expansion vessel size?

[moogwai]

evenin lads...finally got my uhf heating workin along side my main heating zone for the house and have noticed my prv is weeping now with pressure on boiler normal....a friend of mine had this problem with a similar set up and fitted an extra expansion vessel on the return from his uhf heating manifold and it cured his problem so i'm going to do the same....what size vessel would i need?? the 2 zones on the uhf are around 15m square...would a 12 litre vessel be ok? cheers.

 

[SimonJohns]

If the system volume is known, expansion vessels can be sized with the formula:

V= eC
1 – p1
p2

V The total volume or nominal size of the expansion vessel. It is not the acceptance volume. (See product description).
C The total volume of water in the system (litres)
P1 The fill pressure of the system in Bars absolute (atmospheric or gauge pressure plus one Bar).
P2 The setting of the expansion/pressure relief valve in Bars absolute atmospheric or gauge pressure plus one Bar).
e The expansion factor that relates to the maximum system requirements.

Expansion factor ‘e’ Temperature °C
0.0324 85

if you want to get it exact lol

 

[moogwai]

If the system volume is known, expansion vessels can be sized with the formula:

V= eC
1 – p1
p2

V The total volume or nominal size of the expansion vessel. It is not the acceptance volume. (See product description).
C The total volume of water in the system (litres)
P1 The fill pressure of the system in Bars absolute (atmospheric or gauge pressure plus one Bar).
P2 The setting of the expansion/pressure relief valve in Bars absolute atmospheric or gauge pressure plus one Bar).
e The expansion factor that relates to the maximum system requirements.

Expansion factor ‘e’ Temperature °C
0.0324 85

if you want to get it exact lol

wtf.....youve just melted my melon!!! the system only has 6 rads on plus the new ufh circuit....wouldve thought that the ev in boiler wouldve been ok as ive seen 15 rads on a ecotec before...so will a 12 litre suffice?? lol

 

[SimonJohns]

probably yes, but i would be doing a check first

i would go there when system is cold and filled to its normal working pressure
fire the heating on max and everything on
observe pressure
if there is a rise of 1bar+ i would be upsizing the size of the EV

if i didnt have a pressure rise, i would just replace the prv

 

[moogwai]

its my house mate...and the boiler is only 3 months old and only started weeping when i fired the ufh up.....like i say dunno wether its coincidence...a fella i know who i installed a boiler for had the same problem on a brand new ferroli (not my choice of boiler, i know they are rubbish) and he got the underfloor heating guys back, they installed an extra ev and the problem was solved.

 

[dancinplumba]

is boiler gauge reading right ?

 

[moogwai]

just checked display and gauge both reading 1.6 bar...with everything off....only started weeping with only ufh on.

 

[secret squirrel]

LOL and you were expecting a simple answer, do the maths.......:hand:

Putting it on wouldn't do any harm, if unsure oversize it.

I'd put it on.

 

[dancinplumba]

just checked display and gauge both reading 1.6 bar...with everything off....only started weeping with only ufh on.

had one last week same prob, changed press sensor, all ok now, if pressure not going up to 3 bar why is prv passing ?
dont need extra ev imo unless you have done simons formula lol

 

[moogwai]

LOL and you were expecting a simple answer, do the maths.......:hand:

Putting it on wouldn't do any harm, if unsure oversize it.

I'd put it on.

lol....tell me about it!!!

 

[moogwai]

had one last week same prob, changed press sensor, all ok now, if pressure not going up to 3 bar why is prv passing ?
dont need extra ev imo unless you have done simons formula lol

i gonna wallop everything on tomorrow and see what happens...might just give vaillant a call and see what they say.

 

[dancinplumba]

check pressure in ev as well

 

[moogwai]

check pressure in ev as well

will do...cant see it bein a problem though...like i say boiler is only 3 months old....but ya never know. thanks pal.

 

[Dannypipe]

So the only expansion vessel is the one in the boiler?

Then you almost certainly will need to put another one in. Doing the calculations is all well and good, but they can make the brain ache. If its easy to install an additional 12 litre vessel, and it's in your house. I would do it, and see if that works.

In the mean time, I would check the pressure of the vessel in the boiler, as if its dead flat then you could save yourself a few quid, and just re-charge it.

 

[Best]

Seen fairly new expansion vessels with air pressure gone, or very low, so I would always check them, - even brand new.

 

[Dannypipe]

Yup - Me too. Had brand new vessels which are almost flat. If he's just relying on the vessel in the system boiler, it might be under sized.

 

[moogwai]

thanks for the input lads....i put in a 12 litre ev this morning and all seems well :rockon:

 

[Dannypipe]

I knew it!! Well done mate.

 

[dancinplumba]

i carnt understand why stickin extra ev on has cured it when pressure doesnt get to 3 bar yet prv passing ??

 

[moogwai]

i carnt understand why stickin extra ev on has cured it when pressure doesnt get to 3 bar yet prv passing ??

agree.....but it did the trick on my system and my mates....strange indeed!

 

[Dannypipe]

System doesn’t get to 3 bar when he's watching it....but I bet it gets around 3 bar and the PRV is discharging. Its just a case of watching the gauge for long enough to watch it get that high.

The trouble with UFH is it's a high volume of water, but a much lower temperature thanks to the blending valve. When mixed with a conventional radiator system, you can get caught out. My gut instinct was additional expansion was required.

I take your point though Dancin.

 

[stevetheplumber]

there may be a lot of water in the ufh but its not getting very hot so doesnt expand as much as rad system

 

[Dannypipe]

That was my point!! Its harder to size the expnsion requirments when you have a mixed system as you need to do two calulations. Easier to just over spec it on a small domestic install.

 

[zzzjim]

Screed is a good fit to plastic pipe , does the plastic do any expanding !

Thinking plastic could expand more than water ? ( I know nothing about UFH )