Hello there,

I would be really grateful for some feedback on a couple of possible systems we have been offered:

i) 2x Thermomax DF100-20 evac. tubes (4sqm) plus OSO 380l unvented solar twin coil cylinder,
or
ii) 2x Thermomax DF100-30 evac. tubes (6sqm) plus Schueco 750l thermal store (vented).

The cost of installation is about £3k and £5k respectively. The system of choice would be installed at the same time as a new Worcester Bosch 30CDI. We are insulating roofspaces and walls where possible and are adding a second woodburner. The house is a Victorian cottage in Kent that has more than tripled in size before we bought a year ago.

What I find interesting(!) is the low estimated savings on the two schemes: 3200 kw/h/year and 5700kw/h/year, which equates to about £140 and £200 p.a. All credit to our installer, who prefers to be conservative in his estimate, but is this a realistic figure?

The collectors will be mounted onto a flat roof with the individual tubes angled at 25 degrees. Installer recommended FPs (lots) but we needed planning permission and the neighbours said they would object vigorously, so for a quiet life and a bit more certainty we chose evacs. (less than 200mm above the roof surface, so permitted development).

On a more general subject, is it thought that the cost of solar water heating equipment will rise? Also, do people rateThermomax?

It would be great to hear any opinions before we take the plunge.

Hey Jim,


Many thanks for all the info. Maybe the cheaper less complicated system makes more sense, or maybe none at all at present, and we wait until grants go up and costs of SHW equipment goes down. Any thoughts on that?

Draught proofing is high on the "to do" list. The new stove we are putting in (when we get it-by Christmas next year if we are lucky) will be a space heater only, because it is going into an enlarged kitchen that will then serve as the hub of the house. A back boiler might compromise its room heating ability.

I will now go and hit the links you kindly provided.

Picky

£5k for 6 square metres?

Is this a real world price? Apologies if asking a dumb question, I thought evacs were a lot, lot cheaper than that from some of the other conversations on this board.

Thanks Tony. So that's a lower bound of £500 a square metre fully installed?

Those prices look reasonable for that kit, but I'd consider 3K to be an upper bound for an installed 4m2 system. 6m2 plus a tank twice as big I'd expect to be more like 1000 more, as opposed to 2000, but maybe that schueco tank is a fancy one (I'm not familiar with the brand) and is pushing the price up.

Those figures for collected heat actually look a bit high to me, but in the right ballpark. My spreadsheet suggests around 2700kWh/yr for 4m2 ish of panels (40x47mm tubes, 30 degree roof, 153 degree azimuth). Detail design of piping, heat exchanger, stratification, insulation are all important in terms of getting an efficient system.

Our annual DHW use is only about 1500kWh though, so from our point of view that 2700kWh is 'plenty'. How much you need depends how much DHW you use.

Thermomax are indeed a long-standing UK manufacturer (South Wales, recently bought by Kingspan), so the tubes have good delivery-miles numbers. However whilst I have no personal experience, I do know they have had reliability issues due to their special 'cut-off' bimetallic heat-connector magic thingie. I know of several examples of 10-20yr old thermomax installations where all/most the tubes have stopped working. It may be that current production has fixed this issue, but in general it's safe to say the chinese tubes are almost certain to be more reliable as they have less to go wrong.

Soltrac are currently doing a comparitive study which includes both Thermomax and standard chinese (Navitron) panels:
http://www.soltrac.co.uk/research.htm results were 'imminient' this summer, but I don't see any on their site yet.

I wonder why you need planning permission for flat plates but not tubes - that seems bizarre.

Posted By: PickyThanks for that, Wookey. With regard to reliabilty issues, apparently Thermomax have a 25 year warranty, so if we use them, we'll have a (tenuous) safety net. What is the guarantee on Navitron tubes?

I think you're confusing PV panels with solar thermal - 25 years is standard for PV but I'd be very surprised if anyone would give you 25 years on a thermal panel.

Navitron guarantee is 2yr. That's one reason they are cheap (they don't have to take out insurance for the guarantee). Details are on their FAQ. I think most of the other suppliers take the same approach. I didn't even ask what guarantee I got from Eco-nomical (even cheaper panels, slightly better design IMHO) but they were happy to replace a couple of damaged parts for free, so I'm happy to recommend them as suppliers.

I guess it depends if you want to pay for the peace of mind of a guarantee or if you don't mind taking a risk of having to buy a few replacement tubes in 15 years time (currently GBP 15 each). In both designs the only bit that might realisitically fail is the heat pipe (lose vacuum or in Thermomax's case have the bimetallic bit stop sproinging). Take a look at the design. There really isn't much to go wrong, and at GBP 320 for a complete replacement it's not something I'm losing sleep over. And it's a very standard bit of kit - you can get a replacement from any number of suppliers.