New two-port valve - water hammer on opening

[dylan_t42]

We had a new gas boiler fitted to an existing Y-plan system back in July but, after several months on DHW only, we turned the CH on and now get a loud water hammer from the new DHW two-port valve OPENING when the pressurised hot water tank calls for heat.

We can provoke the bang by manually opening the DHW two-port via the lever, but how do we stop this noisy bang & rattle happening every time the HW tank feels it’s less than a toasty 55ºC?

Investigations so far:
The hammer-on-open noise varies in strength, but…
- only occurs after the two-port has been in a closed position, as the tank thermostat reaches its target temp
- and only when the CH has also been active
The existing three-port valve is operating quietly.
No issue over the summer on just DHW, and no issue in October with CH alone when we switched off the DHW.
Timing initially seemed random, including outside hours of operation (06:00-22:00) & independent of whether or not the pump running.
Both the new pump & new two-port valve have their arrows pointing in the right direction.
The heating circuit has a new expansion tank showing a very steady 1.2bar since installation.
There’s a new automatic de-aerator, we’ve bled off air from two existing float valves, as well as the new pump’s bleed function.
We didn’t have this issue before the new boiler and other components were fitted.

Any ideas on how to fix this?

TIA

 

[Madrab]

If it's an existing Y plan and then a 2 port's been added, is it an unvented? Not that that should make any difference though TBH.

If the 2 port is jumpy/sticky and causing water hammer then it's probably faulty, that's the first thing I'd suspect.

 

[dylan_t42]

Thanks Rob - seems reasonable, though the new two-port was a direct replacement for a similar Honeywell unit.

 

[cross thread]

Is it fitted through the valve going the correct way

 

[dylan_t42]

Yep - we checked the arrows cast into the brass body

 

[Johntheo5]

Is/was the 2 port valve installed on the coil inlet to the HW cylinder as I think its supposed to be with a Y system since the MPV defaults (power off) to be open to (port B) the cylinder coil.

 

[dilalio]

New boiler correctly piped to flow and return?

 

[Madrab]

Is/was the 2 port valve installed on the coil inlet to the HW cylinder as I think its supposed to be with a Y system since the MPV defaults (power off) to be open to (port B) the cylinder coil.

Usually that's only in play on a Y plan when the cylinder is changed to an unvented and the 2 port is wired through the HL as a fail safe. Not sure it was every required on Gravity HW?

We never used to install any 2 ports on a Y plan gravity system, otherwise it would just have been easier to make it an S plan and remove the troublesome Mid Pos valves altogether.

 

[Johntheo5]

Apparently the new 2 port was a direct replacement for a similar one?, post #3

 

[Madrab]

is it an unvented

@dylan_t42

 

[dylan_t42]

>Madrab said:
>is it an unveiled
Yes, unvented, with a pressure vessel indicating 1.2 bar
Presume the original installer didn’t go S-plan as it’s always been unvented.
(Should have checked back over lunch!)

Is/was the 2 port valve installed on the coil inlet to the HW cylinder as I think its supposed to be with a Y system since the MPV defaults (power off) to be open to (port B) the cylinder coil.

Yes & that’s how the MPV/three-port valve sets itself by default

New boiler correctly piped to flow and return?

Yes, definitely hot flows out to the pump & cooler/cold on the return into the boiler

Mrs.T42 has found a couple of references to bangs & rattles due to hammer-on-open - one found replacing the unit didn’t work, and the other fixed it by reducing the pump speed - but we’re already on the slowest setting (c1) for the Milo