Combi boiler expansion vessels

[Alfie25]

I've been sizing up my system requirements for a combi boiler, looking at how many rads I will need, replacing singles with doubles etc, being sure that the combi boiler is of a large enough kW to cope. Whilst reading a few topics on this site I've come across the occasional problem of the expansion vessel in the combi being too small for the volume of water in the rads, and the system losing pressure as water is expelled out the prv.

I thought I just had to look at BTUs/Watts and tally up all the radiators then make sure the boiler was big enough to cope. Surely this is linked to the size of the expansion vessel, but obviously not if people are having problems with the expansion vessel being too small for a system. Am I missing something here?

 

[Steelmasons]

Most boilers with integral vessels will require an additional vessel to be fitted.

Most manufactutes will state this in the instruction manual , how to size the vessel is another matter to which manufactures eire on the small side.

 

[Dan Robinson]

Anything more than half a dozen rads and your boiler's vessel will be too small. Bunnies are particularly prone to this.

 

[Alfie25]

Interesting, as I'd expect most people have more than half a dozen radiators, although I've never seen a secondary expansion vessel fitted.

 

[Agile]

I've been sizing up my system requirements for a combi boiler, looking at how many rads I will need, replacing singles with doubles etc, being sure that the combi boiler is of a large enough kW to cope.

Unfortunately you are wasting your time! That's why you need a competent gas registered installer to advise you.

A combi boiler is rated for its hot water delivery with a minimum power of 24 kW.

24 kW will easily heat a six bedroom detached house! Is yours at least that large?

Its a very simple matter to add a second expansion vessel. Again your installer will advise! Usually required if the water content is more than about 80 litres.

Tony

 

[Dan Robinson]

Interesting, as I'd expect most people have more than half a dozen radiators, although I've never seen a secondary expansion vessel fitted.

Depends where you are. In London I think you'd be surprised.

 

[Agile]

He says he is in Durham!

There are many large houses there but probably mostly still on open vented systems.

According to Paul there is not much money about there so that usually means many unmodernised properties.

Tony

 

[Alfie25]

A combi boiler is rated for its hot water delivery with a minimum power of 24 kW.

24 kW will easily heat a six bedroom detached house! Is yours at least that large?

Its a very simple matter to add a second expansion vessel. Again your installer will advise! Usually required if the water content is more than about 80 litres.

Tony

Yes I'm aware most combis are 24kW or bigger. My rads will add up to approx 10kW so well within the 24kW capability of the boiler but you've hit the nail on the head mentioning the 80 litres point though, as this was basically what my question was all about. And aye, I'm up in the County of Durham, it's a 3 bed detached house, 1980's build currently with the original open vented system (albeit with a more recent boiler).

 

[Agile]

With the normal pressed steel panels it would rarely ever be needed to have a second EXV in a three bed semi.

Particularly in a relatively modern house with good insulation.

Its likely to be a cavity wall and getting that insulated is not only beneficial by reducing the heat required by about 20% but also gives a very good return from gas savings.

Tony

 

[Alfie25]

With the normal pressed steel panels it would rarely ever be needed to have a second EXV in a three bed semi.

Particularly in a relatively modern house with good insulation.

Its likely to be a cavity wall and getting that insulated is not only beneficial by reducing the heat required by about 20% but also gives a very good return from gas savings.

Tony

What relevance is the insulation of the house? I thought the size of the rads and the boiler output dictated the expansion required rather than the insulation. Or is it just the case that the longer the radiators are on (e.g. due to bad insulation) and the hotter they get then there is more requirement for a bigger/2nd expansion vessel?

Nevertheless I thought the advice was to get good sized rads, and set boiler up high to get the rads as hot as possible, as quickly as possible as opposed to having the boiler output set to say 3 quarters? Wouldn't this therefore result in maximum expansion, regardless of the insulation value of the house? I know if this advice is followed with a combi that rads can get unbearably hot to touch after the boiler has been on for only 15 minutes. In short, does the water expand more and more the longer the radiators are on, even though full heat is reached quite quickly?

As an aside, it's a 3 bed detached, not a semi, so I don't benefit from any heat from next door!

 

[Dan Robinson]

The better the insulation the lower the heat loss.

The lower the heat loss with the same size rads the lower you can run the boiler setpoint.

The lower the setpoint the lower the expansion.

 

[Agile]

And the lower you run the flow temperature the more efficient the boiler runs!

Turning it up to max is the best way to make its operation as inefficient as you can!

 

[Alfie25]

I do seem to read conflicting things on here. Tony, what are your opinions on radiators? Is it best to over-spec them and go bigger, rather than go for close to the BTU requirement (taken from a couple of different sources)

 

[Dan Robinson]

Not sure what conflicts you see?

Bigger rads=lower running temps=more efficient boiler operation.


We are all in agreement on that.

 

[Agile]

The only conflicts I see are those between what Alfie thinks and what we tell him!

Rads are best sized so that adequate heat out put is achieved with a flow temperature of about 65 C so that when well set up the return will be close to 50 C and enables the boiler to be well condensing!

But that's only possible when replacing them all or at least adding extra rads.

I do like seeing things which are working efficiently.