Replaced Pump - boiler still slapping and banging

[buckmoy]

Hiya guys

Replaced a pump last night (with some help from a plumber with a blowtorch).

I then drained the system from the draincock (opening rads on the way down), added some Fernox or equivalent to the empty F&E tank, and refilled.

Bled the radiators as the water came back up, bled the pump and started the circulation again.

There is water in the F&E tank (about half full).

Switched the boiler on and it sparked and ran silent, sweet as a lamb for about 20 mins. Rads started heating up then I heard the banging again - switched off the boiler before it tripped itself.

Any ideas what this might be? One thing I noticed is that the F&E tank vent seems to making noise when the pump is spinning.

Now when I try and switch the boiler on it bangs straight away.

Pump is on maximum speed.

Other history:
October - boiler banging - bloke replaced 3 way valve and pump - so the pump only lasted a few months...!

 

[kevplumb]

are the valves open under the boiler

 

[buckmoy]

kev

Not sure. Will check when I get home - hopefully the post won't have died by then...

From memory:

There are no valves under the boiler (a Thorn Apollo) - it runs from the kitchen up to an attic room where the pump / 3 way valve / F&E tank is.

All the valves up there there are open - except a bypass/return valve I don't entirely understand which is half open/shut.

The only thing near the boiler is a draincock on the return pipe (shut - never touched it) and all the boiler gubbins underneath its main shell - the pilot light pizo igniter etc.

 

[kevplumb]

did you open the three port valve when you refilled

and are you sure its ok

sounds like a circulation problem

are the pump valves open

 

[buckmoy]

Kev

When I refilled I had the the CH/HW switched to "off" so the pump wasn't spinning.

Not sure what position that means the 3 way valve would have been in (its "default off"? the "last known good"?)

Should I have fettled the manual switch on the side of the 3 way valve?

if so - what setting should that be in when I refill.

The valves above and below the pump are both open.

Whats confusing me is that system was fine for about 20 minutes when it first went going.

Thanks! Long live oil filled radiators, luke warm showers and electricity bills...

 

[kevplumb]

lock the valve mid pos when filling

are sure there isnt a bleed on the hw side as well

 

[buckmoy]

hmm so a bleed for the HW as well as all the rads.

Last thing I did after bleeding the upstairs rads after refilling was to bleed the pump - but that wouldn't have necessarily bled the HW circuit would it..

Where might a HW bleed be?

Anyway - if the worst comes to the worst I can always drain and refill the entire system whenever I want to switch it on - and every 20 mins thereafter...

 

[kevplumb]

Where might a HW bleed be?

usually near the cylinder

 

[buckmoy]

OK

Couldn't find a HW bleed.

3 way valve is making all the right noises and the boiler is staying lit long enough for me too see that the right side of it is getting hot for the right setting.

When i first switched it on having been off all day - the F&E vent made a massive racket for a while. I tried the glass of water trick - it sucked it up at first (about 1/3 pint) when the pump was first switched on, then didn't seem to be doing anything - but the noise was coming from there.

I turned pump down to lowest setting and put boiler on "low". Its been on CH only for about 5 minutes now - and settled into a regular drone noise (1 peak per second approx) - which can almost be felt underfoot. I bled the nearest rad and it was bubbling in time with the drone.

F&E tank vent has quitened down though.

Not sure - have the uneasy feeling its about to blow..

 

[keego]

Try it on hot water only, shut the bypass & turn the pump on full. If there is any air trapped on that side that should clear it, & don't forget to put the bypass back the way you found it.

 

[felix]

Are you sure the droning noise is really coming from the boiler? I'll rephrase that. Are you sure the noise is being GENERATED in the boiler? I ask this because I've heard such a noise before. It echoed round the whole system but I traced it to a thermostatic radiator valve (TRV) which was fitted at the wrong end.

Some of these valves claim to be bi-directional but many are not. You MUST fit them at the flow end of the radiator. If you put one at the return end then, at some critical valve opening, it can rattle like a road drill. The fact that your heating ran silently for 20 minutes is a clue. Perhaps that's how long it took for the rogue valve to warm up.

Next time you hear that noise, run round any TRVs you have and give them a slight tweak. This will make the noisy one go quiet - for a while.

Of course, if you have no TRVs then my theory falls flat.

 

[htgeng]

Some of these valves claim to be bi-directional but many are not. You MUST fit them at the flow end of the radiator. If you put one at the return end then, at some critical valve opening, it can rattle like a road drill. The fact that your heating ran silently for 20 minutes is a clue. Perhaps that's how long it took for the rogue valve to warm up.

interesting. i have recently done some work on a system and now I'm getting a lot of whooshing and slapping Of course the laydee is blaming me in a sarcastic way. Its a really ancient old boiler (not the woman). I think I got an airlock in it when I drained down last week, although the system water is black as ink so its likely sludged up to hell too... oh joys!

 

[buckmoy]

Excellent stuff - thank you very much for the ideas.

Keego - I'll try that when I get back. nice one.

Felix - the system was working fine for about 4 years since I moved in - so not sure therefore if its a TRV on the wrong end of a rad. However, one of the TRVs is spinning freely and has no effect (no predictable effect anyway) on the temp of the rad its governing - could this be causing all the circulation problems? Presumably if I shut all rads and lockshields there should still be a CH circuit (albeit a short one) that water can pump round - so would a single TRV knacker things?

By the way, the droning was coming from the area that the pump / 3 way / F&E stuff was in.

Whats happened now is that, with the pump on medium and the boiler on low: when the boiler sparks the pump starts getting noisier up to the point at which the boiler's thermostat tells it to stop heating for a while. If I put the boiler on high I suspect it will do the banging thing again.

I think you're right though Felix , the 20 quiet minutes must be a clue...any one care to wager a guess?



Thanks again for the help.

 

[felix]

I think the TRV theory is a dead duck. The answer obviously lies in the vicinity of the pump and valve but both seem to be working. It sounds like they're not very far below the feed and expansion pipes and so a horrible thought is surfacing here. You shouldn't need a blowtorch to change a pump so what did he do? Did he move pipes around? What I'm thinking is that your pump is sucking air down the vent in great gulps. Pumps aren't designed to run in air so you get loud noises and poor circulation. After a while, maybe twenty minutes from cold, the pump can no longer get the heat out of the boiler so it shuts down.

Are you forever bleeding radiators? Where is all that air coming from? You'll need to study the pipe layout. In particular you must establish where the vent and feed join the system. If they're on opposite sides of the pump, get that plumber back and stick his blowtorch in the appropriate waste outlet!

PS: If you close all the radiators and HW is off, the only route for the water is through that valve that's half on. That's your bypass. It's purpose is to allow a SMALL flow of water even if all other paths are closed. To set it up you turn off HW and all radiators then listen to the boiler. It will cycle on and off. If the bypass flow is insufficient it will 'kettle' just before switching off. Having more bypass than you need reduces the flow to your radiators (when they're back on) and causes the boiler to cycle unnecessarily. You can't set it up until your other problem is fixed.

 

[buckmoy]

I don't have to bleed the radiators at all. (I've tried but theres no air).

The situation now is:

CH+HW. Noisy. Kettling. Boiler fires up about once a minute, for about 20 secs a time.

Haven't touched the bypass.

Boiler set on low, pump on medium.

Rads not getting very hot.

Any more ideas? Except for log fires etc.