Using a Solar Water Heater to Preheat Water for a Tankless Direct Vent Water Heater

One often overlooked method of decreasing energy costs and increasing energy efficiency that can be used with a direct vent water heater, is preheating the water with a solar powered water heater. In fact, during the summer months, it’s quite possible to provide all of your hot water needs by only using the solar powered water heater. This is especially true in homes located in the southern states where the sun is much stronger.

But even if you’re not in the sunbelt, you can achieve very good results, and substantially lower your energy costs, by incorporating a solar water heater into your homes hot water system. And what’s even better is that you can do this with cheap, or sometimes even free materials. For instance, you can easily build a low cost flat panel solar hot water heater in a weekend. Combine that with an old hot water heater that someone was throwing away for a storage tank, you’ve got a low buck solar preheater that will more than pay for itself in a short period of time.

Here’s a fairly good example from instructables of one way to utilize a solar water preheater with your direct vent water heater. This example can be used with either a tankless direct vent water heater, or a conventional storage type water heater, but the plumbing layout and arrangement may be different. NOTE: This is ONLY an example for educational purposes and should not be used for construction. If you’re unable to design such a system on your own, seek professional assistance.

A common rule of thumb is that for every ten degrees of temperature that you don’t heat, you’ll save between 3-5% in energy costs. So for every 10 degrees you can PREHEAT your water, lets say you’ll save 4% in overall water heating costs. This may not sound like much, but if you take into account that city water temperatures often average 50 degrees farenheit or less, and well water temperatures are often much lower, you can start to see where preheating makes a lot of sense.

In the above instructable example, the author states that he’s achieving a 20 degree temperature increase, which in our case would amount to about an 8% savings in energy. And that’s from a homemade solar water heater that can be improved substantially with a few minor changes. It’s worth noting though that he wasn’t using copper pipe in the collector, and achieved this temperature rise with CPVC PIPE! So his design seems to be rather good.

As you can see, preheating your hot water has a very significant impact on your overall hot water heating costs. And if you can buld a simple homemade solar powered hot water heater for little to no cost, it would be well worth your while. With a good design, you could potentially save 10-15% or more of your annual hot water heating costs on top of the 30% savings from using a tankless water heater.

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