Link-Up Heating Systems
When two boilers are used in a heating system they should be linked to the system in such a way as not to interact with each other.
This is usually achieved by using pumps for each system together with non-return valves (NRV’s).
When Boiler No.1 and associated pump is switched on the flow of water opens the Non-return valve (NRV) and the heat emitters receive hot water.
Hot water is prevented from ‘back circulating’ to Boiler No.2 by its Non-return valve.
If hot water flows through a cold boiler excess heat is lost since the flue connected to the boiler increases the heat lost by convection.
When an existing open vented solid fuel boiler heats a dwelling and an oil-fired boiler is added then the link-up system can get complicated as shown below.
The diagram below shows changes made to the heating system.
The primary flow and return to the Hot Water cyclinder is changed from a gravity to a pumped circulation.
The new flow and return pipes from the new Oil-fired boiler are connected in to the existing system at a common flow and a common flow return pipe junction.
A new vent pipe is added.
The system is now piped so that either boiler and pump will heat the radiators and the hot water cylinder.
If a heating system has two boilers, each boiler should be separately vented.
Sometimes it is preferable to have gravity circulation for a solid fuel boiler to dissipate heat, together with a heat leak radiator.
This complicates matters and sometimes it is difficult to resolve as shown below.
The system shown below will not heat the Hot Water cylinder and heat leak radiator from the Oil-fired boiler and it is preferable to adopt the previous system.
A twin coil Hot Water Cylinder is a useful appliance in these circumstances.
Also two Feed & Expansion tanks are better so that the cold feed pipes are separated.
It is safe practice to add a safety valve to Solid Fuel boilers.
This system is shown below.