Best Water Heater Under the Sun!

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.

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 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 interruptions. 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 may be 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:

Recommendations for Solar Hydronic Systems

Solar applications place special demands on a water heater. However, there are a number of steps you can take to avoid problems.

Don't undersize. Unless you live in the Barbados, don't even consider a tank of less than 75 gallons. More solar problems result from overheating and system stress than anything else. A larger tank will give you more storage and provide a buffer for thermal stress as it is less likely to go through extreme temperature swings. You will also store more energy; remember, you store BTU's NOT degrees. Also, don't get sucked into over-sizing the collector area/system for space heating. The summer thermal stress will be a constant service issue even with and including any heat dump. Your savings are in domestic hot water, it is used 365 days a year.

Set it up right: The Marathon's thermostat is designed with an upper limit of about 175F. The BOTTOM thermostat is adjustable to this temperature, which means the entire tank can be heated to this level. The TOP thermostat has an ECO setting of 160F for safety reasons. During vacations and peak solar times, the tank could easily reach this temperature. You may want to consider a high temperature top thermostat which goes to the 190F. Just order Special Use/Dairy Thermostat Part number SP313070.

If you are using a single upper element as back-up, upgrade to the titanium if there are any issues. The top element is fused to protect the tank and designed for quick heat transfer, but the bottom element is the workhorse. Because of this, the top is copper and the bottom is titanium. As a replacement part you can install a titanium upper element.

Replace the boiler drain with a full port ball valve if there is high sediment. The ¾" boiler drain on the heater actually gets reduced to about 3⁄8" inside of the valve. If you are using the drain as a solar port, replace the valve with a 2" brass nipple, tee, and full port ball valve (these remain a full ¾" throughout the body). This will allow you to open the valve quickly and blow out any sediment in a matter of seconds. Marathon has a full port ball valve for these applications with hose threads on the end and is available as part number SP330000.

Do a water test. Lime (Calcium carbonate) is the enemy of hydronic heating systems particularly where there are small orifices involved such as flat plate heat exchangers. Inexpensive hardness test strips are fine for this and will give you a much better idea of what you are dealing with and how to mitigate it. Water conditioners such as Scaltrol, and conventional salt-based conditioners are both effective in prolonging life of the equipment.

Protect against freezing. 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.

Do not go cheap on your sealants. Due to its chemical make-up, propylene glycol will leak in places water will not. Typically, it will seep through Teflon tape and many connections over time. Pipe sealants such as SOS #10 (blue) or Permatex (non-hardening) are good choices.

Expansion woes. If the system is on municipal water with a backflow prevention valve (typical) you may need to install an expansion tank on the water system if one is not there. As the water is heated in a large solar tank it will expand. If there is no well tank or expansion tank it will create pressures well in excess of 100psi and cause the T&P to weep or relieve. This is more pronounced in solar systems due to the tank volume and temperature swings not present in a standard water heater. Any closed loop solar system will also have its own expansion tank.

Temper the water. Solar water heater's temperatures fluctuate far more than any other. Avoid the risk of scalding by installing a good quality tempering valve on your system.

Take advantage of industry experience. 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? You have an investment that can pay dividends for years if properly done.