Marathon Water Heaters: For Contractors

Solar Collectors at our eastern office provide all the hot water needs for the facility. No small task considering part of the office is rented by a cleaning service that does over 20 loads of laundry a week in this upstate NY facility.

With propane sky high, natural gas at record highs, fuel oil rising and electric rates following suit, there is renewed interest in solar domestic hot water systems. The technology is sound and well proven. Those that invested in this technology back in the early ‘80s when federal tax credits were available have enjoyed thousands of dollars of savings. The good news is the sun is still there along with a new 30% federal tax credit.

There are a number of systems available:

Thermosiphon Systems. These systems heat water or antifreeze fluid and the fluid rises by natural convection from collectors to the storage tank, which is placed at a higher level. Although heat transfer is not as rapid as other systems, no pumps are required. In thermosiphon systems fluid movement, and therefore heat transfer, increases with temperature, so these systems are most efficient in areas with high levels of solar radiation such as the deep south and Caribbean.

Direct-Circulation Systems. These systems pump water from storage to collectors during sunny hours. Freeze protection can be obtained by recirculating hot water from the storage tank, or by draining the water back to a reservoir in the home (drain-down). Since the recirculation system increases energy use while flushing reduces the hours of operation, direct-circulation systems are used only in areas where freezing temperatures are infrequent. special caution must be used however in the event of a power failure. Collectors can reach stagnation temperatures of over 300F and when the power returns the super heated water can ruin storage tanks.

Drain-Down Systems. Treated or untreated water is circulated through a closed loop, and heat is transferred to potable water through a heat exchanger (HEX). When no solar heat is available, the collector fluid is drained into a reservoir by gravity to avoid freezing.

Closed Loop or Indirect Water-Heating Systems. These are some of the most prevalent solar water heaters. They use the sun to heat water or a non-toxic antifreeze solution in the collector. The most commonly used fluid for freeze protection is propylene glycol – the same material used in many of your foods. When the controller senses the collector is hotter than the water in the storage tank, a circulation pump turns on, through a HEX (heat exchanger) then the tank.

As the system is not operating at night or during rainy days (it does work to a degree even when cloudy!) it is essential the tank be larger than normal and very well insulated. Many solar dealers and manufacturers have found that Marathon is the ideal solar tank for these applications as it is the most efficient with the lowest heat loss. Some such as Energy Works of Maine are even packaging their systems with the Marathon.
	Pictured at left is the Energy Works Marathon package they use with Apricus Collectors.
At the right is the packaged system manufactured by Purist Energy LLC

Sizing:

For rough calculations, assume 20 sq ft of collector area for the first 2 people in the home and 8 sq ft for each additional person if your are in the sunbelt. If you are in northern climates, add 12 sq ft per additional person.

To calculate your tank capacity generally 1.5 gallons of storage per square foot of collector works well in the north and a minimum of 2 gallons per square foot in high sun areas (too deal with excess heat).

Caution: Improper sizing or can cause severe overheating. Your Marathon warranty will be voided by storage temperatures exceeding 185 degrees F. This may occur for improper sizing or direct circulation systems that can send a surge of super-heated water to the tank following power interuptions. If you are unsure of sizing or installation, use a qualified contractor!

Retrofits and Upgrades:

It is also possible to use the Marathon to replace an existing failed steel solar tank. In the picture on the left, the contractor is using a vertical HEX (“Hot Rod”) that is connected to the tank (MR105) through the top element port and drain valve. Other heat exchangers maybe used and they are usually connected to the cold water dip tube (from HEX) and drain valve (to HEX).

Note: Bushings are available from Marathon to adapt the element ports to 1"NPT.

If you are thinking of going solar here are some things to consider:

Use a reputable dealer. Do they have experience with solar hot water? Do they know how to protect against both overheating and freezing? Are they familiar with Marathon?
Size it correctly; Most people are not aware of the btus a solar system can produce. Improperly sized systems cause more issues than any other. Usually this is due to the collector array being too large for the tank. You are far better having the storage being a little larger than needed than undersized. We strongly recommend using at least an 85 or 105 gallon model for most solar applications.
If your temperature can drop below freezing, you must use antifreeze in a closed loop system, or a drainback system. The antifreeze should be propylene glycol solution suitable for solar and at a solution strength designed for your area. NEVER use ethylene glycol or car antifreeze – it is very toxic.
Solar tanks and loops undergo significant temperature ranges which cause pressure fluctuations. Expansion tanks must be added to any closed loop systems and usually to the tank if none is present.
If you are unsure of the sizing or design, purchase a package system and have it installed by a professional. There are numerous dealers that have taken the guess work out of the system design. Their experience and support can be invaluable.