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

 

[dilalio]

How well is the pipework supported /clipped?

New pump will be stronger than old pump, which may have been weak enough not to shock the pipework.

 

[Johntheo5]

>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!)


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


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

Is it a 6M Wilo Yonos Pico or a 8M you have installed?.
If its the 6M then C1 (which you are runing on?) is 2.1M, the other CC curves are C2 is 3.8M & C3 is 6.0M ( I have this pump)
if its the 8M version C1 is 3.1M, C2 is 5.6M & C3 is 7.7M

You also stated that UV cylinder is "indicating 1.2bar", the same as the primary circuit? or are you just quoting the primary pressure in both cases,

What make of UV cylinder?, some of these make some very weird sounds from time to time even though doesn't seem to be anything to do with your problem.

Don't know if the 2 port end switch is used in this set up but maybe worth removing the switched live that drives it open and then just manually try it in different settings to replicate the noisy operation above.

 

[dylan_t42]

Morning,

How well is the pipework supported /clipped?
New pump will be stronger than old pump, which may have been weak enough not to shock the pipework.

There’s definitely some loose clips on the wall & it’s possible that there’s unsecured pipe work under the floor, but the previous setup never made this noise.
The bang also occurs without the pump running - both during the day & out of hours.

Is it a 6M Wilo Yonos Pico or a 8M you have installed?.

Not sure - the label says 25/1-6 130 (ROW) so maybe the 6M? A small unit with variable power between 4-40W & currently operating at 12W & 0.6m3/h - so fairly low flow & definitely less than our previous Grunfoss lump.

You also stated that UV cylinder is "indicating 1.2bar", the same as the primary circuit? or are you just quoting the primary pressure in both cases,

The pressure gauge is only on the primary circuit. The Megaflow cylinder should be limited to around 3 bar but there’s no gauge, and it’s never made much noise.

Don't know if the 2 port end switch is used in this set up but maybe worth removing the switched live that drives it open and then just manually try it in different settings to replicate the noisy operation above.

Not sure of the exact wiring setup, but we can provoke the noise manually by pushing the two-port to open once the tank is fully heated. After doing that, if we adjust the tank thermostat it opens & closes the two-port without any more banging.

Wonder what happens if we do that while the pump is running…