I think its time for me to change the water heater in the house. However, my house is double storey and I think the way the plumber do my piping is thru ground floor then only 1st floor. Our bathrooms are upstairs…anyone has any good suggestion?
Water Heater. Reliable and cost efficient!

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Check your local gas suppliers too, if you can get gas connected then on-demand gas heaters are pretty darn good too! :)
I can vouch for that. We have Rinnai Infinity with controllers. Dial up 39C, turn on the hot tap, shower for infinite period of time.
my house's heater is a gas heater. so what brand are reliable?
when you say local gas supplier, you mean origin?
There's often multiple; as you say, Origin, AGL etc, etc…depends on your area!
It only cost me $126 to get gas mains connection (street to house) from AGL…then I supplied my own appliances, hardware & plumbing! They used to have specials on these too but not any more unfortunately, so I just got some tradies I know to source it all for me! ;)
Solar….
That way it HAS to be coming from the roof down!Perhaps, but not with a gas boosted system. The solar feeds a Continuous Flow gas heater usually mounted at ground level. You need seriously long copper runs with a gas boosted system and upstairs bathrooms, but the gas bumps the temp up until the warm solar water finally makes it way down to there.
You might want to look into heat pump water heaters, they are more efficient.