Sealed-system or Open vented system?
Open-vented system - There is a header tank in the loft. This fills the boiler & radiators & keeps the system full.
The header tank has a ball valve inside it that controls the water level in the tank & the heating system. This allows any water lose from the heating system to be made up without any user input.
Open vented systems do have some problems -
1) The ball valve seizes can stick closed – The float valve just needs freeing & the system will then re-fill.
2) The cold fill pipe can get blocked - The blocked section a pipework needs to be located, removed & replaced.
3) Pumping over – It is good practice to position the pump so that it gives positive pressure within the circuit. This ensure that air is not drawn into the system via microscopic leaks. The pump position is even more critical within a fully pumped system because of it's position in relation to the cold feed & vent pipe.
Primary open safety vent -
In a fully pumped central heating system there will be an open safety vent. This should rise to a minimum height of 450mm above the water level in the header tank to allow for any pressure surge effect created by the pump.
Sealed-system - This replaces the open vent pipe with a pressure relief valve. It must be installed on all types of sealed systems. For domestic use it must.
1 Be non adjustable.
2 Be set to discharge at a system pressure of 3 bar.
3 Have a manual test feature.
4 Connect to a full bore discharge pipe.
5 Have a metal discharge pipe with a minimum diameter the same as the valve outlet.
A pressure gauge must also be installed to help with filling and topping up the system. It must give a reading of between 0 and 4 bar.
Filling and pressurizing the system is done through a filling loop that consists of.
1 A flexible braided hose
2 A stop valve
3 A double check valve
The boiler used on a sealed system must also have a high limit stat fitted so in the event of a control thermostat failure there is added protection.
An expansion vessel is used in a sealed system to replace the feed and expansion tank that is used in an open vented set up.
It takes up the increase in volume of system water when the water is heated.
The vessel needs to be installed close to the suction side of the pump to ensure that the system works under positive pressure.
In most domestic setting this is achieved by installing a combi boiler.
One of the few problems is explained below -
1) System pressure loss - Heating systems can lose water from tiny leaks or component failure. Keeping an eye on the system pressure is recommended. 1.5bar is a good average pressure to maintain although this cannot always be maintained by the home owner if there is a fault.
If the system loses too much pressure then it will close down as a safety precaution.