Heat Exchanger

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A single-wall, tube-in-tube heat exchanger can easily be built from readily available parts.

Most solar hot water systems that operate in freezing conditions use a heat exchanger. A heat exchanger is a device that allows a hot fluid to heat a cooler fluid without the two fluids mixing. I am a plumber by trade, but basic soldering skills are all you need to construct your own simple heat exchanger. In this article, I'll show you how to build a simple, single-wall, tube-in-tube heat exchanger for use with drainback solar domestic hot water (SDHW) systems.

Heat exchangers are used in both closed loop antifreeze and drainback type SDHW systems. I have also used them in wood-fired heating systems, radiant floor house heating systems with a domestic water heater, and custom-built spas. For more information on all types of heat exchangers and their use in solar domestic hot water systems, refer to HP92, "Heat Exchangers for Solar Water Heating."

A Pipe Inside a Pipe

The tube-in-tube exchanger described here is simply "a pipe inside a pipe." This is accomplished by a very small and easy modification to a standard copper pipe fitting.

The fitting is called a "reducing coupler" or "coupling" (sometimes called a "bell reducer" because of its shape). But it must have the right kind of "stops." Normally this fitting is used to join two pipes of different diameters, and there are "stops" to prevent the smaller pipe from going too deep into its socket.

Bert Echt ready to solder together a heat exchanger.

Bert Echt ready to solder together a heat exchanger.

heat exchanger how-to

Typical TUbe-In-TUbe Heat Exchanger

Hot Collector Loop Fluid In

Hot Collector Loop Fluid In

Inner Pipe: Passes all the way through, contains collector loop fluid

Outer Pipe: 10+ feet long, contains potable water


Two (one on each end), with stop nubs filed off

Cooler Collector

Loop Fluid Out

Cooler Collector

Loop Fluid Out

Hot Potable Water Out

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