[Greenbuilding] Baxi Luna question

[George J. Nesbitt]

Lawrence Lile wrote:
> Tankless should be more efficient than tanked, but a badly designed
> plumbing system will make either kind of system annoying.
Should be more efficient, but often not for several reasons.
1st because you now never run out of hot water so you use more.
2nd because every time the water heater turns on and off it wastes
energy (cycle losses).
3rd because it takes some time to reach steady state efficiency (maximum
efficiency).
4th because they are tested and rated with long water draws so it runs
mostly at maximum efficiency. (Energy Factor or Recovery Efficiency).
Most/many draws are shorter.
5th efficiency goes down as the incoming water temperature goes up.

Lawrence Lile wrote:
> but a badly designed plumbing system will make either kind of system annoying.
     How true. Too large diameter pipes, too long of runs (especially
central manifold parallel pipe systems), hard elbows instead of long
sweep, tees, too little insulation, no heat traps, pipes outside of 
conditioned space, pipes under concrete slabs, water heater at one end 
of the house and fixtures at the other end, etc....
     Then we add in low flow fixtures and the water and time waste goes
up even more. The slower the water flows the more water is wasted. It
takes a larger volume of water because more heat is lost to the pipes
and some of the hot water gets to the fixtures with still cold water
(ever notice you have some hot water but not hot enough yet?). A
tankless wastes even more water because of the delay in the burner
turning on, and the warm up phase. A Tankless needs a minimum flow rate 
to turn on of.5-.75 gpm (sorry I am a Merican, and not a Can add in 
metric'ian), and have an upper limit of how many gpm it can deliver at a 
given difference of incomeing to outgoing water temperature.

     So what is/are the solutions? not removing the flow restrict r's so
you waste even more water. Not adding a small electric tank before the
tankless or at the far end of the house. Solar hot water preheat would
displace gas/electric use, but I'm not sure a gas tankless is the best
way for backup (see above).

     The best way to solve wasting time and water is a recirculation
pump. But most will waste energy and money needlessly and void the
warrentee on a tankless water heater. The only one that won't is a
D'mand pump, made by ACT/Metlund www.gothotwater.com, also sold under
the Taco and Upinor/Wisbro brands. It works by pressing a button or
motion sensor (hardwire or wireless), and pumps the water very fast, and
shuts off when the hot water gets to where it is needed. It won't void
the residential warrenttee on a tankless. You can retrofit it using the
cold water line as a return, or install it with a dedicated return line
(if you have access or in new construction).
     Energy Star only gives this product a credit, no other pump or
distribution system.