pressure issue on sealed central heating system

[carlos21]

I have just redone our heating system from open vented to sealed system, everything apart from the radiators was replumbed ( due to renovation).

it is an old oil boiler( not combi)  

I have filled the system to 1bar when cold, but when the system is hot and running the pressure doesn't rise much, is this a problem??

everything seems to be working ok, ie hot water tank is hot , all the rads heat up ok.

my thoughts is the expansion vessel is too big (18L) as I didn't really calculate it, I just erred on the big side.

is the lack of pressure a problem or would the system function much better with a higher pressure?

any thoughts welcome,

thanks carl

[JohnMo]

36 minutes ago, carlos21 said:

have filled the system to 1bar when cold, but when the system is hot and running the pressure doesn't rise much, is this a problem??

It shouldn't rise too much, if it does that's an issue. Your system will have an expansion vessel this has a rubber bladder in it to absorb the extra water volume.

 

Extra pressure doesn't aid anything, 1 bar is fine. You pressurise to help remove air from the system, any air vents or bottle traps should be opened to release any air a week or so after the start of the system, then closed again.

[Nickfromwales]

On 26/01/2026 at 22:47, carlos21 said:

I have just redone our heating system from open vented to sealed system, everything apart from the radiators was replumbed ( due to renovation).

it is an old oil boiler( not combi)  

I have filled the system to 1bar when cold, but when the system is hot and running the pressure doesn't rise much, is this a problem??

everything seems to be working ok, ie hot water tank is hot , all the rads heat up ok.

my thoughts is the expansion vessel is to big (18L) as I didn't really calculate it, I just erred on the big side.

is the lack of pressure a problem or would the system function much better with a higher pressure?

any thoughts welcome,

thanks carl

18L is a big expansion vessel, so that's just absorbing the expansion and not letting it reflect much in the reading on the pressure gauge.

 

It's all gold mate, as you've done the right thing by over-sizing vs under-sizing, and this will mean you won’t need to top up the pressure as often. My 2 cents is that you can never have too much expansion.

 

Do you know you have to treat the system with inhibitor after these works, to prevent corrosion? Also, do you know you will have to check the system pressure every 3-6 months, or possibly more frequently, as part of switching out from auto-fill from the tanks to regular maintenance of a sealed and pressurised system?

 

[carlos21]

thanks for the replies, il keep an eye on it ,plus i must add more rads once the renovation bit is done so i guess that may change the pressure a bit.

i should probably get the boiler serviced so i will ask the plumber to cast his eye over it.

yes i have some corrosion inhibitor, but i must drain down again due to a sticking drain cock that i must replace.

thanks again carl

[Nickfromwales]

2 hours ago, carlos21 said:

thanks for the replies, il keep an eye on it ,plus i must add more rads once the renovation bit is done so i guess that may change the pressure a bit.

i should probably get the boiler serviced so i will ask the plumber to cast his eye over it.

yes i have some corrosion inhibitor, but i must drain down again due to a sticking drain cock that i must replace.

thanks again carl

If it’s going to be a number of weeks or months then lose the tenner and dose with inhibitor immediately. 
 

You’d be surprised how quickly the new fresh oxygenated water kicks off the corrosion process again.