Hot Water

In the NE US heating hot water for inside use (washing, bathing, kitchen etc.) is the second energy drain after the house heat.  In more moderate climes, e.g. Australia, it is #1,

Solar hot water systems are almost a no-brainer in a moderate climate, and well worth it even in colder climes.  Of course where temperatures fall below freezing the solar panels can’t use plain water, but need an antifreeze mixture and then a heat exchanger to transfer the heat from collector fluid to household water.

Oz solar HW system - ugly, but efficient

Oz solar HW system - ugly, but efficient

Another alternative to save energy is to use instant hot water heaters.  here is a quick pro/con list:


  • Efficiency = Cost savings on energy
  • Water savings
  • Saving on piping – IF the heater is installed in new construction, can be placed close to outlets, gas goes there anyway – e.g. where a kitchen and bath are close together and the heater can be placed between them.  Just a single cold water line needs to be run to the unit, you can save a lot of copper if the layout is designed well.
  • Comfort – hot water quickly, rather than waiting for pipe to clear.
  • Only turns on when there is sufficient water flow – so will not drain a tank full of hot water every few hours if a faucet is dripping, or left on slightly by mistake.  (but – see the equivalent con)
  • You never run out of hot water – unlimited showers for all.
  • Smaller space – no tank
  • Probably lasts longer – no tank replacement


  • Cost of unit, plus cost of installation can be multiple the cost of a “standard” tank heater.
  • Needs good water and gas supply to the unit, retrofitting using existing 1/2″ lines is not a good idea
  • These units are sealed and need an air inlet and exhaust to the the outside – the same as high efficiency furnaces.  It should not be a problem, but ideally the unit should be on an outside wall.
  • If you leave a faucet running by mistake, or a pipe bursts (say the hot water feed to the washer) then the unit will supply beautiful hot water continually – and continue to burn massive amounts of gas to do so.  The old tank heater will do the same, but burn much less gas.  (I’ll have to do some research to see if there is some emergency off system available)

The bottom line: go for it in new construction.  It could be tricky if you are replacing a tank unit in an existing house.

“Hybrid” hot water heaters seem to be hitting the street.  Basically these are tank type heaters with an air-source heat pump attached.  The heat pump uses the heat in the air, using a reverse refrigeration process, to preheat the water in the tank and so save on heating the water.  GE ( advertises a model  claiming savings of $320 per year.   The downside is of course the cost (about $1500), installation and as far as i can see it only applies to electric water heating systems.

An alternative approach is to use a stand alone heat pump in conjunction with your existing tank style heater (electric or gas – but the savings are really greater for electric heaters).  A unit from North Road Technologies is an example.  Upside: acts as a dehumidifier (and makes distilled water, which you either save or have to get rid of somehow), works as a 6000BTU air conditioner , qualifies for tax credits, saves money, claims are for 50-70% savings on an electric unit, easy installation, your existing tank stays right there.  Downside: as usual, the cost: $1400.

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