Solar Options: Combi + Thermal Store or Twin Coil Cylinder?

[smokeyjon]

Hi There
As you can see I'm a newbie, and am also not a plumber (oops). I hope you don't mind but I thought this might be a good place to get some unbiased advice on the installation of a new central heating/water heating system incorporating solar thermal panels. I'd be grateful for any comments which'd help me out. This is our situation:
- 3 bed mid-terrace house, probably loft conversion in next 5 years to give 1 more large bed (or 2 smaller) + 2nd bathroom.
- currently 2 people (the family thing's bound to happen sooner or later!)
- Current system comprises of 15yr old central heating boiler and separate multipoint boiler giving instant hot water (both nearing end of life), and electric shower (which also needs rewiring as it's overrated for its wiring!)

We're a bit ECO, so want to install solar, despite its significant payback period. My options seem to be a system boiler and unvented twin coil cylinder, or a solar compatible combi + thermal store to preheat DHW (both in loft and + solar panels). I'm leaning towards the combi option as we might not want to add the panels for a couple of years (ideally when converting the loft) and the preheated water option sounds more sensible for two of us as our water use is currently fairly low and irregular), but the solar installers say that the twincoil system is more efficient (but are unable to explain why!). Unless someone can convince me otherwise I want to go the combi route, but I am finding that there are only a couple of suppliers that can confirm that their boilers happily accept preheated water (eg ALFA, ATMOS, apparently also Viessman). All plumbers we've dealt with recommend Worcester Bosch or VAILLANT for reliability and aftersales reasons, but neither of these manufacturers seem to be able to confirm that their combis accept pre-heat.

Does anyone know anything about ALFA or ATMOS boilers or know of any others that will happily take preheated water (up to 60 deg), ideally without invalidating the guarantee?

Thanks in advance for any info. Sorry that was such a wordy post!!
John

[CharterGAS]

there is argument for both side of thermal store vs twin or 3 coil cylinders
i personally believe a thermal store makes more sense but it has been brought to my attention that here may be an issue with over heat with the panels.

Depending what the 2 coils are used for ie solar gain and boiler boosting hot water there is little use currently at having any gain on the heating side.

The units which can accept pre heated water are
ATAG
Viessmann (confirm with manufacture)
Alpha (but they use a valve)
if atmos do it be very careful of these i do not like their design!!! but that s my opinion
worcester-bosh = rubbish
Vaillant = ok


Use weather compensation to go on your heating side and this wil make quite a difference to gas usage and comfort!!!!


check out
ATAG - q solar - Atag verwarming | Comfort with economy
or Viessmann- 343F -www.viessmann.co.uk (this unit id for new builds)

[smokeyjon]

Thanks for your reply Chartergas. I'll have a look at the ATAG system. I've never heard of ATAG though, so it would be another step into the unknown. To be honest I hadn't heard of Alpha or Atmos before this though (and have now heard 2 negative comments about Alpha, and now 1 about Atmos!!). What is it about their design that you don't like - the boiler or the solar system? (system diagram here http://www.atmos.uk.com/core_files/productDoc(114).pdf)

There seem to be some significant differences between the Alpha and Atmos systems. These include the fact that the Alpha supplies hot water directly from the preheated cylinder, whereas the atmos uses a heat exchanger in the cylinder. I have read elsewhere in relation to legionella that the latter is probably preferable.

it has been brought to my attention that here may be an issue with over heat with the panels.

- Could this explain the strange drainback heat exchanger on the Alpha?

(system diagram at this link: http://www.alpha-innovation.co.uk/products/images/solarsmart/additional/alpha_solarsmart_system_diagram.gif

Interesting what you say about Worceser Bosch (as they seem so popular), but their staff say that they're not happy to accept pre-heated water anyway. I've also been told the same about Vaillant, and haven't managed to find out anything about Viessman yet. I'll let you know if I find anything out!

I hear what you say about limited gains with the central heating (I've heard it's better with underfloor heating), but wondered if running heating through the coil could have some benefit at some times, and could have the opposite effect of heating the store at night so that it's nice and warm for my morning shower?

Oh and, excuse my ignorance, but what is weather compensation and how does it work?

Sorry for all the questions!
Cheers again, John