HR92's Knocking - Water Hammer?

[walking13]

Hi There

I have just had a new system boiler installed (swtiched from combi) and installed new radiators

The HR92's were all working fine prior to the switch.

The new radiators all have honeywell valencia bi-directional flow valves fitted. Some are on return, some are on flow.

There are still two old valves on the system from radiators that were not that old and did not need to be replaced.

The issue is that when the heating cools or moves to off we get knocking from one of the HR92's that reverberates throughout the house and other radiators. Upon trying to isolate which radiator is causing the issue, by switching the valve fully off the knocking then moves to another radiator. (so far 3)

Valves are all new so I cannot imagine more than one of them are faulty, so is this:

- An issue with valves being randomly fitted on both return and flow (didn't think this was an issue on bi-directional).
- A pump issue?
- Or maybe faulty valve somewhere that needs a trial and error to see which one is causing the problem?

I currently have all HR92's removed for the last few days and have had no issues so the system itself seems fine.

Thanks in advance

[Richard]

Pump speed is probably too fast (resulting in high DP across the valve seat and narrow DT across the radiator), but really bad practice to put TRV's on the return (even if they are bi-directional).

[GeorgeZip]

Thanks, this is a really informative answer which I think I'll talk to my CH chap about.

I've just bought a 16 radiator setup from you with valves and am having the same symptoms as OP with radiators knocking when TRV is barely open.

He just replaced the valves location for location on radiators and didn't mention about return or not. I'll checkout the original valves to see if they're directional or not.

I asked him when he quoted to installed a bypass valve too, like I had on my previous EH system in last house, buy I was told there was no need if I left at least one radiator open.

[Richard]

I asked him when he quoted to installed a bypass valve too, like I had on my previous EH system in last house, buy I was told there was no need if I left at least one radiator open.

This all depend if minimum flow rate required by the pump or the boiler heat exchanger can be satisfied by this individual radiator? If you need 300 litres per hour of flow and the radiator is designed to give 50 litres per hour, you are going to be at a deficit of 250 litres that needs to go somewhere? He has given you bad advise, get another engineer who knows what they are talking about.

[GeorgeZip]

Thanks for taking the time to respond back to me.

I've got a WB 42CDi so it's not the smallest of boilers. The radiator that the chap suggested I leave is pretty much the smallest radiator in the house and it's on 10mm piping whereas the majority of the system the radiators are over twice the size with 15mm piping.

Yeah I'd already decided to not have him back. I'll make enquiries about having the ABV installed with others once I've had recommendations. In the mean time I've taken the HR92's off the largest radiator, in a room which is one zone with three other radiators, and left it wide open. Hopefully that might stop some of the other radiators from hammering away