16-11-2016, 10:29 AM
Domestic hot water heating system with cold water storage tank and non pressurised cylinder heated by electric immersion heater and other heat source.
The cylinder fills from the cold storage into the base of the cylinder.
The hot water is taken from the top of the cylinder along with an expansion pipe which discharges into the cold storage tank BUT ABOVE THE MAXIMUM WATER LEVEL IN THE TANK EVEN UNDER OVERFILLING CONDITION.
So if the cylinder boils for whatever reason, steam will vent over the cold tank water surface not into the water. In this condition the cold water tank continues to keep the cylinder full of water at all times. The water level will be kept topped up by the mains cold feed into the tank. In this condition considerable steam at zero pressure will be created but the system is fail safe even if the cold feed is interrupted. In that event the immersion element will eventually melt and fail when it is no longer immersed, disconnecting the power.
The only way that the storage tank can be heated to any degree is if the expansion pipe end dips into the water and a circulation is achieved, the steam being condensed so that there is no cold water makeup.
In this condition the cold storage will eventually boil, weakening a plastic tank considerably.
So if properly installed, the system is fail safe.
If the hot water cylinder is a sealed pressure cylinder, a whole new set of dangers arise necessitating precision thermal and pressure controls to maintain a safe operating condition.
The cylinder fills from the cold storage into the base of the cylinder.
The hot water is taken from the top of the cylinder along with an expansion pipe which discharges into the cold storage tank BUT ABOVE THE MAXIMUM WATER LEVEL IN THE TANK EVEN UNDER OVERFILLING CONDITION.
So if the cylinder boils for whatever reason, steam will vent over the cold tank water surface not into the water. In this condition the cold water tank continues to keep the cylinder full of water at all times. The water level will be kept topped up by the mains cold feed into the tank. In this condition considerable steam at zero pressure will be created but the system is fail safe even if the cold feed is interrupted. In that event the immersion element will eventually melt and fail when it is no longer immersed, disconnecting the power.
The only way that the storage tank can be heated to any degree is if the expansion pipe end dips into the water and a circulation is achieved, the steam being condensed so that there is no cold water makeup.
In this condition the cold storage will eventually boil, weakening a plastic tank considerably.
So if properly installed, the system is fail safe.
If the hot water cylinder is a sealed pressure cylinder, a whole new set of dangers arise necessitating precision thermal and pressure controls to maintain a safe operating condition.
Boater Sam.
