(please pay attention to the "Advisory Notes for the purchase of showers for bathing with WELS labels)
Generally, the water heaters available in market can be divided into three main types, which are 1) Storage type, 2) Flow-controlled type (can adjust the water temperature, with or without Hi/Low temperature mode), and 3) Pressure-controlled type (can adjust by Hi/Low temperature mode only).
Except under special condition, the storage type water heaters are generally compatible to water-efficient showers of grades 1 to 3.
In general, all levels of water-efficient showers are compatible to the flow-controlled type water heaters. However, when the actual flow rate of the shower is very low (i.e. below 7 liters/minute) or under low water pressure condition (such as the top 3 floors of a buildings without pressurisation installation), the hot water may be a bit over-heated due to the influence of hot weather on the operation of individual water heaters.
There are two types of pressure-controlled water heater which are gas-driven and electricity-driven. Generally, most water-efficient showers are compatible to the gas-driven water heater. However, when the actual flow rate of the shower is very low or under low water pressure, the hot water may be a bit over-heated due to the influence of hot weather to the operation of individual water heaters. The electricity-driven water heaters which require high flow rate to operate are not compatible to water-efficient showers of grades 1 to 3.
All the above information is for reference only, there may be different operational results for individual water heater/ shower products.