boiler rising in pressure

[andymck1]

Hi all
I know this has been done many times and I've searched the forum and found some excellent info.
But with this one I'm a little confused.
The pressure set on the boiler starts at 1 bar and rises as the central heating gets going to 3 bar plus. (if I have the boiler set very low I can keep it below 3 bar put if I set the temp high it goes above 3 bar) Is a Worcester bosch 24i junior by the way
So I opened a radiator to release the boiler pressure to zero. (its in a caravan so not a lot of water in the system I guess)
When I put a pressure gauge reader on the EV valve there is no pressure. So I tried pumping the EV up as I pump I can see the pressure gauge on the boiler rise and fall to zero so air is going in( I think). I pump for a while, then open a rad to let of water there must be some pressure as water comes out of the rad. I continued to do this for some time but every time I measure the pressure on the EV there is nothing - so I assume that the seal has gone and its leaking and the air is going into the boiler/water.
I then re - pressured the central heating water system to 1 bar and then measure the EV and it measures 1 bar - I started up the heating and it raised to 2.5 3 bar pretty quick
Again I measured the same pressure on top of the EV - ie it follows the boiler pressure, but no water comes out of the valve ? only air, and as I let air out the boiler pressure also drops?
This is confusing me a little as I expected water if the EV had failed and clearly some air is in the system as it escapes when the valve is pressed.
Is it possible for the air to be sat on top of the water even though the EV has failed and becomes pressurised as the boiler heats up - the more I think about this the more I convince myself this is the case. If I let all the air out and filled with water then maybe then I would see water in the valve?
I have assumed there is no blockage as when I pump I see the boiler gauge move (am I correct to assume this?)

Thanks for any help you can provide me - really appreciate it

 

[ianmcd]

last time I am doing this , sorry that I sound grumpy but it is in the FAQ and people constantly dont read the proper instructions, there is only ONE way to re-charge your expansion vessel, and you can try as many times that you want to do it differently it wont work, you have to DRAIN THE BOILER, LEAVE A DRAIN POINT OPEN THEN PUMP THE EXPANSION VESSEL UP TO 0.8 BAR, close the drain now pressurise the system, there is no other way to do it, if you cant pressurise the vessel it needs replaced , if you try to do it without a drain point open you are completely wasting your and my time

 

[andymck1]

Many thanks for speedy response Ian - I did read the FAQ instruction and followed it - but it did not say drain the boiler and leave drain point open whilst pumping up the EV - I will try that as I can see why that may be easier

 

[ianmcd]

you have to leave the drain point open as you re -pressurise the system , you would be surprised how many so called heating engineers dont get that , the excess water has to go somewhere

 

[AGAS]

Many thanks for speedy response Ian - I did read the FAQ instruction and followed it - but it did not say drain the boiler and leave drain point open whilst pumping up the EV - I will try that as I can see why that may be easier

YES IT DOES

 

[ianmcd]

Many thanks for speedy response Ian - I did read the FAQ instruction and followed it - but it did not say drain the boiler and leave drain point open whilst pumping up the EV - I will try that as I can see why that may be easier

YES IT DOES

I know mate but it doesnt matter how many times you tell people and I include supposed gas engineers in that

 

[AGAS]

i haven't read it to be honest , but that is one of the most important part of the method , so can't imagine it not being mentioned

 

[andymck1]

NO IT DOESN'T , and perhaps its best to read it before commenting!!!

 

[Aquaheat]

@andymck1 @AGAS

YES IT DOES MATE!!! We know this site like the back of our hands.

May I suggest that it is you that reads through things properly before commenting

Jon

1) REPRESSURISING THE PRESSURE VESSEL

NB, Pressure Vessel = Expansion Vessel.

Turn the boiler off at the mains.

Find the PV valve, which looks like a car tyre valve with just a threaded part sticking out. The PV is often red. They can usually be accessed from the top of the boiler or behind the front drop-panel. Some boilers such as Ravenheat can be impossible to access!

If water comes out when you press the central pin, you need a new PV

Find a drain cock on the pipes somewhere, or on the boiler. DO NOT use the pressure relief valve, or it WILL leak afterwards. If you can't find one you can use a radiator bleed point, but that's in a pinch, because you'll be letting several litres of water out.
Drain enough water from the system to drop the water pressure as read on the boiler gauge, to Zero. Close the drain cock.

Use any (eg car or bike) air pump to pump up the PV to eg 10 psi (0.7 bar).
Some boiler instructions or those with knowledge about the individual system might give different figures. It isn't critical but don't go above 1 bar 15psi for a boiler in a 2/3 floor house.

Remove and refit your pump to see how much the pressure drops when you remove it and inevitably lose some air.
If the pump doesn't have a built in gauge, pump it up a bit high, then keep taking readings with a push-on gauge. Every time you do, the pressure will drop a bit.

It's important that you don't rely on the reading that you get as you pump into a "closed" system - as the wet side starts to pressurise it'll give you a meaningless reading on the dry side you're pumping into.

When you get to 10psi, undo your drain cock again. If there was still water in the PV, your air pressure will force it out. If that happens, recheck the pressure and pump more, and repeat until undoing the drain cock doesn't give you a spurt of water. You can now rely on your air pressure reading.

The PV may be say 10 litres. If it was full of water you have to displace all of that water from the system side, by your pumping up the air in the vessel. That's a whole bucket of water!

Fill the boiler water back up to 1 bar pressure, slowly, and switch the boiler back on.

The air shouldn't be able to get into the radiator system because of the rubber diaphragm in the PV.
If you bleed your radiators, you will have to top up the water in the boiler via the filling loop, but not the air in the PV.


DO NOT let water into a hot boiler. Let it cool first.

DO NOT let water in too quickly. 20 seconds for 0.5 to 1 bar should be OK.

DO NOT take the water pressure above 1.5 bar- even if the system is warm. If it doesn't cure your "Low Pressure" indication, the switch may have stuck. Leave it an hour or two - water may force its way through dirt to get to the switch and turn it on.

DO NOT allow the water pressure to go too high when refilling Turn it off early to see if the gauge keeps rising a bit.

NEVER use the Pressure Relief Valve to let water out of a system – it will leak afterwards!

If the pressure relief valve was leaking before you repressurised the PV, it will probably continue to do so and need replacing.

 

[ChrisR]

You can leave it open if you want to, but it depends where it is. Sometimes they glug water out and let air in if you leave them open. You don't want that.
Once the air in the EV has expanded enough that the rubber diaphragm can't move any further, ie it's hard up against the sides and water connectiion hole, then you won't be pushing more water out. With the DC open , that could happen quite early.
So if you leave the DC(or vent) shut,
pump, then open it, water comes out,
close it, pump a bit more, etc, then when you try again
if NO water comes out, you can leave the DC/vent shut.

The precise method isn't critical, as long as you understand and take account of what can go on, and go wrong.

 

[andymck1]

thanks Chris appreciate your response.

 

[jeff the gasman]

Many thanks for speedy response Ian - I did read the FAQ instruction and followed it - but it did not say drain the boiler and leave drain point open whilst pumping up the EV - I will try that as I can see why that may be easier

YES IT DOES

WHERE DOES IT GO............
I love science.

 

[ChrisR]

Where does what go? The water in the boiler?
There isn't any to go anywhere, once the vessel's empty of water.

A low pressure in the EV will push all the water out of an open dc.

It's (fairly) simple to work out the optimum precharge pressure - it's not obvious.
Going too high, is the usual error.
It's a balance between stored water and expansion allowance. Would it help if I showed some sums to illustrate?