Drain Heat Exchanger

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How It Works

A drain heat exchanger is a crossflow heat exchanger. Water to either the shower or hot water cold feed is prewarmed by the warm drainwater, thus reducing energy use and causing a hot tank of water to last longer.

There are 2 main types, non-storage and storage. Storage exchangers include a body of water that stores recovered heat so that simultaneous flow in both pipes is not needed to recover and use the heat. Storage units are bulkier.

More explanation

Options

Non-storage type simpler to plumb

When retrofitting, existing British houses often don't have a convenient space for storage type exchangers. The main use for a non-storage exchanger is with showers. Other applications will see little return because they don't use hot water in and warm water out at the same time.

Therefore a non-storage exchanger need only heat the cold water to the shower, which minimises plumbing.

Horizontal or vertical

When retrofitting there may only be room for a horizontal drain pipe rather than vertical. This reduces heat recovery per length significantly. With vertical pipes, the falling water forms a film on the surface of the pipe for better heat conduction. Performance is thus better for upstairs showers and houses with basements. Digging a small pit for the exchanger is one option.

Length

In some cases there may only be space for a reduced length of drain pipe to be used. This will reduce heat return, but not by as much as the reduction in length. Each additional inch of length returns ever less energy.

It might be possible to use a wider bore exchanger to increase heat transfer surface in these situations.

Payback

Value & payback depend on:

• Frequency and quantity of hot water use simultaneous with warm water waste.
• Installation cost
• Efficiency of the exchanger
• Comparison with other options such as eg replacing the HW tank, or spending the money on loft insulation instead.
• Value of secondary benefits such as less problem with showers going cold.

Other benefits

Drain heat exchangers reduce the amount of hot water used by a shower. Because the shower is mixing hot with lukewarm rather than hot with cold, less hot is used for the same output temperature & flow. This means that a given installed hot water capacity will supply a shower for longer with a drain heat exchanger fitted.

This may make a drain heat exchanger an attractive alternative to fitting a larger hot water tank. It is often cheaper to install and reduces running costs.

Wait times for the shower to get hot are reduced when the exchanger preheats the shower cold feed.

Safety

Mains water is normally at positive pressure, but there are situations in which that pressure is occasionally lost. Hence measures are required to avoid the possibility of contamination in the event of damage to the exchanger. Double walled construction should be used, with 2 walls between mains and waste water, and the failure of any join should not cause contamination risk. These requirements are met by the wrapped microbore around central pipe design.

DIY designs

For now you'll need to make your own. They're simply a counterflow heat exhanger, with the drain pipe having a straight unimpeded flow. There are many possible ways to design these.

Design No. 1

• copper waste pipe
• 4 parallel microbore pipes are wound round the drain pipe in parallel and soldered in place.
• The remaining gaps can be filled with silicone, since this conducts heat better than trapped air.
• Silicone loaded with metal powder will have better heat tansfer. If using copper, neutral cure silicone might be needed (not yet tested).
• The microbores are connected to a 4 way manifold at each end. The use of 4 parallel paths allows much more water flow than with a single microbore path 4x as long.
• With a 4 microbore exchanger, a valve is fitted across the cold water path, and adjusted to give enough cold water flow. Use of 8 parallelled microbores is another option to avoid limiting cold flow.

See Also

• Making Heat Exchangers
• Making a storage type Heat Exchanger
• FAQ
• More info - commercial site
• Wiki Contents
• Wiki Subject Categories
• US Dept of Energy