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My earlier post entitled Honeywell ABV setting raised several issues and a number of well-informed comments (thank you). One topic in particular seems worth pursuing on its own, hence the present post. Your comments will be appreciated.
Widely-used domestic CH pumps such as the Grundfos UPS2 15-50/60 have selectable modes of operation: either a constant head for most of the flow range (including no-flow) or modulation of the head inversely to the system resistance (as TRVs warm up and shut, or DHW cylinders hit their temperature set-point, the head is decreased – proportional pressure).
Concurrently, modern gas boilers typically require pump overrun in order to cool the HX when demand for heat ceases. When overrunning with motorised valves shut (or, conceivably, with all the TRVs shut), the pump needs to bypass the heating circuit via an ABV*. Typical ABVs are pressure driven: they open when the head-loss across them exceeds the pressure set by the installing engineer. But since (qv above) pumps either have a constant head for at least the lower part of their flow range (including no-flow) or have a head that is automatically reduced at low-flow or no-flow, it appears impossible to use such an ABV successfully. Worse, when the pump is in proportional pressure mode, a pressure-driven ABV works the wrong way and may shut off the flow just when it is needed for pump overrun!
Unless the boiler itself can provide a signal that pump overrun is about to be enabled – which could then operate a motorised bypass valve, the obvious solution is a constant-flow ABV to achieve a flow rate of say 3 or 4LPM.
And there lies the problem.
The Swedish Maric constant-flow valves are mostly limited to 60degC; the 100degC exceptions have a head-loss starting at 1.4bar (say 14m head – beyond the typical domestic CH system). There is a low-pressure version starting at 0.4 bar (4m head) but this is limited to 60degC.
The alternative I have found to date is Cottam & Preedy’s CP961 (aka ‘Wondervalve’), an isolator ball-valve containing a restrictor cartridge. The low-pressure version can have a 4litres/min restrictor, covers 0.2–4 bar head-loss and is limited to 85degC; and these criteria may well suit the typical domestic CH system. C&P’s literature does however only describe the CP961 as a restrictor (or filter) for DHW, and I am wondering what the snags are for using one as an ABV: perhaps the downstream end must be open (like a tap)? I realise that to avoid flow restriction, gate-valves are preferred to ball-valves in CH pipework, but in this case restriction clearly isn’t problem!
I rang C&P last week them and then emailed, and await a reply.
How do others get round this problem?
* - unless the boiler flow/return are hydraulically decoupled from the heating circuit flow/return by a low-loss header or closely-spaced tees
Widely-used domestic CH pumps such as the Grundfos UPS2 15-50/60 have selectable modes of operation: either a constant head for most of the flow range (including no-flow) or modulation of the head inversely to the system resistance (as TRVs warm up and shut, or DHW cylinders hit their temperature set-point, the head is decreased – proportional pressure).
Concurrently, modern gas boilers typically require pump overrun in order to cool the HX when demand for heat ceases. When overrunning with motorised valves shut (or, conceivably, with all the TRVs shut), the pump needs to bypass the heating circuit via an ABV*. Typical ABVs are pressure driven: they open when the head-loss across them exceeds the pressure set by the installing engineer. But since (qv above) pumps either have a constant head for at least the lower part of their flow range (including no-flow) or have a head that is automatically reduced at low-flow or no-flow, it appears impossible to use such an ABV successfully. Worse, when the pump is in proportional pressure mode, a pressure-driven ABV works the wrong way and may shut off the flow just when it is needed for pump overrun!
Unless the boiler itself can provide a signal that pump overrun is about to be enabled – which could then operate a motorised bypass valve, the obvious solution is a constant-flow ABV to achieve a flow rate of say 3 or 4LPM.
And there lies the problem.
The Swedish Maric constant-flow valves are mostly limited to 60degC; the 100degC exceptions have a head-loss starting at 1.4bar (say 14m head – beyond the typical domestic CH system). There is a low-pressure version starting at 0.4 bar (4m head) but this is limited to 60degC.
The alternative I have found to date is Cottam & Preedy’s CP961 (aka ‘Wondervalve’), an isolator ball-valve containing a restrictor cartridge. The low-pressure version can have a 4litres/min restrictor, covers 0.2–4 bar head-loss and is limited to 85degC; and these criteria may well suit the typical domestic CH system. C&P’s literature does however only describe the CP961 as a restrictor (or filter) for DHW, and I am wondering what the snags are for using one as an ABV: perhaps the downstream end must be open (like a tap)? I realise that to avoid flow restriction, gate-valves are preferred to ball-valves in CH pipework, but in this case restriction clearly isn’t problem!
I rang C&P last week them and then emailed, and await a reply.
How do others get round this problem?
* - unless the boiler flow/return are hydraulically decoupled from the heating circuit flow/return by a low-loss header or closely-spaced tees
).
