Popcorn noise from boiler

With the aluminium measurement plates for the IR thermometer I get a consistent 6 degree figure.
That would imply a flow rate of approx 0.7 litre/sec which, using the screwfix graph, would need the pump set to 3. But you say this is with the pump set to 2. Stranger and stranger.

Have you had a chance to check the temp drops with the pump on all three speeds and all rad valves open? Don't have the HW circuit open when you do this.

I am a bit suspicious of the aluminium plates used for measurement. I can see why you are doing it - to provide a consistent emissivity. Try wrapping some black insulating tapes round the pipe for a few inches and measure off that. Due to the size of the area over which an IR measures - it's like a torch beam - you have to be almost in contact with the pipe to get an accurate reading.
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Time has been short this week but I'm going to do exactly as you suggest this weekend and see what I can learn. Will report back.


An IR thermometer reads the IR which is emitted from the iten being measured. Its calibrated for 0.95 but thats not usually every found in practice except perhaps something which is matt black.

I would always use a contact thermometer.

Hi Tony, Agreed the IR thermometer is not the last word in accuracy but it is providing very repeatable results (the contact plates I have made up for the pipes are painted matt black so this does help).

I did a very basic test to get some correlation by taping a contact thermometer (a meat thermometer TBH) to each pipe for 10 mins last night and they did agree with the IR one to within about 0.5 of a degree.


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Okay, bit of an update. I spent many hours last night messing around with the system and worked out a few things. I'll try to break it down into the three main parts of the discussion on this post.

1. Balancing the system. Finally made some sense of it. After five iterations round the loop I managed to get a 9.8 degrees temperature drop across the system (much better than the 6 degrees I had before). To achieve this I had to do one simple thing, let the system warm up for 3 hours!!!! Only once the send temp exceeded 72 degrees did I start to get sensible results. Also, after 3 hours the max temp of the send pipe only reached 75 degrees (way short of the specified 82 degrees max temp). This got me to thinking, why does it take so long to warm up?

2. Measured the gas flow to the boiler (simply by measuring gas usage on the gas meter over 10 mins). In Low flame mode (used during summer) I measured 33.6 cubic feet per hour. This is almost exactly in agreement with the 34 cubic feet specified in the boiler manual.

However, when I put the boiler into high flame mode (normal mode) I only measure 43 cubic feet per hour. According to the boiler manual the minimum gas rate in High flame mode should be 58 cubic feet per hour. So, it looks like my boiler is only generating about 10kW. The High flame rate can be adjusted by turning an internal screw and should be set during comisioning of the boiler. So I guess I can try adjusting it up the way but I wonder why it was set so low. I guess the burner is okay since the low flame mode is using the correct amount of gas.

3. The popping noise that kicked this post off! Traced this back to the bypass valve itself using the screwdriver stethoscope method. Essentially, when the valve is open, it generated a pining/ticking noise that reverberates through the whole system and sounds like a popping noise when heard via the boiler. Has anyone heard of a bypass valve making a noise like this before?

Cheers, for all the help so far.

We always said the bypass would be the cause of the noise.

You seem to be attacking the problem ( if any ) from a wrong direction.

To get a larger differential you have to slow down the flow. Thats done by closing the lockshields. Typically they should be only open about half to one turn but this depends on the type fittted.

The flow temperature will be held down if your differential is too low.

Yeah, the bypass should have been my first port of call but it's brand new so I thought it was more likely that I had done something wrong. It's a pain as I'll have to drain the system again so I can change it over.

Has anyone else seen a bypass valve make this kind of noise?

Regarding your other comments, I have turned all lockshields down to between 0.25 and 1 turn during balancing but, to be honest, they have always been around that level. I do now have a differential of 10 degrees across the system.

Despite this, it still takes over two hours for the water in the system to reach 75 degrees and it never gets close the rated max temp for the boiler of 82 degrees regardless of how long the system is left running with it's 'stat at full (even on a warm day like yesterday). This is why I looked into the gas usage.

The system was only able to get the house up to 14 degrees during the really cold spell this winter so I'm pretty sure something is wrong with it (as the boiler and radiators are, in theory, correctly sized).

So far, I have replaced the broken HW zone valve that was previously jammed open and this has made a huge difference (as the hot water loop was taing all the flow previously). However, with the boiler unable to hit it's max temperature I'm still concerned something is not as it should be.

Surely such low gas usage is an indicator that the boiler is not generating enough heat?


Have you checked inlet gas pressure at the boiler? What about the pressure at the meter when boiler is running
Unfortunately, I don't have a gas pressure measurement tool as I'm just a DIYer.

While I'm happy to work with the water and electricity (though not at the same time) I'm inclined to accept that I might need a proper gas trained plumber to look much further.

I have tried turning the brass nut to control burner pressure (as described on p22 of the manual below, but this seemed to make no difference.



as mickyg has said you are barking up the wrong tree ffs. The water cooled burner is blocked; personally I'd just change the boiler as not worth fixing.
Get someone in fast. Probably a simple fault that will take 10 mins to diagnose.

You've now adjusted the gas valve nut and without doing working and burner pressure checks have potentialy left the boiler unsafe.

Also be aware this is one of the worst boiler designs ever...you could write a book on the numerous design faults. It will need a thorough safety checkover...most have long since been scrapped. Don't waste any money on it :)
mickyg reckoned the heat exchanger was blocked but you think the water cooled burner is blocked. Are you referring to the same part?


mickyg reckoned the heat exchanger was blocked but you think the water cooled burner is blocked. Are you referring to the same part?



there are 3 heat exchangers on this, the water cooled burner is one of them and it will be this that is blocked as mickyg has already stated.

Don't **** about with boilers if you don't have a clue; they are not toys ffs. Adjusting the gas valve without a manometer is foolish to say the least.
Yeah agreed. I have returned the adjuster nut returned to its initial position and will leave that well alone.

I think it is time to get a proper gas fitter in to give it a once over.

Out of curiosity, which 28kW boilers would people recommend to replace this? I could consider a combi provided it was quiet and provided decent hot water pressure? My old flat had a worcester 24i jnr and the water heater was very weak.

Thought I'd let people know how this story ended.

Nothing was wrong with the boiler at all! Turns out when the gas main was installed (back in 1978) the chump who did it did not put a bit of tape over the pipe when he passed it up through the hole in the ceiling. Little bit of plaster fell inside and, hey presto, pipe was partially blocked for the next 32 years.

Now that the boiler is producing more than 8kW, the flow/return temps are spot on and the radiators are piping hot.

What really ****es me off about this is that the previous owner had the boiler installed by British gas back in 2000 and had it serviced annually. Since there is no way the peice of plaster could have found its way into the pipe after the gas main was installed it must have always been there and the british gas engineers would have measured the same pressure drop over the pipe that I did. So, either they didn't bother to check the burner pressure properly or they did, and decided to do nothing about it even when they installed a new boiler.

As a result, an elderly couple paid £2k for a new boiler install and still had to live in a freezing cold house. Shower of b*****ds!

Oh yes, a new bypass valve fixed the popcorn noise. God knows what was wrong with the last one but the whole system now runs beautifully and quietly.


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