Solar thermal panel is exporting my tank heat!

TheSaint_2

Hi, everything you say is correct. The coil for the panel is at the bottom. I have done as you suggest already and set the panel on a timer to limit any export. The pump can only run now when it is getting sunlight. 

What I see is the top tank sensor drops in temp as soon as the pump turns on. The pump comes on when the panel is effectively cold (20 degrees) and the tank sensor is reading 45. Then the panel heats up to 40 ish and the tank stats cooling.

I think im just going to book a full service and they can look to see if anything is faulty at the same time.

Cheers 

zeupater

TheSaint_2 said:

Hi, everything you say is correct. The coil for the panel is at the bottom. I have done as you suggest already and set the panel on a timer to limit any export. The pump can only run now when it is getting sunlight. 

What I see is the top tank sensor drops in temp as soon as the pump turns on. The pump comes on when the panel is effectively cold (20 degrees) and the tank sensor is reading 45. Then the panel heats up to 40 ish and the tank stats cooling.

I think im just going to book a full service and they can look to see if anything is faulty at the same time.

Cheers 

Hi

Because the ST thermal coil is lower in the cylinder than the GCH coil. heat provided by the GCH will create a stratified layer of hot water between the GCH coil an the top of the cylinder and will not (significantly) affect the water column temperature between itself and the lower ST coil ... even if the solar thermal controller was faulty and caused the system to circulate water between the cylinder and the panel at night the only heatloss would be between the top of the lower coil and the bottom of the cylinder ...

Regarding " set the panel on a timer to limit any export" ... it's not the solar thermal that needs to be limited by the use of a timer, it's the GCH boiler hot water provision ... also, the point "The pump can only run now when it is getting sunlight" ... does this mean that your ST pump has been observed running when it's dark outside? ...

The following is particularly interesting .... "What I see is the top tank sensor drops in temp as soon as the pump turns on. The pump comes on when the panel is effectively cold (20 degrees) and the tank sensor is reading 45. Then the panel heats up to 40 ish and the tank stats cooling." .... as the cylinder top temperature shouldn't be influenced by the pump at all, unless you also have a destratification pump installed, which according to what we've discussed (pump in loft) doesn't seem to be the case ...

Effectively, there are a couple of issues that I can think of that could be causing the entire water column to cool, these being the system is syphoning between the DHW cylinder and it's heater tank (probably a stuck non return valve), or the solar coil in the cylinder has lost integrity and the solar pump is pumping not only the heating circuit, but circulating the whole cylinder & header tank contents. Either way, have you checked whether the cold water header tank in the loft (assuming it's an indirect vented system & there is one!) is actually cold & not being heated from the cylinder, if not can you check the next time the issue happens ...

We experience thermal syphoning between the cylinder and the header tank years ago which resulted from a combination of pipe run design and a failed non-return valve on the whole house DHW thermostatic mixing valve .... this was fixed by removing the TMV (non-return) valve (which was a flimsy plastic insert) and adding both a beefy in line valve & an anti-syphoning loop ... however, although this manifested as increased overnight cylinder cooling, the entire cylinder being at the same temperature, and extended running of the ST pump to heat a greater water volume (cylinder+header tank), it didn't cause an immediate temperature change at the cylinder top sensor ...

Anyway, if the solar coil in the cylinder has failed it would have caused the contents of the collector circuit to mix with the DHW you use, in which case any antifreeze additive would have run out of your hot water taps by now .... have you ever had the smell of antifreeze (glycol) when running hot water? - if so this would tend to confirm this as being the likely cause ...

HTH - Z

#Edit - +"(non-return)"

TheSaint_2

Hi, we haven't had any smell of antifreeze, but the system has been like this since we moved in two years ago. If it has happened I expect it happened before we moved in. It sounds like I need to get a professional round to inspect it. Thanks for your help.

Just to clarify, yes the solar pump runs when it is dark. Or it did until I put the whole system on a timer. 

I will check out the tank in the loft as well. Cheers.