Solar hot water issue

[XONE]

Right, Have a 20 tube pannel, And can't, even on a super sunny day, get more than luke warm water from the tap. What's up with that? The pannel is run into a twin coil tank which is also connected to a rayburn. Getting tank tempretures of around 350C and pannel temps of about 450C Can anyone help?

[dimengineer]

How big is your tank? If its <200litres you have a problem. If its >400 you may not have.

Are all your tubes intact?

[XONE]

I believe the tank is 172 litres.

All tubes are intact.

[wookey]

I think you've missed a decimal point or two there. 350C and 450C is rather too hot.

[dhaslam]

8.6 litres per tube isquite a lot but on a very sunny day you should get reasonable temperatures.  Is there any circulation to the Rayburn or any other heat loss from the system?   

[cirrusvfr]

Just to get some more basic details...

What direction does the panel face ?  What angle is it mounted at ?  are there any shading issues ?

[emacinty]

What way is your pump pumping?

If it's pumping down the way, you may have air in the pump which means it won't actually move the water.

If you feel the pipes entering and leaving the cylinder coil, is one of them just as hot as the pipe leaving the pump? Does the temperature near the pump seem to you to match what the sensors are telling you about the manifold? 40C is touchable. 50C is getting uncomfortable. 60C and above are only for those with asbestos hands.

I have an automatic vent on the last elbow before my return heads down for the pump, though there's a valve since I don't trust it not to breathe in air. I open the valve every couple of weeks to make sure it's clear, after making sure the pressure is 1.3 bar or more.

Until I put in the vent, I got very hot manifold temperatures, and hours of pumping - but for nothing since the water wasn't moving. Yes, I know the pump should ideally not pump downwards, but that was an installation constraint.

Good luck troubleshooting!

cheers

Ewan

[XONE]

The panel faces directly south, not sure of it's angle, the roof has a reasonable steep pitch. There is no air in the system. And the temps are right 35oC and 45oC. The Rayburn has a pump for the radiators and relys on thermo siphoning to heat the tank.

[Drawmer]

If you turn off the solar circulating pump when there's full sun on the panel, can you watch the panel collector temp climb steadily?

If not, there's a problem with the panel or sensor or something in that area.

If the temperature at the panel climbs nicely with the pump off, then something is happening to your heat after it's been collected at the panel.

Is the hot water continuously being drawn off? Have you tried leaving the hot tap off all day to see if the tank warms up?

Is it something to do with the Rayburn circuit; is that taking the heat away as quick as you put it in from the solar? Maybe there's a way that the Rayburn radiator circuit could be thermo siphoning the heat out of the tank? Are there any radiators higher than the tank?

[mick]

I also have 20 x 47mm tubes on 172 lt cylinder (Note - it is important to state which tubes 20 x 58mm = 30 x 47mm in power output).

20 (x 47mm) tubes is insufficient - this was my first installation and I followed guidelines (and wanted to save money) but quickly found out that, except in ideal conditions, our water use outstripped its output. Luckily I have gas water heating as well so most of the time the solar is pre-heating for final top-up by the gas. Factored in is also must be water use - we have heavy use presently but even when (if) we revert to planned use I now know 8.5 lt per tube is insufficient. Think about it - cylinder of cold or cool water - solar input heats from bottom up so the now oft observed mixing takes place leaving a cylinder nearer an average temperature rather than hot at the top, cool at the bottom. You draw off HW and the bottom of the cylinder is filled with cold - solar loop drops accordingly to output modest temperature only. If you could use no hot water for a couple of days, and assuming heat losses are under control, you would see much higher temps but the same as above will occur once you start use again. On the plus side your panel will be working at maximum efficiency even though output insufficient for volume.

The solution, beyond either Rayburn or immersion on almost everyday, is either 10 or twenty more tubes (for our professional installs we work as close to 5 lt per tube as practicable) or.....

add in an extra heat exchanger to supply the top of the cylinder first.  You could fit a retro coil if you have a top mounted immersion heater and re-pipe so it goes there first (or is diverted there then switches to proper solar coil when top is hot). Paulh Boats is a fan of top mounted retrocoils see - http://www.navitron.org.uk/forum/index.php/topic,6054.msg61550.html#msg61550 + rest of that thread especially Langstroth and chickensoup's contributions  + search retro coil, etc..

Or you could add in an external HE like Wookey (search Wookey side-arm PHE) or a Willis (search Willis).

For all other readers still planning - work on 5 to 6 lts per 47mm tube.

HTH

Mick.

[Drawmer]

Just as an aside to follow on from Mick's post..

My set up spec is at the bottom of each post.

There are only two of us in the house, we are low water users.

Last Sunday here was really sunny all day. Very little water draw off during the day and we had 210L at 72C top and bottom at the end of the day.
Today, on the other hand, the pump has run for a total of only 3 hours, the top of the tank was 52C at the end of the day, so the boiler will have to top up the heat for my shower in the morning. Since by the time we have washed up after supper, and the inevitable loss overnight, it will have dropped further.

The relationship between the panel output and the size of storage will determine your average temperature achievable, and the amount of draw-off during the day will overcome an undersized system. Ours works for us (just) because we use little water during the day. If my wife washed up after every cup of tea and each meal, we'd never get any hot water! We have trained ourselves to be more frugal with the hot tap.

[dhaslam]

In rough figures you are heating  172litres by 30C  which is 6Kwh ( 172X 30 / 860).  OK the 30 degrees bit is a guess, allowing for some water drawn off.  Normally the tank will approach the panel temperature but won't if water is constantly drawn off, even in small amounts.     My best output from 60 tubes this month was 21 Kwh so  6Kwh wouldn't be too far wrong for 20 tubes on a good day.       

[Drawmer]

XONE, today would be a good day to test the rate of rise in the collector with the pump off. Just don't go away and leave it with the pump turned off though!

If you can persuade the household; try placing an embargo on the hot tap all day end see what sort of temps you get top and bottom at the end of the day.

Is there any chance of the radiator circuit thermo siphoning heat out of the cylinder?

Best of luck tracking your lack of heat gain.