Originally Posted by carlager
Thanx for your replies.
The pump has an openflow of 4.5 GPM , the water is 75C when heated and the boiler has a capacity of 23l.
Is it possible to say anything about the best size of the relief with this information
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None of the above helps.
Pressure ratings come in a number of flavours, for example "working pressure" which is the pressure the system is rated for and usually allows for metal fatigue, corrosion
, temperature effects, etc. "burst pressure" is the pressure at which the pressure vessel will rupture. Your relief valve should be set for somewhere slightly above "working pressure" and definitely considerably below "burst pressure".
If you cannot find a working pressure rating marked on the appliance try to contact the manufacturer. Due to the danger
associated with bursting pressure vessels it is pretty well standard that manufacturers clearly mark or label them appropriately with pressure data.
If you cannot do any of the above and it is a constantly pressurized system you could put a gauge on the system, observe the pressure where the pump cuts out and put in a relief valve set to say 5psi above this pressure.