Expansion Vessel Sizing.

Westford

Hi all..
It seems that this topic has come up on here quite a few time, but there are many different flavours of the query, and I haven't been able to use any to confidently get an answer that applies to me.
So my problem is that I'm having by boiler cuts out due to low pressure. I've done the usual check for leaks, but nothing is coming up (Thankfully). At this stage, after watching the system closely, and catching and measuring the water released by the pressure valve, I am reasonable sure that my expansion vessel (on the heating circuit) is incorrectly sized.
But rather than simple go shopping for "a bigger one"... I'd like to be confident that (a) I'm not wasting my money, and (b) the size I get is adequate.
So the heating circuit includes:
29KW Boiler with 70 litre water capacity
Buffer/Storage tank of 1000 litre
UFH for about 160m2 (I've estimated this @ 135 litres.... but could be way off)
Rads ( 2 x large doubles : 3 x Medium Doubles : 4 x Small doubles)
HW Tank is about 300 litre (but no idea of the actual capacity of exchange coil in it).
Additional piping: Unknown (but there is a bit of a external run between house and boiler-house)
The boilers operating pressure is listed as up to 2.5 bar and pressure release is a 3bar release valve.
The temp set points on the boiler (driven from stats on the 1000l buffer tank is ON @ 60 & OFF @ 80.


Any guidance that will get me to a correctly sized vessel would be approciated... Or even you you have a comparable setup, that is running OK for you; please let me know the capacity of your vessel.


Thanks in advance

Dtp1979

What size is your current vessel? What pressure is the gauge at when the system is cold?

Westford

Dtp1979 wrote: »

What size is your current vessel? What pressure is the gauge at when the system is cold?

Current one is 50 litre.
Pressure when cold is the issue... Sometimes it can be 1.5 bar... But then it can be 0.5 bar or less other times (at this pressure the holier will stop/not start). Worse in the colder weather.
I'm not sure of the correct calculation method... But in some attempts at working it out, I come up with just over 100 litres for the expansion requirement.
Plus.. I've seen a few package offers on the web (one price for all components)... And those that are similar to mine, seem to include a 150 litre vessel.
But 150 l vessels are not cheap.... Sonwod hate to buy one on a guess or assumption.

Dtp1979

Westford wrote: »

Current one is 50 litre.
Pressure when cold is the issue... Sometimes it can be 1.5 bar... But then it can be 0.5 bar or less other times (at this pressure the holier will stop/not start). Worse in the colder weather.
I'm not sure of the correct calculation method... But in some attempts at working it out, I come up with just over 100 litres for the expansion requirement.
Plus.. I've seen a few package offers on the web (one price for all components)... And those that are similar to mine, seem to include a 150 litre vessel.
But 150 l vessels are not cheap.... Sonwod hate to buy one on a guess or assumption.

Did you try recharging the vessel you have?

Westford

Dtp1979 wrote: »

Did you try recharging the vessel you have?

Yes... I've recharged but the Vessel doesn't seem to be dropping pressure. It's a new one... Previous one had leaks in the actual metal (seemed to be rusting from the inside.... Despite only been a few years old).

Calahonda52

What follows may be 100% rubbish.

Is this a new problem?

Perhaps the Pressure relief valve is faulty?
Maybe replace it?
Maybe everything is shutting down on the flow from the boiler and the boiler pump is blasting out...

Also could you jury rig it to be an open system with a small header tank above the highest point, maybe tap into a rad line.
You need to do this with care so if you don't feel competent don't do it

Dtp1979

Westford wrote: »

Yes... I've recharged but the Vessel doesn't seem to be dropping pressure. It's a new one... Previous one had leaks in the actual metal (seemed to be rusting from the inside.... Despite only been a few years old).

Water expands upto boiling point 4% in volume. For longevity, systems are sized 11% of total volume of system. Going by volumes stated, you have about 1,300 litres or so. This would need 150 litres of expansion, however, system is operating at much lower temp than a full rad system. I would install a 100 litre vessel or just add another 50 litre vessel beside the existing one if it is in good order.

Westford

Calahonda52 wrote: »

What follows may be 100% rubbish.

Is this a new problem?

Perhaps the Pressure relief valve is faulty?
Maybe replace it?
Maybe everything is shutting down on the flow from the boiler and the boiler pump is blasting out...

Also could you jury rig it to be an open system with a small header tank above the highest point, maybe tap into a rad line.
You need to do this with care so if you don't feel competent don't do it

Thanks calahonda.
Pressure valve was also replaced.
I supposed there is other stuff I could do/test.... But at this point I wanted to check the vessel size using calculations (before trying anything else).

Westford

Dtp1979 wrote: »

Water expands upto boiling point 4% in volume. For longevity, systems are sized 11% of total volume of system. Going by volumes stated, you have about 1,300 litres or so. This would need 150 litres of expansion, however, system is operating at much lower temp than a full rad system. I would install a 100 litre vessel or just add another 50 litre vessel beside the existing one if it is in good order.

Thanks Dtp1979...
Ok so it sounds like your figures are pointing to an undersized vessel.
And that my dodgy calculation that give me a answer of 150 l might be about right.
Thanks for taking the time to post.... Really appreciate it.