Sensor placement

[Gixer]

Well today was a disappointment, my first day of no solar input, zero, zilch !!

The panels got to a max of 19deg all day, umm in September hope this is a rare thing?

I have a feeling that my panels are getting much hotter than the sensor would suggest, not today of course  

Take yesterday, nice and sunny for a change, my controler tells me the panels are 35 deg, but if I touch the silver manifold casing it feels much hotter than that and the pipes are really hot to the touch.
Has anyone had probs with the sensor not picking up the heat properly in the sensor tube?
I'm thinking of attaching the sensor directly to the 22mm exit pipe and see if that helps performance at all.

Any views on this?

[CeeBee]

Don't get too hung up about expecting high temeperatures at the panel (unless the pump is permanently off!). In normal use, manifold heats to some amount higher than cylinder (whatever is set on controller), pump comes on, and thereafter manifold and cylinder just rise together as the cylinder heats up. OK - that's hugely simplified - there might be some turning on and off, depending on flow rate, amount of solar input, etc. You don't expect (or want) to see (say) manifold at 70 and cylinder at 20 - that would mean the system is failing to transfer heat from the panel properly.

But you should have got more than 19 so long as there was even a bit of sun (well unless the cylinder was say 10, and pump was on, in which case 19 might be OK). So what can be going on? Not sure myself how the sensor pocket relates to the pipework in the panel - I assume it's meant to position the sensor near the manifold. If you've got easy access to the sensor, then perhaps try sticking it in boiling water and observe reading on controller (i.e. does it work, is it wired properly?). Next, is there actually water in the manifold (and not air)? If there's no water in there, the heat will have trouble getting from the heat-pipe tips to the sensor. Any more bright ideas from installers (which I'm not)?

[Gixer]

Hi, I'm not looking for higher temps,
I think the sensor is not picking the true temp as the panel feels very hot to the touch. whilst the controller thinks the temp is quite a lot lower so won't run the pump.

When the pump does run I can feel a slug of hot water come down the pipe and that certainly feels hotter than the display would suggest.

I just wondered if anyone has observed similar?

Next time the sun come out I'll test with my digital temp probe.
When it comes out !!

[wookey]

My panels have never been as cold as 19 apart from overnight - that does seem suspiciously low for a daytime max. I've had one day of zero input since april (4th or 5th Sept - weasel peed it down all day. Conversely I got my hottest tank temp of the year so far on 13th Sept (87C). (This is because I only had 20 tubes from april to august - 40 for last month or so).

[Gixer]

OK, currently I have 60 tubes in on the west side, on the east there are two panels but no tubes as yet as they are not plumbed in. Just a loop of pipe making the circuit.

This darn rain is preventing me finishing the installation on that side.

Maybe I'll have another go at bleeding just in case some air has found it's way to the panels

[Gixer]

Bled it this morning, no air.

Took a bare tube outside for half hr, stone cold.

Pulled a tube from a panel and measured the manifold directly, 16 deg.

So we really are not getting any UV through this cloud. Funny really as it seems quite bright and the clouds are not black, just thick.

My query on the sensor placement will have to wait till we get some sun to investigate further.

[Gnidrolog]

Gixer

My sensor is currently showing 24C. 20 tubes SE + 20 tubes SW. Location Essex/Suffolk border, sky is dark clouds. Not a good day for solar!

Gnidrolog

[Gixer]

It's not just the hills that are suffering then, I'm in the mountains of Snowdonia.

What a miserable year...or two 

[Gixer]

Wahey, the sun has come out to play, finally.

Found a couple of problems.

3 20 tube panels, 25m run of pipe.

First bash of heat.
store 21 deg
panels pumped at 26 (it's set for +5 diff to switch on and +2 in an effort to make it run for longer)
Pump switched on panels temp went to 29 deg whilst pumping (it has always done this)
Digital thermometer placed halfway along pipe run to tank hit a peak of 32 deg which at this point it stopped pumping

So, the main problem is the hot water is not getting to the tank before the pump stops, it has to sit in the pipes and wait for the next pumping cycle in which time it cools down a bit. This seems to be repeated as the day goes on.

Now then, how do I solve all this, as far as I can see most of the heat generated is never getting to the tank.

[dhaslam]

Are the pipes in a place where you can wrap a lot of insulation around them?  You can use any insulation one the high temperature stuff covers the pipe.   You could get cooling losses down to very small amounts.   

[Gixer]

Hi, I used armaflex all the way with no joints, this comes with its own high quality insulation.
All the same it still cools down, a lot to start with but this seems to lessen after a few pumps.
But still, there are losses I'd rather not have.

Perhaps I'll increase the pump speed and see if that helps, currently set to 3lts/min for the 3 panels

[CeeBee]

I also observe that when the panel is initially getting going (so you almost inevitably get this start/stop behaviour) that the panel sensor rises immediately after the pump turns on - panel sensor is usually (hopefully?) at the end where water leaves the panels, so it's easy to imagine that there is hotter water in the centre of the manifold which the sensor doesn't get to see until after a few seconds pumping.

I've never got to the bottom of whether people usually see start/stop pumping even when the system has settled down in good sun - because I certainly don't - my system very quickly settles into continuous pumping.

3x20 tube panels should produce a serious quantity of heat, ballpark 3kW (?) - after a bit of start/stop while the circulating fluid is warmed up you'd really hope it would get going properly - you shouldn't be able to lose that much from the pipework, bus as dhaslam says, insulation is all important.

I wonder, to try to stop it turning off again so quickly, if it's worth trying a higher 'on' delta-T (I've had mine at 16 at times, can't remember what it's on at the moment - lower than 16 - maybe 10) so that when the pump comes on, there's more energy in the system (higher delta-T shouldn't be a problem - if the panel doesn't reach very high temperatures with the pump off, then the energy's not enough to be worth having anyway). Also maybe a lower flow-rate, so that cool water in the 'return' pipe to the panel doesn't quickly reach the sensor (mind you, you say you're already on 3l/min - that's what I'm on for just 1 panel).

But both these suggestions are still just fiddling about to avoid on/off while it's getting going. Either way, I'd hope to see it settle into more continuous running, with delta-T converging on whatever difference is needed so that the cylinder coil dissipates the same power as the panel is gathering.

[kristen]

Does anyone use a bypass valve at the tank end?

What I am thinking is:

Panel sensor detects sufficient heat, and turns the pump on, but the pipe starts out full of cold water.  A sensor at the tank end opens a bypass valve and just loops the cold water back, and thus this cold water doesn't go into the thermal store.

When the hot water arrives at tank the sensor closes the bypass valve allowing this hot water into the tank.

Pump stays on IF the sensor at the tank end is "hot", OR the panel sensor is "hot".

(If panel is hot, but tank end sensor is cold, then valve is opened to bypass the tank again)

[Gixer]

Finally the panel temp is rising as it's pumping

Slowly but at least it is...will see how it goes over the day.

Mind you without the East side connected it was always going to be a bit tough on it.
Hopefully that will be rectified this week sometime.

[CeeBee]

Does anyone use a bypass valve at the tank end?

Sounds quite a nice idea, though whether saving enough to justify fitting it, I don't know. Be careful though, or you'll have the "minutiae" police round i.e. M*rtin :-) (ref. some other recent thread). One of the folk with a home-grown controller and sensor network wouild be best placed to try such a thing.