Hi,

I was just wondering what temperatures people are registering on their panels this week?

We have two Joules solar thermal panels on our south facing roof, and they’ve topped out at about 75c today. Ambient temp is 29c. Tank is at 68c.

I seem to remember last year, when it was this warm, that the panels were a lot hotter, at 120c and the tank was approaching 85c. I’m just wondering if there is potentially an issue resulting in them not getting as hot? Certainly feels as hot as last year, but not seeing that on the panels.

The 350l gledhill thermal store tank this morning was at 45c before the sun hit the panels, post showers!

Cheers

Ben

Posted By: BenMJoules solar thermal panels

There seem to be several types?

I seem to remember last year, when it was this warm, that the panels were a lot hotter

The panel temperature will depend a lot more on how sunny it is rather than the outdoor temperature.

The 350l gledhill thermal store tank this morning was at 45c before the sun hit the panels, post showers!

Depends how many showers whether that is good or poor :) Also top or bottom of tank? We have a 250 L tank and take two showers and the centre and above of the tank is always above 60°C in summer. FWIW we use PV rather than solar thermal to heat it.

If your 350l TS heats up from 45 to 68 degC that's
350* (68-45) * 4.2/3600 = 9.4 kWh delivered by the panels (not bad?)

But if you use two short showers and a kid's bath (guessing max 200l , 38degC @ point of use, 10degC mains) that's
200 * (38-10) * 4.2/3600 = 6.5 kWh

Can you investigate why the discrepancy, where's the missing heat gone?

Are you somewhere using (losing?) more hot water than that guess, maybe a bit more than last year?

What is the tank standing losses - has some insulation got removed somewhere?

Are some of the temperature measurements losing accuracy somehow?

Obvs if the daily kWh usage is really that much less than is being collected, then the TS will get hotter every day until it hits its thermostat, then the pump will stop and the panels will be left to get superheated.



On a separate tack, if you have an immersion and heat the TS to about 85degC, then a non-resetting safety cutout should be triggered, disabling the immersion (even if the heat wasn't from the immersion).
If this didn't happen, check the safety cutout hasn't quietly failed. Mine was corroded up, and the cheap replacement I bought didn't work either, I had to get a decent branded one. There have been occasional fatal accidents with vented DHW systems overheating.

Posted By: GreenPaddyHas the recirc pump speed been turned up (manually, or by the controller)? That could account for there being a lesser difference between panel and tank.

Have the the ON/OFF set points for pump start/stop been altered? Again that could alter the tank/panel diff in temps.

Maybe the thermocouple at the panel is failing/inbuilt offset? so it's actually 100oC but reading 70oC. Switch off the recirc pump, let the temp climb at the panel, then switch recirc pump back on. Will only give a very gross indication.

So tried turning the pump off my lowering the Store Max to about 40c (below the actual tank temp). The temperature in the panels did start to climb, got to about 100c and I switched the pump back on.

When the pump came back on, the flow rate was 0 on the gauge next the pump, some gurgling from the feed from the solar. Panel temps continued to rise and then suddenly dropped to about 70c. When this happened, the flow rate when back to normal (4 l / min). Not sure if the panels overheated and it blew off the excess heat into the glycol collector. Not crawled in to check.

Panel is down to 62c and dropping. Tank is now at 49c and rising slowly.

It’s 28c and bright sunshine all day today. Really not sure what this solar system is doing. A call the installer tomorrow, I think.

Posted By: djh

Posted By: BenMWhen the pump came back on, the flow rate was 0 on the gauge next the pump, some gurgling from the feed from the solar. Panel temps continued to rise and then suddenly dropped to about 70c. When this happened, the flow rate when back to normal (4 l / min). Not sure if the panels overheated and it blew off the excess heat into the glycol collector. Not crawled in to check.

Perhaps sounds like an airlock? Which may have been moved or not.

Might well be an airlock. Would that cause lower temps on the panels?

It's really warm and sunny today (already 30c outside), and the panels are only at 63c. Tank started the day at 40c (with only one shower taken prior), and it's got 46c so far.

Not sure how to bleed the system, guessing it's not like bleeding a radiator!

Posted By: djh

Posted By: BenMMight well be an airlock. Would that cause lower temps on the panels?

Dunno, you haven't described how your system is plumbed.

Not sure how to bleed the system, guessing it's not like bleeding a radiator!

I'm no expert on solar thermal so somebody else will have to answer, but again, they will need to know how your system is plumbed. Where is/are the bleed valve(s), for example? A drawing would be a good idea.

Diagram attached now. The solar has it's own coil within the tank. The tank itself is open vented. The only addition missing is the solid fuel stove that also inputs, direct feed (no coil).

I'm not yet clear why you think you've got a problem?

The panel is running at 20oC higher than your cyl temp.
When the pump is switched off, the panel temp rises.
When the pump is switched back on, the panel temp falls.

If there was low/no fluid flow, the panel temp would rise (which it is not)
If there was poor heat exchange at the cyl, the panel temp would rise (which it is not).

The sun is def hitting the panel; the fluid is taking that energy away; so it seems like you are expecting a hotter cylinder.

If the energy is getting transferred to the cyl (which it must be since the panel is not getting very hot), then either you're using the energy, or you're getting duff info from the temp sensors, or your expectations do not match the actual energy transfers.

Check temp sensor accuracies (to 1 or 2 oC would be fine). As pointed out above, could there be an unwanted energy draw (rads drawing from the cyl by "gravity" is not uncommon, though you'd likely have noted a few warm rads).

You're starting from quite low cyl temps 40's oC. What energy input are you expecting from the panels? If the morning cyl start temp were say 55oC, then you'd be running everything hotter. Panels in say 80's or 90's oC, cyl 55 to 80oC. Perhaps that's the scenario you've mentally logged from before. You're better running cooler, if it gives you adequate water temps, but you'll still have similar energy input either way.

Posted By: revorThere may be a vent at the highest point (manual or auto) or near the pumping station a micro de-bubbler which is an auto device. If the glycol is not pumping around then I would have expected the panels to have got very hot.
Check if the pipes to your boiler are getting hot you could have gravity circulation through the boiler robbing the heat, I had this problem until I fitted a gravity check valve in the boiler circuit. For you to have this problem guess one of your system boiler control valves is by passing. Does your control system give you temperatures at different points in the HS that may help diagnosis.

The boiler feed and return pipes connecting to the thermal store tank are hot. As are the back boiler pipes. I can't see any gravity check valves on these. In saying that, they aren't hot down at the actually boiler and back boiler.

Posted By: GreenPaddyI'm not yet clear why you think you've got a problem?

I think you may be right, it's just I remember last year the panels getting up to 120c and the tank peaking at 85c, with the same level of showers and baths etc. I should have explained that we had them originally installed in July last year, then taken down as we had work down on the roof, stored in the basement then put back up again in the spring this year. Original install and re-install down by the same people. Not sure if that would make any difference.

We have turned off the gas boiler, perhaps that's an error?