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Yes, the pressure at the ERV is less than 2.43 bar (safety setting from CIBSE), in mode B with both pumps at max, system at max temp, and DPBV closed. Thanks.

Thanks George, I’ll need to calculate pressure at ERV for that scenario. I’m away from the office now until Monday, so will calculate then; and update you. Thanks for all your help.

Thanks George for the guidance. The EV is sized for expansion from cold fill to max design operating temperature. Thanks for advice regarding location of EV for keeping boiler in liquid phase and NPSH for pump 1. The ERV is a pressure relief valve set at 3 bar as a final safety device, should...

Thanks George. Manifolds A and B are for radiators and U for underfloor heating. So any leaks would be to atmosphere. The EV has a butyl rubber diaphragm whose lifespan can be reduced by exposure to higher temperatures. The EV is at a high point (TU is higher; with a NRV), so the anti-g loop is...

Thanks George. The flow is greater in mode B than mode A; also the flow varies within mode A as the electrothermic valves open and close. Expansion Vessel blanketing pressure is fixed. If the Expansion Vessel is connected between the boiler and pump 1, I think that would work as the variations...

Thanks George for the advice and guidance. The expansion vessel is at the bottom of the diagram mark EV. (TU is a top-up unit, mainly for commissioning and maintenance.) The expansion vessel is pre-charged with Nitrogen. I think I see the issue, in mode B this EV would come under pressure from...

Thanks George for the guidance. The check valve is after the combined exit from the 2 banks of electrothermic valve heating banks, apologies for the mistake, I have corrected the detailed P&ID. Sequence when transitioning from mode A to mode B: 00 secs: [tab]zone valve A open (90°), [tab]zone...

Thank you LittleInch for the reply, that's encouraging. Diagram of system: Zone Valves A and B are exclusive, if one's open the other will be closed. Zone Valve response time open-closed 75 secs Pump B would be switched through Zone Valve B, so would not operate until Zone Valve B is 85° open...

Could two centrifugal pumps of the same type be used in series in constant pressure mode, where the pumps are set to deliver different heads please? The system is a closed circuit, which can operate in two different modes: Mode A: Only pump 1 operates, zone valve closed for circuit with pump 2...