Solar Water Heating Systems with Gas Backup

Traditionally domestic hot water solar systems have had an electric backup. It has been relatively easy to situate an electrical element of sufficient power the correct distance from the top of the tank to give adequate backup.  With Natural Gas or LPG taking over the nations water heaters, we need to look at how to get the best efficiencies from solar systems using a gas backup.
Example of multiple backup methods are shown below. These systems combine linking methods which can be used independently in a variety of ways.
Gas Boilers

While there are some very efficient gas domestic hot water units on the market, many boilers, particularly if they have a central flue, suffer from high stand-by heat losses.  If we use one of these gas boilers as part of the solar storage, and raise it's temperature above it's normal 130ºF, the losses will become extreme.
Connect the solar tank in series, with a tempering valve reducing the temperature of the water entering the boiler to 130ºF
1.  Solar Tank 4.  Gas Flue
2.  Gas Boiler 5.  Hot Water to Taps
3.  Tempering Valve 6.  Cold Feed

Instantaneous Gas Water Heaters

One of the most exciting attributes of gas is it's ability to provide instant hot water. Instantaneous gas water heaters, also known as tankless water heaters, have been on the market for many years.  They generally have a fixed heat output, and use flow restrictions to achieve high temperatures from very cold groundwater. This can be a problem if they are fed with solar preheated water.
Recently a number of solar models have been introduced.  These units check the temperature of the water entering the unit and modulate the flame accordingly.  If the inlet temperature is high enough, the heater does not ignite at all. 
This allows us to install the most efficient solar storage system, where the solar energy takes care of all the standby losses          
1.  Solar tank 4.  Cold Feed
2.  Instant Gas Heater 5.  Hot Water to Taps
3.  Tampering Valve 6.  Gas Feed

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Columbia, Maryland 21045
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