Mary Fisher wrote:

Ours doesn't. We don't need to drain it.

Why? http://www.solartwin.com/questions_a...hp#reliability

Mary



Mary,

I read that article yesterday, then thought about it.

Your system wouldn't suit me.

I can't be sure exactly how much hot water we use each day.

When I have a shower, I don't want the temperature to be down at ...
what was it? 31 centigrade? when I finish. So the temp in the top of
the tank needs to be 40 plus. Better 45.

Your system pulls in cold water from the bottom of the tank, warms it a
bit, then dumps it back in the top. Where it will mix with the *hotter*
water left over from the last boiler run. And ruin the stratification.

What *would* work would be a second cylinder to preheat the water before
delivering it to the hot one, which would be gas (or something) heated.
By raising the input temperature to the hot tank from... er... 8.5
degrees (I just went and measured it) to 31 I'm saving 23 degrees worth
of heating. I then need to dump in another 40 degrees or so to get it
to a legionella-safe 60. Lets make it easy and bring it up by only 35,
which means the solar is going to save me 40% of my gas bill.

Of the part that is for hot water only.

OK, in summer it'll be more effective - but in winter it'll be doing
nothing much, or freezing wouldn't be an issue.

My last summer quarter gas bill was £65 - no heating, so that tells me
the hot water cost. I'll save 40% of that, by 4 quarters, about £100
per year.

Remember that installed cost is going to have to include a 2nd cylinder
to even get that efficiency...

Now I can get 5% on investments with no problem. So if the installed
cost is over £2000 *including* the 2nd cylinder - forget it. Better
things to do with my money. Say buying a really fuel efficient car for
the commute.

Andy.