Author Topic: Central heating pumping over  (Read 1864 times)

Central heating pumping over
« on: December 22, 2018, 10:28:14 am »
Any heating pros out there?

 CH fully pumped with F and E and cylinder, is pumping over and driving me bonkers. New rads throughout last  year, F and E tank cleaned out twice,  new Alpha 2L 15/60 pump last year, cleaned system using  Fernox F3, plenty of inhibitor, and Sentinel boiler silencer (was kettling). Pump speed 2 constant or lower variable constant pressure (running on 1 constant produces kettling).  All Rads 10 rads of 14 panels amd two towel rails are fully hot, boiler is now quiet, water feed and air separator are clear and use off strong neodyme magnet produces no surprises in the cylinder cupboard,  but system is pumping over significantly. Is not related to water cylinder as occurs on heating cycle and water is clear. Ball cock seems to be ok. Flow temp seems to be ok using food thermometer.

Is it time for DS40?

Hard water area.

Any ideas?
Get a bicycle. You will never regret it, if you live- Mark Twain

Re: Central heating pumping over
« Reply #1 on: December 22, 2018, 10:51:18 am »
sounds like there is air (or gas anyway) in the system.  This expands when it gets hot and pushes water out. When the pump runs if there is a significant pressure change (due to pressure drops in the pipe runs) in the air-containing part of the system then again the level in the tank can go up an down and/or you get pumping over.

So have someone eyeball the tank whilst the pump is cycled on and off (system cold) in the first instance. Then do the same thing with the boiler running.

If the level changes enough, and there isn't enough static head in the system, and the cold feed is badly placed, you can end up in a situation where the pump is drawing air in (eg through the cold feed) when it is running.


There may well be radiators that need bleeding out (and NB there is nearly always some air rather than no air trapped in the top of a radiator, so bleeding when the pump is running is different to bleeding when it is not). If there is active corrosion in the system then gas will be generated continuously; test the inhibition by using a nail in a jam jar full of progressively diluted sample fluid.

By shutting off both valves on individual radiators, you can identify which ones contain air and which ones don't, even if you can't bleed the radiator properly for some reason.

hth

cheers

SoreTween

  • Most of me survived the Pennine Bridleway.
Re: Central heating pumping over
« Reply #2 on: December 22, 2018, 11:16:36 am »
Air is the most likely culprit, can you hear any air moving or bangs from the pump?

I've been fighting our system to get the air out and learned two lessons.
1) Only bleed the rads when the system is not running.  Bleeding with the pump running can result in air pulled into the rad instead of expelled.  A length of clear pipe with a few inches of water in it pressed to the vent revealed the water moving the wrong way.
2) If you have an auto vent keep it screwed firmly shut and only open it to let air out when the system is not running.  Since I had air trouble I left mine open thinking it would help.  In fact the auto vent was the source of the air in the first place.

Ours wasn't expelling much water but the expansion tank was full of warm water to the overflow so we must have been losing some.
2018 targets: Survive
There is only one infinite resource in this universe; human stupidity.

Re: Central heating pumping over
« Reply #3 on: December 22, 2018, 11:27:24 am »
Thanks both have bled the rads when cold not much came out. We do have an auto vent will look at that.
Get a bicycle. You will never regret it, if you live- Mark Twain

Re: Central heating pumping over
« Reply #4 on: December 22, 2018, 11:49:49 am »
Have just read that AAVs can draw air in when pumping so have closed the cap on ours which is on the cylinder coil circuit (and which replaced a previous broken manual model). Thanks. Be interesting to see if that is the culprit.
Get a bicycle. You will never regret it, if you live- Mark Twain

Re: Central heating pumping over
« Reply #5 on: December 22, 2018, 12:24:26 pm »
Mmm still pumpiing over when boiler gets to temp. I am stumped.
Get a bicycle. You will never regret it, if you live- Mark Twain

SoreTween

  • Most of me survived the Pennine Bridleway.
Re: Central heating pumping over
« Reply #6 on: December 22, 2018, 01:08:59 pm »
Splurts, stream or gusher?
2018 targets: Survive
There is only one infinite resource in this universe; human stupidity.

Re: Central heating pumping over
« Reply #7 on: December 22, 2018, 01:46:18 pm »
Dripping but significantly so at high temp. Boiler is set to 65 deg for efficient condensating and legionella. Don't think its a pinhole in the cylinder as water is clear and pumping over occurs when water heating is switched off. Dont think its the boiler stat as flow temps fine. Wonder if there is an air lock somewhere, but if so, lord knows where.


Get a bicycle. You will never regret it, if you live- Mark Twain

Aunt Maud

  • Le Flâneur.
Re: Central heating pumping over
« Reply #8 on: December 22, 2018, 04:09:24 pm »
Please explain what pumping over is, cheers.

Re: Central heating pumping over
« Reply #9 on: December 22, 2018, 05:55:29 pm »
fluid coming out of the vent into the header tank = 'pumping over'

one of the things I have done (for diagnostic purposes) is to fit a blanking cap over the vent pipe, which I have previously modified to have a presta valve in it.

With the fill valve and air bleeds closed also, a good system can be subjected to either vacuum or (modest) pressure.  This allows leaks to be found more easily and it also allows the volume of gas in the system to be estimated; the way pressure builds  in the system (i.e. how many pump strokes) when it is pumped up using a bicycle pump can be compared with a bicycle tyre of known volume.


cheers

Aunt Maud

  • Le Flâneur.
Re: Central heating pumping over
« Reply #10 on: December 22, 2018, 06:04:24 pm »
Thanks.

SoreTween

  • Most of me survived the Pennine Bridleway.
Re: Central heating pumping over
« Reply #11 on: December 22, 2018, 07:06:27 pm »
An airlock is still the most likely explanation.  The restriction on regular flow resulting in back pressure looking for somewhere to go.  Most of the system is in good, recent fettle so my money is still on that AAV letting air in which still needs to be cleared.  What is the layout?  For example mine is:

Floor mounted boiler on ground floor.
Single set of horizontals under 1st floor floor.
Pump upstairs almost at the highest point [1]
Drops and risers from the horizontals mostly to single rad up & down.
One instance of three rads off the same drop.
Three port valve.
Bypass valve at the tank. (on instruction from the plumber kept almost closed but I found opening it with the system stopped helped clear air from the return side)
Two downstairs rads permanently on to provide base load, the rest on TRVs
AAV on the HW side only 2

I would set the controls to HW only and then use the tank stat to (10/20 sec mark space) start / stop / start / stop / start the system.  Open the AAV while stopped and repeat until you are reasonably confident the primary loop is clear of air.  Next shut off all the upstairs rads that have TRVs and run the CH until all the downstairs rads are cooking.  If any don't get close to boiler flow temp shut off the ones that do using the lock shields.  Be mindful of where is the air you are clearing going, is it reaching the primary loop and getting to the AAV? Once downstairs is all flowing well set the TRVs to normal values and let the rooms warm up. Next open one upstairs rad TRV and lock shield fully and ensure the feeds to / from that are clear, stop the system and bleed that rad, bleed the primary loop.  Repeat.

This assumes a system basically like mine.  If you have horizontals below floors on 2 levels you need to treat both floors as upstairs above.

I've had a lot of practice clearing airlocks recently.  Only since the penny dropped about the damn AAV has that done any good.

1  No longer true.  Our upstairs pump being almost at the highest point was filling with air and the pump would gradually stall.  I had to stop the pump, fiddle the 3way over to HW and then bang the pipes to get some air out.  Then, start/stop the pump umpteen times until a reasonable volume of air had been expelled.  I recently had a second pump fitted downstairs just above the boiler the theory being when the original pump stalled due to being full of air the new pump would keep things moving enough to clear it.  This worked.  I straight away got air in the only upstairs rad taller than the top of the pump & 3way arrangement.  I had never previously got any air in any rad. The downside of the new pump was that even more air was getting into the system. Setting the TRV full on to the rad now collecting air made it collect lots of air.  As soon as I set the TRV back to normal the air would gather in the pump again.  That led me to concentrate on finding where the air was getting in.  Previously I'd assumed the warm water getting pushed up to the header tank was becoming aerated.  With the only upstairs tall rad now a player in the game I looked for pinholes in that.  Eventually I started experimenting with the clear tube and found that rads suck if they are bled with the pump running and that the AAV sucks in more ways than one.
2 Got that part already.
2018 targets: Survive
There is only one infinite resource in this universe; human stupidity.

Re: Central heating pumping over
« Reply #12 on: January 01, 2019, 12:21:21 pm »
Well........Still driving me batty. Closing all downstairs rads and maxing out the pump yesterday produced no pumping over initially and this for quite some time i.e. a couple of hours.  All rads really hot. Eureka he thought but ........no. I opened a towel rail upstairs in order to ensure dry towels this pm and then the overflow started cascading like a good un, I mean lots. So something is happening. Debris in the sytem moving around perhaps? I turned pump down a notch and it reduced to constantly dripping slowly. Pump is on constant as against variable speed constant pressure. Boiler started kettling a bit this am with upstairs rads still closed.

System was well dosed with inhibitor so annoying to see it diluting daily. Incidentally, auto bypass seems to be operating properly.

This am. Have now closed all downstairs rads, whacked the room stat up a bit, all upper trvs are maxed out radiators nice and hot and no pumping over. Very very odd.
HW circuit seems fine and is set to operate once only a day.
Loath to drain down as it always causes lock out problems but may have to clean system again and put up with it.
Have opened and closed the AAV each time. Currently closed.
Get a bicycle. You will never regret it, if you live- Mark Twain

Re: Central heating pumping over
« Reply #13 on: January 01, 2019, 10:11:07 pm »
the pumping over you saw when the towel rail was opened is 100% consistent with said towel rail being full of air. When the valve is opened the trapped air gets hot and then expands, pushing water out of the system.

It might be that the towel rail (and/or other radiators) 'gather' trapped air/corrosion gas product that gets pumped round the system. This can happen more easily (even when said radiators are 'off') if the valves are plumbed in wrongly, i.e. the used valve/TRV is on the exit rather than entry to any given radiator. If the valves are correctly positioned the collection of gas (circulating in the pipework rather than being generated in that particular radiator) is restricted to the spur in the pipe run between the main feed pipe  and that particular radiator.

Note also that under certain conditions the position and nature of 'T' pieces in combination with the flow rates can mean that you get a negative pressure on some radiators when the inlet valve is closed and the system is pumping. This can suck air in through open bleed valves and (of course) automatic bleeds can also suck air in.

cheers

Re: Central heating pumping over
« Reply #14 on: January 01, 2019, 10:29:47 pm »
Have bled all radiators when heating off and all are fine with none giving evidence of trapped air. Ho hum.
Get a bicycle. You will never regret it, if you live- Mark Twain

Re: Central heating pumping over
« Reply #15 on: January 01, 2019, 10:39:59 pm »
if there is trapped air somewhere in the system (and/or corrosion generates gas and/or  air is drawn in somewhere...) then it will get pumped round the system until it finds a way to escape or it 'gathers' somewhere.  It sounds quite likely that it 'gathered' in the towel rail  for some reason, then expanded when it got hot.

The only other possibility is that once there was flow through the towel rail, air was being drawn into the system somewhere.

cheers
 

Re: Central heating pumping over
« Reply #16 on: January 02, 2019, 10:14:37 pm »
Well for the life of me I can't find any air trapped in system. have bled rads until I am blue in the face and rads glow in the dark. Boiler was a bit clanky until recently but boiler silencer has stopped that (it was repositioned last year and had a new pcb). but it does whistle a bit and we did power flush it last year and the condensate pipe split ( a known fault with Vaillants). Have ordered some x400 will try this for a month and then x800 if nesc.  If I have a sub floor leak it is not showing itself. as yet. Ground floor circs are buried in the screed but are protected. I do not really want to go for a sealed system.
Get a bicycle. You will never regret it, if you live- Mark Twain

Aunt Maud

  • Le Flâneur.
Re: Central heating pumping over
« Reply #17 on: January 03, 2019, 06:59:28 am »
I've got a sealed system and it's never been a problem. Why don't you want one?

Re: Central heating pumping over
« Reply #18 on: January 03, 2019, 12:01:46 pm »
if (for example) the system is gradually drawing air in when running, and it is gathered in an unused/occasionally used radiator or pipe spur, the symptoms you would see could well be

a) when the system is running 'normally' there might be a slow dribble through the overflow ( corresponding roughly to the rate at which cold air is drawn into the system; there would only be a few hot bubbles at any one time)

b) when you open the valves to normally unused radiators (which may have gathered gas in the meantime, either inside or in the pipe spur leading to each one), there will be a sudden expansion in the gas (it will go from cold to hot) and this will push water out of the overflow.

If the system is merrily corroding internally, gas will be generated continuously and may give similar symptoms. Corrosion may produce hydrogen in which case the bleed gas from the radiators will certainly smell different and may be inflammable.

The system will draw air in if the pressure is low enough at the pump entry. If you can do, fit a 'T' near the pump entry and (temporarily) fit a clear plastic hose to this, configured as a second overflow.  This will also operate as a pressure indicator (manometer) at the pump entry.  If there is enough resistance in the pipework (which will vary with the number of radiators that are working at any one time) you may find that the pressure reduction at the pump entry exceeds the static head of water that is pressurising the system.  This is quite likely to occur with an upstairs pump, and if it does the system may be drawing in air eg via the cold feed, auto bleeder, etc.

cheers

Re: Central heating pumping over
« Reply #19 on: January 03, 2019, 12:11:07 pm »
I've got a sealed system and it's never been a problem. Why don't you want one?
I meant a combi.  A sealed system may be the answer.

Brucey there are a couple of redundant spurs where rads have been removed and sealed. Ho hum.
Get a bicycle. You will never regret it, if you live- Mark Twain

Aunt Maud

  • Le Flâneur.
Re: Central heating pumping over
« Reply #20 on: January 03, 2019, 04:11:36 pm »
À la Brucey,When did you last dose the system with corrosion inhibitor ?

'Scuse the crap typing, I've consumed snaps.

Re: Central heating pumping over
« Reply #21 on: January 03, 2019, 04:34:50 pm »
Recently. It has had 7 or so litres of Fernox MB1 in there.
Get a bicycle. You will never regret it, if you live- Mark Twain

Gattopardo

  • Lord of the sith
  • Overseaing the building of the death star
Re: Central heating pumping over
« Reply #22 on: January 03, 2019, 04:43:15 pm »
7 litres, how big was the system.


Re: Central heating pumping over
« Reply #23 on: January 03, 2019, 09:08:49 pm »
MB1 is not concentrated and takes a 4 litre container for a typical system of 8-10 rads. I have 10 rads with 14 panels (one double rad being 2m long) and a couple of towel rails and of course system is pumping over. The idea of converting to a sealed system is becoming more attractive but even these, it seems, can have problems on older systems where an increased circulation pressure of 1.5 bar can find weaknesses.
Get a bicycle. You will never regret it, if you live- Mark Twain

Re: Central heating pumping over
« Reply #24 on: January 04, 2019, 10:37:14 am »
Made an interesting discovery a few mins ago. Upstairs towel rad valve on side where lock shield would  normally go starting coming off and letting by when when opening by hand. removing the flat cap resulted in finding that the valve is of two part design with a valve stem the inner nut of which was loose and adrift. Have nipped both inner and outer nuts up. Could this be the source of air getting in I wonder? We will see. Stand by to be disappointed.  ;)
Get a bicycle. You will never regret it, if you live- Mark Twain