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

[aerod]

My secondary hot water circulation (i.e. returning the water from the furthest tap to a tapping about a quarter of the way down the cylinder) 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 ...?
Without an NRV the worst that would happen is that, when the pump is off, the hot water system would be fed by both flow from the top of the cylinder and the return from a short way down the cylinder, but resistance through the pump would minimise the latter. What's the problem with this? What have I missed?

 

[Dan Robinson]

Convection

 

[aerod]

I understand convection but don't see how that would cause a problem without an NRV. Any chance of a few more words?

 

[Dan Robinson]

Energy wastage.

Nrv is there to prevent convection once the timer and stat have been satisfied.

 

[aerod]

OK, 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?

 

[Nozzle]

Prevents reverse rotation of the pump, and by doing so reduces the strain on the pump motor when it starts.

I'm surprised the NRV is on the suction side though, if it gets wedged partly closed the cavitation will kill the impellor and volute. It's more normal to have an NRV on the discharge side. Then, the worse that happens is the pump runs at shut-off which while not great long-term, at least it doesn't cause pitting/errosion from flashing off.

Nozzle

 

[Dan Robinson]

OK, but the NRV prevents flow out of the circulation return tapping on the cylinder.

Not only.

 

[aerod]

.. but also?
(this is starting to remind me of Peter Cook and Dudley Moore's "not only but also" shows, such as:

)

 

[aerod]

Prevents reverse rotation of the pump, and by doing so reduces the strain on the pump motor when it starts.
Nozzle

Have you got a reference for that? It's surprising that the starting load would be significantly affected by even a moderate reverse flow.

 

[OvertonPlumber]

Most non return valves used on secondarys arent fit for purpose. Plastic internals.
A proper one like this will not clatter https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/252311090440

 

[Dan Robinson]

Most non return valves used on secondarys arent fit for purpose. Plastic internals.
A proper one like this will not clatter https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/252311090440

Also, most secondary pumps are massively oversized.

 

[Hot&Cold]

Without an NRV the worst that would happen is that, when the pump is off, the hot water system would be fed by both flow from the top of the cylinder and the return from a short way down the cylinder, but resistance through the pump would minimise the latter. What's the problem with this? What have I missed?

There are many systems where secondary circulation is not actually required,often installed by self installers with knowledge gained from google .

There again on some installations secondary circulation is essential but often omitted.love google .

 

[Nozzle]

Have you got a reference for that? It's surprising that the starting load would be significantly affected by even a moderate reverse flow.

http://www.marinesite.info/2017/01/...osed-during-starting-of-centrifugal-pump.html

Nozzle

 

[aerod]

Thanks for the link, but it's not clear that it's dealing with comparable flows.

I've now had an answer from Worcester-Bosch 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."

 

[Hot&Cold]

I've now had an answer from Worcester-Bosch to the same question:

great stuff.You have it all sorted out without a visit to your property.

bye