Would 70 tubes be too many???

[stevie_f_2004]

I currently have 30 tubes (58mm) on the roof and looking to upgrade to 70 nr this year. Concern that I have, will 70 just be too much. I have a 300litre tank and average temperatures top to bottom during sunny days last year was 40deg. The family are pretty hard on water usage therfore is this about right??

Rgds

Stephen

[Brandon]

I would argue that 70 is over the top, by rights 40 ought to be about right, if doing it yourself I would try 50 first.

We put 90 onto a 300l heat store that dumps to a small pool, and the store heats very quickly when the sun comes out.  Until the pool was plumbed we dumped to 2# 210l water butts, and they got to 58^o in the first day.

[stevie_f_2004]

What I am thinking is putting a TMV on the outlet of teh storage cylinder therefore reducing the volume of hotwater required. This would give the benefit of the overcapacity of the tubes raising the water temperature yet giving protection of the 65Deg temperatures and "smoothing" peaks and troughs of sunshine/cloud cover...

 

[Baz]

The problem is the tank will boil on sunny summer days.

[KLD]

The problem is the tank will boil on sunny summer days.

A heat dump is an obvious MUST HAVE on such an over-spec'ed system.
If access to the roof (?) is easy, I'd side with Brandon and go for 50 tubes first and see. Upgrade to more if you really need to.  50 tubes translate to 4L/per 47mm equivalent tube, which should be plenty under "normal" conditions (south facing installation etc.)
OTOH, if access requires a scaffold and / or hiring external people, it's likely that a stepwise approach becomes uneconomical, in which case I'd take 70tubes and make sure the heat dump can handle the extra capacity.

Klaus

[stevie_f_2004]

Access is fine it is the postage costs that are the problem £100+ each time .....What I might do is install the 2 extra manfolds i.e. 2x20 and install 10 in each and see how it performs and add inteh tubes if necessary but at least the facility is there to be added /subtracted......

[dhaslam]

I would say that that the number is about right.    There are probably about thirty days in the year that you will have  a big surplus  but  for the rest of the year the extra  capacity will be useful.     

[stevie_f_2004]

Well finally ordered another 36 tubes which will take the system to 66 in total. Looking to install these Mid October 2010. Hopefully this will Knock the gas bill to absolute minimum. So far I have had £450 back year ended Sept 2009 and £150 year ended Sept 2010 from the Utility bill.  (New Boiler fitted Nov 2008 and Solar April 2009 30 tubes). Delighted with the saving so far but with the extra tubes I am hoping for at least another £200-£250 saving throughout the  year (Should be noted that Tumble drier went last year and I never bothered replacing!!!)

Rgds

SF

[greentangerine]

I have 50 tubes on a 300l store - installed it in March and has provided a huge amount of hot water this year so far.  Gas usage has plummeted to almost nothing over the summer months.

I have the store limit set to 95C and it would go into stagnation often during June/July so I think it is sized about right.  It has hit high 80s this month on good days.

Just about to fit a wood burner with back boiler to top it up over the winter months.

[oliver90owner]

You are going to have 66 x 58mm tubes heating a 300 litre tank? 

I would suggest a bigger tank.  Your average temperatures, top to bottom, are pretty meaningless - the tank will always be cold at the bottom - so an average of 40 degrees might mean a 25 degree rise (average - incoming).  If that is so, you should expect about a 55 degree rise (theoretically).

You don't actually say but seems like a largish family?

What you could do is check the temperature rise when no water is drawn from the system.  If working correctly (easily checked for energy conversion) you could simply extrapolate/redraw for the extra tubes and be able to calculate volumes of hot water generated on a good day.

Things like angle and aspect would be good too.  If southerly, OK, if east/west, you may need to opt for a split collector.

RAB

[rogeriko]

to heat the whole tank top to bottom just put a circulator pump connecting top to bottom and a thermostat on the top so that when the top reaches 60 to 70 degrees the pump runs. this way on sunny days you get 300 litres hot water not just the top 50 litres

[brackwell]

You seem to be getting through a colossal amount of hot water. With your 300L tank you could do with more tubes but dont forget the heat dump.

Whats this top to bottom temp difference?  My solar circuit (160L tank 30x 47) is hot in at roughly 40% up from the base and cold out at the base of the tank. In the morning there is obviously a temp difference from top to bottom but as soon as the thermal starts the bottom heats up obviously lagging the middle tank. The middle and bottom climb in unison to the the point where the middle is hotter than the top and then this temp starts to climb. At the end of the day the top to bottom temp difference is just a few degrees!  Are other tanks configured differently?   There have been some excellent investigations in to this phenomenon on this forum its a pity they are not held in a kind of reference library.

Ken

[stevie_f_2004]

There are only 3 of us in the house....Wife takes at least two showers per day....3 year old takes a bath and I take one shower. All the water usage is down to us having a "power" shower. The pump can pass forward over 30litres of combined water per minute (twin arrangement). Before everyone starts having a go...I already know

[stevie_f_2004]

to heat the whole tank top to bottom just put a circulator pump connecting top to bottom and a thermostat on the top so that when the top reaches 60 to 70 degrees the pump runs. this way on sunny days you get 300 litres hot water not just the top 50 litres

The solar is connected to the lower coil in my tank and therefore heats the whole tank. The upper coil i.e. circa 150litres is heated by a gas condensing boiler.

[stevie_f_2004]

You are going to have 66 x 58mm tubes heating a 300 litre tank? 



RAB

Yes that is the intention 66x58mm Tubes.Thing is Rab I am located in Central Scotland and we don't get that much sun in these parts, hence the number of tubes combined with the high water usage.