Long time off this forum, but hoping the plumbing brains still frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the cylinder is in the last few minutes of it's heat-up cycle (before the boiler cuts out) - water hammer occurs close to (or in) the Megaflo cylinder.

Water hammer never occurs if CH is completely off (no matter how the hot water is used or not used), nor does water hammer occur if CH set to radiators only.

Water hammer occurs occasionally and to a lesser degree, again in the last few-minutes of cylinder heat-up, if both cylinder heating and radiators are on.

By touch, the water hammer seems to be affecting both the CH inlet and outlet pipes to the cylinder equally (and sometimes also the cold mains water inlet to the cylinder, but not the hot water outlet).

The water hammer can almost always be stopped by simultaneously switching on heating to the radiators (and temporarily stopped by drawing off some hot water).

Gas CH, modern house (c. 12 years), Megaflow, S-plan, cold water mains pressure up to 6 bar. Seems to be a standard megaflow configuration with genuine Heatrae Sadia parts.


SO FAR:

All pipes seems reasonably well clipped.
Bled air from rads.
CH water seems clean.
Checked S-plan (2 port) valves have correct direction of flow.
CH system when cold pressure 1.2 - 1.5 Barr, when hot pressure 1.8-2.0 Barr..
Mains water cold 4.5 - 6.0 Barr, hot water 3.0 - 4.5 Barr.
Have checked that the 8 Barr temp/pressure discharge valve, and 10 Barr safety valve both manually operate, and discharge to tundish.
Have regenerated air pocket as per instructions on the Megaflow cylinder.



I haven't yet tried turning off the cold water inlet to the cylinder, and letting the boiler do a how-water warm-up cycle with the Megaflow depressurised - to attempt to prove the problem is definitely CH side, and not mains-water side (incidentally the quarter-turn valve just before pressure-reducing valve is frozen in the fully-on position, so have to use the under-sink main cold-water stopcock to isolate the hot-water system).



Could it be pump overun at the boiler, and the pressure-operated CH-circuit bypass valve not operating correctly? My impression (but not certainty) is the that the 2-port valve to the cylinder is still open when water hammer occurs.

On Friday, 6 January 2017 19:39:53 UTC, wrote:

Clarification - I should have made clear that the water hammer (when it occurs in a nearly-fully-heated cylinder) is not the few-Hz bang-bang-bang, but more more like more-or-less continuous-tone vibration noise that randomly rises and falls in loudness, randomly dropping to barely audible for a few seconds only to increase again, and continue until the boiler finally cuts out.

On 06/01/2017 20:13, wrote:
On Friday, 6 January 2017 19:39:53 UTC, wrote:

Clarification - I should have made clear that the water hammer (when
it occurs in a nearly-fully-heated cylinder) is not the few-Hz
bang-bang-bang, but more more like more-or-less continuous-tone
vibration noise that randomly rises and falls in loudness, randomly
dropping to barely audible for a few seconds only to increase again,
and continue until the boiler finally cuts out.


So this is a hammer in the CH primary system, not in the DHW take off
from the cylinder?

Has it always done this, or is it new?

What is the set point temp on the cylinder stat?

What kind of circulator pump is it? (and what speed is it set to?)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Friday, 6 January 2017 21:00:27 UTC, John Rumm wrote:

So this is a hammer in the CH primary system, not in the DHW take off
from the cylinder?

Has it always done this, or is it new?

What is the set point temp on the cylinder stat?

What kind of circulator pump is it? (and what speed is it set to?)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/


Thanks for your interest, John.

1) I *think* it's in the CH primary - hence me planning to try depressuring the hot water side and let it go through a heat up cycle to be certain.

I would say for definite that the cylinder HW outlet is not affected by vibration. Trouble is the noise is of a pitch, and reflected around in an airing cupboard, that makes it hard to localise - except by touch.

By touch I can say CH inlet and outlet to the cylinder affected equally, cold feed affected sometimes.

2) The noise issue has (I'm told) re-occurred from time-to-time over the 12 years (approx) of usage. Allegedly no problems in the early years, but has come and gone spontaneously after that.

3) Cylinder stat set to its mid-point and what I would describe as a normal domestic HW temperature at the tap (sorry, no thermometer immediately available).

4) Circulator pump is built into the boiler (Ideal icos) and is non-adjustable single-speed.

wrote:
Long time off this forum, but hoping the plumbing brains still frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the cylinder is
in the last few minutes of it's heat-up cycle (before the boiler cuts
out) - water hammer occurs close to (or in) the Megaflo cylinder.

Water hammer never occurs if CH is completely off (no matter how the hot
water is used or not used), nor does water hammer occur if CH set to radiators only.

Water hammer occurs occasionally and to a lesser degree, again in the
last few-minutes of cylinder heat-up, if both cylinder heating and radiators are on.

By touch, the water hammer seems to be affecting both the CH inlet and
outlet pipes to the cylinder equally (and sometimes also the cold mains
water inlet to the cylinder, but not the hot water outlet).

The water hammer can almost always be stopped by simultaneously switching
on heating to the radiators (and temporarily stopped by drawing off some hot water).

Gas CH, modern house (c. 12 years), Megaflow, S-plan, cold water mains
pressure up to 6 bar. Seems to be a standard megaflow configuration with
genuine Heatrae Sadia parts.


SO FAR:

All pipes seems reasonably well clipped.
Bled air from rads.
CH water seems clean.
Checked S-plan (2 port) valves have correct direction of flow.
CH system when cold pressure 1.2 - 1.5 Barr, when hot pressure 1.8-2.0 Barr.
Mains water cold 4.5 - 6.0 Barr, hot water 3.0 - 4.5 Barr.
Have checked that the 8 Barr temp/pressure discharge valve, and 10 Barr
safety valve both manually operate, and discharge to tundish.
Have regenerated air pocket as per instructions on the Megaflow cylinder.



I haven't yet tried turning off the cold water inlet to the cylinder, and
letting the boiler do a how-water warm-up cycle with the Megaflow
depressurised - to attempt to prove the problem is definitely CH side,
and not mains-water side (incidentally the quarter-turn valve just before
pressure-reducing valve is frozen in the fully-on position, so have to
use the under-sink main cold-water stopcock to isolate the hot-water system).



Could it be pump overun at the boiler, and the pressure-operated
CH-circuit bypass valve not operating correctly? My impression (but not
certainty) is the that the 2-port valve to the cylinder is still open
when water hammer occurs.


Do you have a bypass valve that allows water to circulate back to the
boiler after the HW cylinder has reached its set temperature and the
motorised valve has closed?

I'm guessing that the noise relates to this.

Tim

--
Please don't feed the trolls

On Friday, 6 January 2017 21:53:03 UTC, Tim+ wrote:


Do you have a bypass valve that allows water to circulate back to the
boiler after the HW cylinder has reached its set temperature and the
motorised valve has closed?

I'm guessing that the noise relates to this.

Tim

--
Please don't feed the trolls

Yes, there is one, and I'm wondering if that's the issue too.

However it isn't my impression that the motorised valve has closed - but I will check this again to be certain.

On Friday, 6 January 2017 21:58:27 UTC, wrote:
On Friday, 6 January 2017 21:53:03 UTC, Tim+ wrote:


Do you have a bypass valve that allows water to circulate back to the
boiler after the HW cylinder has reached its set temperature and the
motorised valve has closed?

I'm guessing that the noise relates to this.

Yes, there is one, and I'm wondering if that's the issue too.

However it isn't my impression that the motorised valve has closed - but I will check this again to be certain.

Water hammer can be solved with an upward pointing blanked off bit of pipe. It fills with dissolved gas, acting like a capacitor.


NT

On Friday, 6 January 2017 22:10:49 UTC, wrote:
On Friday, 6 January 2017 21:58:27 UTC, wrote:
On Friday, 6 January 2017 21:53:03 UTC, Tim+ wrote:


Do you have a bypass valve that allows water to circulate back to the
boiler after the HW cylinder has reached its set temperature and the
motorised valve has closed?

I'm guessing that the noise relates to this.

Yes, there is one, and I'm wondering if that's the issue too.

However it isn't my impression that the motorised valve has closed - but I will check this again to be certain.

Water hammer can be solved with an upward pointing blanked off bit of pipe. It fills with dissolved gas, acting like a capacitor.


NT

Yes, there is one of those in a piping "top loop" near the cylinder.

wrote:
On Friday, 6 January 2017 22:10:49 UTC, wrote:
On Friday, 6 January 2017 21:58:27 UTC, wrote:
On Friday, 6 January 2017 21:53:03 UTC, Tim+ wrote:


Do you have a bypass valve that allows water to circulate back to the
boiler after the HW cylinder has reached its set temperature and the
motorised valve has closed?

I'm guessing that the noise relates to this.

Yes, there is one, and I'm wondering if that's the issue too.

However it isn't my impression that the motorised valve has closed -
but I will check this again to be certain.

Water hammer can be solved with an upward pointing blanked off bit of
pipe. It fills with dissolved gas, acting like a capacitor.


NT

Yes, there is one of those in a piping "top loop" near the cylinder.


It can also be caused by too much "dead leg" volume. Might be worth
bleeding all the air out of it.

Tim

--
Please don't feed the trolls

On 06/01/2017 21:46, wrote:

2) The noise issue has (I'm told) re-occurred from time-to-time over
the 12 years (approx) of usage. Allegedly no problems in the early
years, but has come and gone spontaneously after that.

3) Cylinder stat set to its mid-point and what I would describe as a
normal domestic HW temperature at the tap (sorry, no thermometer
immediately available).

4) Circulator pump is built into the boiler (Ideal icos) and is
non-adjustable single-speed.

Could it actually be bearing noise or vibration from the pump itself
that is just "ringing" the coil in the cylinder?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Friday, 6 January 2017 23:52:23 UTC, John Rumm wrote:
On 06/01/2017 21:46, wrote:

2) The noise issue has (I'm told) re-occurred from time-to-time over
the 12 years (approx) of usage. Allegedly no problems in the early
years, but has come and gone spontaneously after that.

3) Cylinder stat set to its mid-point and what I would describe as a
normal domestic HW temperature at the tap (sorry, no thermometer
immediately available).

4) Circulator pump is built into the boiler (Ideal icos) and is
non-adjustable single-speed.

Could it actually be bearing noise or vibration from the pump itself
that is just "ringing" the coil in the cylinder?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Yes - because of the pattern of vibration I had indeed wondered about 'ringing' of the coil.

One theory I entertained was that as the tank coils got hotter that they were expanding and becoming loose in whatever restrains them (I've never sawn a megaflow in half to determine all its secrets).

However Tim+ suggests that it relates to the bypass valve, and that would rather fit with an ill-defined pattern of occurrence of noise dependent on the state of each individual TRV.

Perhaps a combination of the two - a coil prone to resonance, and a bypass valve issue that triggers it?

On Friday, 6 January 2017 21:46:04 UTC, wrote:
On Friday, 6 January 2017 21:00:27 UTC, John Rumm wrote:

So this is a hammer in the CH primary system, not in the DHW take off
from the cylinder?

Has it always done this, or is it new?

What is the set point temp on the cylinder stat?

What kind of circulator pump is it? (and what speed is it set to?)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/


Thanks for your interest, John.

1) I *think* it's in the CH primary - hence me planning to try depressuring the hot water side and let it go through a heat up cycle to be certain.

I would say for definite that the cylinder HW outlet is not affected by vibration. Trouble is the noise is of a pitch, and reflected around in an airing cupboard, that makes it hard to localise - except by touch.

By touch I can say CH inlet and outlet to the cylinder affected equally, cold feed affected sometimes.

2) The noise issue has (I'm told) re-occurred from time-to-time over the 12 years (approx) of usage. Allegedly no problems in the early years, but has come and gone spontaneously after that.


The best way to pinpoint a noise is with a listening stick.
This can be a big screwdriver. You put the blade on the suspected source and press the handle against your ear (or rather the area just in front of your ear, not the actual ear hole.)

Try throttling one of the pump isolation valve (the down stream one).

Try opening and closing radiator valves.

You can get "flutter" on the obturating device on any valve due to wear. Zone valves are specially suspect.
I think this last may be your problem as you say the noise depends on shutting heating on/off. This is usually done by means of an "electric/zone valve".

Using listening stick, check the pressure relief valve inside the boiler.
(This maintains constant pressure in the system by opening as the radiator valves close.)

It is on a pipe linking flow and return pipes. (Adjusting this might help with the problem)

On 06/01/2017 19:39, wrote:
Long time off this forum, but hoping the plumbing brains still
frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the
cylinder is in the last few minutes of it's heat-up cycle (before the
boiler cuts out) - water hammer occurs close to (or in) the Megaflo
cylinder.

Last few minutes suggests the boiler may be in bypass mode. There should
be a bypass valve and I would suspect that. Some are adjustable and if
this is too I would be tempted to alter it's position to see if the
hammer stops or gets worse.

The other possible is your 2-port valve. This could be 'bouncing'
internally and causing the hammer. Most valves have a lever to manually
operate it. If you open it during a hammer episode and it goes away it
confirms it could be this valve, but unfortunately doesn't rule out the
bypass valve.

On 07/01/2017 09:46, wrote:
On Friday, 6 January 2017 23:52:23 UTC, John Rumm wrote:
On 06/01/2017 21:46, wrote:

2) The noise issue has (I'm told) re-occurred from time-to-time
over the 12 years (approx) of usage. Allegedly no problems in the
early years, but has come and gone spontaneously after that.

3) Cylinder stat set to its mid-point and what I would describe
as a normal domestic HW temperature at the tap (sorry, no
thermometer immediately available).

4) Circulator pump is built into the boiler (Ideal icos) and is
non-adjustable single-speed.

Could it actually be bearing noise or vibration from the pump
itself that is just "ringing" the coil in the cylinder?


Yes - because of the pattern of vibration I had indeed wondered about
'ringing' of the coil.

One theory I entertained was that as the tank coils got hotter that
they were expanding and becoming loose in whatever restrains them
(I've never sawn a megaflow in half to determine all its secrets).

You could wait for the noise and then go have a listen / feel of the
pump to find out.

However Tim+ suggests that it relates to the bypass valve, and that
would rather fit with an ill-defined pattern of occurrence of noise
dependent on the state of each individual TRV.

Spring loaded bypass valve would also be possible, adjusting its setting
while the noise is there would probably influence it.

If the noise happens when the rads are off, then the TRV settings ought
not come into play.

Perhaps a combination of the two - a coil prone to resonance, and a
bypass valve issue that triggers it?

Yup certainly could be.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

wrote in message
...
On Friday, 6 January 2017 19:39:53 UTC, wrote:

Clarification - I should have made clear that the water hammer (when it
occurs in a nearly-fully-heated cylinder) is not the few-Hz bang-bang-bang,
but more more like more-or-less continuous-tone vibration noise that
randomly rises and falls in loudness, randomly dropping to barely audible
for a few seconds only to increase again, and continue until the boiler
finally cuts out.

Is there anything as simple as a ball-cock in a cold-water header tank in
your system? I only ask because I had just such a water-hammer sound as you
describe, like a low cello note. This was just after I repaired the plastic
ball which had got half full of water, but was now so light it vibrated. I
cured it by clipping a submerged metal plate to the arm of the ball cock by
a clothes peg to act as a damper.
--
Dave W

On Sunday, 8 January 2017 11:10:08 UTC, Dave W wrote:
wrote in message
...
On Friday, 6 January 2017 19:39:53 UTC, wrote:

Clarification - I should have made clear that the water hammer (when it
occurs in a nearly-fully-heated cylinder) is not the few-Hz bang-bang-bang,
but more more like more-or-less continuous-tone vibration noise that
randomly rises and falls in loudness, randomly dropping to barely audible
for a few seconds only to increase again, and continue until the boiler
finally cuts out.

Is there anything as simple as a ball-cock in a cold-water header tank in
your system? I only ask because I had just such a water-hammer sound as you
describe, like a low cello note. This was just after I repaired the plastic
ball which had got half full of water, but was now so light it vibrated. I
cured it by clipping a submerged metal plate to the arm of the ball cock by
a clothes peg to act as a damper.
--
Dave W

No, it's an entirely sealed system. CH has filling loop, and expansion tank in boiler. Hot water system is mains pressure with an air pocket in the top of the Megaflow cylinder.

On Friday, 6 January 2017 19:39:53 UTC, wrote:
Long time off this forum, but hoping the plumbing brains still frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the cylinder is in the last few minutes of it's heat-up cycle (before the boiler cuts out) - water hammer occurs close to (or in) the Megaflo cylinder.

Water hammer never occurs if CH is completely off (no matter how the hot water is used or not used), nor does water hammer occur if CH set to radiators only.

Water hammer occurs occasionally and to a lesser degree, again in the last few-minutes of cylinder heat-up, if both cylinder heating and radiators are on.

By touch, the water hammer seems to be affecting both the CH inlet and outlet pipes to the cylinder equally (and sometimes also the cold mains water inlet to the cylinder, but not the hot water outlet).

The water hammer can almost always be stopped by simultaneously switching on heating to the radiators (and temporarily stopped by drawing off some hot water).

Gas CH, modern house (c. 12 years), Megaflow, S-plan, cold water mains pressure up to 6 bar. Seems to be a standard megaflow configuration with genuine Heatrae Sadia parts.


SO FAR:

All pipes seems reasonably well clipped.
Bled air from rads.
CH water seems clean.
Checked S-plan (2 port) valves have correct direction of flow.
CH system when cold pressure 1.2 - 1.5 Barr, when hot pressure 1.8-2.0 Barr.

On 13/01/2017 19:16, wrote:

That pretty much leaves the pressure reducing valve on the cold water
inlet side, and/or the 8-bar/90c safety valve.

I believe that safety valve also contains a non-return valve.

The one on my Vaillant does not appear to have any additional non return
on the side mounted over temp / pressure valve - it relies on the
tundish for that. Its basically a rubber faced piston held against a
flat brass mating surface by spring tension.

Could the water hammer be some sort of interaction between the
air-pocket in the cylinder and a defective non-return valve?

Have you checked there is no let by into the tundish? I noticed with
mine that if one turns taps off abruptly, it can shock a tiny amount of
water past the side mounted over temp/pressure valve. Over time that
built up some scale on it and then it started letting by continuously. I
had to descale it and polish the brace facing part to get it to resel
properly.


I'm getting to the point where "replace every component near the
tank" is sounding a sensible course of action.

You may find some of the controls rather pricey alas!

New ideas/theories very welcome.

Change in the volume of the air pocket? (Is it in a bladder on the
megaflow?[1]) - you could try adding a bit more air charge to it and see
if that makes a difference.

[1] The unistor has a separate expansion vessel externally.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Friday, 13 January 2017 22:37:44 UTC, John Rumm wrote:
On 13/01/2017 19:16, wrote:

That pretty much leaves the pressure reducing valve on the cold water
inlet side, and/or the 8-bar/90c safety valve.

I believe that safety valve also contains a non-return valve.

The one on my Vaillant does not appear to have any additional non return
on the side mounted over temp / pressure valve - it relies on the
tundish for that. Its basically a rubber faced piston held against a
flat brass mating surface by spring tension.

Could the water hammer be some sort of interaction between the
air-pocket in the cylinder and a defective non-return valve?

Have you checked there is no let by into the tundish? I noticed with
mine that if one turns taps off abruptly, it can shock a tiny amount of
water past the side mounted over temp/pressure valve. Over time that
built up some scale on it and then it started letting by continuously. I
had to descale it and polish the brace facing part to get it to resel
properly.


I'm getting to the point where "replace every component near the
tank" is sounding a sensible course of action.

You may find some of the controls rather pricey alas!

New ideas/theories very welcome.

Change in the volume of the air pocket? (Is it in a bladder on the
megaflow?[1]) - you could try adding a bit more air charge to it and see
if that makes a difference.

[1] The unistor has a separate expansion vessel externally.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Ah - sorry, I was being unclear.

According to the Megaflow documentation, the non-return valve is inline with the cold water feed into the cylinder,
i.e. pressure-reducing-valve - safety-valve (with non-return on inlet side) - tank-inlet.

I'm guessing the intent here is that as the pressure rises inside the cylinder, it can't create back-pressure on the pressure reducing valve.

My (current) theory is that this non-return valve isn't effective, and the water-hammer is some sort of resonance between the cylinder air-pocket and back into the cold supply.

The tundish here had been wet recently, but now seems to be completely dry since I regenerated the air-pocket in the cylinder as per the instructions on the megaflow.

(the air pocket AIUI is simply the top portion of the cylinder, with the hot outlet pipe dipping sufficiently deep into the cylinder that the air has no way out.This would tally with their instructions to turn off the cold supply, open the farthest hot tap, and then hold open the safety valve - until it stops gurgling - to regenerate the air pocket)

On Friday, 13 January 2017 19:16:42 UTC, wrote:
On Friday, 6 January 2017 19:39:53 UTC, wrote:
Long time off this forum, but hoping the plumbing brains still frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the cylinder is in the last few minutes of it's heat-up cycle (before the boiler cuts out) - water hammer occurs close to (or in) the Megaflo cylinder.

Water hammer never occurs if CH is completely off (no matter how the hot water is used or not used), nor does water hammer occur if CH set to radiators only.

Water hammer occurs occasionally and to a lesser degree, again in the last few-minutes of cylinder heat-up, if both cylinder heating and radiators are on.

By touch, the water hammer seems to be affecting both the CH inlet and outlet pipes to the cylinder equally (and sometimes also the cold mains water inlet to the cylinder, but not the hot water outlet).

The water hammer can almost always be stopped by simultaneously switching on heating to the radiators (and temporarily stopped by drawing off some hot water).

Gas CH, modern house (c. 12 years), Megaflow, S-plan, cold water mains pressure up to 6 bar. Seems to be a standard megaflow configuration with genuine Heatrae Sadia parts.


SO FAR:

All pipes seems reasonably well clipped.
Bled air from rads.
CH water seems clean.
Checked S-plan (2 port) valves have correct direction of flow.
CH system when cold pressure 1.2 - 1.5 Barr, when hot pressure 1.8-2.0 Barr.
Mains water cold 4.5 - 6.0 Barr, hot water 3.0 - 4.5 Barr.
Have checked that the 8 Barr temp/pressure discharge valve, and 10 Barr safety valve both manually operate, and discharge to tundish.
Have regenerated air pocket as per instructions on the Megaflow cylinder.

On Friday, 6 January 2017 19:39:53 UTC, wrote:
Long time off this forum, but hoping the plumbing brains still frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the cylinder is in the last few minutes of it's heat-up cycle (before the boiler cuts out) - water hammer occurs close to (or in) the Megaflo cylinder.

Water hammer never occurs if CH is completely off (no matter how the hot water is used or not used), nor does water hammer occur if CH set to radiators only.

Water hammer occurs occasionally and to a lesser degree, again in the last few-minutes of cylinder heat-up, if both cylinder heating and radiators are on.

By touch, the water hammer seems to be affecting both the CH inlet and outlet pipes to the cylinder equally (and sometimes also the cold mains water inlet to the cylinder, but not the hot water outlet).

The water hammer can almost always be stopped by simultaneously switching on heating to the radiators (and temporarily stopped by drawing off some hot water).

Gas CH, modern house (c. 12 years), Megaflow, S-plan, cold water mains pressure up to 6 bar. Seems to be a standard megaflow configuration with genuine Heatrae Sadia parts.


SO FAR:

All pipes seems reasonably well clipped.
Bled air from rads.
CH water seems clean.
Checked S-plan (2 port) valves have correct direction of flow.
CH system when cold pressure 1.2 - 1.5 Barr, when hot pressure 1.8-2.0 Barr.

On 14/01/2017 15:07, wrote:
On Friday, 13 January 2017 22:37:44 UTC, John Rumm wrote:
On 13/01/2017 19:16, wrote:

That pretty much leaves the pressure reducing valve on the cold
water inlet side, and/or the 8-bar/90c safety valve.

I believe that safety valve also contains a non-return valve.

The one on my Vaillant does not appear to have any additional non
return on the side mounted over temp / pressure valve - it relies
on the tundish for that. Its basically a rubber faced piston held
against a flat brass mating surface by spring tension.

Could the water hammer be some sort of interaction between the
air-pocket in the cylinder and a defective non-return valve?

Have you checked there is no let by into the tundish? I noticed
with mine that if one turns taps off abruptly, it can shock a tiny
amount of water past the side mounted over temp/pressure valve.
Over time that built up some scale on it and then it started
letting by continuously. I had to descale it and polish the brace
facing part to get it to resel properly.


I'm getting to the point where "replace every component near the
tank" is sounding a sensible course of action.

You may find some of the controls rather pricey alas!

New ideas/theories very welcome.

Change in the volume of the air pocket? (Is it in a bladder on the
megaflow?[1]) - you could try adding a bit more air charge to it
and see if that makes a difference.

[1] The unistor has a separate expansion vessel externally.


Ah - sorry, I was being unclear.

According to the Megaflow documentation, the non-return valve is
inline with the cold water feed into the cylinder, i.e.
pressure-reducing-valve - safety-valve (with non-return on inlet
side) - tank-inlet.

ok, that's fairly normal on larger cylinders...

I'm guessing the intent here is that as the pressure rises inside the
cylinder, it can't create back-pressure on the pressure reducing
valve.

Yup. (small cylinders sometimes actually deliberately back water into
the supply pipe to handle the expansion)

My (current) theory is that this non-return valve isn't effective,
and the water-hammer is some sort of resonance between the cylinder
air-pocket and back into the cold supply.

The tundish here had been wet recently, but now seems to be
completely dry since I regenerated the air-pocket in the cylinder as
per the instructions on the megaflow.

(the air pocket AIUI is simply the top portion of the cylinder, with
the hot outlet pipe dipping sufficiently deep into the cylinder that
the air has no way out.This would tally with their instructions to
turn off the cold supply, open the farthest hot tap, and then hold
open the safety valve - until it stops gurgling - to regenerate the
air pocket)

That presumably means that the air pocket gets absorbed over time as it
defuses into the water?



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Friday, 13 January 2017 19:16:42 UTC, wrote:
On Friday, 6 January 2017 19:39:53 UTC, wrote:
Long time off this forum, but hoping the plumbing brains still frequent this place!

PROBLEM:

When Central Heating is set to hot water *only*, and when the cylinder is in the last few minutes of it's heat-up cycle (before the boiler cuts out) - water hammer occurs close to (or in) the Megaflo cylinder.

Water hammer never occurs if CH is completely off (no matter how the hot water is used or not used), nor does water hammer occur if CH set to radiators only.

Water hammer occurs occasionally and to a lesser degree, again in the last few-minutes of cylinder heat-up, if both cylinder heating and radiators are on.

By touch, the water hammer seems to be affecting both the CH inlet and outlet pipes to the cylinder equally (and sometimes also the cold mains water inlet to the cylinder, but not the hot water outlet).

The water hammer can almost always be stopped by simultaneously switching on heating to the radiators (and temporarily stopped by drawing off some hot water).

Gas CH, modern house (c. 12 years), Megaflow, S-plan, cold water mains pressure up to 6 bar. Seems to be a standard megaflow configuration with genuine Heatrae Sadia parts.


SO FAR:

All pipes seems reasonably well clipped.
Bled air from rads.
CH water seems clean.
Checked S-plan (2 port) valves have correct direction of flow.
CH system when cold pressure 1.2 - 1.5 Barr, when hot pressure 1.8-2.0 Barr.
Mains water cold 4.5 - 6.0 Barr, hot water 3.0 - 4.5 Barr.
Have checked that the 8 Barr temp/pressure discharge valve, and 10 Barr safety valve both manually operate, and discharge to tundish.
Have regenerated air pocket as per instructions on the Megaflow cylinder.

On Sunday, 15 January 2017 02:36:45 UTC, John Rumm wrote:
On 14/01/2017 15:07, wrote:
On Friday, 13 January 2017 22:37:44 UTC, John Rumm wrote:
On 13/01/2017 19:16, wrote:

That pretty much leaves the pressure reducing valve on the cold
water inlet side, and/or the 8-bar/90c safety valve.

I believe that safety valve also contains a non-return valve.

The one on my Vaillant does not appear to have any additional non
return on the side mounted over temp / pressure valve - it relies
on the tundish for that. Its basically a rubber faced piston held
against a flat brass mating surface by spring tension.

Could the water hammer be some sort of interaction between the
air-pocket in the cylinder and a defective non-return valve?

Have you checked there is no let by into the tundish? I noticed
with mine that if one turns taps off abruptly, it can shock a tiny
amount of water past the side mounted over temp/pressure valve.
Over time that built up some scale on it and then it started
letting by continuously. I had to descale it and polish the brace
facing part to get it to resel properly.


I'm getting to the point where "replace every component near the
tank" is sounding a sensible course of action.

You may find some of the controls rather pricey alas!

New ideas/theories very welcome.

Change in the volume of the air pocket? (Is it in a bladder on the
megaflow?[1]) - you could try adding a bit more air charge to it
and see if that makes a difference.

[1] The unistor has a separate expansion vessel externally.


Ah - sorry, I was being unclear.

According to the Megaflow documentation, the non-return valve is
inline with the cold water feed into the cylinder, i.e.
pressure-reducing-valve - safety-valve (with non-return on inlet
side) - tank-inlet.

ok, that's fairly normal on larger cylinders...

I'm guessing the intent here is that as the pressure rises inside the
cylinder, it can't create back-pressure on the pressure reducing
valve.

Yup. (small cylinders sometimes actually deliberately back water into
the supply pipe to handle the expansion)

My (current) theory is that this non-return valve isn't effective,
and the water-hammer is some sort of resonance between the cylinder
air-pocket and back into the cold supply.

The tundish here had been wet recently, but now seems to be
completely dry since I regenerated the air-pocket in the cylinder as
per the instructions on the megaflow.

(the air pocket AIUI is simply the top portion of the cylinder, with
the hot outlet pipe dipping sufficiently deep into the cylinder that
the air has no way out.This would tally with their instructions to
turn off the cold supply, open the farthest hot tap, and then hold
open the safety valve - until it stops gurgling - to regenerate the
air pocket)

That presumably means that the air pocket gets absorbed over time as it
defuses into the water?



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/




That presumably means that the air pocket gets absorbed over time as it
defuses into the water?

Exactly so.

replying to dom, Michael wrote:
My megaflow system too having the same nature of bang! issue when the
motorise valve in the primary flow get shut off. If any one can suggest a firm
solution please advise.

--
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