What you put in makes all the difference
Isothermal systems - that is systems that boil water to make steam – typically maintain relative humidity levels with one to 5% of an established set point.
By far, the more critical factor is the hardness of the water – the amount of scale-causing minerals that must be removed to make the system work consistently. As water hardness increases, so does the need to skim, drain and flush the system.
Each time a system is skimmed, cold water is introduced to the system to replace what has been removed. Heating cold water slows the production of steam and uses more energy.
When a system is drained or flushed, the delays are longer and the energy use is higher because an entire tank must be refilled and heated. A humidifier could be off line for up to 16 minutes during this process, and the relative humidity can drop by as much as 5% waiting for the water to heat and steam to be available again.
In office buildings, a drop of 5% in relative humidity is not an issue. However, in process-critical environments, it is critical, and that is why humidifiers in those applications are filled with DI/RO water, which has far fewer minerals. Of less importance than water hardness is a responsive energy source.
In equivalent conditions, a system with a modulating valve, such as a boiler-steam heat exchanger, has a small edge over electric and gas isothermal systems because of the infinite variability of a modulating steam valve. However, in practice, the difference between the system is minimal and not a basis on which to choose one system over another.
Low mineral content means low maintenance
The lower the mineral content in the water, the less maintenance an isothermal system requires. Mineral buildup can cause humidifiers to malfunction: heater coils can fail prematurely, heat exchanger output is reduced by scale build-up, conductivity probe systems that measure water levels quit working, and drain valves get plugged. The plastic cylinders used in some systems may need to be replaced.
DI/RO water has the lowest mineral content, but it is too expensive unless needed for high-purity humidification or to meet strict performance requirements in applications like semiconductor manufacturing.
It is OK to use hard water in an isothermal humidifier, but the trade off is the need for regular inspection and cleaning and an awareness that the relative humidity will fluctuate.
The easiest and most cost-effective solution often is to soften the fill water, reducing maintenance costs and improving efficiency.