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SPACE HEATING : Direct Hot Air Heating, Heat Pump Preheat, Air Radiant Floor Heating
WATER HEATING: Water Preheat
EQUIPMENT HEATING: Clothes Dryer Air Preheat, Heat Pump Heating
COMMERCIAL: Industrial, Commercial, Agricultural
How to Use Your Heater Boards for Water Preheating
Most solar water heating systems are actually water “preheating” systems. They preheat water that enters the house from the well or city water main, before it enters the regular water heater. The solar heat is collected from panels, as either heated air, or heated water, or a heated antifreeze solution. The solar heat in the air or water/antifreeze is then passed to the household water supply by an air-to-water heat exchanger or a liquid-to-water heat exchanger. This household water is stored in a storage tank that then feeds into the regular household hot water heater. The regular hot water heater delivers water to the sinks and showers and laundry, etc. See image below.
In most of the US, the household water comes into the house from the city water main, or the well, at between 45 and 65 degrees F. This cold water moves into a solar preheat tank where the solar heated air or water/antifreeze fluid heats it from its cold temperature to as much as 140 F. In the early morning hours, the solar heat delivered from the panels is quite low, but the morning hot water use is quite high. About ½ of the daily hot water load is in the early morning. So, solar heating panels can not instantaneously provide all the heat necessary to meet the morning hot water needs, without some storage of heat collected throughout the previous day.
Heater Board Solar Hot Water Pre-heating download .pdf
To provide this storage, the solar preheat tank takes in the coldest water (~50F) and preheats it with solar heat, throughout the day, and stores the preheated water until it is needed by the regular hot water heater. When water is drawn from a faucet or shower, water moves out of the regular water heater at about 120F and is replaced by warm/hot water from the preheat tank. The water drawn from the preheat tank is replaced by cold city/well water. For example, if solar heat preheats the water from 55 to 90F, then the regular tank only has to heat the water from 90F to 120F, cutting the electric/gas/oil heating load by more than half.
During the sunny, warm summer, the preheat tank might easily reach 120F by the end of the day, eliminating most of the hot water heating load. During the coldest winter days, the air (or water) from a collector might not be warmer than 70F. While that’s too cold to directly heat the air in a house, it is still warm enough to preheat water coming into the house 45F. So a solar air heating, hot water preheat system can make use of solar heated air year round. (Even when the delivered air temperature is too cold for direct space heating).
There are two ways to install such a system. The first way uses an integral tank and heat exchanger. The second way uses a copper coil outside the tank to transfer heat from the air to water in the coil, and a water pump to circulate solar heated water from the coil to the preheat tank.
The first way (integral tank and heat exchanger) uses fewer moving parts, costs less, uses parts available from any good sized home center or plumbing supply store, and requires no electrical work. A schematic is shown in the figure above.
The preheat tank to use is a conventional, tank type, gas hot water heater. The tank acts as the preheat storage volume. The internal flue and tank bottom above the burner area act as the heat transfer surfaces for solar heated air to heat the water in the tank. This is exactly the same duty these surfaces perform when gas is burned in a conventional gas hot water heater, only the solar heated air is lower temperature and runs constantly through the day.
However, in this case,
NO GAS IS CONNECTED TO THE TANK.
 
In fact, it is strongly recommended that the internal burner be removed from the tank and the gas valve be removed, or at least labeled to prevent anyone from connecting gas in the future.
The best performance will be achieved by introducing the hottest solar heated air into the flue at the top of the tank and letting the air exit at the bottom of the tank. A 40 gallon residential gas hot water heater will typically have a flue pipe that is 3 inches in diameter. A 3 inch duct can carry solar heated air from the Heater Boards to the top of the tank. The 3 inch sheet metal duct can be clamped over the flue to direct the solar heated air down the flue to the ‘burner’ area. You may need to make a small cut in one end of the 3 inch diameter duct to fit it over the 3” flue pipe. Use metal foil tape or duct tape around the joint and then install a hose clamp over the tape at the top of the flue to secure the duct and flue.
The solar heated air will pass down the flue and out the air outlet at the front of the tank.
You have the choice of collecting and re-using the solar heated air that passes through the outlet of the preheat tank, or you can simply let it exhaust into the room. The air stream will still have some useful heat that can be used for other purposes as it exits the tank. However, in the summer you will probably want this air sent out of the house. So, re-using this solar heated air will add to your savings and keep the room more comfortable in summer.
To collect this exhausted solar air requires that an adapter be built to the front of the air outlet. Gas hot water tanks have a variety of air outlets at the bottom. For older tanks, the air would exit through the door at the front of the burner area. Modern tanks usually have the ‘burner’ door area tightly sealed and have different air inlets on the sides or bottom of the tank. These air inlets have systems to prevent gas flames from burning backwards out through these inlets. Since the solar preheat tank has no gas connected or open flame, these inlets are not required. These inlets include spring loaded baffles, fine mesh screens, and other mechanisms. Whichever system you have on your preheat tank, these systems can be disabled to direct the exiting air to the ‘burner’ door area. At the ‘burner’ door area, you can capture the exiting air in a duct to direct it to any other useful purpose, such as clothes dryer air preheating, combustion air preheating, ventilation air preheating, or crawlspace preheating. During the summer months you can direct this warm air out of the house.
A small section of 3” or 4” rigid metal duct can be installed at the burner door, as shown in the photos. The small duct should be feathered on one end with small cuts around the length to make tabs that can be bent back for attachment. The other end should be crimped to allow attachment of another duct to direct the exhaust flow. The feathered end is placed up to the ‘burner’ opening and between the duct and opening should be taped tight to the perimeter of the ‘burner’ door. Taping the inside of the joint, where the duct is attached to the opening, can be accomplished by reaching inside the duct with several small pieces of tape.
The figure below shows data taken from several days of operation of an air to water preheat tank with integral air to water heat exchanger. The temperatures shown are air temperatures at the top and bottom of the tank and outside air temperature. The air temperature difference between the top and the bottom represents heat given up by the air to the colder water in the tank. At the end of the spring day, the system delivered over 10,000 BTU of heat to the water which was used in the household throughout the day. Higher energy delivery would be expected during the hotter summer months, with higher solar air temperatures.
The air exiting the bottom of the tank was as hot as 90 degrees during the peak hours of these April days. That exhaust air was ducted to the nearby clothes dryer air intake and used for clothes dryer air preheating and for direct space heating and boiler air preheating.
The second approach to water preheating is to use an air to water heating coil and a separate tank and pump. The solar heated air heats water in the coil which is continuously circulated through the coil and to the tank and colder water from the bottom of the tank circulates back to the coil. The coil can be sized to maximize the heat transfer surface between the water and air. This transfers more heat the to the water than the integral tank and heat exchanger, with its fixed heat transfer surface. However, the coil approach adds more parts, more expense, more complexity, and another electrical and piping connection compared to the integral tank and heat exchanger.
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