Drain Heat Exchanger

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Drain Heat Exchangers extract heat from the warm drain water, and use it to prewarm the cold water feed to showers or the hot water cylinder, thus saving energy. Cost & payback depend on the details.


How It Works

A drain heat exchanger is simply a type of heat exchanger. Cold water to either the shower or hot water cylinder is prewarmed with heat extracted from the warm drainwater. These are always crossflow exchangers, which enables them to warm the cold water to a higher temperature than the water that goes down the drain.

As well as reducing energy use, when the heat is routed to the shower cold feed, a cylinder full of hot water lasts longer, since the shower needs less heat from the hot water, and so uses a lower percentage of hot water to achieve the same shower temperature.

There are 2 main types of drain heat exchanger, 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. More explanation


Types

Non-storage

By far the most popular type is the non-storage exchanger. This uses a vertical metal drain pipe several feet long. The water flows down the walls of the pipe as a film flow rather than bulk flow, which improves the heat transfer by about 4:1. Horizontal installation would result in much less energy harvest, and is not recommended.

These exchangers only harvest heat when there is simultaneous flow of warm drain water and cold water that needs warming. Consequently they are most useful with showers. They usually fail to harvest heat from washing machine, dishwasher, etc.

The main use of these is in hotels where the large consumption of hot water results in large payback. In these situations a drain heat exchanger is often able to pay its cost back in a year.

Some interest has been expressed in fitting exchangers to domestic showers. Commercially made exchangers aren't cheap, so these plans normally involve a DIYed heat exchanger.


Storage

A storage type exchanger is typically a large tank of water with 2 large coils of pipe inside. Flow through the drain pipe conducts heat to the body of water, and flow through the cold feed pipe collects this heat.

The advantage of this type is that the heat can be captured from all drain water, since there is no need to have simultaneous flows of warm drain water and warmed cold water.

The water in the tank is heated more at the top than the bottom, and becomes stratified in use. Drain water enters at the top and exits at the bottom, and warmed cold water enters at the bottom and exits at the top, maintaining the counterflow principle. Keeping warmer water over cooler maintains the stratification, enabling the counterflow principle to work, even over considerable time delays.

Storage type exchangers have 2 significant issues. The first is size, they aren't small.

The second issue is that the warmed cold water pipe stays warm for long periods, hence the warmed water output has increased risk of infection. The tank water is also at risk of infection, and some provision to sterilise the entire device regularly is usually part of the plan. This typically involves heating the whole tank up to sterilising temperature with a heating element. This heat is subsequently captured and fed to the hot water cylinder by normal exchanger action, so only some of this heat is lost.


Payback

Payback depends on:

  • Frequency and quantity of hot water use simultaneous with warm water waste.
  • Exchanger & installation costs
  • Efficiency of the exchanger

Also significant are

  • Comparison with other options such as eg replacing the HW cylinder to get more capacity, or spending the money on loft insulation instead.
  • The value of secondary benefits such as less problem with showers going cold.


Extending shower times

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 supplies 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 cylinder.


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 corrosion or 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 microbore design below.


DIY designs

Commercial units are expensive, and most of us will be looking at DIY exchangers. They're simply a counterflow heat exhanger, with the vertical drain pipe having a straight unimpeded flow. There are various possible designs.


Microbore

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  • Copper waste pipe (32/40/50mm)
  • 4 parallel microbore pipes are wound round the drain pipe in parallel,
  • 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 (which would be 4x as long)
  • The microbores are soldered to the drain pipe to achieve thermal conduction.
  • The remaining gaps may be filled with silicone, as it conducts heat better than trapped air.
  • Silicone loaded with metal powder has much better heat tansfer.
  • With a 4 microbore exchanger, less than full cold flow passes through the exchanger. The exchanger and a valve are parallelled and fitted in the cold water line. The valve is adjusted to give adequate cold water flow while maximising the flow through the exchanger.
  • Use of 8 parallelled microbores is an option to avoid limiting cold flow. No valve is then needed.


Water jacket

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This is simply 2 vertical pipes, one inside the other, with the relevant fittings at each end to create a closed cicruit in the outer jacket. The outer jacket must be fixed to the inner pipe somehow. Soldering is relatively easy, but that raises the question of differential thermal expansion and the strength of soldered joints. A compression joint with rubber ring is more able to handle slight diffferential thermal expansion.

Tank exchanger

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| (1)        (2) |
| (2)        (1) |
| (1)        (2) |
| (2)        (1) |
| (1)        (2) |
| (2)        (1) |
|________________|

A tank with 2 coils of plastic pipe acts as a storage heat exchanger. Fresh water travels through pipe 1 from bottom to top. Drain water travels through pipe 2 from top to bottom.

An immersion element is included for periodic sterilisation. Note that bleach would sterilise the captive water, but not the contents of the cold water pipe.


Figures

A traditional mixer shower was measured to see what sort of energy savings were likely.

  • Temp at showerhead: 35C
  • Temp at shower tray drain: 33C
  • Room temp: 19C before shower, 22C after
  • Approx temp at mains water incomer: summer 15C, winter 5C, estimated ave 10C
  • Flow rate 1.5l in 12 seconds = 7.5 litres/minute

Higher temps and flow rates were also selectable. At the above settings & figures, and with an estimated average incoming temp of 10C:

  • Shower temp rise without exchanger 35-10=15C
  • Shower temp rise with ideal heat exchanger 35-33=2C
  • Energy saving with ideal exchanger is therefore 15-2/15 = 87%
  • Real world energy saving is therefore 87% x real exchanger efficiency.

7.5lpm x 10mins = 75l per 10 minute shower

  • Water requires 4.187 kJ/kg per kelvin temp rise.
  • Energy used in joules = 75l x 15C x 4.2kJ = 4800kJ
  • Energy in kWh = 4800 kJ x 2.8e-7 (kWh per joule) = 1.3kWh
  • Energy use per yr for a 3 person household taking 1 shower each a day = 3 x 365 x 1.3 = 1420kWh/yr

Final costs

  • Cost per yr with gas at 4p/kWh, 85% efficient boiler = £69pa, or £1700 per 25yr system life
  • Cost per yr with electricity at 12p/kWh = £176pa, or £4400 per 25yr system life
  • Cost per yr with E7 electricity 6p/kWh = £88pa, or £2200 per 25yr system life

A 50% efficient exchanger would save 50% of these costs.

  • £850 with gas
  • £2200 with electricity
  • £1100 with economy 7 electricity

Cost savings at 39C and 10 lpm:

  • £1400 with Gas
  • £3650 with Electricity
  • £1830 with Economy 7 electricity

Power showers increase the numbers greatly.


See Also