Expansion vessels

Why am i asking this  here, why not !

I have a Vokera Linea 28 boiler which the expansion vessel has gone, well its 16 years  old now. I cant find what size it is, even in the original instructions or on line, anybody  better luck than me.

I will be fitting an external vessel as i cant be bothered to take the boiler off the wall to change the built in one, so i would like to know the original's size as  24 ltr which most places advise for a 4 bed house seem excessive in physical size 

24 litre doesn't sound unreasonable TBH for the size of the original (volumetrically). You could probably get away with 15-20 depending on the size of the load, but the bigger the better unless you are struggling for space.

The manual says the internal vessel is 8 litres.

Remember also, an under sized expansion vessel is one of the primary reasons for early failure.  Fit the bigger one, and fit it once.
 

Lurch said:

24 litre doesn't sound unreasonable TBH for the size of the original (volumetrically). You could probably get away with 15-20 depending on the size of the load, but the bigger the better unless you are struggling for space.

The manual says the internal vessel is 8 litres.

Click to expand...

Thanks Lurch, where did you find the 8 litres, i must have missed it ?

Dave, i know that it doesnt matter if they are too big , but if the 8 litre one has lasted 16 years then  maybe a 12 is all i will fit. A 24 ltr is about 480 tall by 300  dia and will fill the whole space under the boiler currently used for something else.

Most plumbers are surprised this one is still going after 8 years

http://www.vokera.co.uk/wp-content/uploads/2009/04/linea_installation_and_servicing_instructions.pdf

sec 3.6.5 page 6

Also page 3. Thanks all for your help

And/or

http://cdn0.vokera.co.uk/wp-content/uploads/2009/04/linea_24_and_28_installation_and_servicing_instructions.pdf

sec 3.4.2 page 3!

I  got some time to  fit the new external expansion vessel, i went for a 18 ltr one. It is pressurised to about 1.7 bar , i say about as i could not find my pressure gauge and used my car tyre inflator  (the type that plugs into a car cigar lighter) its digital but i dont know how accurate. Using the boilers pressure gauge (the new vessel is connected to the same place as the original, directly onto the back of the gauge) When cold the pressure was 1.2 bar and rises to 1.9-2.0 when hot.

My question is,if all is operating correctly will the pressure now rise to what ever the pressure in the expansion vessel is ?

SLIPSHOD & SLAPDASH said:

My question is,if all is operating correctly will the pressure now rise to what ever the pressure in the expansion vessel is ?

Click to expand...

No.

The pressure in the vessel is factory set, it shouldn't need adjusting or setting before fitting.

I'm with lurch on this the expansion vessel is preset pressure and you pressurise the water to 1.2 bar from boiler. I suspect that you may potentially damage the diaphragm in the new vessel by increasing pressure? 

I have not adjusted it. I tested it once boiler  filled and it was 1.7 bar  cold. My concern is as you have mentioned and was wondering if the pressure has risen to 1.9 as thats what the expansion vessel is   pressurised to ?

I am going to open a drain to let out a bit of water ( dont want to touch the PRV) . My next thought was to let a bit of pressure out the vessel.

Out of interest it seemed 50/50 with new Ex vessels if they come charged or not.

If the system is sealed, and pr-charged to say the same pressure as the pre-charge in the expansion vessel, ignoring for now the expansion vessel.

Then as the water is heated, it will expand, as will the pipework, etc. but, the static pressure in the system will increase, thus increasing the pressure in the water side of the expansion vessel, thus the pressure in the gas side of the expansion vessel, they are just a gas bladder accumulator, and are subject to the same laws of physics as every other system on this planet.

Sidewinder said:

If the system is sealed, and pr-charged to say the same pressure as the pre-charge in the expansion vessel, ignoring for now the expansion vessel.

Then as the water is heated, it will expand, as will the pipework, etc. but, the static pressure in the system will increase, thus increasing the pressure in the water side of the expansion vessel, thus the pressure in the gas side of the expansion vessel, they are just a gas bladder accumulator, and are subject to the same laws of physics as every other system on this planet.

Click to expand...

So from that, as i thought i will let a bit of air out the ex vessel

NO, why let air out?

That does not make sense.

I thought the whole idea of the ex vessel was just that,

To absorb the rise in pressure as the water heats up, thereby, letting air out if the vessel simply means you have to increase the water pressure simply to maintain the status quo, and when it heats up and the pressure rises again, do you let more air out,?

SLIPSHOD & SLAPDASH said:

I have not adjusted it. I tested it once boiler  filled and it was 1.7 bar  cold. My concern is as you have mentioned and was wondering if the pressure has risen to 1.9 as thats what the expansion vessel is   pressurised to ?

Click to expand...

No, the system pressure will increase as it heats up, that is normal. Usually you are looking at 1-1.5 bar cold, and around 2 bar hot if it is all installed and sized correctly. I wouldn't be concerned about anything up to about 2.5 bar when hot.

SLIPSHOD & SLAPDASH said:

I am going to open a drain to let out a bit of water ( dont want to touch the PRV) .

Click to expand...

Best idea, don't use the PRV to drain water out, otherwise you will be replacing it.

SLIPSHOD & SLAPDASH said:

My next thought was to let a bit of pressure out the vessel.

Click to expand...

Why? Stop now before you damage/ruin something. Don't keep randomly opening and closing valves and hoping for the best. Probably best to get someone in at this point if you don't actually know the first thing about heating systems.

SLIPSHOD & SLAPDASH said:

Out of interest it seemed 50/50 with new Ex vessels if they come charged or not.

Click to expand...

I don't know where you are getting this info from, I have never bought one that wasn't correctly charged out of the box.

I do not consider letting some pressure out the ex vessel & also the boiler as they  were at a higher pressure than it said in its specs as random. I did do this last night and made  the vessel 1.5 bar & the boiler 1.2 bar both cold. Right now when hot  the boiler sits at about 1.8 bar, dropping to 1.4 cold. Thanks for your concerns though and am grateful for all the replies.

I am happier asking here than on a plumbers/diy forum.

The pre-charge pressure can only be tested with the boiler drained if it is an internal expansion vessel or with the system drained if it is external. An isolating valve can help here, but must be left open during use.

The vessel should be installed with the inlet on the bottom so that no or little water enters the expansion vessel when pressurised and the diaphragm is not in contact with the water, as close as possible to the boiler (within 3 metres) and on the return pipe, so that most of the system is under positive pressure from the pump.

I know that people are loathe to open the PRV, since you could get dirt on the seating and have trouble getting it to close fully afterwards. But it is one of those safety devices that could get stuck in the closed position and fail to open when needed. Manufacturers usually also say not to drain the boiler using the PRV, but also usually also say to test/open it during servicing. If the system is full of dirt, then that needs remedying before the boiler gets damaged.

Thanks Rich, The new vessel is less than 1m away from the boiler  and fitted as you described. I did fit an isolating valve but tested the pressure with water in the system. Vokera say to use the PRV to drain. I did as i had to empty the old, full  vessel and it took about 2 hours to get it to seal after.

I have no clue what your heating system consists of or how it's configured but for most pressure tanks we get parts like bladders and valves and just recondition them. I've literally just done the one on me borehole system yesterday. I doubt you'd damage it with over pressure as long as it's not above 2 or 3 bar and it shouldn't get damaged if there's zero pressure air either. We usually charge the air to a nominal 1 bar unless it's for a special system that requires a specific charge pressure. You should be able to set the air pressure without draining as long as you release pressure from the water system.