Expansion vessel air pressure?

[vaux_2]

Hi all,

Hoping someone can confirm my pressures are ok on my CH system. I've just fitted an external 8L expansion vessel as the old one was knacked. I can't remove the old integrated one without the boiler coming off the wall so don't know what the pre-charge pressure was. Reading the boiler manual i can't find any mention of the air pressure. Its a Worcester Greenstar 30Si.

I released some air from the new vessel to take it down from the factory 1.5 bar to 0.9 bar, then filled the system to 1 bar. With all rads bled and hot, the pressure is rising to a fraction under 1.5 bar.

Can anyone tell me if this sounds about right? or do i need to tweak my pressures a wee bit to get it correct? I just want to be certain its operating correctly so i don't have any more hassle with this.

Once i know this is all good i can start thinking about upgrading the rads but thats a post for another day.

 

[45yearsagasman]

Why did you drop the pressure in the new vessel? Is an 8 litre vessel big enough for your system?

 

[vaux_2]

The old expansion vessel was 7L. Its a small 3 bedroom bungalow with 8 rads, four single panels and the other 4 are doubles.

Having scoured the web i think the old expansion vessel is 0.8 to 1 bar pre-charge, but hoping to confirm that. Boiler manual says to charge external vessel to same as integrated.

 

[Steelmasons]

Why would you go to the bother of fitting an external vessel only to use an 8 litre?????..why come here asking for advice after the horse has bolted...
Obviously undersized..

 

[vaux_2]

It wasn't that much bother to fit and had to be done.

I don't pretend to know it all, every day's a school day. If it turns out that i need to change it out for a larger vessel, so be it, it only cost £16 but at least my heating is working in the meantime and the peace that gives me from the other half is worth the cost!

Back to my original question, do those pressures look OK...or as you brought up, would the pressure rise on heating indicate a larger vessel is needed?

 

[Steelmasons]

With a correctly sized vessel the system working pressure should exceed vessel pre-charge pressure by around 0.1 bar...yours is clearly wayyyyy undersized..

 

[Nozzle]

Some numbers from steam tables

@20C
998.2071 kg/m3 = 0.00100179612026402136390334230241 m3/kg

@85C
968.6232 kg/m3 = 0.00103239319479442573747975476945 m3/kg

Divide one by the other and you get 1.03 or put in good sensible terms the water in your system expands by 3% when heated from 20C to 85C. Next up - find some data sheets for all of your rads and your boiler and estimate the amount of length of pipe and this is an estimate for how much water your system contains, in litres, m3 or kg it doesn't matter.

If your answer is less than 270 litres, your expansion vessel size is fine - else, it's time to buy a bigger one..

Nozzle

 

[vaux_2]

Not sure if i got your meaning there Steelmasons, at the moment i've got a pre-charge air pressure of 0.9 bar and a system pressure of 1 bar when cold. When hot its rising to just under 1.5 bar. What should the pressure be doing when the system heats up, should it stay more or less constant or should there be a rise in pressure? Were you saying there shouldn't be a rise of more than 0.1 bar when it all heats up?

All of my rads are quite old and my plan is to replace them all in the near future. I can do the sizing for the new rads, then work out the system volume to see what size vessel i'm going to need in the future, but not sure how i can get the volumes of the existing ones, no idea of makes etc. Would the volume of older rads be comparable to new rads of the same size?

 

[Nozzle]

He's saying it should be pressurised when it is cold and hot, and that there shouldn't be a large difference between the cold and hot states. If the pressure rises drastically it's because the expansion vessel has become so full of water (that 3% of the system I mentioned above) that all of a sudden it has no-where to go and so the pressure goes high. Some pressure rise is inevitable as the expanding water is squishing the air bubble like you squishing a balloon. When you do so the pressure rises and this is desirable.

Nozzle

 

[vaux_2]

Ok, i've just done a quick calculation/estimate on volume.

I used current day kudox rad volumes based on the sizes/types i have, then added 20% to the total. This gives me around 40L for the rads. (i would guess my rads are aroun 20 years old and similar looking to older kudox ones)
I have approximately 54m of pipe, assuming all 22m, then adding 20% to the final volume gives me around 20L.
The boiler has a capacity of 4L

This gives 64L in total so not too big.

Also just found this snippet in the boiler manual;
'Where the system volume is more than 100 litres or exceeds 2.65 bar at maximum heating temperature an extra expansion vessel must be fitted...'
That pressure seems way too high...and its in the boiler manual?!?