat the tank (should have plenty of excess for this) as we have a large roof facing due south

Do you think it is worth utilising a Combi, with a tank and a Diverter Valve, (higher cost) rather than just sticking with a traditional boiler / tank set up

What is a WB boiler?

It may be useful if I point out a few things that you may wish to consider. Solar PV is approximately 20% efficient, whilst solar thermal can be about 80% efficient, so for the same energy output, solar PV would require 4 times the roof space. How much roof space do you have? Obviously solar thermal can only produce thermal energy, whilst solar PV is more versatile.

I presume that your pressurised tank is a hot water cylinder.

Have you considered a heat pump?

@derek-m thanks for the reply. Apologies, WB = Worcester Bosch. Yes thermal is more efficient, but PV more versatile (for my needs) as you point out. Yes the tank is hot water cylinder. Have not really considered a heat pump, i need to read up on them some more. I am mindful of them being in-vogue and i don't know of their longevity (or cost, at the moment). Tks again.

Posted by: @z8lccda

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I prefer the idea of solar PV (mainly for use with a electric car) and potentially batteries as well

Understood. Electricity is often regarded as the "highest form" of energy from renewable sources. It retains the potential to be converted into so many other types of energy.

However, let me just lay out the background physics so that others who read this in future can better understand the available options.

At the time of writing (spring'23) almost all PV panels available in the domestic market are based on silicon. When a photon (particle of light) collides with a silicon atom, it releases an electron from the outer-shell. That flow of electrons creates the electric circuit.

The physics (chemistry?) of silicon is such that the theoretical maximum efficiency is 23%.

The following diagram shows steps in the conversion of solar radiation to electricity, based on the latitude of London/Bristol

Posted by: @z8lccda

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I don't really want PV and Thermal on the roof and i think it's probably time to dispose/sell due to age

I have both solar PV and solar-thermal panels on my roof.

Whilst PV panels' output decreases over time, the reduction of output from solar-thermal is minimal.

The efficiency of vacuum-tube solar thermal panels is around 80%.
Much depends on how the heat is extracted, variable-speed pump, and the temperature differentials you set up on the controller.
If you're not getting 80% with the collector in full sunlight, then there's probably something you can do about it!

Having said that, the output of solar-thermal is heat - generally regarded as the "lowest form" of energy.
You don't convert it into any other energy.

If a proportion of the energy derived from your roof is going to be used to heat water in a tank/cylinder, then you're clearly better off retaining a solar-thermal system.

Using electricity from PV panels to heat water is inefficient.
It should only be considered as a last-ditch strategy for energy which would otherwise be discarded.

@z8lccda if you're already favouring PV for your roof space because of wanting to go electric car and batteries (a good plan - having PV optimises both of those) , then I would also strongly suggest you look at heat pump for your heating. that way the whole thing can be electrically driven. with batteries you can buy in cheap overnight electricity in the depths of winter, and with  PV you can  (partially) power the heatpump  at least in shoulder seasons - how much of course depends on your house heat load and how much PV - and your cost to run will come down a lot. Seems like you do have space for a DHW cylinder which is another pretty much mandatory thing for moving to a heat pump from a gas combi.

I do totally agree with @transparent 's point about a good solar thermal being much more efficient than PV surplus for the specific job of heating water. However I found myself in a similar place to you 5 years ago - I had an aging solar thermal system (which was also not a good one, with hindsight) . I chose to rip it out so that I could give all the roof space to PV.  Then since then done batteries, EV, electric kitchen and just recently heat pump. The "highest form of energy" factor for electricity means that the PV can go into battery for later, into a car, for cooking, for space heating (via a heat pump so that its multiplied by the heat pump's efficiency factor or "COP") or DHW heating (again via the heat pump) and finally via the immersion into the DHW cylinder to make it hotter than the heat pump is capable of and thus store those last few spare kWh. All flexibly depending on what you need at the time.

have you worked out how many kWP of panels you can fit? 

Posted by: @z8lccda

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@derek-m thanks for the reply. Apologies, WB = Worcester Bosch. Yes thermal is more efficient, but PV more versatile (for my needs) as you point out. Yes the tank is hot water cylinder. Have not really considered a heat pump, i need to read up on them some more. I am mindful of them being in-vogue and i don't know of their longevity (or cost, at the moment). Tks again.

It is your decision at the end of the day, but I would suggest keeping the solar thermal for the time being and use it for hot water production and possibly to assist with central heating when possible.

That would allow all the solar PV production to be used for EV charging, home use, and heat pump operation.