Well to get greater certainty you need to predict your demand with more accuracy.

That said, do you really want a close to 100% efficient system. Bear in mind the difference in size betwen meeting say 50% of your annual demand and 90% of the demand will be huge - tank size will rise exponentially with increasing efficiency (with consequent stagnation risks).

If you really want to do all this at a high efficiency I would consider a series of linked tanks so you can more accurately detrmine your usage pattern from actual data and then use one or more tanks to manage water quality and quantity.

Personally I would seperate your domestic house demand from the garden demand and address them seperately

Regards

Barney

No streams unfortunately Ted!

The anticipated roof collection area is about 110 sq metres in artificial slate.

The suggested rate of use that seems most commonly quoted is about 5 litres per flush for each toilet and 50 litres per washing machine cycle. There will be two of us living in the house at a minimum with three more members of the family visiting for at least three days per week on average throughout the year. I anticipate at least three or four washing machine cycles per week as young children are involved. I use an automatic watering system in the garden in our current house and from April to October this is on for four daily cycles of approximately 15 minutes each ie. an hour a day in total including the greenhouse. I’ve measured the flow rate from the system using a rigged up water meter and it uses 150 litres an hour. There are also car washes to take into account.

The average monthly rainfall over the last ten years in millimetres is as follows at Aberporth (our nearest weather station):

January 81.22
February 64.07
March 51.6
April 57.94
May 67.65
June 46.56
July 64.35
August 72.88
September 66.14
October 127.24
November 118.05
December 95.74

Recent experience indicates that whilst the annual total is pretty much on trend the distribution is probably even more skewed through the year with longer periods of very low rainfall becoming more common in the spring to mid summer and the autumn periods. This year is a case in point with the lowest rainfall for many years being experienced over the first six months of the year. At other times rainfall can be torrential. It may be that these latest observations are colouring my view and pessimism regarding the tank sizes quoted.

Posted By: Rwatking On the other hand, I'm also conscious that having too large a tank may result in stagnant water with consequent issues in that regard.

We have about 16000 litres of water storage underground, and it seems to remain sweet indefinitely. We use it for everything except drinking and cooking. 6000 litres does sound a bit small to my mind, but then it rains more in west Wales.

Wash the car when it's raining?

Thanks again for the comments.

I'm beginning to firm up around the idea of a larger tank than the 6000 litre one but I'm still not yet sure of just how big.

I have read somewhere during my research (can't remember where!) that some sort of regulation (building control?) limits the size of the tank to 5% of the total water supply. Is this true? If so it seems to kick a larger tank well into the "long grass".

The size of tank you need depends on the distribution of rainfall rather than how much you use. If you could be sure it was going to rain every day you would only need a very small tank. If it only rained once a year you would need a huge tank. Working out anything in between is tricky. Suppose you wanted to survive an 6 week drought. You would need to store that much worth of consumption at least. Even then the tank would need to be full at the start which it might not be. UK average consumption is around 150L a day or 4500L a month. 6 weeks worth would be 6300L.

I worked out your roof could collect on average around 7500L a month but it could be nearer half that in summer...

110sqm at 45 degrees = 78sqm
For june..
78sqm x 0.046m = 3.6 cubic meters or 3600L

If you want to build an inter seasonal tank it would need to be much bigger. There is probably a way to calculate the size but it's not trivial. Perhaps by calculating the shortfall of each month in summer then working out the running total shortfall. That extra capacity would need to be collected in winter and stored eg That's how much water you need to start the summer with. Something like that anyway.

Thanks very much for all that very useful advice.

Taking CWatters approach I think I'm looking at a 15000 litre tank.

Any recommendations for the type of tank? Can anyone recommend any particular rainwater harvesting system with this size of tank available?

Thanks Mike7 but we're less than two miles from Aberporth as the crow flies so I doubt we can get very much more local figures. You are correct, however, to suggest that local micro-climates can introduce variability. Whereas we are only a relatively short distance away, they can be shrouded in a sea mist at the coast when we're basking in sunshine. Some, but less, variability in precipitation I should imagine but, generally, the figures from the weather station at Aberporth are likely to be as good as we can get.

Regards

Roger

You might want to get in touch with someone who has installed a tank ...
We visited a B&B last year in Yeovil
http://www.greenarches.co.uk/
He's likely to be happy to discuss how he's got on with it.

Crazy if you plan to water your veg with it as well.

I suppose if anyone complained your tank was too big you could easily put something in it to reduce the capacity.

This is a British Standard which has no standing in law unless a Regulation states that it does. Is there such a Regulation?

<blockquote><cite>Posted By: ted</cite>
The standard does allow a 'detailed' calculation method that by-passes the 5% rule.</blockquote>

ted, what does that mean?

also if you had an underground tank that was "too big" how is anyone going to prove it?

Naughty Joe

Part G mentions the rainwater standard in passing but does not mandate it. The WRAS byelaws don't apply as it isn't a 'public water supply'.

Joe, the standard lays out three methods of calculating the storage tank size - simplified, intermediate and detailed. Only the intermediate method uses the 5% rule. If you get the data to use the detailed method then the calculated size of tank may be larger, or smaller, than this.

Using the 'detailed' calculation method would be appropriate in circumstances when there is an irregular demand for the water (e.g. garden watering in summer) or where there may be other extenuating circumstances such as a higher than average use of toilets.

For more info see p27 onwards in this presentation: http://www.waterwise.org.uk/images/site/Events/social_housing/john%20griggs%20-%20water%20recycling%20small.pdf

The intermediate method is described on p31/32 and the detailed method on p33.

If its down to usage and more is used in the summer than the winter, as most rainwater tanks would have an overflow why not have several across the hight of the tank with stopcocks so that the amount stored could be matched to the usage at the particular time of year.
By the way what is the recommended storage time for rain water and how do you prevent stagnation in dead corners which must occur even with a cross flow installation?
Peter