Heat dump for solar system

Henrys Grimey Paws

Hi,

We've a hot water solar system in our house that's suffering from stagnation once the tank is heated which is causing leaks to occur. I've had the system leaks fixed but I'm concerned that they'll happen again if the current weather continues and I'm not here to run off some hot water.

I'm looking into getting a heat dump installed as this seems like the obvious solution to our problem, I had a chat with the plumber who fixed the leaks yesterday and he mentioned running pipes in the attic to dissipate the heat but I'd prefer to dump the heat externally rather than heat up our crawlspace of an attic! His solution was to extend the run to the tank which doesn't also seem efficient use of the heat in lower temp parts of the year.

I'd prefer that this dump was only triggered when the hw tank had reached temp and then the solar controller would switch a valve to push fluid through the dump to dissipate the heat. I'm thinking of a radiator that can be exposed to the elements mounted on a north facing wall of the house to expel the heat. Does such a thing exist? Any other tips for my situation are much appreciated!

Thanks in advance.

Wearb

I dont do solar, but often heard of this problem. I wondered if a valve on the hot-water pipe -controlled by a thermostat- could be used to automatically discharge hot-water into a drain. NOTE. This is only an idea and I'm not sure if it's sound.

Biggest problem with the idea, is the waste of water. 😰💧

1874

The best solution might be to have the solar array sized correctly to suit the demand. I'm taking that this is a Solar Thermal setup of some kind, like evacuated tubes?

Is it that no one is present to utilise the heated water for long periods or just for specific times or days? because it seems a waste to dump water/heated water? is the issue only in the recent hot weather or has it occurred previously also?

Would having a larger tank solve the problem? if replacing a tank is costly and the current tank has plenty of life left in it, could a secondary tank that all hotwater flows to and through be a solution? (that assumes there is space to fit one? any colder water in that tank would have to be displaced though? maybe for some use?

Not sure if it would be possible for the hotwater to rise to a tank at a higher level and the displaced cold water fed back to the tank accepting the solar input?? could get complicated and messy. How is the hotwater that is generated now distributed after the tank? by gravity or by a pump?

A place to dump heat might be better to avail of inside? like in a bathroom or a hotpress? sounds like you are thinking of offloading that heat outside.

I still think having a set up sized to the demand is the best set up

Henrys Grimey Paws

Realistically adding an additional tank is a non starter for us, resizing the solar panels would also be too expensive to justify.

The way the system works is that once your hot water tank reaches 60 degrees then the pump no longer operates and the glycol on the roof sits there getting hotter and hotter which causes problems. One solution is to automate the dumping of hot water down the drain, that seems quite wasteful and if water charges ever came in could get quite pricey during hot periods! So the heat needs to go somewhere, I'm not sure how easily I could divert it to our internal radiators or how that would operate plus by definition the times when this is likely the summer so the heat in the house wouldn't be wanted. It seems to me that the only solution is to send it outside....

Del2005

Could you add a switch or somehow force the pump to the come on? It should be easier to modify the control system than the physical system. You'll still need to dump heat but the hot water system should already be setup to prevent it over heating.

Henrys Grimey Paws

Yes, the controller has a setting that will pump through even if the tank is at temp, problem is it will do this up until the tank hits 95 degrees, the tanks emergency pressure valve triggers at 90 degrees. I tried this previously and discovered that the tundish on emergency valve wasn't fitted properly and leaked everywhere! If I'm honest I'd prefer not to be using a safety valve as a functional aspect of the system as I'd prefer the solar system to overheat than have 300 litres of boiling water in the tank.

Essentially I think I need a 3 way valve that gets switched when the solar system goes into stagnation and the pump keeps running the glycol through a radiator of some form to dump the heat. I'm surprised there's not a common solution to this, there's 50 houses in our estate all having issues because there was no thought put into what happens when this system overheats. I assume it must be pretty common across the country in new builds with hw solar panels

John.G

https://www.boards.ie/discussion/comment/117657122#Comment_117657122

Your solar expansion vessel is undersized if its lifting the 6 bar PRV setting (safety valve), when the system stagnates some of the the fluid in the solar collectors vaporizes and blows down some or all the collector contents into the expansion vessel which should be sized properly ( 18 litre is quite common) the collectors can reach temps of 240C but the pressure should still not exceed 3 or 4 bar, your system should also not leak at any of these pressures, certainly not at 6 bar, and should be pressure tested to 10 bar.

Evacuated tubes should IMO always be installed with a dump system, the flat plates which are not as efficient will rarely stagnate if you can set the maximum cylinder temperature 85 to 90C but must have a anti scald (thermostatic mixing valve) valve to reduce the temperature to 60C after it exits the HW cylinder.

I have flat plates with the cylinder temp set to 85C but I also have a very simple heat dump where I utilize the HW cylinder coil to dissipate excess heat via a few of the existing CH radiators. I can isolate (switch off ) the power to the burner (OF boiler) I then latch open the HW zone valve, and turn up a room stat to max, this circulates the water through the coil and rads and keeps the cylinder temp around 60/65C. I very rarely use it but on one of our very hot days recently it maintained the cylinder temp below 65C even with practically no hot water usage. Even today with no HW demand, I switched it on when the cylinder reached 78C and it reduced the bottom half of the cylinder to 59C with no further rise (> 78C) in the top half.