Solar Hot Water: Loop Max Temp?

[shaniac]

Greetings, thanks in advance for all your help over the years.

I just completed my SHW closed loop system and it is working like a charm. When the storage tank temp hits 140 the pump shuts down and the fluid in the loop that is still exposed to the sun continues to heat up. My loop sensor said 222 degrees F the other day, and the panels still had one hour of sun on them.

My question for you is:
1. How hot can the panels get and still be fine?
2. Is 222 excessive or par for the course? Any need to worry???

I have a T/P valve on the panels, just in case, along with a pressure tank on the loop to keep things regulated. I have not added glycol to my loop yet, as I wanted a few weeks of problem free observation to make sure all is well, before I committed to the chemicals.

Your advice / opinions are always welcome.  - S

[DanG]

That is called "stagnation" and is a serious condition you need to do more research on.

If its a closed loop have you provided an expansion tank? if your gauge is correct, raw water and 222F means flashing to steam - bad hoodoo.

I've seen a system that has a plumbed "S" pipe under the panels (in the shade) that is valved to allow passive convection coolant flow to keep panel temps below boiling - valve could open when pump shuts down or any temperature you choose, its a parasitic load but keeps it a safer system...

[B529]

 Shutting off your pump at 140 seems low. What is the max temp rating on your tank? Our tank is rated at 185.

 

[ghurd]

Shutting off your pump at 140 seems low. What is the max temp rating on your tank? Our tank is rated at 185. 

That's exactly what I was thinking.
G-

[huntedheads]

My T&P valve is tamped 210 degrees, never seen it yet.

[frackers]

I've had to have a bypass valve retro-fitted to my new 30 tube system as when it was cutting off the pump at 80c the roof manifold went up to over 220c (yes Celcius, not Fahrenheit). The noises, steam and water leaks when the pump started again was something else... and it had a steam release valve on the roof, guess it just couldn't keep up!!

[shaniac]

Thank you for all of your great help and suggestions on this. First off, I did my homework on stagnation. To resolve it I have set the max water temp from 140 to 160 and by the time I get to about 159 the sun is gone anyway, but all this means is I have bought myself some time till next summer. When the days get longer again I plan on covering up one or more of the three panels until the temp and time seem like a good balance. But again, it is a band-aid, not a good solution. Anyone have any better ideas?

[hydrosun]

I think  the company Butler solutions sells a thermostat opening into a radiator to handle overtemp on solar panels. It's real similar to the setup on a car radiator. They also sell the solar wand heat exchanger that screws into a regular hot water tank. Everything is a bit pricey but might be worthwhile in some situations.
Chris

[shaniac]

Great idea, but yes at $500... wowser, I wonder how it works? If it is always on or just when a certain temp kicks in?

On another note, I have found that on my solar controller ( the basic SunEarth-Steca model) there is a hidden menu that allows me to set the pump to come on if a high loop temp is reached. By default it is 261 degrees. It can be adjusted.

Does anyone feel that 261 is too high?

Thanks as always!!!!

[WindriderNM]

261 is too high. You should try to keep it below the boiling point. Antifreeze will increase the boiling point some so will pressure. If the pressure is released it will start boiling spraying hot water.

NOT GOOD!!!

[hydrosun]

The Butler solution overtemp cap looks just like the cap on a car radiator. It has a spring that when it gets hotter shrinks and allows hot water to go out a side pipe into a radiant loop.
Chris

[GaryGary]

Great idea, but yes at $500... wowser, I wonder how it works? If it is always on or just when a certain temp kicks in?

On another note, I have found that on my solar controller ( the basic SunEarth-Steca model) there is a hidden menu that allows me to set the pump to come on if a high loop temp is reached. By default it is 261 degrees. It can be adjusted.

Does anyone feel that 261 is too high?

Thanks as always!!!!

Hi,
Some of the controllers have a  couple features designed to address overheating the antifreeze -- my Caleffi controller has these two:

- You can set the maximum tank temp to (say) 160F.  This will shut the pump off when the tank gets to 160F.

- Now if the collector temp goes above a value that you set (say) 250F, then the pump will come back on even though it will heat the tank above the 160F.  This allows you to protect the antifreeze in the collector from excessive temps, but it does boost your tank temp higher than you wanted to (probably a good tradeoff).
The pump goes back off when the collector temp drops a few degrees below the 250f (or whatever you set it to).

- Then at night, there is a feature that can be enabled that turns the pump on if the tank temp is over the 160F (or whatever you set).  The idea is that if you used the feature just above to protect the collector during the day, and you heated the tank hotter than then 160F limit, then this feature allows you to pump fluid through the collector at night to cool the tank back down to the 160F (or whatever you set).

My guess is that the Steca has these features. 
http://www.solarhotusa.com/support/literature/files/StecaTR0301UHiddenMenu.pdf

If the tank gets up to the high limit you set frequently, then you maybe your tank volume is a bit small for your collector size?
If you can do it, increasing the collector tilt angle will decrease summer overheating and improve winter performance as well.
Some people use a dump load to (eg hot tub or loop of pipe in the ground) to dump excess heat.


Gary

[just-doug]

a nice system will actually dump the excess heat.when you add the glycol to the system,you must keep it from over heating or it will become acidic and eat the copper up in the system.