Recycling Hot Water

There are a number of different ways to heat domestic water for bathing, washing clothes, dishes and whatever else people need.  Electric Resistive, Natural Gas, Propane, Kerosene, BioMass, Solar, Heat Pumps and even Compost Heaps come to mind.  Obviously, some of these methods make more sense than others.  The fossil fuels are idiotic for reasons I need not go into.  Electric resistive is incredibly wasteful of good quality energy and requires a connection to the utility grid.  The grid is currently powered by coal and natural gas as the majority, and even if the source was more sustainable such as by wind turbine, there are great losses in the transmission of power over long distances and through transformers.  An electrical grid is not a poor idea, but the status quo of the currently implemented technology is.  BioMass can include a lot of things, but the important feature common to them all is they represent ‘present day’ sunshine as opposed to the ancient sunlight store in fossil fuels.  These can be anything from corn husks, bamboo, maple trees, sugar cane, green algae or about anything else that grows on the land or in the oceans.  They can be converted to a standardized fuel source, maybe burned directly in downdraft gasification furnaces similar to these.  Wood gas is good for lots of things; it can even run an internal combustion engine.  Of course a common source of biomass for people in rural regions is firewood and hay bales.  These are often combusted in inefficient boilers, but the technologies are improving rapidly, motivated somewhat by local ordinances against the acrid smoke of poor combustion.  Another use of biomass is in the creation of BioChar.  I won’t go into this too much because I am really excited about the subject and I don’t want to get off topic.  It’s really neat; check it out.  Solar water heating is very sensible and with a small initial investment can provide most if not all of people’s hot water needs in sunny climates.  It’s certainly not a new idea.  Modern forms of solar hot water existed in the 19th century and from what I’ve read were not uncommon in places like Florida and Southern California.  Often, a simple tank painted black supported on a roof or other well exposed area, sometimes built into a 5 sided box with glazing over the south facing portion to trap heat.  Then there are heat pump water heaters; a fascinating subject I wish to further explore.

They can be thought of as the love child of a traditional electric water heater and a window air conditioner.  These nifty machines have been around since at least the 1980s, but like many energy saving devices, most people choose not to make large up front investments without a financial return in sight.  They are expensive.

The idea is this:  It is a vapor compression system which pumps heat out of the air in it’s local environment and discharges it into an insulated water tank maintained at the desired temperature.  Typically, they are going to run on an electric utility hookup, but they can use a fraction of the energy an electric resistive water heater does depending on a number of conditions.  Now because they use heat from the surrounding air, it is important they are located in a warm, humid environment.  Humid because water vapor release a lot of heat energy when it is condensed from the air and because water vapor is the vast majority of air’s heat holding capacity.  Another big factor is the thermostat setting on the tank itself.  Small increases in the desired temperature can come with big consequences in the overall energy usage and can in fact, at times, exceed that of electric resistive water heaters.  The performance of these heat pumps, like any others, is closely dependent on the quality and availability of heat energy from the source and where it is to be moved to.  The ‘quality’ is really just the temperature in this case and the ‘availability’ is the thermal contact and surface area available.  So if such a heat pump water heater is installed in a hot humid attic of someone’s home, then the temperature of the air and the amount of heat available (from the humidity), is much greater than if the water heater were in a cold, dry basement in the winter when the humidity is generally much lower.  In both cases, the hot water temperatures desired are probably the same, but in the case of the basement water heater, much more air must be circulated for a longer period of time in order to maintain the water tank at the desired temperature.  There is simply less heat available in cool, dry air.  Since heat is known to naturally flow from warmer objects to cooler, a bit of work has to be expelled to move heat from cooler objects to warmer.  The greater the temperature difference, the more work has to be done.  In certain conditions, we can move several times more heat units than it takes work units (which is thermodynamically the same thing), to do the job.  That is why a heat pump can use less power than an equivalent sized electric resistive water heater.  The same can be said about home heating.  At small temperature differences, heat pumps can move several dozen times the heat energy that an equivalent electric heat source could provide.  A heat pump water heater may dehumidify and cool a room or attic space while providing all of the hot water needs for a home and do so with a fraction of energy moved.  We generally call this the ‘Coefficient of Performance’ or COP.  At large temperature differences the COP may be one to one or even less.  This could be the case in our basement water heater; the temperature difference is so great between the air in the room and the water in the tank, that it would be better to just convert the electricity running the compressor directly into heat; an option in these machines, resistive heating elements.  In fact, the temperature difference is greater yet because in order to absorb heat from the room, the evaporator (the cold side of the system) has to be several degrees COLDER than the air (a problem exasperated by the small amount of heat carrying humidity) and the condenser (the hot side of the system) must be several degrees warmer than the water in the tank to get any heat to move at all.  These temperature differences depend on the physical construction of the heat exchanger and the material characteristics of the medium (air, water, salt brine, earth, etc.).  The greater temperature ‘spread’, the greater the internal pressures in the machine and thus more work by the compressor.  The same applies to all the heat pumps in your life.  Air conditioners work best when the temperature difference between the room cooled and the outside are close.  Refrigerators perform well when they are full of warm food.  Yes, they consume lots of electricity, but they move a lot of heat for the electricity consumed.  So it can be well understood when my friend wanted to install his heat pump water heater very near the shower of his bathroom so when the hot water is being consumed, it is saturating the room in hot, humid air; a perfect mix for a heat pump.

So what on Earth does all of this have to do with recycling hot water?  Well, not much other than showing the great lengths we go through to generate hot water.  Even if your water is heated by NG or electric resistive heat, it is hard to imagine the infrastructure and energy needed to manufacture the heating device itself and the greater amount of energy expelled in the infrastructure and power generation so you can take a hot bath.  The part I find most difficult to conceive is the amount of this heated water simply gets drained to the sewer while fresh, cold potable water is pumped into the tank to be heated to the desired temperatures and once again discharged when it is no longer is of desirable tepidity.

Now, I don’t know if you have a basement, crawl space or pipe chase where you might have access to the main drain pipe of your home, but if you do; the next time someone in your home is taking a shower or the hot wash cycle on the washing machine drains, wrap your hand around some of the drains in your home to get an idea just how much heat is being dumped down the sewer.  You might be surprised.  Many of our water heaters are set to a temperature of 120-125 degrees Fahrenheit.  I haven’t done any serious study of the drain temperatures in my home, but I’ll bet that the drain water from the shower or sink isn’t much cooler than 90 degrees F.  That is still quite a bit of heat compared to the makeup water feeding the tank at about 60 degrees F.  Certainly something could be done to recover this heat and provide a boost to the incoming water stream.  There are heat exchangers which replace a portion of the drain pipe in a home and provide a copper conduit in which cold water flows through picking up heat from the warm effluent so as to preheat a water heater’s makeup water supply.  These are certainly a good idea, but the recovery is marginal and the up front cost is prohibitive for many costing upwards of $800.

A bit of an aside:  Economic costs do not always reflect the true cost of a technology.  In fact it rarely does.  Solar panels and wind turbines are often sold with the caveat that they will take 15, 20, 30 years to pay for themselves and it is sometimes difficult for people living in a market driven capitalist economy to see such a purchase as ‘rational’ when a much cheaper short term solution are right in front of them and are often more widely promoted considering the faster return for manufacturers and energy providers.  Some investments will not have an economic return quantitatively calculable in the near future and so could be viewed as irrational.  Constructing safe, well built, efficient structures and technology designed to last for generations is a social investment; one which capitalism will not afford us.

Back to water heaters.  So we want to recover this heat going down the drain right?  Well, my interest is obviously in heat pumps.  I certainly like low tech solutions and a vapor compression system is not the simplest machine to construct , but they are one of the only over unity devices known and they are very useful in some situations.  When I run my hand across a warm drain pipe I don’t think of dirty water going down the drain, I think “Jackpot!”.  That, my curious friend, is some primo grade heat available just going to waste.  I see hot water as a very precious thing.  Water has an unusually large capacity to hold heat and being a fluid, it’s not terribly hard to recover it; certainly easier than getting heat from the air.

Back to the air to water heat pump.  These machines are neat and deserve attention, but also deserve improvement in my opinion.  As I explained before, their performance is determined by the environment in which they are installed.  Sometimes, they are well suited to that environment providing cooling/dehumidification and hot water.  Other times, not so much.  I would not label this as ‘appropriate technology’.  Now, if installed to recover humid bathroom heat during a shower, that certainly makes sense.  At the very time hot water is being used, it is readily available floating about the room.  Of course, much of the hot water is still going down the drain, partially recoverable by a heat exchanger.  I want that heat.

I loathe heating and cooling air.  Not only does it require a lot of surface area to exchange heat because the heat capacity of air is relatively low, but it takes a lot of energy to blow it around too.

I’ll wrap this up as succinctly as I can.

I propose, and would someday like to build a heat pump system which recovers and reuses as much of the heat in a domestic hot water system within reason.  This would require, rather than a atmospheric coil absorbing heat from the air, a more passive coil bathed in the warm effluent of a home or apartment building.  Much like the simple copper heat recovery heat exchanger, this setup would provide the most heat to the system when it is being consumed.  The warmer the effluent temperature, the higher the evaporator pressure, the smaller the compression ratio and thus a higher coefficient of performance.  I would think significantly higher than a conventional heat pump water heater.  The warmer the water reaching the evaporator, the higher the performance.  Hot water supply lines are often insulated to provide the warmest water at the spigot; with a recovery system like this it is advisable to insulate the drains as well.  To keep the temperature difference small between the warm effluent water and the evaporator so the compressor does not have to do much extra work, the water would probably have to travel through an insulated tank similar to a water heater tank where the warmest water rises to the top and the coldest water sinks to the bottom; just like a water heater.  A submerged coil in this warm region could absorb heat and pump it into fresh water in the domestic system while influent is deposited somewhere in the middle or the top and effluent runs through a vented overflow sourced at the cold water in the bottom.  Performance of a system such as this could be further improved by the appropriate application of Phase Change Materials with the system.  Bothe tanks, the recovery tank and the domestic hot water tank, could be fitted with a latent heat storage mass or masses capable of absorbing excess heat energy when it is available, then release it when needed; there by increasing the capacity of the heating system, lowering power consumption and/or decreasing the overall size.

It sounds big, complicated and impractical.  Probably the first one would be, but I think such a system could be made that would be low cost, long lasting and very successful.  It could be implemented in existing structures with some ingenuity or configured into new homes as part of an overall energy management philosophy.  Solar water heating in climates where this is an option should be explored first, but in other areas much more consideration must be made for the energy we have on hand.

Another project for another day.

M.C. Pletcher