Expansion Vessel size ?

[Francis.]

I have an underfloor heating system of 720M of 16mm Alpe pipe & only short 2M runs of 28mm pipe to a Ground source heat pump.
Absolute maximum temp rise of 65 degrees more probable 40 degrees.

What size should the expansion vessel be ?

 

[Harvest Fields]

I don't work on UFH Francis. Can any of you other lads help out on this one?

 

[ShaunCorbs]

If I remember right about 2 litres should be fine

Your underfloor supply should tell you how much there Pipework needs

 

[Jones82]

A system expands by 4% when heated but rather than work out the volume of the system I'd fit an 8 or 12 lite vessel. It's always better to have more than you need!

You should also take into account the pressure on the vessel from the head of water above it. For example a vessel in a cellar will have more head / pressure on it than one in a loft.

The manufacturers have technical advise lines you can call so if you have any doubt ask them.

 

[Last Plumber]

As above really. I would probably use an 8.

 

[ShaunCorbs]

Just worked this out

ufh is a total volume of 88 litres
Pipework 2.5 so we will round up to 3
Gshp let's say also 2.5 so round upto 3

So total volume aprox 94 litres

Dived that by 100 = 0.94 * 4 = 3.74 litres needed for expansion (approximately)

 

[Francis.]

Thankyou very much for all your answers. I agree that it's better to be slightly over the top , but totally over the top is unsightly & unnecessary.
As I understand it pressure grows by 1 bar in every 10M dropped so I can't see any problems there.
Many thanks again...

 

[Darren Jackson]

Thankyou very much for all your answers. I agree that it's better to be slightly over the top , but totally over the top is unsightly & unnecessary.
As I understand it pressure grows by 1 bar in every 10M dropped so I can't see any problems there.
Many thanks again...

True that 10M of head produces 1 bar of pressure but that is static head and not influanced by heat. The pressure increase by 4.5% as the heated water expands from 1° to 100°. We normally use 4% in our calculations as the max heating temp of a boiler is aprox 80°. I worked out almost like for like as ShaunCorbs has. So I would be installing a 4ltr. It is only allows a 0.25ltr buffer but given the lower temp it should easily cope. Or a 4.5ltr if there is an unusually amount of head. Just make sure you fit a 3bar PRV aswell.

 

[Francis.]

Isn't 3 bar a bit low for the PRV ? The underfloor pipes have been tested to 10 Bar & left on test at over 6Bar for weeks.
The incoming main now has a pressure reducing valve set at just above 3Bar, this will only connect during initial commissioning & maybe during maintenance, so would you expect the prv to blow at this time ?

 

[Darren Jackson]

No this is to protect your boiler. The expansion vessel will be pre charged at approximately 1 bar to neutralise the internal pressure of the system which will also be charged to 1 bar + 0.1bar per metre of head. And then there will only be around 0.5-1bar of intetnal expansion under full rate usage. So under normal usage the pressure will be between 1&2 bar. Boilers have a 3bar max rating.

 

[Francis.]

Thankyou very much for that Darren.
I can see that the system would work fine off a header tank but expansion vessels & closed systems seem more the in thing these days.
I'll have to get to reading the ground source pump manual to check on it's recommended working limits.
Many thanks again...