Stagnation occurs when the solar storage tank heats to maximum temperature early in the day; flow through the solar collector is stopped, and the stagnant fluid in the collector gets hotter and hotter. The results in a raised pressure, high temperature condition that can lead to increase maintenance requirements or even damage the system over time by subjecting it to extremes of expansion and contraction. In addition, when anti-freeze used as a heat transfer fluid in solar heating systems is superheated every days for weeks at a time, itbreaks down and becomes acidic, thereby transforming into a corrosive substance that circulates through your system slowly damaging its components.
To prevent this, some method of overheating protection is suggested for those times when there’s simply more hot water capacity than you need.
SunEarth manufactures both passive and active solar water heating systems. Passive solar water heaters, in particular ICS(Integral Collector and Storage), are able to achieve water temperatures of 100ºC or 212ºF. While active SWH systems can reach collector temperatures above 350F. As impressive as these facts may be, it also presents a challenge: how to protect yourself and your water heating system from too much heat and pressure.
First, all residential installations should include a tempering valve, also referred to as a thermostatic mixing valve such as those manufactured by Cash Acme and Watts. This is a mechanical device that is installed downstream of your solar water heater. It is a spring loaded device that automatically mixes hot and cold water down to a set temperature. It does this by bringing together the solar water storage hot line with a cold water line. As the water from the solar storage hot line passing through the mixing valve gets hotter, the mixing valve restricts that line further therefore mixing in more cold water. This all happens automatically.
There are two main types and temperature ranges of thermostatic domestic water mixing valves. They are typically made with ranges of 80ºF – 120ºF and 120ºF – 160ºF. Most people reading this will want a lead free mixing valve with 80ºF – 120ºF range as 110ºF is a comfortable shower for most people. Those who have a long pipe run between their shower and water heater or those who like especially hot shower can open this valve all the way to get 120ºF hot water at the tap. WARNING water above 120F can scald within ** seconds.
You’re protected. What about your SWH system?
All of the components of your water heating system, like your piping and your conventional water heater should be rated to handle water temperature above 180ºF. If any component (like a tankless water heater) is not rated for that temperature, then plumb in your mixing valve between that component and your solar water heater.
For the plumbing system pressure is as much as a concern as heat. As the temperature of your water rises, the water expands and increasing pressure on your piping and components. In many municipal and well water systems there are back flow preventers. These are one way or check valves that allow water from the city come into your home but do not allow the water to flow backwards. Whereas systems without back flow preventers allow for the water to expand back out to the street, those with backflow preventers do not allow that and can cause your home’s piping systems to handle the increased pressure. Most plumbing systems can handle this, but pressure fluctuation on a plumbing system create wear and tear and can shorten the working life of the plumbing components. That is why we recommend that expansion tanks be installed with your solar water heating system.
Expansion tanks contain a rubber bladder ithat has pressurized air on one side and water on the other. As the water expands in the plumbing system and the pressure increases, the water pushes its way into the tank which compresses the air on the other side of the bladder. This allows the tank to take the extra expansion instead of your plumbing system and keeps your pressure constant as the temperature in your solar water heaing system r fluctuates.
Built-In Protection
The critical component that all water heaters, solar or conventional, have is a temperature and pressure valve, commonly referred to as a T/P valve, a PRV (although this only refers to the pressure), or a pop off valve. The T/P valve is very important as it protects the solar water heating system and the plumbing system from increase pressure. Conventional water heater valves are set to open at 210ºF or 150PSI. This means that the plumbing system is not subject to too high of pressure. Technically, as a safety device some jurisdictions rely on this valve soley instead icluding an expansion tank, but they are in fact complimentary to one another. With the expansion tank functioning to smooth and regulate pressure fluctuations and the T&P as a safety device.
Every time your T/P valve opens or releases it expels hot water. This allows cool water to enter which cools down your system and lowers the system pressure. Because this does occasionally happen, it is important that your T/P valve is piped while maintain drainage slope to a place where it is safe to drain extremely hot waterIt is also worth noting that you should manually release your T/P valve every six months to ensure that it is working properly.
The T/P valve can release up to 20 gallons when cooling your system and this wastes water (unless it is piped to drain into a pool or otherwise captured) and heat. Under normal operation and if your system is sized correctly this shouldn’t happen often, if ever. However, for some of our customers in places with extremely good solar resources, like parts of the Southwest, this may happen occasionally. For these customers and others who have more intermittent use, we have developed a few options to help manage over production of a SWH system.
Can a Drain-Back system Stagnate?
A drain-back solar water heating system is a non-pressurized volume of water in a closed circuit that, as the name implies, drains back from the panels, down to a drainback reservoir, at the end of every heating cycle.
Drain-back systems offer built-in freeze and overheating protection, because fluid only enters the collectors after they has been heated, and then ‘drains back’ when the collectors cool below their operational range. This in itself ensures that freezing of solar fluid does not occur. While additionally providing protection of the solar fluid from stagnation temperatures during the summer when the solar storage tank is fully heated.
However, there are some unique design requirements to this type of system. One is the need for a high head pump (and the slightly higher initial cost and daily operating costs that go with it) because, unlike a pressurized closed loop system, the pump must be powerful enough to push water from the solar storage tank, against gravity, up to the panels. A pump this size requires approximately 245 watts during operation. As a comparison, a standard solar circulator in a pressurized closed loop system uses approximately 45 watts.
Also, the solar collector array must be installed at a slight angle to drain. A minimum slope of ¼” per foot must be designed into the support structure to guarantee that all the liquid in the collector drains back to the drainback reservoir. If your solar array is clearly visible from below, this can give your system an “off level” look.. In addition, if you’re installing panels in a ground mount application, a drain-back system may lack sufficient fall to the drainback reservoir.