Single Water Hammer in c/h pipes when h/water valve closes

steveoldgit · 25 Aug 2009

My c/h system is 10mm, and hot water system 15mm added later (years ago) with one Honeywell open/closed valve on rads circuit and one on hot water circuit.
Since fitting a new pump 2+ years ago (direct replacement as near as I could get) there is a single water hammer each time the hot water system valve closes. This didn't happen with the old pump, and nothing else has changed.
The pump stops pushing before the valve closes water off, as it should do.
Anyone got any ideas?
It has been suggested that I fit an air bottle/spring to absorb the shock of the water flow being stopped, but if so I don't know sizes or details, or possibility of success.

Thanks

twgas · 25 Aug 2009

in HW valve fitted the right way?

steveoldgit · 25 Aug 2009

Thanks for quick response, Mick. Yes, it is right to circuit diag. hw valve is on flow, rads valve is on return (wierd system because it came about in three stages; ch first, hw later, and boiler change later still. Pump change was years after that)

There are yards of 15mm pipe full of water between the pump (downstairs front by boiler) and hw valve (within inches of the hw tank input upstairs back) , I think that must be a factor, but I don't know how.

Cheers

D_Hailsham · 25 Aug 2009

steveoldgit said:
There are yards of 15mm pipe full of water between the pump (downstairs front by boiler) and hw valve (within inches of the hw tank input upstairs back) , I think that must be a factor, but I don't know how.

What size is the boiler (kw not inches

)

Does the 15mm pipe just serve the upstairs? If so what is their total heat output in kw? (The Stelrad Elite Catalogue is a good guide.

15mm pipe may be too small for the amount of heat it is carrying. So the pump has to be turned up higher, which will generate more noise in the pipe work. There should be an arrow on the valve body showing the direction of flow.

steveoldgit · 25 Aug 2009

Boiler is Ideal RS50 output to water is 14.7 KW.
Salient point is that the water hammer didn't occur in exactly the same system until the Grundfos pump was changed.
I think the pump performance was up a bit on the old pump, but it was the nearest available to the old Grundfos, and I haven't got details of the before/after re pumps.
The pipe to the hw tank is 15mm and was the same with the previous pump, when the fault didn't occur. It serves only the hot water side of the system.
The 15mm system was designed by my late Uncle who designed systems for public bulidings, so I think it was sized OK. The rads are on a 10mm system, and I would expect to have probs with that rather than the 15mm! The pump is turned up to full as the old one was before it was changed.
Yes, I found the arrow, and the valve is the right way round.

Thanks

D_Hailsham · 26 Aug 2009

steveoldgit said:
Boiler is Ideal RS50 output to water is 14.7 KW.

Then the main feed to and from the boiler should be in 22mm pipe

The pipe to the hw tank is 15mm and was the same with the previous pump, when the fault didn't occur. It serves only the hot water side of the system.

Is it 15mm all the way back to the boiler or does it connect into a larger pipe before the boiler? When the boiler is heating only the hot water cylinder, the full output of the boiler is travelling through the 15mm pipe.

A 14.7kW boiler requires a flow rate of 0.32 litres per second. In a 15mm pipe this is equivalent to a speed of 2.2 metres per second. The maximum recommended speed is 1m/s, otherwise you will have a lot of noise.

If, as is usually the case, a gate valve has been put in the return to the boiler, to ensure the cylinder does not get more than its fair share of the heat when the central heating is on, the problem will get worse.

The pump is turned up to full as the old one was before it was changed.

What happens if you turn the pump down?

doitall · 26 Aug 2009

Kin heck

The little ball has gone spongy, loose in the MV.

Change the valve and solve the problem.

steveoldgit · 26 Aug 2009

Mr Hailsham

Thanks.
The pipework entering/leaving boiler is all 22mm, and reduces to 15mm after tees (It's not quite as simple as that, but in essence) so I think the flow rate is OK.
It's not noisy all the time anyway, just when the hw valve closes and the head of water upstream of that valve is stopped.
There is a gate valve, but on the flow side of the hw system just before the valve. This is wide open at the moment - haven't tried closing it a bit, because it was there and set the same before the pump was changed and the hammer started.
I have turned the pump down, and it does reduce the single hammer somewhat, but it's still there. I've turned it back up to full because the hammers been going for 2+ years now, without seeming to affect anything else, and turning it down just makes the rads heat up slower in winter.

doitall

Thanks.
That does sound feasible.
I suppose it could just be coincidence that it started when the pump was changed, since the new pump has a bit more poke that the old one, and showed up an existing weakness.
Practically, would this mean that the the ball jumps shut too quickly instead of powering smoothly home, do you reckon?

Thanks both very much.

doitall · 26 Aug 2009

No it means the ball is loose in danger of falling to bits even.

Take the actuator off and turn the shaft by hand, You'll soon know what I mean.

With the system running.

D_Hailsham · 26 Aug 2009

steveoldgit said:
It's not noisy all the time anyway, just when the hw valve closes and the head of water upstream of that valve is stopped.

Is there a bypass? Does the position (open/closed) of the heating valve make any difference?

There is a gate valve, but on the flow side of the hw system just before the valve. This is wide open at the moment

It's normally on the return, but still doing the same job. Try closing it down a turn at a time and see what happens. (If you close it too much the hot water will take too long to heat up!)

Nige F · 26 Aug 2009

steveoldgit said:
Mr Hailsham

Thanks.

very much.

That`s Lord Hailsham

steveoldgit · 27 Aug 2009

Nige F said:
steveoldgit said:

Mr Hailsham

Thanks.

very much.

Click to expand...

That`s Lord Hailsham

Sorry, Your Worship

steveoldgit · 28 Aug 2009

Ignoring p###takes for the moment;

1) no, there is no bypass. Don't know why, probably another function of the way the system 'evolved'. When the tank stat stops calling for heat, simultaneously the boiler and pump stop, and the solenoid in the zone valve de-energises, then the spring starts to close the valve, and maybe three seconds later when it seats, you hear the hammer.

2) the gate valve on the flow just upstream of the hw valve closes completely in 4 1/4 turns, so I marked it with tape and closed it down 1/4 turn at a time. It is now 1 1/4 turns down from fully open, and it has reduced the single water hammer. Instead of a bigger echoey booming type noise it is now more of a clonk.

3) I've tested it with the gate valve closed down as described, and when the heating valve is open at the same time (ie rads are heating up), and the hw valve closes, I can't hear any noise. I think last time I did that with the gate wide open there was a hammer, so that's an improvement.

I'll leave it like that for a bit and see whether the water heat-up time is OK etc, and report back.

I would value opinions as to whether 1 1/4 turns closed out of 4 1/4 sounds OK and not OTT.

Thanks and regards

D_Hailsham · 28 Aug 2009

steveoldgit said:
1) no, there is no bypass. Don't know why, probably another function of the way the system 'evolved'. When the tank stat stops calling for heat, simultaneously the boiler and pump stop, and the solenoid in the zone valve de-energises, then the spring starts to close the valve, and maybe three seconds later when it seats, you hear the hammer.

Bypass valves provide a short cut from flow to return after the pump for two reasons: So the pump can circulate water if the boiler has a "pump overrun"; to provide a leakage path when TRVs start to close, other wise the resistance encountered by the pump would be too great and the water would not circulate at the correct speed. Your boiler does not have pump over run, so it's just a case of whether you have TRVs.

I would value opinions as to whether 1 1/4 turns closed out of 4 1/4 sounds OK and not OTT.

The flow through a gate valve is not proportional to how many turn the valve is open. So shutting down 1 1/4 turns does not reduce the flow by 30% but may be 20%.

If you have any means of measuring the pipe temperatures, check the temperatures of the flow and return at the boiler when it has been running for some time; calculate the difference (should be about 11°C)

Then do the same at the cylinder flow and return when HW is on. The difference in temperature should be about the same. Open or close the gate valve to alter the difference (close= increases, open = decrease).

steveoldgit · 28 Aug 2009

Ashamed to admit I had to Google TRV, but now I know, no, I don't have Thermostatic Radiator Valves. We have lived here for about 27 years, and the ch part of the system was already installed when we arrived. It is quite basic but works fine, so I don't want to b###er about with it unless I am assured of success.

That being said, I admit that now that the hw zone valve creates no hammer when the rads zone valve is open does make me wonder whether a bypass would make the difference.

Thanks for the delta T formula for adjusting the gate valve, very good approach. Will do that when I can borrow a thermometer.

Regards

diynot

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Single Water Hammer in c/h pipes when h/water valve closes

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