Too much solar?

[Piwoslaw]

This is probably a dumb question, but...
What happens to your solar hot water installation when you go for a 3-week vacation in the middle of summer? I imagine you can't just turn the pump off or the collector will boil, but with the pump running all that heat is getting dumped into the tank, but there is noone to use it, so it keeps getting hotter...

What measures are taken to ensure that the system does not overheat when not in use, or when the system is oversized?

[Xringer]

I had three large Novan collectors and even in the middle of the summer, the system never got the 84 gallon tank up over 200F.

I'm not sure exactly what happens, but I think the very hot water being pumped up to the collectors, does not pick up much heat.

I think the max output temperature of the collector array is the limit for the system.

Look at it like this, if you have a 200 degree electric hotplate laying out on
the sidewalk, will the sun make it very much hotter?


Anyways, on the freshwater side of my system, there was a pressure & temperature relief-valve & expansion tank. (Required by code).

Over-heating was never a problem.

[Ryland]

I've seen a solar hot water system get the working fluid up to 260F and at that point the system softened a hose that was used to allow some physical movement and it burst.
If stand your panels more upright they will get less summer sun and more winter sun to help them self regulate.
You can also have a dump loop in the ground that is after the hot water storage tank and before the panels, but normally if you just size your storage tank so it's large enough to handle the number of panels you have you should be fine, you can also, instead of having a ground dump loop, have a tempering tank that it dumps heat in to, that 2nd tank will never get as hot as your first tank, but it will help drop the working fluid temp enough that you will extend it's life and that is one of the big reasons for keeping the temp lower is that you don't want to cook your anti-freeze/working fluid.

[knowbodies]

The systems I have seen have a radiator (usually on the north side of the building) where the heat is dumped to once the maximum temperature has been reached in the hot water tank.

[Xringer]

My home heating boiler has a 200 degree Dump switch that turns on my basement heat. (Forced hotwater baseboard).
The sensor switch is about 8" right of the gauge. It's set for 190 these days.

It only clicked on once. I was burning coal and something cause the air-intake to stick in the open posistion.
It was about 2AM and the basement got up to about 90 degrees and the boiler was boiling!!

Scary high pressure too, since the T&P failed to work. I had to release
the pressure by hand. Live steam is very dangerous..


It's a good thing my old monster is made out of steel and not cast iron.. Or else, it could have popped..

[Xringer]

Deleted my reply. I guess that I was replying to spam..

[Solar Mike]

I built a radiator from 1/2 inch copper tube clamped between two sheets of mini-corrugate aluminum sheet, painted black, approx 4 square meters surface area. Placed this under the house, diversion valve in the hot feed from the panels starts moving the hot water through the radiator before entering the hot water cylinder when panel water exceeds 80 Celsius. This self regulates and prevents over heating. Because I built approx 8 sqm panels feeding into a 300 liter cylinder, things tend to over heat in summer. But i get good winter heating. Yearly average is 95% of our hot water is solar.

Cheers
Mike NZ

[Xringer]

Hi Mike and welcome..

It sounds like you created a nice dump-load. Is your system all DIY? 95% is fantastic!
You must get great sunshine down there in NZ..
Please post some pics of your system.

Are you running any PV?

Cheers,
Rich

[Solar Mike]

Yes system is all DIY, controller is a Pic cpu setup of my own design, whole system runs off 12 vdc, charged from small pv panel.
Actually two independent setups, one built first was a prototype and at ground level (2 sqm) so it was easy to get to, has wooden case. Second system roof mounted has alloy cases with welded seams (8 sqm), both use 1mm thick polycarbonate for glazing, been up for 4 years now with no problems.
Location is Wellington NZ, not the best weather approx 2000 sunshine hours PA, very windy often part cloudy

Intend to install 3.6KW PV on ground mounted posts, soon as I can purchase grid tie inverter.

Will link to a couple of images, one shows prototype panel that now feeds a HWC supplying just the kitchen (180L), other mounted on the roof connects to the main HWC (300L). I have some other photo's showing the heat dump etc but they are not online so not sure how to post them.

Sorry system doesnt allow me to post any links to images. So what do I do ?

Cheers
Mike

[Piwoslaw]

Welcome to ER Mike

Originally Posted by Solar Mike

Sorry system doesnt allow me to post any links to images. So what do I do ?

Get up to 5 posts and then link all you want. Post in the Introductions subforum, then start a thread on your system and by then you'll be able to include piccies. I'm eager to see them.