Popcorn noise from boiler


5 Apr 2010
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United Kingdom
I have a Gloworm Energysaver 60 boiler used in an s-plan open vent system. The system uses two Honeywell spring return zone valves for the HW and CH return pipes and a grundfos selectric pump. The boiler is in good condition and the the zone valves are both working correctly.

Recently I replaced the HW zone valve as it was stuck 'open'. I also took the opportunitty swap the the old gate valve that was being used as a fixed bypass valve across the boiler for a honeywell automatic bypass valve. Relevant manuals for my system below:



Bypass Valve:

When the boiler is running, everything works fine and the sytem produces very little noise (just a reassuring hum). Once the water in the system is up to temperature, the boiler thermostat shuts down the burner and continues to run the pump (as it should). Again, this works fine and the system remains good and quiet.

However, when the cylinder and room thermostats shut the system down, the boiler starts to make quite a lot of noise. It sounds for all the world like popcorn being 'popped' in the boiler but, in fact, the noise seems to originate from the pump and simply echoes through the boiler.

When both the room and cylinder thermostats are 'off', both the CH and HW zone valves are obviously closed and the burner in the boiler is shut down. Immediately after the valve closes , the pump still runs (for about 5 mins) to help cool the boiler down a little (I guess). It is during this 5 mins that the noise can be heard.

In this mode, the bypass valve comes into action and opens up to allow water to circulate around the 4m of pipe that enclose the boiler, pump and bypass valve. Having tested the flow and return temperatures under these conditions, it appears that the valve is opening corrctly as the temperature around the whole loop is a constant 70 degrees (ie no temperature differential between send and return). For the record, when operating with the CH zone valve open, there is a normal 10 degree drop across the system.

I have adjusted the bypass valve setting from minimum up to about 0.5bar and the noise does not change. However, if I reduce the pump speed to minimum, the noise stops immediately. I guess the noise did not used to happen because the HW valve was stuck open so the water continued to circluate around the whole HW system all the time.

Since the water temperature is only 70 degrees and the noise stops when the pump stops, I'm pretty sure it's not kettling but I have no idea waht else it could be. Has anyone heard/seen anything like this before?


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Firstly thanks for posting so clearly which puts most others to shame!

There is only one part which is not clear where you say the tewmperature "across" is 70°C. The word "across" indicates a temperature differential.

During overrun with both motor valves closed the temperature on all boiler and bypass pipes should be virtually the same.

My immediate reaction is that the bypass is either not correctly located or is backwards ( or faulty ).

But you sound competent so that would be surprising but does seem the most likely cause. Even very experienced installers sometimes make a vesy basic mistake.

Posting photos of the pipework and bypass in relation to the pump and motor valves might indicate the problem.

Hi Tony, good point. I used the term 'across' quite badly there. What I meant was that the temperature is evenly 70 degrees around the loop (ie, no temperature differential). For this reason (and the gentle hissing noise from the valve as I adjust it's setting to a higher value) I suspect that the valve is working correctly. I'll edit my original post to reflect this.

I have also double checked that the valve is installed in the right direction and it appears to be correct (arow across valve matches direction of arrow on the pump).

I don't have any photos but I can try describe the loop as below:

Boiler send -> input of bypass valve ->
output of bypass valve -> input of pump ->
output of pump -> boiler return

I was initially surpirsed to see the pump on the return side of the boilr but maybe this is normal.

I have read up a lot on bypass valaves and mine does appear to be in the correct position of the system.


It was normal to put the pump on the return until about 30 years ago.

They are still on the return inside boilers.

Your layout does sound correct but a photo might show us something that you have not mentioned or overlooked.

Mobile phone photos can be sent to me or others as a picture message to post on here for you if thats of any help.

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Hi Tony, thanks. I'll try to get a pic tonight when I get in from work.


Hi Tony, sorry for the delay in getting back on this, got a bit sidetracked this week at work.

Anyway, here is a photo of the system. The photos does not show the boiler itself but the two pipes that head up out of the photo are the send and return to the boiler. The return is plumbed to the outlet of the pump.

Stuff I have tested today. Adjusting the bypass valve does not seem to fix the problem but does perhaps reduce the frequency and volume of the 'pops'.

Setting the pump to its lowest setting immediately stops the noise and setting it to its highest (3) makes it much worse.

I've tried working out exactly where the noise comes from but it is very hard to pinpoint (as it resonates through the pipes). However, it's definitely not the boiler and does come from the area of pipes near the bypass valve (though doesn't seem to sound like it's from the valve itself).

Does that give you any ideas?


Why change the pump? It runs silently under all other operating conditions and flow through the system is excellent.

The noise only occurs when the zone valves are closed and the pump is driving water through the bypass valve.

That's right. It runs fine and heats the house well (all very quietly). The noise (like popcorn popping) only occurs when the zone valves are closed and pump is in overrun mode through the bypass valve (which is new). It's a pretty loud and scary noise.

Hi LittlePlum2, thanks for the advice but I would urge you to read my original post to get answers to the questions you asked. I know it's a long post but all the info is there.

In summary though, a fixed bypass valve was already there but I decided to replace this with an auto bypass valve when I replaced the HW zone valve which had become jammed open (since I had to drain down the system). So two things changed at the same time:

1. The HW zone valve was replaced with one that actually closes when HW demand is off.

2. The fixed bypass valve was replaced by an auto one (link to manual in original post).

Since the HW zone valve was previously jammed open, the old zone valve was essentially unused since the HW circuit was always on. So, it is not obvious that the new auto bypass is at fault.

Additionally, since I have adjusted it across its whole range and the noise still occurs (albeit slightly quieter as the valve pressure is increaed) I am no convinced that it is the bypass valve.

I have checked for kids eating butterkist popcorn in the cupbaord but they were just smoking fags and told me to f*ck off. Pesky kids.

iep. is it possible the bypass is reversed?

Useing a long screwdriver, with one end to your ear and other to areas around the pump, listen to noise. Where is it loudest?

PVM- no body can be suttle as your good self. Welcome back after a long absence.
I have checked several times whether the valve is reversed (as it is exactly the type of mistake I am prone to making. But, on this occasion, I am pretty confident that I have installed it correctly.

Good thinking on the screwdriver stethoscope, I use that on my car. Don't know why it did not occur to me to use it on plumbing. Even just listening to it with the naked ear though I can hear that water is definitely passing throug the bypass valve when the pump is in overrun mode. So, I'm confident that the valve is at least partially functional.

I made a recording of the noise but don't seem to be able to uplaod it to the site. does anyone know if this is possible?

Setting the pump to its lowest setting immediately stops the noise and setting it to its highest (3) makes it much worse.
The arrow on the pump should be point upwards and the arrow on the bypass should be pointing to the left.

Are the motorized valves the correct way round? It should be IN at port A and OUT at port B (There may be an arrow, which should point upwards)

Have you balanced the system? How to balance a CH system.
The arrow on the valve points left and the one on the pump upward just as you describe. There is also another 3m of pipework above the pump which gives the correct minimum length for the bypass loop.

The zone valves are definitely correctly fitted too with their arrows pointing upwards.

Not sure if your 'Have you balanced the system?' question is actually part of your signature but, for the record, yes.

I'd really like to get to the bottom of this issue as the noise is genuinley loud as is frankly a bit scary.


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