Requirement Circulation pump for the hot water secondary Return

[Hendrik]

Silly question (and might already been asked 100rds of time) - I am specifying a circulation pump for the hot water secondary (circulation) return to reduce the wait for hot water. HW is coming from unvented cylinder with a secondary return connection.

The secondary return is almost straight line up to the loft which is furthest away from the unvented cylinder and is about 9 meters above the secondary return connection at the tank. Secondary will be 15mm with minimal number of elbows. Unvented cylinder is located in basement.

Does this mean I need a pump with a minimum head of 9m?

Thanks

 

[ShaunCorbs]

No as the pump would suck down to the unvented eg being next to the cylinder

 

[fowlerboi]

No as the pump would suck down to the unvented eg being next to the cylinder

The picture below would still require capability for 9m of head no? The secondary return part may be sucking down from 9m but it still has to get up to 9m to be able to come back down.

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[ShaunCorbs]

No as they equal each other out you would need it for the vertical bit on the right hand side so 3m ish I’m guessing

 

[Hendrik]

No as they equal each other out you would need it for the vertical bit on the right hand side so 3m ish I’m guessing

Thanks for responding

fowlerboi - that picture is about right. There is also a draw off point(s) in the loft.

Of course the HW is under mains pressure so with be pushed up so the secondary is coming down on gravity. So actually the circulation pump circulation pump can be a small pump (minimal head).

Or is the wrong thinking

 

[fowlerboi]

I'm trying to wrap my head round why it doesn't need 9m but hopefully Shaun can explain.

In theory when the water isn't being drawn off you won't have the mains so the loop needs to be done purely by a capable pump

 

[ShaunCorbs]

I'm trying to wrap my head round why it doesn't need 9m but hopefully Shaun can explain.

In theory when the water isn't being drawn off you won't have the mains so the loop needs to be done purely by a capable pump

It’s a closed loop so water falling will siphon the water rising

It would be different if the pump was on the hot outlet it would have to overcome the system resistance to get it going

It would be best if you could avoid going into the loft as you could suffer with air locks / needing air vents as it’s the highest point

 

[fowlerboi]

Do bare in mind also that some plastic pipe manufacturers don't allow their fittings to be used with hot water recirculation systems

 

[ShaunCorbs]

Do bare in mind also that some plastic pipe manufacturers don't allow their fittings to be used with hot water recirculation systems

None I’m aware of copper and mlcp (uponor q and e also)

 

[Hendrik]

The loft has a shower room - so that's where the HW needs to go