Posted By: Justinit seems that every year after very hot extreme cloudless days and lengthy stagnation, I get an air lock in the pump

Perhaps you get some boiling (or close enough to boiling) to cause a bit of steam /bubbles to form which won't easily get reabsorbed which then get trapped in the pump creating the vapour lock.

Can you swap out the pump for a type that doesn’t trap vapour and so solving the PITA problem?

BTW what is the difference between a pump for a solar thermal and one for central heating?

Posted By: Peter_in_HungaryPerhaps you get some boiling (or close enough to boiling) to cause a bit of steam /bubbles to form which won't easily get reabsorbed which then get trapped in the pump creating the vapour lock.

Surely steam will get reabsorbed as soon as it cools below boiling point? Dissolved air won't once expelled, but if there's a continuous problem it implies there's always a fresh supply of water, and I thought solar circuits were normally effectively sealed?

BTW what is the difference between a pump for a solar thermal and one for central heating?

Temperature rating, I imagine. Solar can get a lot hotter than central heating.

<blockquote><cite>Posted By: Peter_in_Hungary</cite><blockquote>

Can you swap out the pump for a type that doesn’t trap vapour and so solving the PITA problem?

</blockquote>

Hi Peter. Well, there's not much choice. It's non-pwm pump.
It's worth noting that manufacturers don't say .."Our pump is very prone to failure with trapped gasses, we suggest you don't buy it.."

Our pump is a Wilo gold RS50 which seems to be a domestic CH circulator. No mention of it being a dedicated solar circulator.

<blockquote><cite>Posted By: philedge</cite>Our pump is a Wilo gold RS50 which seems to be a domestic CH circulator. No mention of it being a dedicated solar circulator.</blockquote>

Ditto here. Wilo Gold RS50. Was installed 17 years ago and still going strong (hopefully not famous last words!). Never had a problem with air in the system or airlocks.

So many replies, thank you
@ Peter & Phil: My understanding is that a standard cheap CH pump may not handle the heat, hence solar version. I believe a budget non-solar specific CH pump would fit just fine. The Wilo Gold R50 is much more like the one I replaced four years ago (can't recall the one it came with the original pumping station). Those traditional pumps do at least have an air bleed screw on the hub bearing (not sure how effective this was, but I used to check it a little, and during the days I had this pump style I had no pump airlock problems).

@Revor, my bubble trap is on the "southbound" (hot) leg. As fitted to the original pump station. It certainly works and I can occasionally extract a few cc of gas, usually only during my re-pump cycles. You say yours is on the cool feed "northbound" just before the pump? Interesting.

@Phil. The pump is on the return from the cylinder coil, yes. "relatively cool" is a relative term, during full-on stagnation (a few of these every summer such as recent all blue days), even the bottom of the coil might reach Smx (Resol speak) max limit which I might greedily set quite high ~68C

@Phil, Re FIT (thank you). I have actually looked into this a couple of days ago and costed that as a means to reduce future maintenance. With my electrician hat on (Yes, though I don't "do" solar PV, it's not onerous, especially for just me). I have costed a separate small stand-alone 3 panels/1.3KWP (bringing me to <=3.68 KWp), and mini inverter and gone through the DNO & ENA docs. Given the down-time I've been suffering with the Thermal, that, with my existing PV diverter may be the best way today.

@Revor, my bubble trap is on the "southbound" (hot) leg. As fitted to the original pump station. It certainly works and I can occasionally extract a few cc of gas, usually only during my re-pump cycles. You say yours is on the cool feed "northbound" just before the pump? Interesting.


Apologies must have been ambiguous. It is the feed to the coil in the thermal store so it is the hot from the panels.

Do you have scope to increase the pressure in the system that will help keep the air in solution?

You do not have to have a liquid leak to indicate air can get in, you can aerate a liquid via a venturi effect.

When filling the system it is best to pump via an external reservoir and circulate for 20 mins to ensure air removed, this was in my install instruction when I fitted our solar thermal

<blockquote><cite>Posted By: philedge</cite>A typical boiler will run at 70 deg C so std CH pumps will be rated much higher than that. If your pump is after the coil then the cylinder is going to limit the coil temp to a little above cylinder temp. Stagnation shouldn't see the coil getting to hot as it's the collector temp that goes through the roof?? Our cylinder can get to 70 if we're not using much water in the summer but that's not stagnation it's just a system working well:)

Is your pump orientated vertically so air will rise through it?

For your PV remember that DNO limits are set by the inverter(s) not the number of panels so your G98 3.68kw limit is what the inverter is limited to, not how much your panels can generate. That means you can have a much larger array than 3.68kw. The FIT rules are different and based on panel capacity!! If you are going for a separate system then you can max that out just keeping to the DNOs overall limit.</blockquote>

Well when it all gets to 70 (or whatever I set Smx), pump turns off and tubes can quickly reach 120+, I've seen 140. Hence stagnation.

Over time (15 years since install) I have noted the delta between the bottom of the tank and the header increase, so less efficient heat dumping to the tank, not because of incorrect pump speed, but most likely the solar coil will be limescaled (we have very hard water). - Still doesn't stop it topping right out on the blue days.

Yes, the pump is vertical, but still seems to trap bubbles. Very annoying that it has no airbleed. If run it with bottom end in a bucket of water, it sits there doing nearly nothing till I prime it. Your suggestion of changing the pump is getting tempting, since I had no problems till I fitted this Wilo Yonos Para solar thermal replacement pump about four years ago.

Re the PV, that's a good point. Odd way that it altered. I've had a clamber round the roof with my tape measure and probably could fit 3 x 440W panels in place of the thermal array.
My primary inverter couldn't handle more in the string though, so I'd be most likely fitting another alongside, not feeding the generation meter, (which also means the FIT doesn't get complicated). So I'd have panels <3.68, but the two inverters together would be >3.68. Hmm,.. conv with DNO needed.

More than 2 bar would be doable but in practice difficult with DIY lash-up drill pump. A proper pump station though would.

A friend of mine pressurised and topped up his system using a pump up garden sprayer. Don't know what pressure he got.

<blockquote><cite>Posted By: philedge</cite>We have a Resol controller and there's a feature called collector cooling which stops stagnation. If I remember correctly it runs the pump when the collector gets very hot but before it boils.

I pumped in the solar fluid with a garden sprayer but used to mains to add water and pressurise things.

If you're only having 3 panels on your second inverter just check that the 3 panels will give a high enough voltage to start the inverters MPPT.</blockquote>

The collector cooling setting works to a point, it can put stress on things circulating very hot water. I have used it in the past.
Another thing I did in the past during summer was set a big increase in the DTo and DTf (start and stop differentials) from 2&4 C to 17 &20. This was to deliberately make the heat transfer less efficient through the daytime (running the panels hotte and reducing the coil efficiency) , to delay the point where the tank was hot . Orec and CMx also allow for some cooling but it reaches a limit, the variability wiinter-summer being so immense.

Justin - I have an adapted garden sprayer to top up my solar thermal system. I used it after I made some mods to the system. IIRC I could easily achieve 2 bar pressure.

My bog standard WILO GOLD c/h pump is on the return (hence cooled solar fluid) and in a horizontal position. I used to get a problem with stagnation too with temperatures of 120 - 130C possible. I got around this by installing a heat exchanger with a cooling fan (think car radiator system). When the return temperature reaches 60C a thermostat switches the fan on and blows the hot air out to atmosphere. I have never had a stagnation problem since!

Mention of 'gold' pumps suggests that you're looking for a bronze bodied pump intended for DHW circulation i.e. exposed to constant fresh oxygen which would corrode a CH pump body - hence much more expensive?

Thanks. It's a Wilo Yonos Para solar pump, which I fitted and seems to me much worse then the old pump (original Resol pump station, and can't recall what sort that was). Yonos seems to trap gas and airlock much worse then old one.

Posted By: JustinI'm back here after many years, pleased to see the forum still running.

Ditto! Just back on here after goodness knows how many years of absence, now also trying to sort a solar thermal system, that a number of people have said needs flushing, but obviously doesn't....

Self installed about 10 years ago whilst reroofing the house. Single Grant solar collector, Resol controller and a Megaflo solar pre-heat cylinder. All has worked amazingly well till a few weeks ago, when performance dropped off a cliff. Collector temperature struggles to get up over 60 degrees C, even in these tropical heatwave conditions. When the pump starts as commanded by the controller, the collector temperature briefly rises, then drops like a stone, so quickly, even the controller cannot regulate the pump speed down fast enough, and soon stops the pump because the collector temperature is now in the 40's

On the advice of the supplier, its been flushed and cleaned with solar cleaner. Not a hoot of difference. Fluid that came out was reasonably clean, perhaps with a mild tinge of discolouration - No 'bits' in it.

Flowrate on the pump station remains normal, and the pressure never dropped. Water freely flushes through the system both with a pump or even just mains pressure.

Grant suggested an issue with the PT1000 temperature sensor for the collector, which was changed with a spare I have, with no difference.

No air in the system, free flow but terrible performance. Its like there is a white sheet been laid over the collector to shield it from the sun.

The glass has been washed and the aluminium sheet with the selective coating inspected through the glass. That sheet is a bit mottled and undulates gently. I guess being aluminium, over the years, the heat has warped it slightly. Can that selective coating go 'bad' and reflect all the heat back out of the panel again? That's what the effect feels like.

Just now, after a week of scorchio weather, even by Orkney standards, the HW is up to 48 degrees C. Its been 100% sunshine all day today, and its not really gained any degrees since this morning. Up till a few weeks ago, a day like this would have the HW up to over 60 degrees easily with a day, and over a course of a few days, banging on the door of 75 unless I had the store cooling active. Now, it can't even get it over 50 degrees,

Grant themselves not really very sure - Hard to get them past the idea of a blockage, when the flow runs freely, and the panel temperature quickly drops when the pump starts. All day today, its basically started the pump, run it for a few minutes, the temperature drops, pump stops and it spends another 10 minutes heating up till the pump starts again. Repeat all day....

Next step is to replace the collector, but that is a pain as this is an inroof install. To get to the top of the collector, I would need to take off the ridge tiles, 3 rows of slates and the top hood / flashing to get to the top pipe (collector mounted landscape).

Any thoughts?

Thats the gist of what I was concluding as well, until I spoke to Grant tech support. My thought that there must be a copper plate under the selective coating with the copper serpentine tube brazed onto the back of it. But Grant tell me there is no thermal joint between the copper serpentine and the aluminium sheet. As described to me, the serpentine is just "in there, in the box" and it would be trickier to join that to aluminium.

Today, with another day of wall to wall sunshine - Us Orcadians are not used to this.... - the HW temperature has gained nearly 4 degrees and is up to 52 degrees.

Unfortunately, I think at the very least it needs to be opened up a look to see what is going on inside. Might need to see if there is any way I can 'adapt' the top hood / flashing to get into the pipe connection and fixing brackets, without needing to remove slates and ridge tiles. Certainly, when it goes back together again, it will have a bespoke flashing / hood that can be more readily opened up!

Hope will be to lift the front glass off and get inside it in situ, rather than take the whole panel down off the roof. Just need cooler days for it - Was up there yesterday and setting a hand on the Welsh slates was like setting a hand on a barbecue....