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a écrit :
This thread is from uk.diy, and is kinda relevant for a.e.h too.

Ed Sirett wrote:

Putting the question the other way around, could you say what the total
installed price for a marginally justified system would be - with a 20
year payback say.

If you'd payed a lot (£2,000 say) for the system then I have no doubt
you'd not recover the cost. What might be the threshhold?

Good question. I'll have a go.

Lets say it saves £20 a year heating dhw only.

I'm curious to how you figure £20 a year in savings... (not having all
the information, the inital posts are no longer....)

If I look at dshw system here in France, I have;

min cover : 50% (lets be conservative) = 1000kWh electric avoided
average cover : 65% (for the most proactive region) = 1500kWh electric
avoided
electricity costs : 0.11€/kWh

so yearly savings = 110€ to 165€

(the rest of the equation here is, for flat plate hot water collectors
cost (installed, all taxes included) : 5500€
subsidies : tax credit 50% materiel costs (say 50% of 4000€)
local subsidies - bring total subsidy up to the 80% EC limit in many
regions)
current loan rates : approx 4% insurance included
current savings account interest rates : approx 4% (best case) approx
1.5% (average case))

Lets say we want it to break even after 10 years, interest is 5% and we
optimistically ignore the risk of system failure, underperformance or
repair for the moment.

A system that does this in 10 years is worth paying £157 for now.

From this emerges one clear conclusion: we need either a lot more
saving or a lot less cost than commercial solar dhw systems deliver.

If one got a bit desperate and justified a 20 year payback, system
value would rise to £251. But I dont think thats justifiable in truth,
nor is the extra spend allowance very significant. Either will make a
cheap system and nothing more.

Now, since almost all solar dhw setups fail miserably to pay their way,
lets see how low cost we can get. Start with a dirt cheap direct
drain-down system for summer use only. This preheats the header tank,
which of course must be hot water safe in this design. That rules out
use of plastic header tanks where already fitted. Metal headers are ok,
a plastic one would need replacement.

Collector and loft piping:
100' garden hosepipe: £20
sheet of green house polythene: £? Lets say we use a £1 poundland
plastic patio table cover set
4x8 sheet 7mm WBP ply £? rough guess £5
stainless roof fixings £? guess £5
black paint £1

CH pump £30

loft piping insulation: cardboard and/or rags £0, sellotape to hold in
place temporarily £0.50, string to fix it permanently £0.50.
header tank insulation: as above, no more spend needed
connecting 2 pipe ends to header tank: £2
1mm cable from lighting jbox to pump £1-2
FCU & pattress £2
panel thermostat, bimetal, £5

HW header tank if metal one not already present: £ not sure, and will
vary according to source

Now, if you had to buy all the above new, total cost is:
£73 + maybe a new header tank

The numbers look quite a bit better if you've got various bits lying
around already, or can use chuckouts to build it from. Then it might
actually be worth the 2 days work.

But given the low value overall of such systems, I dont think this is
the way to go. More sensible to go for a system that delivers much
more, such as flat panel solar space heating, which is cheaper, easier,
requires no roof access, and delivers more payback.

Solar flat panel space heating is little more than a frame, polythene,
black mesh cloth and holes in the wall. I'll let someone to play with
the numbers if they wish.

NT

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