expansion vessel on central heating

[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.

 

[chris watkins]

So what are you going to charge this 25 L vessel (& the system) up to then alex ??

P.S. what is the Kw rating of the boiler.

 

[SimonG]

2.5 bar is not normal on an oil boiler when upto temp. Cold fill pressure should be about the 1 bar mark, although some combis/system boilers have an integral pressure switch that cuts off at about .5 bar and won't reset until pressured to 1.5 bar.

Personally I don't like to see the system pressure near or above 2 bar when running at full temp. The expansion vessel will be shot in a crack.

 

[tamz]

So what are you going to charge this 25 L vessel (& the system) up to then alex ??

P.S. what is the Kw rating of the boiler.

One about this size should cover it if he can find somewhere to put it :wink:

 

[alexmurchison]

thanks for all the replies. There is space as its in a utility of its own, so floor standing is not an issue, as if im not mistaken 25lt is the largest wall mounted there is ? system pressure will be between 1 and 1.5bar as normal, shall leave expansion with initial setting , run up and see what the pressure reads, then can adjust accordingly. Taking a reading from a worcester manual, on setting up filling etc, says fill system to 1.5bar, doesnt really state pressures in expansion vessels but reads on to say max pressure whilst up to temp, should not exceed 2.65bar I totally agree that this does not leave much margin for the blow off pressure of 3 bar at the PRV. 2 - 2.25bar is more like what i would expect to see if original pressure was set at 1.5bar on a large ish system. appreciate all the advice and comments on this topic. cheers

 

[Howsie]

Your location does make me smile every time I look at it.

 

[alexmurchison]

Your location does make me smile every time I look at it.

lol, you could never take ALL the ****s out of S****HORPE even if you were the genie in the lamp !!!:tounge_smile:

 

[chris watkins]

thanks for all the replies. There is space as its in a utility of its own, so floor standing is not an issue, as if im not mistaken 25lt is the largest wall mounted there is ? system pressure will be between 1 and 1.5bar as normal, shall leave expansion with initial setting , run up and see what the pressure reads, then can adjust accordingly. Taking a reading from a worcester manual, on setting up filling etc, says fill system to 1.5bar, doesnt really state pressures in expansion vessels but reads on to say max pressure whilst up to temp, should not exceed 2.65bar I totally agree that this does not leave much margin for the blow off pressure of 3 bar at the PRV. 2 - 2.25bar is more like what i would expect to see if original pressure was set at 1.5bar on a large ish system. appreciate all the advice and comments on this topic. cheers

Alex, enough is enough for me on this thread, lots of people on here have tried to push you in the right direction but it is clear you have no understanding at all of what the cold fill pressure of a heating system should be - it is not just 1 to 1.5 bar it is based on the head of water required above the vessel, nor do you understand what the pressure of the gas charge in the vessel should be - again it is not just what the manufactures supply it pre-charged to, it needs to be set to a pressure to support the system head pressure when cold. The pressure settings of both the system & vessel have a dramatic effect on how effective the expansion vessel is & therefore the size required. I don't know where you did your training but if you are going to work on these systems you really do need to understand what is required & how things work.

 

[alexmurchison]

Alex, enough is enough for me on this thread, lots of people on here have tried to push you in the right direction but it is clear you have no understanding at all of what the cold fill pressure of a heating system should be - it is not just 1 to 1.5 bar it is based on the head of water required above the vessel, nor do you understand what the pressure of the gas charge in the vessel should be - again it is not just what the manufactures supply it pre-charged to, it needs to be set to a pressure to support the system head pressure when cold. The pressure settings of both the system & vessel have a dramatic effect on how effective the expansion vessel is & therefore the size required. I don't know where you did your training but if you are going to work on these systems you really do need to understand what is required & how things work.

OBVIOUSLY NOT THE SAME PLACE AS YOU . Not sure how you read my posts, but what part of I shall run and adjust accordingly doesnt make sense.
As stated a few times by various contributors, most boilers will not operate below say 0.5bar fill pressure, filling the system any higher than 1.5bar would almost certainly see the prv pass by !! the expansion vessel is the main factor as if its sized correctly you would not have to compensate by letting the air out to reduce pressure etc. ! My original post as with all posts on this forum are usually from people (like me) who would like a little friendly advice, not sarcastic comments. I have fully read your posts and appreciate them. im all for a little humour and sometimes the easiest of problems for some poeple just need a little more detail and explanation. sorry if i just seem to be the back street bodger that i presume you think i am !

 

[TBServices]

Chris is a well established contributor of the forum, and is always helpful to new posters, even when many of the rest of us can't be bothered wasting our time. He's tried to help as best he can, but you seem incapable of grasping his advice.

I'd suggest adjusting your attitude.

 

[alexmurchison]

my attitude ??????? from where do you think i have an attitude. Im the one who was asking for advice, not making sarcastic comments. I have not dissmissed any ones advice, simply asked more in depth. And it doesnt mean that somone who comments on everyones post is always right either. and surely anyone who can read would see i have taken head with advice and said i shall adjust setting / pressures to suit !!!

 

[tom609]

whos hats been chewed

 

[Mikefas]

A working pressure of 2.5 bar is too high, try lowering the pressure and make sure the vessel is isolated from the water supply when you do this. 1 bar cold is plenty and if you have any more than 2 bar hot then youre vessel size is too small. Also whats with the 32mm flow and 22mm return? no value in having different flow and return sizes

 

[alexmurchison]

A working pressure of 2.5 bar is too high, try lowering the pressure and make sure the vessel is isolated from the water supply when you do this. 1 bar cold is plenty and if you have any more than 2 bar hot then youre vessel size is too small. Also whats with the 32mm flow and 22mm return? no value in having different flow and return sizes

Hi there, thank you for the input. yes its a strange set up, not sure date of 1st installation, boiler is around 15 years old, so mismatch of pipework, also still pipes from an old solid fuel link up, looking again, 28mm flow to downstairs and up then for some reason the return has around 10" of 22m at the boiler. Would be easier to re do the lot, but hey, cant convince them .

But either way it worked for umpteen years so main objective is stop the over pressurization. I still think i will go for the next size tank after doing a few calcs, shall try it at 1 bar fill, like you and others have mentioned