What pressure for tank?

oceanclub

I bled our bathroom towel radiator and found I had to top up the tank. No bother except I then had to do it three days in a row when the heating didn't come on.

Maybe I'm not topping up enough. Our tank has a red marker at 1.5 bars:

I always presumed this is the max pressure you should aim for even with the heating on. So I tend to fill it when cold to .75.

But is the red line the max pressure when cold? Should I be topping up to 1 bar or more?

P.

John.G

Most Oil and gas boilers have their PRVs (safety valves) set at 3 bar so topping up to 1.5 bar when cold is OK as the pressure should then not rise higher than 2 to 2.3 bar when system heats up. Grant oil fired boilers are one exception that I know of, their PRV lifts at 2.5 bar so jf one of these is fitted maybe a max cold fill pressure of 1.25 bar.

oceanclub

John.G wrote: »

Most Oil and gas boilers have their PRVs (safety valves) set at 3 bar so topping up to 1.5 bar when cold is OK as the pressure should then not rise higher than 2 to 2.3 bar when system heats up. Grant oil fired boilers are one exception that I know of, their PRV lifts at 2.5 bar so jf one of these is fitted maybe a max cold fill pressure of 1.25 bar.

Thanks. So on these tanks the red indicator normally indicates the _cold_ max pressure?

P.

John.G

Probably I would think as they can be used as a marker as well and are settable I think.
Another very important item is the pre charge air pressure in the E.vessel before charging it from the water side, if your boiler PRV is 3 bar then this pre charge pressure would normally be 1 bar, you then use a filling pressure (cold) of 1.5 bar so this gives a litre or two of water reserve to allow for any air venting etc, if, for any reason the pre charge pressure is 1.5 bar, then with filling pressure of 1.5 bar there will be no reserve which leads to frequent top ups, a pre charge and filling pressure of 1 bar will have the same effect. So your filling pressure of 0.75 bar doesn't really tell you anything re any potential leaks etc. You should normally only have to top up once/twice a year after any air venting is complete.

Wearb

oceanclub wrote: »

Thanks. So on these tanks the red indicator normally indicates the _cold_ max pressure?

P.

If it hasn't been accidentally or deliberately moved from original setting, it shows the cold recommended pressure.

oceanclub

Wearb wrote: »

If it hasn't been accidentally or deliberately moved from original setting, it shows the cold recommended pressure.

Ah thanks. Bit weird they would use red for that!

P.

John.G

Not a great picture but this is the type typically installed on the (mostly Gas) Boiler Front, it gives a green operating sector between ~ boiler low pressure cut out and PRV setting, some boilers now AFAIK have a high pressure cut out as well.

oceanclub

As a followup - I've now topped up the tank 4 or 5 times and each time by morning it's down to almost zero:



The tank was topped up to 1 last night.

This only happened after I bled that bathroom radiator.

Any idea what to do before I call in a plumber?

Wearb

You could have a leak or a problem with your expansion vessel. Either way its a plumber.

Edit: Are you topping up when system hot and then checking when cold? That would cause a lower cold reading.

John.G

When you next top up watch the pressure when the system is heating up, if the expansion vessel has failed or has lost its pre charge (air) pressure then the pressure will rise to 3 bar and lift the PRV.
If the expansion vessel is external just press the schrader valve with your finger nail and if water comes out then a failed E.vessel, if no pressure then needs recharging.

oceanclub

By coincidence someone from Bord Gais was around to inspect the boiler and yup it is the expansion vessel. Thanks for the info, all.

P.