pressure in boiler rising

[seanydee]

Hi,
I am installing a new bathroom at home and have had to remove a radiator to make room for a wider bath. I plan on installing a narrower towel radiator so just closed the trv and lockshield valves, loosened the connections and removed the rad no problem. Anyway the rad is still removed as the floor boards were rotten under the bath so i've been sorting those out. This morning there were drips coming from the trv opening when the heating was on so i put a blanking cap on the valve. Anyway this evening the others rads weren't heating up (the rad is still off, rotten joists too) so i went into the loft to check the boiler and the pressure gauge was at zero and the insulation under the boiler was saturated. I opened the fill valve (the boiler is a RAvenheat combi) and filled to 1 bar. About 15 minutes later i went to check the boiler again, mostly to see if it was leaking and the pressure was up to 1.5 bar so i switched the heating off, the pressure then stabilised. Is the problem because of the missing rad so that the water isn't circulating fully but stopping at the blanked trv? If so can i just run a length of copper pipe between the 2 valves, seal with compression joints and open the lockshield the same number of turns that i used to close it?

 

[tincup]

Hi
The pressure will go up when the heating is on but fall back when it cools.
If the pressure still rises when the boiler is not on, then its likely the filling loop valves are passing. Make sure they are closed properly.

HTH

 

[MANDATE]

If you set the pressure to 1bar (cold) then put the heating on the pressure is expected to rise. The water when heated expands. the increase in volume has to go into the expansion vessel. Here it pushes against the diaphragm and compresses the air until its held in equalibrium . ie air pressure and water pressure both increase.
Providing the vessel is empty of water, giving max amount of air the pressure should rise about 1 to 1.5 bar.
Removing a radiator should have no effect. If you bleed a radiator the pressure will fall. when you fit a new radiator and open the valves again pressure will fall. A leaky air valve on the expansion vessel will lower the air pressure allowing diaphragm to take up different position allowing more water in than intended and again pressure will fall.
Any leak in the heating system will result in pressure falling.
A faulty pressure relief valve not sealing correctly due to dirt on valve seat will result in pressure falling

 

[seanydee]

The pressure did drop back down to 1bar after i turned off the heating but i haven't had this problem before, especially what seems to have been a major loss of water, i assumed that this was caused by the pressure getting to such a level that a pressure release valve opened. Should the heating system work normally even if a radiator is removed, breaking the circuit of water?

 

[MANDATE]

The circuit of water has not been broken. Water goes along the flow pipe and back along the return pipe. Each radiator is fed off the flow pipe through the radiator and then back into the return pipe. So water flowing in the flow and return pipes is continuous.
The difference between closing a radiator and removing a radiator then replacing it is the latter needs the water replacing whereas the closed radiator just needs opening.
If water pressure is rising too much due to insufficient volume of air for compression (or blocked connection pipe) then the PRV will open at 3 bar. The pressure will instantly drop to just above zero when water is discharged from the discharge pipe and topping up will be needed.
It's the norm for the water pressure to rise from about 1 bar up to 2 bar or even a little more. It all depends on the volume of water in the system and the size of the expansion vessel.
You will note that there is little rise in pressure when using Hot Water because only a small volume of the system water is used through a heat exchanger so the increase in volume is small.

 

[Mikefromlondon]

My experience with a Ravenheat 820/20 combi boiler is that when some water is taken out, i.e. when you closed your trv or lockshield valves, you said that the boiler was empty as water had leaked away, and you then had to cap the trv, re-filled water again to 1 bar, the chances are that the water underneath your boiler was result of the automatic air purge valve, its vent sticks out from the top of the boiler and often this air purge valve won't seat properly and will leak when re-filled after it has purges the air, so a water leak is possible from the top. Similar to pressure relief valve, which often will leak if you operate it as any debri in the system may make the valve seat seal properly.

However, be careful as in my wife's flat, this water runs down the sides of the boiler cabinet and goes straight into the electrics, lead by wiring, and caused arcing in the mains ON/OFF switch! Very Dodggy , just make sure the top air vent is dry and is not leaking.

Obviously this vlave opens (the float within the valve assembly drops as the system water leaks away, and then does not always re-seats well.