The Ariston GL6+ mini-tank water heater has two mounting options: the GL6PLUS can be mounted either vertically or horizontally.
In contrast, the other two Ariston models -- the GL2.5 and the GL4 -- can only be mounted vertically.
When the Ariston GL6 PLUS is mounted vertically it holds 5.1 gallons of hot water. When it is mounted on its side it holds 7.1 gallons of hot water.
A common question regarding the Ariston GL6 is whether it can also be mounted on its back.
The answer is NO.
The T&P relief valve must always be on the top of the heater where the temperatures are hottest. With the heater on its back the T&P would be on the side, measuring a lower temperature.
Inside the heater, the cold water tube is longer than the hot water tube. This allows for "stratification" of water within the tank - cold water is delivered to the bottom of the tank and hot water (like hot air) rises. With the tank upright or on its side, the cold water is delivered to the lower portion of the tank and hot water is drawn off the top.
If the heater were placed on its back, cold water inlet and hot water outlet would be at the same level. The water at the top of the tank would be nice and hot, but you'd never be able to access it. The cold water would practically flow right through to the hot water side and result in lukewarm water at best.