What size expansion vessel do I need?

[Deansplit]

My UV pressurised system was previously fed by a header tank, but now it's being changed to a closed system with a filler loop. I understand that I need a vessel for the CH now as well as I the one I have on the HW. Is there a specific size I require or does it go by the volume of rads on the circuit?

Cheers

 

[kevplumb]

the size of the EV depends on the water content of the system

 

[Deansplit]

Is there a science to calculating it - I can measure all rads and currently have all pipes exposed so can measure pipe volume (presuming there's a calculation for that too?)

 

[kevplumb]

have a look at http://www.mikrofill.co.uk/index.php?wcId=15&

btw you cant oversize an av

 

[newbuildinstaller]

you should know that its impossible to oversize the vessel but you can undersize them so always go bigger than you work out to be sure

 

[bengasman]

1.5 - 2 litres per rad should do the job

 

[Boilerman2]

No so easy to Guess with UFH, I don't know the size of your property, but with UFH, if more than 3 Bedrooms go for 18Ltr if 5 or more Bedrooms go for 24Ltr (Better too big than Too small)

 

[Biggles..]

Rule of thumb, 1litre per kw of boiler output.

 

[onlyfitidealboilers]

most combi boilers have an 8 ltr vessel. so 1.5 to 2.0 per rad seem far too much to me

 

[Deansplit]

It's a big place - 3 story Victorian, 5 beds + office plus 2 reception room and a 45m2 kitchen diner (this part is UFH) 2750sq'. Does this help? Oh, and I'm putting a rad into the loft which is size of a med bedroom.
Ta

 

[Boilerman2]

24 Litre at least!!

 

[Terrywookfit]

most combi boilers have an 8 ltr vessel. so 1.5 to 2.0 per rad seem far too much to me

Thats because it's the biggest one that can be squeezed in.
Most combi boilers will only have a 12 kw load max on the heating so they get away with it.
Try installing a full heating load say 24 kw on a 8l vessel.
System will be running at 2.5 bar when hot, and the EV will be sh@gged in no time.
Not to mention the Prv, iso valves and various seals etc.

 

[5TIMES]

work out how much water is in your system then multiply it by 0.074


so 300litres multiply by 0.074- 22.1 litre expansion vessel is whats required.

most combi boilers will give a central heating output of 24kw...

 

[Deansplit]

Now that's the answer that I'm looking for, OK, so next step is how do I work out the volume of water in the system? This is what I mentioned on my initial post - knew there'd be some sort of science behind this1

 

[bengasman]

... so next step is how do I work out the volume of water in the system?

In real life, you don't. It's not worth the hassle.
To do that, you need to pull up the floors and measure all the pipes. And it doesn't really matter if the pressure fluctuates a bit, as long as it is not too much. Average size house, any ev over 10 litres will do the job.