If you're after gas heater, how about 6.9 star rating?
If definitely that you need a new gas heater, get one with high efficiency.
This particular model is built on top of it's 6star rated tankless model, and top it up to 6.9star! You can hardly get any other water heater that surpasses 6.9star, except Rinnai at 7star-rated.
The 6.9star rating is achieved by improving heat exchange efficiency in the unit, through capturing heat from hot exhaust to pre-heat cold inlet water.
And also, max temp of 50C from heater is another reason energy is conserved, as lower piping temp will lower temp delta between pipes and grounding ambient temp, thus less heat loss. Correct me if this is wrong.
I haven't got more gas bills to make sensible opinion, and that has to wait till May. However, with this new heater, as well as some other changes to how we use hot water (eg. make sure turning tap to pure cold if no hot water is needed, adjust water tempering valve to suitable temp so that no shower tap mixing is required), I am confident to have lower gas bill, just a matter of numbers.
Having said, and again, if you are after a gas water heater, i would recommend Bosch C21.
However, if you are not constraining your choices to only gas heater, then, solar water heater is another better option worth considering, frankly. As nothing beats solar power.
(Updates on 11thMay2017)
As disclosed earlier, I am confident that this water heater system is providing some level of savings, and more will come when I receive gas bill for May.
With attached photo on May's bill, you will notice that my last three consecutive gas usage are approx. 3000units (Nov2016), 600units (Feb2017) and 612units (May2017).
And the water heater system changed over in Oct2016.
That implies my old hot water tank was severely consuming a lot of gas to stay warm 24/7.
Hope this hard fact helps in your decision making.
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