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Pressurised Solar System - heat not reaching tank

Postby jrec149 » Fri May 09, 2008 7:12 pm

We have (and have had for 9 years) an evacuated tube solar water heating system and it has been temperamental from time to time. The pump has had to be replaced and a maintenance man persuaded to come to re-pressurise the system. Now, it seems, a different problem:
- the sun is shining
- the tank is not heating
- the pressure guage seems OK (red arrow on 3.5 and black on 2.1 - maybe these are high?)
- the pipe with the fluid in (can't remember what the fluid is) red hot to pump
- pump also red hot
- pipe from pump hot for about a metre (running horizontally from pump)
- where the pipe turns down towards the tank (or actuallyjust before) the pipe temperature drops away rapidly
- vertical pipe is cold water temperature

My guess is that is there is some kind of airlock in the system that can be bled off somehow.....

I have no idea if my guess is correct and if it is how to do it and what risks I run (loss of all pressure for example) and how to minimise these risks....

Help appreciated (all the other fuel costs are rising but the sun keeps on shining!)

Thanks all
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Postby sparx » Wed May 14, 2008 7:41 pm

Hi, seems like pump has failed again,
ours here, and most others I have seen, use a WILO brand pump which looks like a standard central heating circulator BUT they are specials due to higher temps involved with this type of system. Do you have model number & type to compare new pump with original?
We find that one problem with the evacuated tube type is they are too efficient on a hot/sunny day, so tank up to max temp. by midday, then when pump shuts off collector temp. rises to way over boiling point of fluid even allowing for high pressure so if then we use a lot of hot water from tank controls prevent pump running until evening temp. drop.
There is then not enough time to replenish tank heat before dark!
Our system pressure is only 1.0-1.5 bar, maybe not high enough?
there does seem to be little practical info about, BTW ours is from
'Smart Energy' and is approx 4 years old.
regards sparx
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Postby jrec149 » Thu May 15, 2008 4:25 pm

Thanks Sparx, think your analysis is correct - seems as thoughthe pump installed as replacement last time (on the hot flow side) is a central heating pump rated to 82 degrees Celsius and likely to (eventually) fail at higher temps. Some people seem to think that the pump should be ont the cold return side to avoid these kinds of issues (and the ones you refer to) having pump replaced and fluid replaced (seems as though there is no airlock or absence of fluid so can't (?) have evaporated according to the guy who came to prepare a quote). At least we found the PRV (on the roof)! Wish I knoew how to reduce the current pressure to see if that has any effect (and for future reference as to be honest I think I am guilty of pushing the pressure up by accident when I opened the black valve on what I now know to be the filling loop (which itself is incorrectly (?) permanently attached to the cold water main)).

Bit of an info dump there.....anyone able to add anything please do (including suggested replacement pump models and possible costs plus how long does it take to drain, refill and recharge such a system?)

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