Water pre-heat with excess solar before combi?

a1234555

Hello, I know that with full systems (with an existing water tank) there is a way to heat the water with excess solar. However, we have a combi boiler and I was wondering whether there is such a thing of pre-heating the water before it goes into the combi (so that we use less gas to heat the water from cold to hot, using excess solar energy)? Does such a set-up exist and is there a name for it?

CSI_Yorkshire

Several ways that it could be done - but I don't know of any widespread, common or cost-efficient method.

CliveOfIndia

It probably could be done, but I suspect it would be expensive.  The whole point of a combi is that it takes in cold water straight from the mains and heats it on the fly.  I imagine you'd have to come up with a pre-heated storage tank of some sort, then feed that into the boiler.  But then you'd probably need some sort of a pump as well, since combi's require a decent mains pressure to work - a straightforward gravity feed from a storage tank probably wouldn't give sufficient pressure.

I'm sure it could be done with a bit of ingenuity, but whether you'd recoup the initial costs within a reasonable time-frame is debatable.

CSI_Yorkshire

CliveOfIndia said:

It probably could be done, but I suspect it would be expensive.  The whole point of a combi is that it takes in cold water straight from the mains and heats it on the fly.  I imagine you'd have to come up with a pre-heated storage tank of some sort, then feed that into the boiler.  

Or you'd have to have the solar system heating on the fly as well - like trace heating of the incoming mains on a whopping scale - but then you can only put in whatever is the excess from the solar at that time (which is likely to be a lot less than the rating of the combi).

grumbler

CliveOfIndia said:

 I imagine you'd have to come up with a pre-heated storage tank of some sort, then feed that into the boiler.  But then you'd probably need some sort of a pump as well, since combi's require a decent mains pressure to work - a straightforward gravity feed from a storage tank probably wouldn't give sufficient pressure.

I can imagine a mains pressure cylinder - sort of a system boiler in reverse, but yes, this defeats the main point of a combi - no cylinder.

ThisIsWeird

I wonder how a combi would cope with receiving already-warm/hot water?

But, as said above, it looks as tho' you'd need a mains-pressurised immersion-heated storage vessel, and that's PartG or summat (ie, needs specific qualifications).

I can't see any other approach - heated trace wire, etc - giving you a useful amount of heated water; it would be flushed through in seconds.

ComicGeek

You can get waste water heat recovery units (wwhr) that collect waste heat from showers and use it to preheat the cold feed to the shower. Some can even be connected back to central stores for more general use. 

That might give you the same result of reducing gas usage to heat hot water, as showers are likely to be the highest usage in households.

They can either be shower tray units, or vertical pipes adjacent to soil stacks, or units that fit below baths. Not that easy to retrofit into existing bathrooms though as it needs changes to water pipework and drainage to work.

ThisIsWeird

Ah, good idea CG.
A small heat store (the smallest indirect cylinder you can find), vented and PV immersion-heated, with the supply to the combi passing via its heat-exchange coil.
Ie, a conventional hot cylinder tank in reverse.

chris_n

Some combis have plastic parts on the inlet side and are unable to take warmed water coming in, some are all metal and are fine. The manual for the boiler normally states a range of incoming temperatures. Bear in mind with the idea above that you will get a slug of hot water at tank temperature then some slightly warmed water as the coil is not efficient enough to transfer sufficient heat quickly and the coil is likely in the bottom of the tank.

FFHillbilly

if you have access to 3-5metres of the cold water pipe leading up to the combi you could wrap a trace heating element around it, and use an energy diverter to divert any excess solar into heating the pipe.
that is right off the top of my head so I'm not sure if it would work well. the trace heating is such a small load it's hardly worth buying an energy diverter (eg myenergy eddi £375)
have you not got an export tariff? 

Section62

I think most of the above ideas would give rise to justifiable concern over the control of Legionella bacteria.

It would break the basic principle of plumbing of keeping cold water cold and hot water hot.  The pre-heat would add an unknown and variable amount of heat to water which is supposed to be cold, without any certainty that it will be heated to a temperature where Legionella would be killed.  The water could easily be heated and maintained for some length of time at a temperature where Legionella thrive.

Some take the view that Legionella poses no significant risk.  But the 'rules' of plumbing are designed to take a systematic approach to Legionella control and to ignore those would be unwise.