RogerL said:
........
Both the Truma Ultrastore water heater and the Alde water heater section are designed to have a small amount of air in their uppermost part to minimise surging - in addition to any surge damper fitted - I can't remember what the Carver Cascades did.....
The Cascade also had an air cap.
As cold water is heated it expands. To allow space for the expansion to take place in the water heaters, thery have an air cap, as the air can be squashed. The effect of this is to raise the pressure inside the heater to rise above that of the cold water supply.
Especially for systems operated by a pressure switch, when a hot tap is opened the higher pressure of the hot water will cause it to be delivered instantly as a surge until the tank pressure drops to just below the pressure of the cold water supply. Only when the pressure in the tank drops enough to allow cold water to be addmitted will the pressure in the cold system start to drop enough to activate the pump.
To overcome this Surge dampers are added to the cold pipework. These mimic the air cap in the hot tank, and provide a compliant air spring which is activated by the pressure delivery from the pump. They are not perfect, as they never achieve the same high pressure as the hot tank, but with careful selection of the damper size, the surge effcet can be largely eliminated. The cold damper needs to have about a 50% or larger volume that the hot tanks air cap.