Why have an NRV with a secondary hot water circulation pump?

[nothanks]

My secondary hot water circulation is quite noisy and I suspect the NRV might be rattling - leads me to wonder why there needs to be an NRV in the return line, before the pump ...?

[tabbypurr]

Otherwise when a diferent pump runs, water will bkacflow through the DHW circuit. Which is actually not an issue, but the CH circuits do need an NRV otherwise DHW heating would heat the rads in summer at reduced flow.

NT

[nothanks]

I think we're at crossed purposes. I'm talking about a *secondary* (i.e. hot water to the taps) circulation pump that returns the water from the furthest tap to a tapping about a quarter of the way down the cylinder.

[tabbypurr]

Ah. I can't think of any point to an NRV there. Unless someone thought gravity circulation might happen, which would result in pipe water being below legionella safe temp for hours every day.

NT

[John Rumm]

Its presumably there to prevent gravity circulation resulting in the water in the cylinder bleeding heat into the tap circuit when its not required. (assuming the secondary pump is on a timer or has some other mechanism to stop it running 24 hours a day)

[nothanks]

Hmmm, maybe, but the NRV prevents flow out of the circulation return tapping on the cylinder. For water to convect that way wouldn't it be necessary for the temperature at the tapping to be higher than at the top of the cylinder?

[tabbypurr]

maybe it wasn't thought through that far.

NT

[nothanks]

Worcester-Bosch have just replied to the same question: "The NRV prevents the formation of a "pocket" of stagnant water at the top of the cylinder by guaranteeing flow out of the top of the cylinder only. This prevents stratification and potential legionella growth by allowing flow through the full body of the cylinder." This sounds sufficiently plausible that I will replace the NRV rather than just removing it.