Expansion vessel on Vaillant combi boiler

[greenrobbie]

Seeking advice from the knowledgeable. We had a new CH system installed 3 years ago, consisting of a Vaillant combi boiler and 13 radiators spread throughout the house. The original installers, who as far as I can tell did an excellent job, aren't interested in servicing it, so we have only just had a first inspection and service by a heating engineer from another company. As far as I can tell, he too is knowledgeable and did a thorough job.

He tells us that the expansion vessel on the combi boiler is not really sufficient to regulate the pressure in a system with 13 radiators, and ideally we should have an additional expansion tank fitted that would keep the pressure within the system at a more constant level, which would reduce wear and tear on the system. He added that the manufacturers know and design for the hot water supply to taps, but cannot know the size of the heating system that is run from the boiler, and therefore the expansion tank supplied is not sufficent in all cases.

All this sounds plausible, but is his advice to have an additional expansion tank fitted likely to be correct? Has anyone else come across this before?
A quick Google search hasn't yielded any advice on this, so TIA for any advice from any other Lemon Fools.

greenrobbie

[staffordian]

Not an exact answer, but the expansion vessel on our Ideal Isar combi boiler failed a few years ago and I had an external one installed rather than go to the trouble and far higher expense of getting an engineer to dismantle the boiler and fit an Ideal one.

We've subsequently had a replacement boiler and the engineer installing it was quite happy to leave the external vessel in place.

So whilst it may not do any good, it certainly doesn't appear to do any harm.

[Mike4]

Seeking advice from the knowledgeable. We had a new CH system installed 3 years ago, consisting of a Vaillant combi boiler and 13 radiators spread throughout the house. The original installers, who as far as I can tell did an excellent job, aren't interested in servicing it, so we have only just had a first inspection and service by a heating engineer from another company. As far as I can tell, he too is knowledgeable and did a thorough job.

He tells us that the expansion vessel on the combi boiler is not really sufficient to regulate the pressure in a system with 13 radiators, and ideally we should have an additional expansion tank fitted that would keep the pressure within the system at a more constant level, which would reduce wear and tear on the system. He added that the manufacturers know and design for the hot water supply to taps, but cannot know the size of the heating system that is run from the boiler, and therefore the expansion tank supplied is not sufficent in all cases.

All this sounds plausible, but is his advice to have an additional expansion tank fitted likely to be correct? Has anyone else come across this before?
A quick Google search hasn't yielded any advice on this, so TIA for any advice from any other Lemon Fools.

greenrobbie

As I understand it, the expansion vessel needs to be sized according to the heat output of the boiler, not the number of radiators. Consequently system boiler manufacturers fit expansion vessels correctly sized for the output of each of their boilers.

What your chap is driving at but hasn't explained well is that firstly, expansion vessels slowly lose their nitrogen charge, meaning their capacity reduces with age. Re-charging the vessel (with air) is part of the annual service but is often skipped by a service technician as it doubles the time taken to carry out a service. And secondly that manufacturers (Vaillant included) fit the correct sized vessel rather than an oversized vessel so there is no spare 'headroom', meaning that after a while the initially adequate vessel becomes inadequate.

So fitting a second, external vessel is a Good Idea but partly defeats the point of paying extra for a system boiler in the first place.

Hope that helps...

[Itsallaguess]

As I understand it, the expansion vessel needs to be sized according to the heat output of the boiler, not the number of radiators.

The system water volume is also part of the expansion-vessel sizing calculation...

Some examples -

Expansion Vessel Size Calculation -

In order to calculate the expansion vessel size, you need to know the following values:

V - water volume of the entire heating system (boiler, piping, heating radiators, other devices)
Tmax - max. operating temperature of the heating system [C°]
ph,dov - max. working pressure in the heating system [bar]
H - elevation of the heating system's highest point above the expansion vessel [m].
ph,min - min. required pressure (set by the boiler manufacturer) [bar]

https://www.regulus.eu/en/vypocet-velikosti-expanzni-nadoby



Expansion vessels for closed heating systems -

Knowing the following data:

The pre-charge pressure
The set pressure for the safety valve
The difference in level
The volume of the fluid in the system
The maximum working temperature of the system
The possible percentage of antifreeze glycol in the system fluid

it’s possible to calculate the required volume of the expansion vessel.

https://zilmet.it/en/heating-vessels

The second link above has an on-line sizing calculator where it's clear that the individual system water volume figure plays a large part in the subsequent size of the expansion vessel...

Cheers,

Itsallaguess

[Mike4]

Thanks. Every day is a skool day!

I was rather parroting what the boiler manus say when asked why they size their expansion vessels so finely. The make the argument that radiator water volume is proportional to radiator output, pipework volume is loosely proportional to total radiator output, and it is completely pointless hanging more radiator onto a boiler than that boiler is capable of heating up. Hence their assertion expansion vessel size is broadly a function of boiler output.

But yes I agree if you put a system with larger than typical water volume onto a system boiler for a boiler of that size, the EV will turn out to be too small.

[greenrobbie]

Many thanks for advice proferred. Given that it's not going to do any harm, may do some good, and provides backup already installed for if/when the original expansion vessel gives up the ghost, we are going to have the secondary expansion vessel installed. One good thing that's come out of this is we appear to have found a reliable and knowledgeable local firm that we can use for any plumbing or central heating work in the future, without entering into an expensive contract with BG, Homeserve or the like.

I took to the owner in my initial phone call, when he raised the subject of Stoicism, and the writings of Marcus Aurelius. Not sure how that came about, but a pleasant surprise to be discussing Stoic philosophy after we had arranged for a boiler service!

greenrobbie

[jaizan]

All of the water in the system can expand, so ALL of it should be considered when sizing the expansion vessel, as stated above.

I fitted a second one to my system years ago myself. Purchased from Screwfix for a very reasonable price.