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15-02-2012, 14:20   #1
fitzie79
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Replacing PT1000 sensor on Solar Panel

Recently I have noticed that the reading on my solar panel is incorrect. On recent nights when it has been cold outside it is reading 12/13degC whereas the air temperature can be no more that 3/4deg. I measured the resistance going into the panel and it corresponded to a temperature of 12deg (1046Ohm) so the panel doesn't seem to be the problem (although it does intermittently read temperatures exceeding 120deg...), There are 2 joints in the cable coming back to the panel and I measured the resistance at each of these points as approx 1046Ohm so it appears that the problem is with the PT1000 sensor itself.

I am going to order a new PT1000 cable but want to ensure that there would be no issues with what I order. Do all cables have standard connectors? Is it a simple task to replace the sensor? I am going to try to order a 10m long cable if possible to remove at least one of the joints.

Also. looking at the piping a lot of the insulatation surrounding the solar pipes has pulled away from the system so I need to fix this as well. Are there good online suppliers of this insulation - I presume it needs to be special high temperature insulation?
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15-02-2012, 17:00   #2
rayh
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Originally Posted by fitzie79 View Post
Recently I have noticed that the reading on my solar panel is incorrect. On recent nights when it has been cold outside it is reading 12/13degC whereas the air temperature can be no more that 3/4deg. I measured the resistance going into the panel and it corresponded to a temperature of 12deg (1046Ohm) so the panel doesn't seem to be the problem (although it does intermittently read temperatures exceeding 120deg...), There are 2 joints in the cable coming back to the panel and I measured the resistance at each of these points as approx 1046Ohm so it appears that the problem is with the PT1000 sensor itself.

I am going to order a new PT1000 cable but want to ensure that there would be no issues with what I order. Do all cables have standard connectors? Is it a simple task to replace the sensor? I am going to try to order a 10m long cable if possible to remove at least one of the joints.

Also. looking at the piping a lot of the insulatation surrounding the solar pipes has pulled away from the system so I need to fix this as well. Are there good online suppliers of this insulation - I presume it needs to be special high temperature insulation?
Just 2 comments:
1. Had this problem myself and found that the problem was with controller. Might be best to check sender against another controller first.
2. PT1000 can come in various sizes - be careful if you have small receiver and how it seals
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15-02-2012, 19:11   #3
fitzie79
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thanks for the feedback. should the fact that the sensor is measuring an incorrect value not suggest that the sensor has a problem? i think i will end up replacing the controller anyway but don't want to do this unless it is absolutely necessary as it'll cost me over a hundred euro
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15-02-2012, 20:38   #4
rayh
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thanks for the feedback. should the fact that the sensor is measuring an incorrect value not suggest that the sensor has a problem? i think i will end up replacing the controller anyway but don't want to do this unless it is absolutely necessary as it'll cost me over a hundred euro
Have a look at http://www.regin.se/dmdocuments/resi...rs_prsh_en.pdf These units dont give much problems
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16-02-2012, 08:00   #5
fitzie79
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looking up the table my reading of 1046 ohm at the pt1000 (with it disconnected from the controller) corresponds to 12degC which is what the controller was reading. however the air temperature was only 3/4deg so this suggests that the pt1000 has the problem?

sounds like i'll need to get up on the roof to look at the pt1000 sensor to see what size it is and what size the seal is
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16-02-2012, 19:33   #6
rayh
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Originally Posted by fitzie79 View Post
looking up the table my reading of 1046 ohm at the pt1000 (with it disconnected from the controller) corresponds to 12degC which is what the controller was reading. however the air temperature was only 3/4deg so this suggests that the pt1000 has the problem?

sounds like i'll need to get up on the roof to look at the pt1000 sensor to see what size it is and what size the seal is
While a sensor failure may seem the obvious, I am not convinced how these units can fail and produce a +10 DegC offset.
Therefore it is my opinion that your controller is faulty and causing heat to passed to the sensor. Infact this fault can cause your panels to become heat sink at night.
Can I suggest that at dusk when the sun has declined, that you power the unit down for a few hours and then check if your controller is reading the expected outside temps. Also check the temps of the pipes leading to the panels to see you are getting any thermosyphoning in the system - is one pipe hotter than the other or higher than expected.
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18-02-2012, 15:58   #7
quentingargan
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I doubt if the PT1000 is faulty. Do you have tubes or flatplate? I would expect tubes to have a temperature higher than ambient for some considerable time. If your cylinder is up to, say, 40C, then the pump will stop when the panel temperature falls to 44C. The temperature will fall slowly, perhaps to 12 degrees after a few hours.

If you have flatplates, then the probe should be a lot closer to ambient. UNLESS your non-return valve is faulty. OR if your panel has the anti-freeze mode working.

If you have the anti-freeze mode on, then when the panel falls below 4C, the pump will run for a minute or so, sending water from the bottom of your cylinder up to prevent it from freezing.

If your non-return valve is faulty, then some heat will escape from your cylinder up to the panel.

But I have seldom known a PT1000 sensor to simply fail, and never with a slightly inaccurate reading.
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20-02-2012, 20:15   #8
fitzie79
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i have flat plates

the non-return valve being faulty is an interesting possibility. with the sunshine over the weekend the bottom of the tank reached about 45 to 50 degrees (it would normally be about 23) i noticed that on saturday and sunday night the reading on the collector was 22 to 23 degrees( instead of 12 to 13).

if the heat from the bottom of the cylinder was escaping then this might explain what i am seeing. how would i determine if this was definitely the problem?
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20-02-2012, 20:53   #9
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Quote:
Originally Posted by fitzie79 View Post
i have flat plates

the non-return valve being faulty is an interesting possibility. with the sunshine over the weekend the bottom of the tank reached about 45 to 50 degrees (it would normally be about 23) i noticed that on saturday and sunday night the reading on the collector was 22 to 23 degrees( instead of 12 to 13).

if the heat from the bottom of the cylinder was escaping then this might explain what i am seeing. how would i determine if this was definitely the problem?
If there is heat flowing around the system, it will be on the flow side, so it is more difficult as there should be no exposed pipework on that side.

Any chance that the pump station was put on the flow side? I've seen cylinders marked the wrong way around.

Is the temperature at the bottom of the cylinder falling off before you have used much hot water??
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01-03-2012, 13:37   #10
rayh
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Originally Posted by fitzie79 View Post
looking up the table my reading of 1046 ohm at the pt1000 (with it disconnected from the controller) corresponds to 12degC which is what the controller was reading. however the air temperature was only 3/4deg so this suggests that the pt1000 has the problem?

sounds like i'll need to get up on the roof to look at the pt1000 sensor to see what size it is and what size the seal is
Any update on your PT1000 sensors problem?
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01-03-2012, 19:40   #11
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busy week so haven't gotten back to it - hopefully over the weekend....
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07-03-2012, 20:31   #12
fitzie79
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Originally Posted by quentingargan View Post
If there is heat flowing around the system, it will be on the flow side, so it is more difficult as there should be no exposed pipework on that side.

Any chance that the pump station was put on the flow side? I've seen cylinders marked the wrong way around.

Is the temperature at the bottom of the cylinder falling off before you have used much hot water??
what do you mean by the flow side? i've attached a picture of the piping in the hot press. there are markings for flow and return for the solar coil. should i measure the temp of the pipe coming from the flow or return pipe or should the temperature be measured back by where the pipes go into the panels on top of the garage roof?



how could i determine if the pump station was put on the flow side
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07-03-2012, 21:59   #13
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Quote:
Originally Posted by fitzie79 View Post
what do you mean by the flow side? i've attached a picture of the piping in the hot press. there are markings for flow and return for the solar coil. should i measure the temp of the pipe coming from the flow or return pipe or should the temperature be measured back by where the pipes go into the panels on top of the garage roof?



how could i determine if the pump station was put on the flow side
Basically, after a hot day, once the pump goes off, the pipework should cool off. If there is heat in either pipe, then something is wrong.

The flow side should come off the top of the panel (if it is a flatplate with a top and bottom fitting), and heat from the panel flows from here to the top of the cylinder coil. The return should be from the bottom of the cylinder coil, back up to the bottom of the panel, via the pump station.

There is a one-way valve in the pump-station to prevent natural siphoning of heat. Otherwise, hot water in the cylinder coil would rise through the coil, on up the pipework into the panel, cooling in the panel and falling back down through the pumpstation.

Quite a few cylinders were sold with the coils marked the wrong way around.

It would be my best guess that this, and not the PT1000 is the reason your panel is reading high temperatures after the sun has gone down, but it is hard to diagnose these things on the www.

If there is heat in the pipework after about 8.00pm on a sunny day this week, either plumbing is wrong, or your non-return valve has failed (or possibly you are pumping hot water up to the panel because the holiday mode in your controller is switched on.
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08-03-2012, 10:17   #14
dathi
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hi i have seen this on a few systems that are not working as expected. if you look at the picture of the cylinder where the two pipes for the solar coil leave the cylinder, one pipe is going straight up. this allows the the hot water in the cylinder to thermosyphon up the pipe as far as the non return valve, this being made of brass heats up and transfers the heat to the other side of the non return valve where it makes its way to the roof. or worse still its on the other side of pipe work and there is no non return valve to stop the heat thermosyphoning to the roof. the solution to the problem is to make the two pipes travel down for 100mm from the cylinder before they start to rise to the roof this stops any water in the cylinder from thermosyphoning
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08-03-2012, 10:40   #15
saibhne
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I agree with Dathi, I have seen this in a number of installations where heat is lost through thermal movement despite the presence of non return valves. Plumbing pipes downwards at the cylinder does work.

S.
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