expansion vessel on central heating

[alexmurchison]

hi guys. This is maybr a sizing issue but thought i would put it to others to get a second opinion. Went to service a boiler the other day then afterwards noticed that the water pressure was down to almost zero (sealed system) i repressurized and checked all rads were bled . All fine. left system running. Couple of days later got called back after customer stated water was coming out the prv on top of the expansion vessel. I checked over, and replaced prv, as common problem. I checked the pressure in the vessel and adjusted to Mi. Re pressurised and left. Now again its blowing off through the prv. So now im thinking that the vessel is NOT big enough for the system.
To clarify I only pressurized the vesssel to 1.5bar as per instructions. The water in system was pressured to around 1.5bar aswe... The vessel is an18litre vessel. There are 16 rads of various sizes. Most over 1200mm wide so quite a big system. All motorized valaves and pumps have been checked and the system has been power flushed so no blockages. As far as i can gather the customer said that the water pressure gauge has always been very low, so im thinking a problem thats existed for a while ??

Any opinions would be appreciated, thanks in advance

 

[tom609]

why not size up the system then you wil know wether the exp vessel is to small could be a faulty vessel

 

[Best]

May need next size vessel at a guess. With 16 rads probably a 24 litre.
Are you sure the vessel actually took the air charge?
Also, the water pressure may be more than the gauge on the system reads, as those gauges are often faulty.

 

[Alpha man]

Is it a combi or heat only boiler? Have had a couple of combis with really high CMW pressure which had split the DHW heat exchanger that kept over pressurising the system!

 

[buckley plumb]

is the pressure going above 3 bar when the htg is on .have you replaced the prv valve could have bad seating

 

[chris watkins]

What happened on the gauge when you finish your work & ran the boiler / system up to temp ? any problems with size of vessel or charge etc are easily spotted by watching the gauge !!
BTW an 18 L vessel @ 1.5 bar (15m) will take expansion from a boiler of up to 19.25 kW's.
Does it need to be at 1.5 bar ?

 

[obseen16]

if expansion vessel is one of those like the worcester greenstar juniors where its attached via a flexi there could be a blockage there so rendering the expansion vessel useless as no water can expand into it. ive seen it on quite a few boilers with the flexi connextions blocked up with sediment.

 

[Terry_the_piper]

if expansion vessel is

one of those like the worcester greenstar juniors where its attached via a flexi there could be a blockage there so rendering the expansion vessel useless as no water can expand into it. ive seen it on quite a few boilers with the flexi connextions blocked up with sediment.

The old classic ff's same but easy enough to sort. On bigger systems I tend to only put in 1 bar. Volume of expansion is obviously that much greater.

 

[tom609]

if expansion vessel is one of those like the worcester greenstar juniors where its attached via a flexi there could be a blockage there so rendering the expansion vessel useless as no water can expand into it. ive seen it on quite a few boilers with the flexi connextions blocked up with sediment.

good call not had it myself but worth thinking about

 

[alexmurchison]

hi, thanks for all the replies, some interesting points.
Its a heat only boiler with flow pipes at 32mm and return of 22mm The expansion vessel is one of the wall mounted types with a flexi to a union on the primary. Yes i changed the prv, did the same. When i ran it up , it obviously got up to temp and the pressure gauge was at around 2.5bar, so pretty normal. but when i have returned to see it leaking its been just under 3, and the prv blows off at 3 bar. The vessel had 1.5bar as per the data plate as from new. I tested by my own gauge via schrader valve to confirm. The filling system is upstairs near the cylinder, where there is a pressure gauge. Then downstairs where the boiler is on the expansion vessel there is another gauage which read the same. tried it at 1 bar aswell, did the same, gradually went up to 3 bar then blows off. judging by the staining on the vessel it must of done it before till the point where no waqter was left to come out

 

[tom609]

work out the volume ot water in the system and i will indicate vessel problem or not

 

[chris watkins]

hi, thanks for all the replies, some interesting points.
Its a heat only boiler with flow pipes at 32mm and return of 22mm The expansion vessel is one of the wall mounted types with a flexi to a union on the primary. Yes i changed the prv, did the same. When i ran it up , it obviously got up to temp and the pressure gauge was at around 2.5bar, so pretty normal. but when i have returned to see it leaking its been just under 3, and the prv blows off at 3 bar. The vessel had 1.5bar as per the data plate as from new. I tested by my own gauge via schrader valve to confirm. The filling system is upstairs near the cylinder, where there is a pressure gauge. Then downstairs where the boiler is on the expansion vessel there is another gauage which read the same. tried it at 1 bar aswell, did the same, gradually went up to 3 bar then blows off. judging by the staining on the vessel it must of done it before till the point where no waqter was left to come out

With all due respect alex you may want to read up on how to set these up & what the cold fill pressure should be, 2.5 is not normal & although they put a pre-change in the vessels we need to adjust that to suit the installation.
Or If you have questions post away, I am sure they will be answered.

 

[alexmurchison]

on a system of this size it must exceed 250 litres, although only a guestimate

 

[Reg Man]

Expansion vessel size should = 4% of the system volume? Any problems before hand?

 

[alexmurchison]

hey chris, these are of course oil boilers that i work on, minimum pressure on a sealed oil boiler system would be 1 bar, 1.5 bar is the upper limit, once they are up to temp it is very normal to see the pressure get to just under 2.5bar. Anything over would of course indicate a problem.

 

[alexmurchison]

Expansion vessel size should = 4% of the system volume? Any problems before hand?

hi reg man, when i first attended the pressure on the gauges were basically zero, so no pressure in system, and some rads not getting hot. I pressurized and bled rads. one point on here which i never did check was any blockages in flexi , although wouldnt of thought so, if water can come out the prv

 

[chris watkins]

Expansion vessel size should = 4% of the system volume? Any problems before hand?

Sorry, Reg Man but you are getting confused with the old F&E, because of the way the expansion vessel works it is a bit more than that & based on the static head pressure, safety valve setting sytem volume etc. RWC have very good info on this. Expansion Vessels

 

[R1CKY]

Are you sure the diaphragm hasn't ruptured in the vessel? Pressurise the system to 2.5 bar and bleed all of the air out of the vessel. If water comes out, the vessel is dead.

A quick way to gauge a vessels condition is to support its weight, if it is very heavy, it is likely to be dead also! If it is undersized, I'd add another

 

[chris watkins]

hey chris, these are of course oil boilers that i work on, minimum pressure on a sealed oil boiler system would be 1 bar, 1.5 bar is the upper limit, once they are up to temp it is very normal to see the pressure get to just under 2.5bar. Anything over would of course indicate a problem.

Hi Alex, I have very limited experience of oil boilers & the system requirements but as far as I am aware there are no real different's between a sealed gas boiler system & an oil one. A lot engineers seem to put a fixed amount at cold fill (either 1 - 1.5 bar) without considering the static head requirements above the vessel / gauge, what is the distance between the vessel / gauge & the highest rads / heating pipework ? This alone with the minimum pressure requirements of the boiler should be used to set the cold fill pressure, in turn this pressure is used to set the pressure inside the expansion vessel. Then as the water is heated & expansion takes place the pressure in the system rises slightly, water flow into the vessel & the system pressure is controlled because there is a greater volume. The rise on the gauge if things are set correctly should be no more than 0.5 bar, on the average system.
So having it rise to 2.5 is putting a lot of strain on the system & vessel, it is normally a reason to investigate what is going wrong.
Hope that helps.

 

[alexmurchison]

hi chris, i understand what you are saying, thanks for the info. Just been again, as only next door. The expansion vessel was initial pressure setting of 0.5bar, (not 1.5bar like i commented before) tested vessel by pressurizing and pushing up the scrader valve, no waqter present so presume diaphragm is ok. The vessel is Tee,d off the gravity circuit for the hot water to coil. Heating side is of course pumped. Plenty of DHW so circuit is obviously flowing ( no restrictions in flow or return to boiler) There is a tub under the vessel to gauge the water lost, it was overflowing so a good 40 litres must of come out and the pressure gauge was reading approx 2.8bar ! still high with preesure and water lost !

I think we can eliminate blockages in pipework, or air locks. full heating system has never worked so well, very hot rads throughout and good hot water at cylinder.
Boiler is downstairs and cylinder is up stairs so average of 2.4metre to plus height to top of upstairs rads maybe 1metre. The longest run is the biggest "??" there are four bedrooms then a bathroom in a row so pipework must be 20 - 25metres plus another 10metre angled off this then down to the boiler.
the boiler itself is only on a low setting so its not overheating and boiling the water, the boiler has never clicked out, so not a temp issue, (apart from rate of expansion)
Im leaning toward a bigger tank, and playing around with the pressures, just a costly excercise if doesent cure it.

 

[tamz]

Here is a link to a very good simulator that shows you what happens when you change the pre-charge pressure fill pressure etc. Just play around with the numbers.
http://www.mikrofill.co.uk/index.php?wcId=15&

 

[Reg Man]

Sorry, Reg Man but you are getting confused with the old F&E, because of the way the expansion vessel works it is a bit more than that & based on the static head pressure, safety valve setting sytem volume etc. RWC have very good info on this. Expansion Vessels

Thanks Chris. Quote from your link:

The main purpose of an expansion vessel is to compensate for the increase in volume of water due to the varying water temperatures in hot water or heating systems. When water is heated it expands and as water is not compressible this increased volume will create a rise in pressure within the system. As an example, water being heated from 0°C-100°C will increase by approx 4.5%.

Thats where I was coming from. I Agree with you that it is not the only consideration.

 

[lame plumber]

hi chris, i understand what you are saying, thanks for the info. Just been again, as only next door. The expansion vessel was initial pressure setting of 0.5bar, (not 1.5bar like i commented before) tested vessel by pressurizing and pushing up the scrader valve, no waqter present so presume diaphragm is ok. The vessel is Tee,d off the gravity circuit for the hot water to coil. Heating side is of course pumped. Plenty of DHW so circuit is obviously flowing ( no restrictions in flow or return to boiler) There is a tub under the vessel to gauge the water lost, it was overflowing so a good 40 litres must of come out and the pressure gauge was reading approx 2.8bar ! still high with preesure and water lost !

I think we can eliminate blockages in pipework, or air locks. full heating system has never worked so well, very hot rads throughout and good hot water at cylinder.
Boiler is downstairs and cylinder is up stairs so average of 2.4metre to plus height to top of upstairs rads maybe 1metre. The longest run is the biggest "??" there are four bedrooms then a bathroom in a row so pipework must be 20 - 25metres plus another 10metre angled off this then down to the boiler.
the boiler itself is only on a low setting so its not overheating and boiling the water, the boiler has never clicked out, so not a temp issue, (apart from rate of expansion)
Im leaning toward a bigger tank, and playing around with the pressures, just a costly excercise if doesent cure it.

I'm confused and a little worried that you are discussing a system boiler whilst referring to gravity feed pipework to cylinder, the 2 dont go together!

 

[alexmurchison]

boiler is a conventional type boiler with NO internal componants (pump / diverters / expansion vessels etc) There is a hot water cylinder upstairs. Next to the boiler is 2 zone valves the heating circulator and the expansion vessel with prv and pressure gauge. (sealed system obviously) hence the requirement for the expansion vessel.

looked at a few sizing theories but all vary !!
averaging 10lt per rad then 10lt for boiler, then averaging 0.5lt per metre for 28mm pipework. anyyhow, roughly guessed at 150mtrs of pipework, not all is 28mm so bit on the exsessive side really. 16 rads then 10 lt for boiler
so 170lts plus pipework capacity maybe 75lt total of 245lts then using one example of 10% of this figure to determine expansion vessel size. which next biggest size is 25lt expansion vessel. Anyones take on this method ?? If it makes any sense at all. thanks guys

 

[Best]

You can't go wrong with too large a vessel, so always go with a larger size if any doubt. Very little difference in money.