Constant flow automatic bypass valves

[decomplexity]

Re John.g's reply: spot on!
I was writing identical comments but was beaten to the draw.
The underlying problem is that a pump such as any in the Grundfos domestic range running in proportional-pressure mode has its lowest pressure at lowest flow (or no flow) - typically when all the TRVs shut on a hot day or when there the CH + DHW demand ceases and the boiler tells the pump to overrun for a few minutes. A pressure-driven ABV set to this pressure will simply open all the time.
And when I looked at the Tacosetter's tech spec, it appeared to operate as a fixed valve with a flow meter. Perhaps the flow set-point is resilient to pressure changes but this wasn't obvious from the spec: perhaps someone more familiar with them can correct me.

 

[John.g]

Re John.g's reply: spot on!
I was writing identical comments but was beaten to the draw.
The underlying problem is that a pump such as any in the Grundfos domestic range running in proportional-pressure mode has its lowest pressure at lowest flow (or no flow) - typically when all the TRVs shut on a hot day or when there the CH + DHW demand ceases and the boiler tells the pump to overrun for a few minutes. A pressure-driven ABV set to this pressure will simply open all the time.
And when I looked at the Tacosetter's tech spec, it appeared to operate as a fixed valve with a flow meter. Perhaps the flow set-point is resilient to pressure changes but this wasn't obvious from the spec: perhaps someone more familiar with them can correct me.

Could this be Honeywell's answer to the Caleffi?.

 

[ShaunCorbs]

Could this be Honeywell's answer to the Caleffi?.

No as it’s a trv

 

[chris watkins]

Pressure Independent Control Valve (PICV)
All leading manufacturers now do their own version, but not so sure how you would use it in a by-pass.
Very cleaver but I am with Shaun too much for a domestic system.

 

[ShaunCorbs]

Sorry I’m lost why is this a problem if you don’t want a bypass valve install a bypass rad eg airing cupboard

Depending on the sys it’s more to go wrong and service it’s only a domestic system

 

[decomplexity]

The Honeywell VT5005T referred to (in a link) by John.g is indeed
a more flexible version of the Caleffi Autoflow 127. The Caleffi valves (the 127 and its big brothers the 121-126) have changeable fixed flow-rate cartridges whereas the flow-rate of the V5005T is set by the installer and can be varied.
It is horses for courses: the V5005T is twice the price of a 127 but you get 'variability' for your money. However, the V5005T is intended for controlling the flow and temperature of its inputs - it is not just a flow limiter, and the temperature control is regulated by its actuator.
Of technical interest - overkill for a domestic installation - are Honeywell’s electrical actuators (e.g. the M4410E/K) that can be bolted on, such that the V5005T operation can be overridden proportionately (i.e. not just on/off) by an external controller with variable (PWM) output. In an ideal world the V5005T would be normally off (no flow), and the boiler would have a suitable PWM output that would enable it to signal to the V5005T actuator when input flow rate was falling towards the minimum.
(Whether the VT5005T can be set to ‘no-flow’ until remotely actuated I don’t know and the documentation I have seen doesn't say)