I live in the uk and I have noticed that tankless (combi in the uk) have
a compleatly diferent flue systems in the us , is there a reasion that
air intake is not inc in a twin wall tube like here in the uk/eu ?.
Tankless water heaters in the US are referred to as multi-points in the UK -
as opposed to the over or under sink single points which only did one tap.
The US tend to have high flowrate models, and they are available in the UK,
made by Rinnai. These only give hot water.
A "combi" does give instantly heated hot water, but also heats a wet
(hydronic) heating system too, all out of the same box. The concentric flue
systems on combis, although some can be converted to two pipe flue system of
many, many meters in length, are available in the US. In Europe the
standard temperature rise is 35C, so everyone knows what compare with. In
the US you have to figure it out yourself when comparing, as makers give
spurious, differing and at times misleading figures on the all important
flowrate.
The Japanese tankless (multi-points), give the highest flowrates, with the
highest combi being around 21 litres/minute @ 35C temperature rise. Some
Takagi's and Rinnais can get to 28 or so. The top of the domestic ranges
are virtually commercial water heaters, the gas burner ratings are so high.
Some are using condensing technology, but most do not having, by UK
standards, low efficiencies. It is mandatory in the UK for combis to be
condensing units.
As the cold water mains temperature decreases in winter the flowrate drops
off. A difference from +2C to +18C water temperatures improves the flowrate
substantially. A marketed two bathroom tankless in winter will probably
only cope with only bathroom simultaneously.
Combis in Europe are cheap as they are knocked out by the millions. Some
wise heating engineers use two cheap combis to bump the flowrate, which ends
up cheaper and a higher flowrate than the likes of a Rinnai.
