# Replacing boiler expansion vessel



## flanajb (15 Oct 2013)

Hi,

I posted about our boiler woes 2 years back and have only just decided I need to sort it out. Given the problem only happens when we turn the central heating on I am putting my money on the problem being the internal expansion vessel.

Now what I want to know is whether I can fit an external vessel to the system and somehow isolate the internal vessel so that it is not used by the boiler.

The reason I think it is the expansion vessel is that when the central heating comes on the boiler starts heating up a greater volume of water than it does when just in hot water heating mode. This greater volume then results in a great expanded mass when hot compared to when in just hot water mode.

Whilst my theory might hold true, what I still don't understand is where the water in the sealed system is going as I don't see water pouring out of the pressure relief pipe on the outside of the house.

Now, if my plumbing knowledge is correct, water can only leave a sealed system in 4 ways

1. Through the pressure relief pipe on the outside of the house. Would see dripping if the pressure in system > pressure relief valve setting

2. Cracked heat exchanger causing water to leak out

3. Leaking joint or rad valve

4. Perforated coil in hot water tank. This would affect hot water mode so this can be ruled out

Have I missed anything off here?

A right royal pain as we have sold the house and I cannot afford to have the now obsolete boiler replaced.


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## monkeybiter (15 Oct 2013)

We were continually topping up our system when using CH. Had an external vessel fitted, the internal vessel is left connected, but I now run it at a lower pressure, I was possibly running it at too high a pressure which may have caused the water loss in the past. Now operated at 1BarG rather than 2 with no noticable losses.


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## kostello (15 Oct 2013)

If the heat exchanger is leaking and losing water and you have a condensing boiler it will go down the condensate pipe and you will never be any the wiser.

When our boiler had similar problems I fitted ball valves on the flow and return, isolated the boiler from the rest of the system and filled it to 2 bar and let it over night.

Sure enough the pressure dropped......

When the heat exchanger was removed I pressure tested it with air in a bucket of water.... There was a tiny pin hole in the pipe........

I would never have seen it with the naked eye..


The pressure vessel should also have the pressure checked and topped up regularly details will be in the boiler manual. If there isn't enough air in the vessel there is no space for the water to expand into and it would exit through the pressure valve.

Hope this helps


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## John Brown (15 Oct 2013)

On some boilers you can re-pressurize the expansion vessel using a car foot pump or similar, I believe.
(I say I believe - I have actually done it myself, but I don't know if the advice is valid for all boilers)


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## RogerS (17 Oct 2013)

Cor...what a kerfuffle these sealed systems are! Indirect vented for me every time ...


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## flanajb (17 Oct 2013)

kostello":nyjajeqe said:


> If the heat exchanger is leaking and losing water and you have a condensing boiler it will go down the condensate pipe and you will never be any the wiser.
> 
> When our boiler had similar problems I fitted ball valves on the flow and return, isolated the boiler from the rest of the system and filled it to 2 bar and let it over night.
> 
> ...



I think I need to do a boiler pressure test before anything else. I can see what you are saying regarding a leaking heat exchanger. The leaking water will just be vapourized off and you won't notice it.


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## RogerS (17 Oct 2013)

I have one of these. Well worth it for testing your system.

http://www.screwfix.com/p/monument-dry- ... -kit/72940


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## tomatwark (17 Oct 2013)

Our boiler was losing pressure, it was the expansion vessel, the diaphragm had perished over the years.

Our plumber priced up the proper one for the boiler and also fitting an external one, there was not a lot in it price wise once he allowed for altering the the pipe work.

The one for the boiler was about £90 ( an external one was about £30 ) and it took about 20 mins to fit. ( he clipped off the pipe to the vessel disconnected it took the vessel off and fitted the new one and then topped up the system) 

Tom


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## FarrellMackennon (8 Jan 2014)

An external vessel can be connected, but it is better to have a central heating plumber to check on it as well as the connections of your heating system. Otherwise, if you do it on your own, you might loosen some wirings and mechanisms that might prove to be more costly in the end.


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## MMUK (8 Jan 2014)

FarrellMackennon":2m1trqb8 said:


> An external vessel can be connected, but it is better to have a central heating plumber to check on it as well as the connections of your heating system. Otherwise, if you do it on your own, you might loosen some wirings and mechanisms that might prove to be more costly in the end.



Only three months late :wink:


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## RogerS (8 Jan 2014)

He must be working flat out :-"


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## flanajb (19 Aug 2014)

Haha. Only getting around to this now :-o

I love the technical post from the xpert plumber 



> ... Otherwise, if you do it on your own, you might loosen some wirings and mechanisms that might prove to be more costly in the end.


 Very technical explanation. I better take care as I don't wan't to loosen any wirings or mechanisms. 

And I wonder why I have zero confidence in accreditation :|


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## Eric The Viking (21 Aug 2014)

On a slightly serious note: you really, really don't need to run the system at any greater pressure than the minimum needed to keep the boiler's pressure sensor happy. In the case of mine it's 1.5 bar. More than that gives the system ample excuse to leak anywhere it feels like it.

I fitted a second expansion vessel externally to ours, because it's a big system with a lot of water in it (inefficient big old rads in places). And the boiler is at the top of the house, so a measured 1.5 bar there is about 3 bar at the bottom of the system. I set the external vessel to 1.5 bar (as close as I could - overpressure than dropped it down, hoping to ensure it wasn't under) and left the internal one alone. System then pressurised to 1.5 bar, it works nicely. In fact it's been far better behaved since I modded it than before.


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## flanajb (22 Aug 2014)

Ok, so I fitted the new 18 litre expansion vessel. I didn't fit it to the return side of the boiler as there is no where easy to do it, so I tee'd off the feed in the airing cupboard just before the 2 motorised valves for the heating and hot water circuits. Too be honest, I don't really see it matters where you site the expansion vessel. Might mean the water is hotter and the pressure a bit higher than at the return?

I turned the central heating on, but still got a 1 bar rise in pressure from cold to hot with the central heating switched on. I checked the pressure of the vessel and it was 1.5 bar. So am still confused why I am getting such a pressure gradient from cold to hot. Could it actually be that I have fitted the vessel in the wrong place?

Thanks


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## Eric The Viking (22 Aug 2014)

What pressure is the system running at? If it's ABOVE 1.5 bar at the expansion vessel, the vessel will have no practical effect (the air inside will be compressed as the system is filled. 

One reason they're fitted close to the boiler is so that the boiler's pressure gauge is also measuring the pressure at the expansion vessel (flow and return don't have a lot of bearing on this - it'll inevitably be slightly lower on the return side). 

If they're in different places, you'll have to guess the pressure at the vessel:

- 32ft water = 1 atmosphere (1 bar). 
- subtract the height difference (as a proportion of 32ft) from boiler to vessel (if vessel is upstairs), to get pressure at vessel.
- more importantly, ADD the pressure in the vessel to the height difference, to see what the boiler pressure will be when the vessel starts to operate. Below that pressure, as you add water it will just increase the pressure, and at the point where the vessel kicks-in there will be a 'step' in behaviour - more water won't change the pressure (it goes into the vessel, compressing the diaphraghm instead). When you use up the vessel's capacity it will go back to pressure increasing rapidly when you open the filling loop.

HTH,

E.


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## flanajb (22 Aug 2014)

Eric The Viking":8rjiisf8 said:


> What pressure is the system running at? If it's ABOVE 1.5 bar at the expansion vessel, the vessel will have no practical effect (the air inside will be compressed as the system is filled.
> 
> One reason they're fitted close to the boiler is so that the boiler's pressure gauge is also measuring the pressure at the expansion vessel (flow and return don't have a lot of bearing on this - it'll inevitably be slightly lower on the return side).
> 
> ...


Many thanks for your detailed reply. Given that the pressure gauge is measuring the pressure on the return side it could well be that the pressure at the new expansion vessel is actually > 1.5 bar and as you say the diaphragm has already been compressed. I suspect that a trial and error approach might be called for here. Inflating the expansion vessel to different pressures and see what works. 

When I had the system on yesterday, I could feel that the lower part of the expansion vessel was hot, but the top was cold. If the whole vessel is being used I would expect the whole vessel to be hot. Rudimentary way of deducing what is going on, but potentially provides an indication.


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## Eric The Viking (22 Aug 2014)

flanajb":1qw932yw said:


> I suspect that a trial and error approach might be called for here. Inflating the expansion vessel to different pressures and see what works.
> 
> When I had the system on yesterday, I could feel that the lower part of the expansion vessel was hot, but the top was cold. If the whole vessel is being used I would expect the whole vessel to be hot. Rudimentary way of deducing what is going on, but potentially provides an indication.



That sounds pretty normal. You don't want it anywhere near full - still room to expand if the pressure rises for some reason.

Mine doesn't stay at one pressure. It drops when it's cold - i'd say about 1/2 bar difference. The trick is to keep it above 1.5 bar (cold) at the boiler, otherwise the thing won't fire up, as it has a pressure interlock (so nobody fires it up empty, I presume).


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