Pressure rising from 0 to over 3 bar when heating is on

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Hi,

I have a sealed system with a dualstream hot water tank and an accumulator, and a Worcester Bosch system condensing boiler. This was all installed just under 12 months ago.

The house is a three storey victorian house with 13 radiators. Boiler is downstairs in the kitchen, hot water tank and accumulator are in the loft.

Hot water is working fine.

When switching the heating back on last week I kept an eye on the pressure gauge. Basically what happens is that it's fine when the hot water is on, but once the heating goes on it goes up to 3bar+, the pressure release valve activates and some water is expelled from the system. When the heating goes off the pressure then drops to 0.

I refill the system when cold, it holds the pressure at 1 bar, then same thing happens when heating goes on.

If I don't refill the system when cold, it stays at 0, same thing happens when heating goes on (rises from 0 to 3bar+)

After reading various posts on here about similar problems I checked the expansion vessel to see if it had failed/dropped pressure, but checking with a tyre pressure gauge (while the system pressure read 0) the expansion vessel pressure was around 3 bar.

As there doesn't seem to be a problem with the expansion vessel can anyone suggest what else may be wrong?

Many thanks in advance!
 
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check the faq's for the correct method to re-pressurise the expansion vessel.

either it's flat/burst or the pipe to it is blocked.
 
When you try to repressurise. Does the system pressure rise rapidly from 0? It does sound like the expansion vessel. Drop the system pressure, a press the shreider valve in, do you get any water out at all? Turn the filling loop on and keep the valve pressed, your looking for water.

To re charge, I isolate boiler, cold system, no pressure and charge the expansion to 1.5bar
 
Hi,

thanks for the replies - I have checked the expansion vessel and it appears to be fine - no water from the valve and the pressure reading is 3.5bar. I've also taken the pipe to the expansion vessel off and checked that for blockages, again all fine.

Only thing I can think now is that the vessel isn't big enough for the system (although it is the one that came with the dualstream and should be sized correctly for that), I have 13 radiators in the house, all traditional style column radiators, which hold more water than conventional ones i think.

Am thinking of fitting a second expansion vessel on a T to see if that helps, any thoughts on that plan?

Thanks again!
 
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A pre-charge pressure in the expansion vessel of 3.5bar is far too much. The system water can't expand into the vessel because it will vent through the pressure relief valve first. If the boiler is downstairs in a 2 storey house, 0.8 bar is the correct pre-charge pressure. Lower the system pressure to zero before adjusting the pre-charge.
 
3.5 bar is the supplied pre-charge in the domestic accumulator not the heating/boiler system expansion vessel which would not have been supplied with the Dualstream system you need to look for the red expansion vessel then release the pressure to zero before checking or adjusting the pre-charged air.

If this installation is under 12 months old contact the installer.

If this system worked ok last winter then the vessel will probably be the correct size so this would point to being a loss of air pre-charge or blocked link pipe between the system and the vessel.
 
If the pressure in the expansion vessel is zero it indicates that the expansion tank has no air in it or, that the expansion tank is full of water and is pushing the diaphram up against the pressure test valve. If this is the case you will need to empty the expansion vessel of water by manually opening the pressure relief valve, usually a quarter turn anti-clockwise of the knob of this valve will set it to open, then by using a foot pump connected to the shrieder valve (air inlet valve), inject air into the expansion vessel until all water is expelled from the vessel through the preesssure relief pipe. When alll the water is expelled shut the pressure relief valve and then pump up the vessel to 1 bar. If you have a 12v battery and a electic tyre pump instead of a foot pump it would save you a lot of energy.

The cuase of this problem is usually becuase the valve on the water filling loop by-passing or a fualty vessel.

Before doing this it would be wise to turn of the heating flow and return valves on the boiler.
 
It could be that the boiler's builtin vessel is not big enough to cope with the volume of water contained in your large system so an additional vessel may need to be added near to the boiler on the htg return
check the M.I.'s for further info on vessel sizing P.S. the air pre-charge should be approx 0-2Bar LESS than the Cold fill Pressure! ;)
 
Thanks to all of you for the replies.

I've added a second expansion vessel along with the existing one today, run the heating and the pressure has stayed stable between 1 and 1.25 bar, so it looks like as suspected the system was just too big for the existing vessel.

I can only think that it worked last year as I had some standard radiators in the house which I have since replaced with column rads, and this has increased the amount of water in the system overall.

Added some more inhibitor as well to be on the safe side and make up for the topping up over the past few weeks.

Hopefully all well now :)
 
I just like to add if the pressure relief valve has been opened for any reason you should replace it as they have a knife edge seal that is increadibly easily damaged by debris and will leak at odd intervals once opened.

I speak from hard earned experience here.
 
iirc the pre charge on the accumulators should be 1.5 bar greater that the HWS system working pressure. So if you have a HWS/CWS working pressure of 2bar then the 3.5 bar you had was correct. Also the accumulators are there to aid the CWS feed to the HWS and has nothing to do with the CH. I get the feeling that you are trying to check/charge the wrong vessel. The vessel will be red and situated within your boiler.
 

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