Solar Heated Hot Water Cylinder Heat Loss Overnight

[BobRK]

Hello,

I have a Kingspan Thermomax solar heating system installed which works well.

It comprises a 20 tube array on the roof, a Dual Stream pump station in the loft, an SC100 solar controller and a 200 litre hot water cylinder in the airing cupboard upstairs.

However, overnight the temperature at the bottom of the hot water cylinder goes down until it is just 2 to 3 degrees higher than the manifold temperature on the roof. This happens irrespective of the outside air temperature. Consequently, our supply of hot water disappears quicker than it should.

It looks like the system is syphoning heat from the bottom of the cylinder to the manifold until there is enough daylight for the system to start producing heat again.

So far I have found that the pump is NOT running overnight, there doesn't appear to be a non return valve anywhere unless there is one in the pump station and the controller correctly displays the temperatures at the manifold and at the top and bottom of the cylinder.

Any thoughts on tests that I could run, possible causes and/or changes I could make would be very much appreciated.

Thanks for your interest.

BobRK.

[JohnMo]

You need to form a heat trap at the cylinder, otherwise a thermo syphon just starts as soon as the pump goes off. A heat trap is a vertical drop in the pipe for 150mm, then horizontal for about 150mm and then vertical. You need this on both pipes connectors to the cylinder  coil

[Redbeard]

I have not come across @JohnMo's solution before, but there would normally in any case be a non-return valve performing this function. They can fail though or, as you describe (to my surprise), be left out!

[JohnMo]

1 hour ago, Redbeard said:

I have not come across @JohnMo's solution before, but there would normally in any case be a non-return valve performing this function. They can fail though or, as you describe (to my surprise), be left out!

I came across heat traps when I was looking for a solution to similar issue I had. Found they are mandatory in most of North America.

 

[BobRK]

Hi JohnMo and Redbeard, Many thanks for your interest.

My hot water cylinder is a traditional one. Its about lagged 200 litres cylinder with an immersion heater a little under half way down the cylinder. there are 2 heating coils: the top one heated by the oil fired boiler and the bottom one is the solar heated one.

The solar heated coil is just above floor level. The solar heater manifold  is approximated at the height of the upstairs ceiling with pump station approxiamtely 1 metre above the mainfold. Due to the roof levels and the house design the pipes from the manifold drop first down about a metre, run horizontally through a cavity wall and then rise back up to connect to the pump station.

If there should be a non-return valve where should it be position? I think a 2 port valve would do the same job as a non return valve but again where to position it and how connect it to the controller?

Thanks again for your interest.

 

[JohnMo]

What you haven't mentioned is how the puppies run that are connected to the heating coil. If they go upwards or horizontal you will get thermal gradient and it will extract heat from the cylinder on a continuous basis as you describe. 

 

A photo of your cylinder would be helpful.

[BobRK]

Hi JohnMo, Many thanks for your continued interest.

 

Apologies for the long delay: holiday and family issues have taken up a lot of my time lately.

 

Difficult to get a full view of my hot water tank due to the width of the landing.

 

However, the photo shows (from the top) boiler feed, boiler return (both grey lagged), solar heating feed, solar heating return (both black lagged) and the copper cold water feed from the cold water storage tank. Just above the solar heating return is the black cable from the tank temperature sensor. NB: the white sensor to the right controls boiler.

 

The solar heating pipes run horizontal for approximately 40 cm and then vertically up to the solar heating pump station.

 

As mentioned previously there doesn't appear to be a non-return valve anywhere.

 

The most convenient place to install a NRV would be in the vertical piping inside the airing cupboard but are such fitings available and would this be the best position to install them?

 

Any further help/comments would be much appreciated.

 

Thanks.

 

 

 

 

[JohnMo]

5 minutes ago, BobRK said:

solar heating feed, solar heating return (both black lagged)

Looks to me they would quite easily setup a thermosiphon. Can you replicate the dog leg as seen on the upper pipe in grey, so pipes drop about 150mm before going horizontal and upwards. A check valve may not fix it if remote.

[Redbeard]

On 27/07/2024 at 10:44, JohnMo said:

What you haven't mentioned is how the puppies run that are connected to the heating coil.

I worry about those puppies! Maybe that's where the dog('s)-leg comes in...

[JohnMo]

4 minutes ago, Redbeard said:

I worry about those puppies! Maybe that's where the dog('s)-leg comes in...

That would be it, (not checking what I'm typing)

[scottishjohn]

when i had solar thermal system

 40thermoax tubes and 300litre tank  which ran dhw  and uderfloor heating  to a degree with a vlave to block it off if water temp was too low for ufh 

I had a similar  problem ,though hot as bad 

just fitted a  valve  with stat which stopped flow if panels were not hot enough to add to tank

problem solved