If we were all to take cold showers and baths, wash our dishes with cold water, and do laundry with cold water, it would be better for the environment and we’d spend less on our utilities. But let’s be realistic; a hot shower feels pretty good. It feels even better with solar hot water. We can use the sun’s energy to reduce our need for fossil fuels and reduce our utility bills.Solar water heating is one of the most efficient ways to utilize the sun’s energy. Solar water heaters need little space on your roof and offset the majority of your water heating costs.

“the average household spends $400-$600 per year on water heating – making it the second largest energy expenditure behind heating and cooling”

Solar water heaters are not a new technology. In 1891, Clarence Kemp patented the first solar water heater in the US. It was a passive, direct, system which is great for its simplicity but not appropriate for climates where temperatures drop below freezing.

At Dogwood Solar, we focus only on solar water systems which are appropriate for our climate in Missouri. Our systems are closed loop, which means the heat transfer fluid that circulates through the solar collector never mixes with the potable water. Instead the heat transfer fluid flows through a heat exchanger in the storage tank where the potable water is heated to temperatures as high as 180 degrees F. Our systems are active, which means a pump circulates the heat transfer fluid.

We install both pressurized glycol and drain-back solar water heaters. Depending on your specific needs, one may be a better fit than the other for you. We are happy to explain in-depth the advantages to each type of system when preparing a design for you.

 Drain-back Solar Water Heater


Easily sized for space heating, simplified overheat protection, great for variable heating loads

Pressurized Glycol Solar Water Heater


Ideal for retrofits to existing homes, extremely quiet, allows the use of roof-integrated collectors for a beautiful installation

How does a Solar Water Heater work?

The sun’s radiant energy heats the solar collector. The solar collector’s black absorber plate warms the heat transfer fluid inside. A pump circulates the heated transfer fluid through a heat exchanger in the storage tank where the potable water is heated. A pump returns the cooled heat transfer fluid back to the collector to repeat the cycle.

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Solar Collector

Generally, the solar collectors are installed on the roof. This collector is a well insulated aluminum frame with a glass front. The glass is tempered for strength and low iron/low reflectivity allowing more energy into the collector. Inside the collector a series of pipes run along the back of an absorber plate. The absorber plate is painted black and has a selective surface coating to increase the heat transfer from the sun to the heat transfer fluid. When the system is on, the heat transfer fluid flows through these pipes collecting solar heat from the absorber plate.

Heat Exchanger

The heat exchanger is the point at which the heat from the sun is transferred to the water storage tank. An internal heat exchanger sits in the bottom of the storage tank. This type of heat exchanger consists of a long, coiled length of pipe with fins to help efficiently transfer heat from the heat transfer fluid to the potable water in the tank.

Storage Tank

Think of the solar storage tank like a battery. The storage tank holds heated for later use. Because the heat source [the sun] is available most days, we only stockpile enough hot water for a few days. There is no advantage to significant overproduction of hot water. The super-insulated storage tank minimizes standby loss and keeps the hot water available for use throughout the day and night.

Differential Controller

The differential controller is the brains of a solar water heater. The controller works by constantly measuring the temperature at the rooftop collector and in the storage tank. When the differential controller senses that the temperature of the collector is significantly warmer than the temperature of the water in the storage tank it turns on the pump to circulate the heat transfer fluid through the collector and then through the exchanger. The pump shuts off when the differential controller senses that the temperature of the collector has dropped below that of the water in the storage tank. This cycle happens automatically throughout the day depending on how much sun is available and how much hot water is being used.


In most cases, a small, low wattage variable speed pump is used to circulate the heat transfer fluid through the collector and the heat exchanger. In a pressurized glycol system it does not take much pressure to move the heat transfer fluid. Using a small variable speed pump is beneficial as it takes only a small amount of electricity to move heat from the sun to your storage tank.

Heat Transfer Fluid

Water is the most efficient heat transfer fluid. We utilize a mixture of distilled water and propylene glycol. Propylene glycol is a food-safe antifreeze. We use the antifreeze to allow us to operate the system year round even in Missouri’s climate. We only use DowFrost HD Propylene Glycol. This type of glycol has special additives to ensure it will work as expected for years as it is exposed to the temperature extremes produced by a solar water heater. Never permit the use of ethylene glycol [the same antifreeze used in cars] in your system. This antifreeze is highly toxic and not appropriate for use on a solar water heater.


Until recently, there was no reliable, cost effective way to monitor system performance. We now include an internet-based monitoring system with all new solar hot water installations. This gives you the ability to view and share your system performance with friends and family. An added benefit is if your system is not performing as expected, we will receive an alert indicating a potential problem and we will contact you if further action is required.


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