Hot Water Usage & Overheating

Normally, most properly sized systems will not overheat. Generally speaking, a tank requires 1 Duda Solar vacuum tube for every 10 liters of water to be heated. This will generally heat the water from cold water inlet temperatures to 50°C (122°F) each day. In the winter, there will be less available heat due to less efficiency and less sunlight. Those who lean towards having the maximum amount of their heating needs produced by the sun, like very hot water for dishwashing or those who are trying to get off the grid completely, may oversize their heating power by 2 to 3 times, which may lead to overheating, especially in the summer.

When Overheating Occurs

Active solar water systems are closed loop. As the temperature of the working fluid increases, so will the pressure. An expansion vessel is used to relieve this added pressure. Should the system shut down the pump in tank overheat protection mode, this expansion vessel is essential for preventing too much pressure build up in the system. As fluid evaporates in the manifold from the excess heat, it pushes the remaining fluid in the pipes down into the expansion vessel. Once all of the fluid has been pushed into the expansion vessel, the system will remain stagnant until the collector cools down enough for the vapor to condense back to a liquid. It will then resume normal operation once the tank temperature protection mode is no longer in effect. Usually once an active system shuts down, it will not resume collecting of heat until the next day, except towards the end of the day when there is not much heat left to collect.

There are two problems with allowing the working fluid to boil off. The first problem is the obvious, that is, that no more heat can be collected during the day once the stagnation temperature is achieved. Because the pressure is too high within the collector, the pump cannot circulate the fluid. Also, if it could force it, there is a chance of damaging the equipment due to the extreme temperatures. Therefore, if it is common for hot water to be needed during the heating period of the day, it may be advisable to install a heat dissipater.

The other problem is that if propylene glycol is used as an antifreeze in the working fluid, it will become more rapidly acidic when above 280°F. The good news is only a small part of the working fluid is exposed to these high temperatures. The vapor inside of the collector pushes the fluid out of the collector and most of it into the expansion tank. For short periods of time, this is not a problem, as propylene glycol with corrosion inhibitors helps prevent corrosion to the piping. But if a system is oversized in the summer and overheats everyday, this can become a potential issue without a regular change of the working fluid. Typically the fluid should be changed every 3-5 years. If overheating is common, it may need to be changed every 2-3 years, depending on how hot and for how long.

Ways to Prevent Overheating

Sizing for the Seasons

The simplest way to prevent overheating from being a problem is to not oversize the heating power of the system too much. If the main concern is to avoid using backup heating, it is better to install a larger system so that the tank capacity cannot be exhausted by your typical usage. The water will not be as hot, but the amount of heat able to be stored will increase. Should not much water be used for some time, it will be difficult for the solar water heater to overheat the tank as there is a lot of water which needs to be heated before overheating can occur. The best advantage to a larger tank rather than over-sizing the heating power is that the systems run more efficiently with low ΔT, and therefore more hot water can be obtained overall.

If a system is oversized on its heating power, one method to avoid the overheating is to angle the collector more towards the horizon so that it is winter efficient. In the winter months, the little extra gains to be made will add up to the amount of heating power needed in the winter time, and the losses from too high of an angle in the summer will help make up for the extra heat which is not needed.

A system can also be oversized by adding reflectors to the collectors rather than adding excessive amounts of tubes. The reflectors can add around 50% more sunlight energy to the tubes, during the winter when the extra energy is needed. During the summertime, the reflectors can be removed to minimize overheating when the energy is not needed as much.

If the system is oversized and the maximum amount of energy is desired year-round, the system's working fluid may be changed on each season change. When freezing is likely, the working fluid should be an appropriate mixture of inhibited propylene glycol. However, when the warmer months approach and freezing is no longer an issue, the glycol can be purged out, bottled and saved for later use, and the working fluid can be changed to pure water. When pure water is the working fluid, overheating will cause a system shut-down, but it will not become more acidic from boiling off into steam.

For those who prefer to oversize their systems to use minimal amounts of backup heating, read on.

Collector Overheat Protection Method

By far the best way to prevent overheating in an active solar system is to use the collector overheat protection mode on the controller even when a system is oversized in a high sun output region. When the summer months begin to overheat the water heater tank, the maximum tank temperature should be set to the minimum desired water temperature, ie: 50°C. Then the max collector temperature should be set to a high temperature prior to boiling off of the working fluid such as 110°C. When the tank reaches the maximum temperature, the pump will shut down. Then, when the collector reaches the max collector temperature, the pump will resume to prevent overheating of the collector and begin heating the tank again. Since the collector is much less efficient when it is very hot it will actually act as a heat dissipater for the solar system. During the summer months, most excess heat will bleed off to the ambient air while still heating the tank a little bit, depending on how oversized the system is.

The above method may require a little attention from the user at first to learn how much excess heat needs to be shed off during the day. Depending on how oversized the system is, the max tank temperature may need to be set lower than the desired tank temperature to allow for more heating during the overheat protection mode of the collector. When the system is in this mode, it is actually more efficient in electrical use since the pump only runs when there are extreme amounts of heat available.

Tank Recirculation Mode

Our controllers also have a function which will cool the tank at night by using the collector as a heat dissipater when there is no sun available. If a system is found to receive too much heat during a few months of the year, this mode can be enabled to dump heat from the bottom of the tank through the night to the ambient air through the night.

This function is also utilized in the vacation mode to discharge heat collected during the day when there may not be anyone in the home to use any of the hot water. Vacation mode works well for short trips out of the home. For extended vacations, it is best to skip the vacation mode and use a combination of the collector overheat protection mode with the tank reciruclation mode. Set the max tank temperature to the lowest setting so that collector overheat protection will always be enabled and then the tank recirculation will bleed off any heat obtained at night.

Heat Dissipation

A heat dissipater allows for rejection of excess gained heat when the solar water heating tank has reached its maximum allowed temperature. The advantage to this type of set-up is that when the tank reaches maximum temperature, the system does not need to shut down. Instead, the hot working fluid is diverted to the heat dissipater, which releases heat back to the ambient air. When some of the hot water in the tank is used, cold water fills into the bottom and then the system can direct the working fluid back to the tank to heat the cold water in the bottom of the tank.

The controllers used in solar water heating systems have an overheat protection mode option. When this mode is activated, the controller outputs a current to a secondary pump or switching device. Typically, when this function is used, solenoid valves are the best way to divert the fluid. A tee should be placed on the outlet of the solar collector, with a normally open solenoid valve connected to the primary solar loop, and a normally closed solenoid valve on the diverting loop. When the overheat protection mode is activated, it triggers both valves, closing the main loop and opening the secondary, which allows the fluid to pass through the heat dissipater first prior to circulating through the rest of the system and back to the collector.

Another method for heat dumping is to use a diverting thermostatic mixing valve. The valve can be set to a maximum fluid temperature. When the temperature of the working fluid reaches this temperature, the fluid will automatically divert to the loop which contains the heat dissipater to release some heat. This type of set-up allows for some electrical savings over the solenoid valve method. However, the disadvantage to this set-up is that if solar irradiation is high, the working fluid may exceed this maximum temperature prior to the tank reaching its maximum temperature, thus dumping heat prematurely. There is more control when solenoid valves are utilized since the controller can measure the temperatures and determine when the switch should be made based on the tank temperature.