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hi guys hope you are well.

ive got a 4kw solar system that is doing fantastically well (2300 kw produced since february). ive cut my electricity bill by 75%!! im looking to see what other efficiencies i can get and thought i could get my gas bill reduced as well.

i have a combi boiler (potterton performa 24 eco he)

is it possible to get a system so that a hot water tank is constantly kept hot by the solar system during the day which would also work with the combi boiler we have - the idea being that the free solar gets us a tank full of free hot water all the time, and if needs be the heat to the shower / taps can be topped up by the combi if needed on demand. i have plenty of space in the loft for a storage tank and believe there is plumbing going up there already because thats where the old immersion heater was.

i hope this makes sense!

if its possible - what do i need and any idea of costs please??

thanks in advance.
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It's possible with a direct unvented cylinder in you have at least 1 bar incoming water main pressure. You would need an heatbank or new mains if it's less than one bar. You would also need something like the Intasol combi solar diverter and possibly something like the immersun switch.

It think it would be impossible to price without seeing the job.
  • Thread Starter Thread Starter
  • #3
thanks for this.

the pressure here is really good (our shower which is straight off the combi is fab) but i dont know what it actually is.

can you give a ballpark for the cost of parts assuming pressure is above a bar?

just want to know if its worth considering.
  • Thread Starter Thread Starter
  • #5
ok, thanks for that. - does anybody have experience with the kind of savings its possible to make doing this please? another very vague question i know!


There is a Worcestor boiler that can take warm water and it just tops up the temperature saving on gas

Gavin A

can you point me in the direction of those please?
I believe it's actually HSE guidance specific to the use of combi boilers with solar water heating, as the water when flowing through the combi isn't heated for long enough to kill off any legionella etc that might have been breeding in the luke warm solar tank.

Though it may also be in the water regs for the same reason I suppose.

not sure of where the document is to be found, but I have read it previously and such guidance definitely exists and does make sense as well from an H&S point of view IMO.

Gavin A

Any hot water storage vessel displaying thermal stratification and maintaining temperatures in the range of 20 to 45
C is very likely to contain legionella bacteria in
significant numbers, unless regular pasteurisation of the entire contents of the vessel
takes place at temperatures of 60OC or above. Solar pre-heat cylinders that are subject
to thermal stratification would be equally susceptible to legionella contamination.
Solar heated vessels that do not reach 60OC and notably those maintaining
temperatures below 45OC would be particularly prone to contamination with high
levels of legionella bacteria.

Where these devices supply water via a combi-boiler in
order to raise the hot water temperature to 60OC, then according to the work of Stout
et al., the boiler would need to maintain the water temperature at 60OC for 3.2 mins to
reduce the concentration of legionella bacteria by 1 log (90%).
In systems where combi-boilers receive water from solar pre-heat cylinders that are not able to raise the water temperature to 60OC or above, then consideration
should be given to programming the system to automatically activate the combi-boiler
to heat the water to 60OC and recirculate this through the pre-heat cylinder, at a preset time daily.
10. This should provide effective pasteurisation of legionella bacteria colonising the
pre-heat cylinder. Where this pasteurisation programme is in place, the volume of
potentially contaminated water that can pass through the heating system before a
temperature of 60OC is reached (approx 20 litres), should then present little risk of
legionella infection. If this pasteurisation programme occurred daily, shorter contact
times of less than one hour may still prove effective in controlling legionella bacteria
in solar pre-heat cylinders, particularly in smaller systems. Alternatively, as legionella
bacteria grow relatively slowly, even when at optimum growth temperatures, then less
frequent cycles of pasteurisation (e.g. twice weekly) where the entire contents of the
vessel are maintained at 60OC or above for at least one hour, may also achieve
satisfactory control of residual legionella bacteria.
I think this is the report for the WRAS that really raised the issue. I'm not entirely sure where it now appears in the guidance though.


The Best way to do it is a thermal store, basically a bog tank of water purely used as a 'heat bank' (think of it as batteries for heat :) ) you can have mutlipe inputs to them, immersion, solar, wood burner, trad boilers etc, and then draw off hot water, central heating via built in coils (heat exchangers) that way you maximise your renewables when they are available and top up only with non-renewables as needed.

Gledhill do the torrent multi fuel store : Torrent MultiFuelStore Solar - Hot water cylinders | Indirect hot water cylinders | Gledhill cylinders | Pressurised hot water cylinders | Domestic hot water cylinders
Though these guys do the same for a lot less :) RM Cylinders - Envirocyl - Heat Pump Cylinders (and will sell directly :) )

Gavin A

so from that, it probably is actually possible, as long as there's some means for ensuring the entire tank get's heated fully at regular intervals eg once or twice a week. I'm actually working up a system for a customer at the moment to work with a combi, so will report back once I've sussed it, as it's a couple of years since I last looked into it.

eta - but worcester's right, a thermal store is likely to be the best way to go, providing you've got the room for it, which my current customer hasn't.


Thermal stores get over the legionella issue - hence why they are popular. - You don't bathe in that water :) it's just a store.

Give RM a chance / opprtunity if you need their type of prodcut - they are really helpful guys on the phone as weelas the good prices - (and we're not connencted to them ! )




Thanks for digging that out Gavin. I understand the point. Not bumped into it to date as all I have been installing with solar are thermal stores.

Gavin A

re willis.

It's just a more expensive, but more prefessional looking version of an external heat exchanger. We've got an external heat exchanger fitted to our second tank, and fitted a couple for SWH customers who didn't want to replace the tank.

They work well, dropping the hot water directly into the top of the tank via thermosyphoning, so reduce the issue of ending up with tanks full of luke warm water instead of half a tank of hot water, but you do need a method of regulating the speed of the water flow through the heat exchanger with temperature if you're going to get useful hot water at over 50 deg the entire time - we've used a specific Thermostatic valve for this purpose that seems to work well, though it's not actually designed for this purpose. This also prevents cold water flowing through it when you're drawing off hot water at the tap.

I think the willis unit might just use the long thin pipes to regulate the flow, but I'm not entirely sure.

One issue though is that the thermosyphoning only really works from the top of the heat exchenger upwards, so once the hot water gets level with this (from the top down), the flow slows a lot, so effectively the bottom 20 cm or so of the tank aren't really used most of the time. It's not so much of a problem though IMO as it operates as a fast recharge / semi combi type system so will rapidly heat the top of the tank to temperature to be usable again instead of having to wait 2-3 hours for usable temperatures with a dual coil set up if you've emptied the tank of hot entirely.
Bruce I'll have a look later today if i can and find you a link or you could try either the ihpe, wras or cibsie website yourself. It was around 2008-9 that we were stopped from feeding water above 23c to combi's prior to that it was a fairly simple job.

You don't need to use either a heatbank or thermal store (provided you have at least 1bar mains pressure) if you're intending to heat the cylinder via an immersion heater as both are far more expensive than a direct unvented cylinder combined with the intasol valve. I have my own heatbank cylinders made in the rm factory and add the other components myself when combing other heat sources.


I would keep it simple. Standard indirect vented/unvented cylinder, coil taken off combi heating via an s plan arrangement. You could run the hot water from the combi to a kitchen tap/utility or similar so that these outlets have instant hot water and this will help reduce de-stratification in the cylinder through the day. Standard immersion can be run off the P.V with combi to boost when P.V doesn't suffice.

You get the best of both worlds, instant hot water in the kitchen at any time from combi and a tank full of hot water for baths/showers in the evening once the P.V has had enough time to heat the capacity of water. The only non standard piece of equipment would be the immersion switch (immersun or similar)

The biggest question is, would the initial outlay be worth the saving of gas consumed by the combi?
See this link Bruce New Page 2 It was 2009 around the time Ferroli lost wras approval for their system which was very similar to the alpha solarsmart which still has approval.

Having thought about it. Someone with a combi boiler wanting to heat water from their pv system should imo use my suggestion first, next would be sambotc's and only use worcester's if there was a possibility of adding another heat source at the same time or in the future. I could of course be seen to be biased but it is the best solution regardless of that.


Hi Solarsavings,

what the reason for specifying Worcester (boiler?)out of interest?


Haha no worries, I mis read your post.

If the potterton performa 24 eco he can take preheated water I agree. Not sure if it can though hence my suggested method.

Have you used the intaeco valve much solarsavings? If so how do you find them and who supplies them to you? (if you could PM a rough idea on price that would be great)


Having had a quick look at the performa HE it appears to be a band B so no full modulation. I would hazard a guess that it isn't suitable for pre heated water as it would work off a set temperature rise :(
I have used them quite a bit (mostly before pv) trade price is a little less than £170 but you can buy Grants combisol valve ? for around £125 and they will do the same job. You could of course make your own using two tmv's as we used to do.

EDIT: for clarity. my use of 1bar is working pressure or if you prefer, open pipe.
Last edited:


How do these blending valves overcome the issues with the pre heat cylinder pasteurisation? If the water is only heated to 25 degrees in the tank through the winter months and then boosted to 60 through the combi in seconds, surely that doesn't meet the required amount of time to safely eliminate legionella?

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