High Boiler Pressure

[IanDB]

I have noticed that my boiler pressure has a tendency to rise gradually once the pump is running. I have sometimes found water underneath the boiler which could indicate that the pressure relief valve has activated (3 bar!). Is this a sign that there is a blockage in the system? If so how do I go about clearing it?

 

[sterose]

My dad has the same problem

He gets up in the morning and has to fill the boiler to 1bar
Turns heating on and pressure releif valve lets some excess water out, but heating works fine after that until he turns it off.
Pressure drops to nothing.
He has to fill it again.

Reason is because the hot water expands = higher pressure, cold water then contracts = vacuum or lower pressure.

I think its the expansion vessel thats broken - because it isnt absorbing the changes in pressure.

 

[sifu]

Sterose is right about the expansion vessel. It'll be a big red or orange vessel on the boiler, look at the exploded diagram in your manual if you have one. I would recommend getting a heating engineer to check it as this is a safety component and there is a particular method for topping up the pressure. There's also the possibility it needs replacing, which can be v. expensive if you have an old boiler.

 

[IanDB]

Thanks for the info. I went back to the manual (if all else fails, read the instructions!) and it gives details of how to check the pressure in the expansion vessel plus guidance on how to re-pressurise. I'm fairly familiar with the process as I have a pressurised water system that requires the occasional air gap re-generation - I should have spotted the similarity in the symptoms.
The expansion vessel is right at the back of the boiler and no doubt will be a pig to access but I'll have a look and post a reply - probably after the weekend as it requires the boiler to be drained down. Joy, oh joy.

 

[sterose]

i think that you can fit an external expansion vessel to the pipework outside your boiler (please correct me...)

 

[oilman]

i think that you can fit an external expansion vessel to the pipework outside your boiler (please correct me...)

Correction not needed, you're right. System might still need draining to allow a connection to be made to install the new one though. I t shuold be at least 10% of the total system volume.

IanDB, your unvented cylinder should have an annual service (if you don't have it done already) and check the guy's certificate (which he must have to work on systems greater than 15 litres) before he starts work.

 

[IanDB]

The system I've got uses a Heatrae Sadia Megaflo water tank. Marvellous system. Loads of hot water at pressure. Had a quick look at the back of the boiler, there is still air in the expansion vessel but I'm going to get someone who really knows what they're doing to have a look and bring forward the annual service.

As far as working on unvented systems goes, where is the borderline? Every six months or so, the air gap in the water tank needs re-generating. This is a very straightforward procedure and all the instructions are written on the side of the tank. Just have to shut the mains off, open a valve and drain the water off until it stops "gurgling", close everything up and let the pressure back on again. The process for the expansion tank seems pretty much the same but I'm not even going to attempt it. However the system is littered with pressure relief valves so you would have to do something pretty idiotic to do some damage - yes, and I bet it has happened!

This post is getting mixed up with the other one about my doppleganger hot water valves but what the hell.