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Open & Sealed Systems

whjtyIt is always recommended to over-size a system expansion vessel to avoid later problems. The point of connection of the expansion vessel into the system is important. The ‘ pressure-neutral point’ of an NRG ZONE is perfectly suitable to allow any appliance open access to the expanding system volume as the system contents heat and cool. The physical location of the vessel can be at any convenient unused port in the NRG ZONE giving maximum flexibility, or can be directly on the inlet side of the heating appliance.

If a system is extended, an expansion vessel of increased volume (or an additional vessel) may be required, unless previous provision has been made for the extension. The vessel charge pressure (1.5 bar standard) should not be less than the static head pressure at the centre of the expansion vessel. For static heads greater than 15 metres, the vessel charge pressure should be increased. As a guide, the expansion vessel is undersized if the pressure gauge indicates 2.65 bar or above when the boiler is at maximum temperature with all radiators or other zone distribution equipment in circulation. In such a case, a larger (or additional) expansion vessel is required. (Note that an appliance safety valve will commence over-pressure discharge at around 3 bar). It is also important that the discharge outlet from either safety valves or Automatic Air Vents (AAV) should never be located directly above the NRG ZONE unit as inadvertent leakage may go unnoticed and corrode the manifold.

When installing an ‘open’ system for a temperature controlled oil or gas boiler it is essential to ensure that the appliance has an unrestricted open vent and separate cold feed. It is also good practice to prevent heat rising in the cold feed pipe. This would warm the expansion tank contents which would evaporate and cause more fresh water to enter the system and damage the ferrous parts. The example adjacent shows how a heat-lock loop could be formed to prevent that occurrence.

The cold feed and expansion pipe may be connected separately to the manifold if the expansion pipe rises adequately above the tank. The example shows a ‘double T’ configuration from connection ‘T2 & T3 on the cold feed and expansion which may be useful by serving two functions; 1) it will create  a vent for the cold feed loop and 2) it will also minimise any inadvertent ‘pitching-over’ of water from the expansion pipe into the tank.


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When installing a solid fuel heating system it is essential to ensure that the appliance has an unrestricted open vent and separate cold feed. Possible solutions are illustrated adjacent.

An expansion header tank is located at the highest point in the system and a cold feed connected through a ‘heat-lock’ loop as shown. The optional by-pass methods shown in these diagrams connect the expansion pipe through a ‘flap type’ non-return valve on a horizontal pipe run and into the return on a the vertical rise from the heat-lock loop. This method is useful when the height of the expansion pipe is restricted.

The optional expansion pipe to return by-pass strategy shown will minimise the possibility of water rising in the expansion pipe and pitching-over into the tank which is a trait commonly found in badly configured open systems.

Both methods shown provide the pre-requisite safe primary gravity fed heat-leak circuit through the DHW cylinder. However, Option one  will make the primary circuit heat available for heating in the NRG ZONE manifold before it heats the Hot Water cylinder. Option two;  will make the primary circuit heat available for heating hot water in the cylinder before it reaches the NRG ZONE manifold. In both instances the appliance’s pump (P1) should be thermostatically controlled by a pipe thermostat fitted on the return to the appliance to turn the pump ‘ON’ above 55°C. This will prevent condensation forming in the fire-chamber which would otherwise damage the appliance.