Summer active heating control?

[Ryland]

Originally Posted by Drake

What would "shading" be aside from tree or overhang? A cover?

It could be a fancy cover on a sliding track or as simple as a heavy tarp that you tie over it, but an overhang is going to give you good passive shading.
Most people however install a dump loop of some sort so they can dump the extra heat in to the ground, doing this still allows you to use you to get solar heated water in the summer without worrying about over heating your system, if you have a pool or a hot tub then they would be the perfect heat dump, otherwise just a loop of pex dug in to the back yard works really well.

[GaryGary]

Originally Posted by Drake

Passive summer heating control is pretty easy to understand - shading. And domestic water heating is needed all year. But how is summer overheating controlled in other active collector systems?

Hi,
When you say "other active collector systems" I guess you are talking about space heating?

The method I like for controlling overheating collectors on space heating or combined space and DHW systems is to mount the collectors vertically. Vertical collectors can be left stagnated all summer if you want to without overheating. The sun is so high in the summer and makes such a large incidence angle with the collector that it won't overheat or damage the collector as long as good materials are used.
This is the system I'm using now:
A Simple DIY Solar Space and Water Heating System
Even with the double glazing, the system is fine through the summer. I have the controller set to turn the pump off when the tank gets to 165F (this insures very long life for the EPDM tank liner). Its a drain back system, so all the fluid drains back to the tank, but even if the collector stayed full of antifreeze, the vertical collector does not get hot enough to damage the antifreeze.

To me the problem with relying on a dump load to protect the collector is that there are a whole bunch of things (controller, pump, valves, power, ...) that can fail and leave you with no protection. I think a dump load is OK as a means to make life easier for the system, but if the system destroys itself with no dump load in place, then you are at the mercy of a failure in any one of a half dozen pieces of equipment.

As another example of how protective high tilt can be, this is the system I used before the one at the link above:
The $1000 Solar Water Heating System
It is a very steeply tilted collector that is made from PEX. PEX is very subject to damage from high temps, but this system never exceeded 230F even when stagnated. There are a full year worth of temperature plots here:
$1000 Solar Water Heating System -- Performance

I like DHW systems with high tilt angle collectors and more than the usual amount of collector area -- this gives you a system where the winter performance is better than typical commercial systems (because of the high tilt working better with low sun and the excess collector area), and that does not overheat in the summer.

Gary

[Ryland]

Originally Posted by GaryGary

To me the problem with relying on a dump load to protect the collector is that there are a whole bunch of things (controller, pump, valves, power, ...) that can fail and leave you with no protection. I think a dump load is OK as a means to make life easier for the system, but if the system destroys itself with no dump load in place, then you are at the mercy of a failure in any one of a half dozen pieces of equipment.

The dump loop should be using the same pumps, sensors, controller and everything as the main loop under your floor that is used for space heating, a single valve switches between the dump loop and the floor loop so your system should be designed well enough and use high enough quality of part and pumps that you will not be left without heat in the middle of the winter, because a system that has a pump fail in the winter will also destroy it's self by over heating as well and that is when I've seen more systems fail is in the winter due to snow being on the PV panel that runs the pump or the thermal expansion being too great going from -20F at night to 200F in the day time that the fluid shrinks and sucks in air or expands to much and bursts at a weak point.

[benpope]

Probably a silly question, but does the pressure get up high enough (20 PSI I think) to use a T&P valve to relieve an overheating system? Also I think there are other valves that open and close just based on temperature. This may be what you guys are already talking about, but I am unsure.

[Ryland]

Originally Posted by benpope

Probably a silly question, but does the pressure get up high enough (20 PSI I think) to use a T&P valve to relieve an overheating system? Also I think there are other valves that open and close just based on temperature. This may be what you guys are already talking about, but I am unsure.

Solar hot water systems should have a pressure valve that is rated for a boiler, not a hot water heater, the hot water heater T&P valve trips at around 150psi and 210 degrees, boilers don't tend to have a temp relieve but instead are set just for pressure, I think 30-50psi is most common.
Solar hot water systems on a sunny day can get very hot, I've seen fluid coming in to a house at 230F in the winter, that is after traveling 200 feet or more underground, so coming out of the panel you can see temps that are pushing the limit of what the antifreeze can even handle.

to control a dump loops you often see a set of ball valves, like are used to turn off the water on the water mains on your house, manual ball valves are cheap or you can get motorized ones to automate your system.