Solar Thermal System Issue: Can anyone help?

[sophibee]

I have a solar thermal system with a Resol Delta Sol BS Plus controller. I have recently had some works done to the system and since then it does not appear to be functioning properly and I wondered if anyone had any ideas about what could be wrong.

The issue is that when the system’s pumps turn off, either because a store has reached its max temperature or because the system is checking its temperature during an Oscillating Break Time, the system (through the collector sensor) no longer detects an increase in temperature at the top of the collectors.

What happens is that the pumps turn off and the collector sensor actually registers a decrease in temperature at the top of the panels. This goes on for as long as the pumps are off and the temperature just falls steadily.

If the pumps are turned back on manually, the flow of the system start circulating again, the temperature reading at the collector sensor then rockets up by 10 – 20 degrees or more and then levels out once the pumps have taken back control, released the heat through the heat exchangers etc.

It’s an issue because it means that (a) the store prioritisation is no longer working properly and (b) the system doesn’t detect when it’s about to overheat and so doesn’t restart the pumps in an emergency fail-safe function (e.g. under the OREC or OCX functions).

It didn’t used to be a problem. On a sunny day, what used to happen is that when the pumps turned off, the collector sensor would detect a steadily rising temperature and then the system behaved as expected, heating the stores in the right priority… with the other system controller functions etc.

Has anyone ever heard of or encountered this issue?

 

[Johntheo5]

I have a solar thermal system with a Resol Delta Sol BS Plus controller. I have recently had some works done to the system and since then it does not appear to be functioning properly and I wondered if anyone had any ideas about what could be wrong.

The issue is that when the system’s pumps turn off, either because a store has reached its max temperature or because the system is checking its temperature during an Oscillating Break Time, the system (through the collector sensor) no longer detects an increase in temperature at the top of the collectors.

What happens is that the pumps turn off and the collector sensor actually registers a decrease in temperature at the top of the panels. This goes on for as long as the pumps are off and the temperature just falls steadily.

If the pumps are turned back on manually, the flow of the system start circulating again, the temperature reading at the collector sensor then rockets up by 10 – 20 degrees or more and then levels out once the pumps have taken back control, released the heat through the heat exchangers etc.

It’s an issue because it means that (a) the store prioritisation is no longer working properly and (b) the system doesn’t detect when it’s about to overheat and so doesn’t restart the pumps in an emergency fail-safe function (e.g. under the OREC or OCX functions).

It didn’t used to be a problem. On a sunny day, what used to happen is that when the pumps turned off, the collector sensor would detect a steadily rising temperature and then the system behaved as expected, heating the stores in the right priority… with the other system controller functions etc.

Has anyone ever heard of or encountered this issue?

Is the collector temperature prope properly inserted in its pocket and pushed fully in?
Was the system drained down and refilled without purging air from the system?
As you probably know all these system work by turning on the solar circ pump when the collector temperature is ~ 6 to 10C higher than the cylinder store temperature (probe located just above the solar coil top), the pump will then stop when the collector temperature fall to ~ 3 to 5C above the cylinder store temperature store temperature or/and the solar controller might control the solar pump speed to maintain say a differential temperature of around 7C, it should continue to run in this fashion until the maximum store temperature is reached at 60C to 80C, the pump then remains off until either HW is drawn off or the collectpor temperature reaches ~ 100/110C, this temperature must also then fall to permit the solar circ pump restarting

 

[sophibee]

I don't think the issue is the collector temperature probe. It wasn't touched as part of the works. I have also put a pipe clamp thermometer next to it and it registers the same thing - a temperature drop at the top of the collectors until such time as the pumps are manually restarted and then it registers a massive rushing temperature increase as the fluid starts recirculating.

The system was drained and refilled. It was done by a professional who said he flushed the system for air. So it should be ok. But I'm struggling for explanations for its behaviour and so I was wondering whether air was the potential issue and it collects at the top of the collector when the pumps stop and stops the collector sensor registering increases in the fluid temperature (because there's then air not fluid in the pipe under the sensor)?

The only other change I have noticed is that we have 3 x circuits with 3 x pumps and 3 x heat exchangers / stores. The way the system used to be, all the pumps were on the return pipe of their circuit and there was a non-return on the flow side. Now we have 1 x circuit with a pump on the return with a non-return on the flow and 2 x circuits with pumps on the flow with a non-return valve on the return side. I can't immediately see how that would be the cause of the issue, but I'm on a steep learning curve to try and understand what's happened....

The controller has a prioritisation system between the stores so every 15 minutes it stops and waits 2 minutes to see if there's a temperature increase in the collector sensor to then determine whether it can heat the priority store etc. On a hot day there was always an increase in that time period. Now there never is, as I say, the temperature falls which is just wrong / inaccurate. I've played around with it a lot in blazing sun and even after 5 minutes it still doesn't think the fluid in the "turned off" system is heating up at the collector sensor. That is until you manually start the pumps and it goes "panic panic panic" as the temperature rockets with the fluid movement.

I so appreciate your thoughts and help.

 

[Johntheo5]

I don't think the issue is the collector temperature probe. It wasn't touched as part of the works. I have also put a pipe clamp thermometer next to it and it registers the same thing - a temperature drop at the top of the collectors until such time as the pumps are manually restarted and then it registers a massive rushing temperature increase as the fluid starts recirculating.

The system was drained and refilled. It was done by a professional who said he flushed the system for air. So it should be ok. But I'm struggling for explanations for its behaviour and so I was wondering whether air was the potential issue and it collects at the top of the collector when the pumps stop and stops the collector sensor registering increases in the fluid temperature (because there's then air not fluid in the pipe under the sensor)?

Trapped air is allways a possibility, on my own self installed flat plate array, the collector temperature starts rising very rapidly say in the morning to trigger the pump.

The only other change I have noticed is that we have 3 x circuits with 3 x pumps and 3 x heat exchangers / stores. The way the system used to be, all the pumps were on the return pipe of their circuit and there was a non-return on the flow side. Now we have 1 x circuit with a pump on the return with a non-return on the flow and 2 x circuits with pumps on the flow with a non-return valve on the return side. I can't immediately see how that would be the cause of the issue, but I'm on a steep learning curve to try and understand what's happened....

Solar circ pumps (normally) pump into the bottom of the collector, up through it and then downwards through the solar coil, it may not make a difference if you change the flow direction but certainly not normal IMO, can you try just running on the traditionally pumped system to see how it reacts,

The controller has a prioritisation system between the stores so every 15 minutes it stops and waits 2 minutes to see if there's a temperature increase in the collector sensor to then determine whether it can heat the priority store etc. On a hot day there was always an increase in that time period. Now there never is, as I say, the temperature falls which is just wrong / inaccurate. I've played around with it a lot in blazing sun and even after 5 minutes it still doesn't think the fluid in the "turned off" system is heating up at the collector sensor. That is until you manually start the pumps and it goes "panic panic panic" as the temperature rockets with the fluid movement.

I so appreciate your thoughts and help.

 

[Agile]

My initial thoughts were that there could be air trapped at the top of the collectors.

You then implied that you "trust" the professional to have correctly expelled the air.

Perhaps I would be less trusting?