XPost: can.internet.highspeed   
   From: Some@Guy.com   
      
   Nobody wrote:   
      
   > > Um, what gas water heaters have any sort of plastic exhaust vent   
   > > pipes?   
   >   
   > MOst energy efficient tankless water heaters.   
      
   On-demand (tankless) water heaters are a crock.   
      
   Facts:   
      
   1) Natural gas prices have been very low the past few years, and there's   
   no indication that's going to change in the next few years.  Anything   
   you spend to reduce natural gas usage will have a proportionately small   
   return on investment given low gas prices.   
      
   2) Conventional hot-water tanks are pretty efficient from a   
   standing-loss standpoint, and what little heat they do radiate can be   
   reduced by a relatively cheap external insulation blanket.  On the other   
   hand, the radiant heat loss from the tank is captured inside your house,   
   the advantage of which is proportional to your northern geographic   
   location (or as a function of altitude).   
      
   3) heat loss from a conventional tank flue is minimal if you have a   
   power-vented system (when the fan isin't turning, it's acting like a   
   baffle preventing air flow through the flue).  I suppose a   
   power-operated shutter could be added to completely close the flue and   
   prevent heat loss when the burner is not on.   
      
   4) efficiency of heat transfer is inversely proportional to the heat   
   gradient.  The burner of an on-demand heater needs to put out 10's of   
   thousands, even 100+ thousand BTU in order to heat incoming water during   
   the water's short residency time inside the heater for the water to   
   reach conventional hot-water temperature (typically 140 to 160 f).  The   
   more north you are, the colder your incoming water supply will be, and   
   the more capacity (in BTU) the burners will need to be to bring the   
   water up to the desired temperature.  Exhaust heat loss from these units   
   is significant while they are operating, and during their off-cycle as   
   they cool down they can't dump much heat energy into the water because   
   there isin't much water stored in the unit.   
      
   Conversely, the burner of a conventional water tank is capable of much   
   less BTU heating, and the heat from the burner has more time to come   
   into contact with the internal tank surface and transfer it's heat into   
   the water.  The exhaust gas temperature in the flue of a conventional   
   heater can be so cool as to require a small electric blower to properly   
   exhaust the gas out the flue.  This is an indication that most of the   
   combustion heat is being transfered into the water and not being   
   exhausted out the chimney.   
      
   In other words, perhaps 50% of the combustion heat of an on-demand   
   heater is actually being transfered to the incoming cold water and the   
   other 50% is being lost in the exhaust, while 80% of the combustion heat   
   is absorbed by the water in a conventional tank.  The difference is that   
   an on-demand heater is on perhaps 30 to 90 minutes per day, while a   
   conventional tank might be on for 4 hours a day.  But remember that when   
   a conventional tank is on, it's burners are using a much smaller amount   
   of gas compared to the on-demand heater.   
      
   5) the efficient use of an on-demand heater is challenged by short   
   hot-water usage events.  In most houses, the hot water lines are   
   minimally insulated and thus the water in them quickly drops to room   
   temperature.  Anyone turning on a hot-water tap in an upstairs bathroom   
   will notice it take 10 to 30 seconds to actually get hot water.  It   
   doesn't matter what type of heater you have (assuming the heater is in   
   the basement).  A short hot-water use event (say, washing your hands)   
   will end up dumping a lot of waste heat out the exhaust when an   
   on-demand heater is signalled to turn on and then soon after turned off   
   to heat the water for that short-use event.   
      
   6) because of the very high heating capability (BTU capacity) of   
   on-demand heaters, the extreme thermal cycling of their internal   
   components will age the unit much faster than a conventional water   
   heater, and they do or will require more maintainence and repair vs a   
   conventional water heater (they have control devices, electronics, etc,   
   that are not present in conventional heaters, and as we all know -   
   electronics and HVAC equipment really don't tend to co-exist very well   
   for the long term).   
      
   7) on-demand heaters have electrical or electronic controls that require   
   a source of AC current.  Thus they will not function during a power   
   failure.  Anyone living in a northern climate that is subject to   
   sporadic winter power failures will not appreciate the lack of hot water   
   during extended outages.   
      
   Conclusion:   
      
   No home owner that has a working conventional gas water heater will ever   
   live long enough to recoup the savings from replacing his existing   
   working heater with an on-demand unit - and it's not a given that there   
   will actually be any measureable savings in gas use.   
      
   What has been observed is that the behavior of occupants change in terms   
   of how they use hot water when a conventional heater is replaced with an   
   on-demand heater, and that change usually results in less hot water use   
   (shorter showers, changes in shower heads, installation of low-flush   
   toilets, etc, insulating hot-water supply lines inside the house) so   
   it's not always clear where the savings come from and why.   
      
   Replacing an old / leaking conventional water heater is very easy for   
   most novice home owners / handymen, and at a cost of only a few hundred   
   dollars, the cost/reward ratio is still heavily in favor of replacing a   
   old conventional water heater with a new conventional unit.   
      
   You will get more bang-for-the-buck by   
      
   1) putting an insulating blanket around your existing or new   
   conventional water heater   
      
   2) insulating as much of the hot water supply lines inside your house as   
   you can reach   
      
   3) use a low-flow shower head   
      
   On-demand water heaters are basically a crock of shit designed to give   
   plumbing and HVAC companies a very lucrative new revenue stream.   
      
   See also:   
      
   http://www.cceci.com/Tankless_WhitePaper.pdf   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)