The more complex and expensive machines are particularly useful where the required pressure is relatively high. The most common pressure is around 10 cm of water : most will cope well on the standard constant level device at up to 10 or 12 cm of water pressure. Adding a heated humidifier will give a similar result to an added 1 cm of water pressure. Heated humidifiers are particularly useful if it gets cold and dry night [eg Ipswich] or if you have grumbling nose problems like sinusitis.