Solar Domestic Hot Water
(DHW)

The sun has been heating water since the dawn of time. So, it is probably not surprising that it is pretty easy to do. Not only is heating water with the sun easy, it is both necessary and relatively expensive. Every household in the United States, with few exceptions, uses large amounts of hot water for cooking, cleaning dishes, personal care and bathing, and other cleaning purposes. A widely accepted rule of thumb for the last few years is that a family of four spends around $800 per year in energy costs to heat water. This number goes up with the cost of fuel and goes down with any efficiency improvements like insulated water tanks or low flow showers and higher efficiency dish washing machines. Since most efficiency improvements are at their end, the cost for Domestic Hot Water (DHW) can be expected to rise with the cost of energy.

If you only consider one solar technology, DHW is the one to consider. Simple evacuated tube systems can be purchased on line that are very easy to mount and use for less than the cost of two years of DHW. Thus, you can reduce your pollution from DHW to near zero immediately and reduce your energy bill from DHW to near zero within two years.

The DHW portion of your energy bill is likely about 50% down to 25%. This means you can reduce your energy costs by 50% to 25% with an investment that is fully recovered in less than two years. Compared to other popular energy improvements like upgrading a dish washer, upgrading a water heater, upgrading you house windows or upgrading your house insulation. Solar DHW is many times more effective and usually costs less. In my opinion, from an energy perspective, only homes built before 1980 should have insulation or window upgrades before solar DHW is installed.

I have had a situation repeat several times in the last few years that is worth mentioning here. Everyone has periodic need to replace their tank based water heaters. About 12 years ago I remodeled a house and put in an "on demand" water heater. This device has no water tank but rather a large burner that is capable of heating a significant flow of water to a desired temperature as you use hot water. The selling points recommending on-demand heaters were these:

1. No water tank to corrode and fail (longer life of on-demand heater).

2. No limit to the amount of hot water available.

3. No possibility of biological (Legionnaire's disease) due to insufficient heat in stale water.

4. Reduced energy consumption by reducing heat loss of holding tank.

5. Smaller space requirement.

Since I installed my system I have had many ask me how I liked my on-demand system as they were considering converting when their tanks failed. I have recommended they not use an on-demand system, but instead put their money into a simple solar conversion. My advise has only been heeded once and greatly appreciated. Once a person who asked me stayed with his tank system. The other times people have not heeded my advise and installed on-demand systems and observed the negative characteristics I am listing now:

1. There is a delay from the time you open a hot water spigot until you get any hot water that is significantly greater than that of a hot water tank. This is due to two things. First, the core of the on-demand unit has to heat up which usually takes about 20 seconds. Second, the on-demand unit has to sense water flow which only takes about 1 second. So, you have to waste 21 seconds of water, energy, and time whenever you use an on-demand system that has been cool for any length of time.

2. On-demand systems know to turn on by sensing water flow. If you want a low flow of hot water for rinsing dishes for example, the On-demand system may not turn on or may not stay on. This means you end up using more hot water than you need in many cases.

3. On-demand systems are significantly more expensive than tank heaters to install because the flue usually needs to be enlarged.

4. Most On-demand systems are open-loop, which means they do not regulate their output temperature, but rather put as much heat in the water as possible. This means hot water temperature varies based upon the temperature of incoming water and the amount of hot water being used throughout the house. You must manually adjust the "remix" valve of an on-demand system seasonally as incoming water gets colder or hotter.

There are circumstances where an on-demand system is nice, like if you use a big whirlpool bath. But generally speaking, if you are thinking about an on-demand water heater conversion, instead think about  a solar DHW solution.

There are many differing technologies and many very qualified installers of these systems throughout the country. In fact, many probably are advertised to the right of this page. See what these people have to offer. Or, search ebay for a tube based hot water system for a quick and easy DIY project that really pays off.