Solar Thermal System: Help! What could have happened?

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.

Have you ever heard of or encountered this issue?

hot_rod

What type of work was done on the system? If work was done on the fluid side, a pump or component replacement, you could have some air in the system and the sensor is not getting an accurate reading.

With the pump off on a sunny day, temperature should go up very quickly, stagnation condition.

It could only drop if the collector or ambient temperature drops.

the sensor never turns off, the control will always be watching that S-1 sensor and by getting into the menus you will read actual temperature condition at it's location.

Is the collector sensor mounted on the collector or into a dry well in a tee fitting?

Are you using them over-heat protection functions?

the Resol sensors come with about 2 meters of wire length, so check the wiring connections at the sensor lead.

The collector sensor leads a rough life, very wide temperature swings, temperatures over 350F possibly. No harm in replacing it occasionally.

The Resol control should tell you by the icons on the screen what the fault is.

the

sophibee

Lots of work was done on the system. It has 3 circuits with 3 pumps, 2 controlled by the Resol controller and 1 which is controlled by a pipe thermostat. Design diagram attached. The system was drained, the pumps and some of the pipework replaced. The only thing which wasn't touched was the tubes themselves, the collector sensor and the stores / heat exchangers.

24.05.08 My updated diagram.pdf

I thought perhaps the collector sensor wasn't working properly but I have manually measured the temperature of the pipes with pipe clamp thermometers and the collector sensor is right - when the pumps stop, apparently the temperature of the pipes at the top and the bottom of the solar thermal tubes drops, which just seems totally bizarre and illogical to me.

I can't work out in my mind what could be happening / where the heat could immediately be going. I wondered if:

a) it could be something to do with air in the system so that when the pumps stop the air collects in the tubes and cools the pipes immediately above and below the solar thermal tubes?

b) something to do with non-return valves not being in the right place so the fluid in the tubes is somehow backing off the solar thermal tubes when not in circulation? But it's under pressure so that shouldn't happen?

c) I've noticed that of the 3 x circuits on one of the circuits the pump is installed on the return side of the store with a non-return valve on the flow side, but on the other 2 x circuits the pump is installed on the flow side of the store with a non-return valve on the return side. Could that be relevant?

hot_rod

If the fluid was drained, I suspect you have some air bubble or pocket in the piping and the sensor is not reading fluid temperature. The sensor needs to be in the fluid stream, in a dry well ideally.

Evac tube type collector can stagnate well over 400F.

There are two common types of evac tube collectors, heat pipe and U tube.

On the U tube style the fluid actually flows down through the piping. These can be a bear to purge all the air out. Hopefully they had a 1/2 hp or larger purge cart to flush the air out of all the various circuits.

With a heat pipe style evac tube, the fluid only flows across the upper header pipe. These pure rather easily as there are no ups and down in the piping, just a straight through header flow path.

Another possibility if the system was not adequately purged is flow stops and the fluid flashes to steam in the tubes or piping. It acts as an air problem. The sensor, being at the high point ends up in an air pocket.

Is there a float type auto vent on the collector arrays?

It's always good to do a second purge after the system heats up. As the heating of the fluid drives air out. Glycol is a stubborn fluid to get air removed from in any case. It could take 20- 30 minutes of purging to get all those loops you have completely air free.

What pressure are you running? What does the pressure read when the system is cold in the morning?

I like to see solar thermal run 45 psi or so. 45 psi= about 292° flash point

•Higher pressures raises the temperature that the glycol can flash to steam. It also makes air purging easier.

The expansion tank pre-charge needs to be set and also assure you have a 70- 90 psi solar relief valve in the system.

I like to set the expansion tank pre-charge 3- 5 psi lower that the pressure you fill to. This allows a small amount of fluid to be stored in the expansion tank. As any air purges out of auto vents, you have some additional fluid to fill that space. It's called a safety seal. By some :)

Is the pool collector acting as a heat dump? If you don't have a load on the collectors when the sun is shinning, they will flash to steam. Rather quickly with evac tubes.

sophibee

This is really helpful. Thank you so much for the advice. We have heat pipe evac tubes. The tubes are old (though still working beautifully) and I haven't seen any auto vents on them.

The circuit pumps (Grundfos single line pumpstations) have little windows with a floating flow rate gauge on them. Those drop to zero when the pumps are off and then pulse up and down when the pump is on (if the reading is the bottom of the float then about 4 litres per minute at 30% pump speed). 2 of the 3 pumps have expansion vessels attached to them. Those have a resting pressure of 3 bar. The system generally doesn't seem to have a pressure gauge on it, so I'm not sure what it's primed to. It used to have a pressure gauge on it but it seems to have been removed.

There used to be air vents about the place on the pipework, albeit they were closed. Now those have been removed, I'm not sure how I would bleed / purge the system to check there's no air pockets in it. Can I do that through the pump stations? It feels like I need to be able to do it at the top of the collector given that's where the sensor is and where the issue appears to be.

Yes - the swimming pool acts as a heat dump in the summer and the central heating circuit acts as the heat dump in the winter. They are supposed to play second priority to the hot water cylinder.

The problem at the moment is that I have a very hot pool and cooler hot water cylinder because the system isn't prioritising the circuits properly because of this issue of it not being able to measure its temperature properly when its pumps are off. Sigh. The number of hours I have spent researching this…

Can you think of anything else it could be other than air?

Again I am so grateful for your expertise.

hot_rod

inside the pump station should be an air purger like this. It will have a manual air bleeder on the side. Be careful not to spray hot air or fluid in your face. This is a PAW brand air purger. Some brands have a chrome plated tube style. All 4 port pump stations have an air purger.

It is good to have a float type at the high point, the collector header. Even if you only use it at start up. The air always makes it way up to the collector, assuming they are above the pump station?

Do you know how to force the pumps on, hand mode? A hand icon will appear in the toolbar and a flashing light to remind you that it is in manual mode.

If so when you have an odd collector temperature reading, force the pump on. To see if it reads more accurately.

The flowmeters should not bounce and it should be a solid stream of glycol running through.

If the pumps run in variable speed mode, the gpm reading may change, but it should not bounce. Bouncing also indicates air in the loops.