Flow through 28mm Honeywell motorised valve

[theSouth]

just picked up a couple of two port valves. I was surprised to see how restrictive the flow seems to be though the valve opening. The ball inside doesn't seem to move too far from the brass to allow water to flow.

Honeywell clearly know more than I do about flow through c/h systems so I'm sure it's fine but can someone explain how this restriction doesn't create a major flow reduction in an otherwise 28mm run (through cylinder)?

If we just assume a hypothetical system for now with no other restrictions (because elbows, boiler, coil in cylinder etc add their own restrictions to flow etc..)

Googles fluid dynamics 101.....

 

[Dan Robinson]

Flow profile is in the paper work.

Are you basing your judgement on the movement using the lever? Or have you powered the valve up to check?

 

[theSouth]

Just a cursory look. Not used Honeywell valves before.

 

[Dan Robinson]

The motor moves the ball further than the lever does.

 

[theSouth]

Ah, thanks.

 

[Agile]

But from a cursory glance into the valves, the inside looks the same on the 28 mm as the 22 mm.

 

[theSouth]

You know, I did wonder about that.

But since my flow is 28 and on to rads is 28 before splitting to two 22's, I thought this would make life easier.

 

[D_Hailsham]

According to Honeywell literature the 28mm valve has a Kv of 8.6. Assuming a differential of 20C and maximum velocity of 1 m/sec, 28mm pipe can handle 45kW. The flow rate would be 1.94 m³/hr, so the pressure drop across the valve would be (1.94/8.6)² bar = 0.05 bar (approx 0.5m H2O).

 

[theSouth]

So.........that's good?

Cheers

 

[D_Hailsham]

when you consider that the pressure drop through a modern condensing boiler can be 2m and the drop round the circuit can be another couple of metres, an extra 0.5m is neither here nor there, You just have to select the correct pump.

Don't forget that I was quoting worst case.

 

[Dan Robinson]

2m? You sized many boilers recently?

 

[D_Hailsham]

2m? You sized many boilers recently?

I may have under-estimated the loss.

Just one example: Vaillant 415 is 1.3m and the 438 is 4.05m. System and Combi boilers are unknown as the pump is incorporated and manufacturers only quotes available head, i.e after the hex loss has been taken into account.

 

[Dan Robinson]

I would say 4m is a more common loss than not.