MCZ/Red Hydro Stoves and boilers - minimum static pressure

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For most of the MCZ / Red 365 Hydro stove and boiler ranges, the installation manuals state that when connecting the appliance to the hydraulic system and filling it, the cold fill pressure should be 1 Bar. This may at first seem to imply that the appliance can only be installed as a sealed system configuration. Conversely, the literature for the Compact range of boilers state that either a sealed system or open vented installation is possible, however if open vented, a minimum static head of 0.3 Bar must be maintained on the boiler. (HOWEVER: See further note below).

In fact, for these other appliances, it is also possible to run open vented: The key factor is ensuring that, as stated for the Compact range, the minimum static head on the appliance is kept at/above a minimum threshold.

The reasoning, as explained by MCZ, is below:


“In short, the open vent solution is always possible when you can guarantee at least 0.3 Bar. The main goal is to avoid cavitation (vapour forming within the circulator) and to do so, it is necessary to maintain at the circulator suction port an adequately high positive pressure (static head) in relation to the vapour pressure of the fluid being handled. The stated minimum value assumes the location is at approximately sea level. For higher altitudes, add 0.1m head/1000 M height increase. For example, if the stove is installed in a mountain region (1000 M) you should have 0.4 Bar. These minimum heads must be respectively increased when handling fluids of higher temperatures or lower densities, higher resistances at the circulator suction side, and in regions of lower atmospheric pressures.”


NOTE: The Compact and Logika range of boilers have a water pressure switch as part of its safety controls. In discussions with MCZ, and from field observations, it transpires that the switch is set to near 0.4 Bar. Given that there will also be an element of differential in their set point, when added to this, the suggested minimum static pressure for these appliances on open vent systems is 0.5 Bar to avoid low pressure alarms from being raised.