As above, the hot water expands, increasing the pressure. The boiler will have an expansion vessel that has a diaphragm inside. As the water expands the diaphragm allows the volume to safely expand. As the pressure drops, the air behind the diaphragm pushes it back.
Only thing to add would be that the EV may need a little adjustment, time for a pre-charge pressure check. Normal pressure swing would be 0 > ~0.3 bar. Any more would indicate that either the EV may be be getting low on pre-charge or the vessel may need to be a bit larger (system size dependent).
When was the last time it was serviced and did they check the vessel?
Not serviced for a few years as the plumber that did it said the servicing doesn't actually do much, it's a glow worm betacom 30c. Thanks for assistance
Not serviced for a few years as the plumber that did it said the servicing doesn't actually do much, it's a glow worm betacom 30c. Thanks for assistance
Get someone who knows what they're talking about to do it next time. Forgot to ask, is this rise in pressure a new thing or has it always risen that much?
Contrary to popular believe boiler servicing, when performed correctly, is a preventative measure, rather than it actually fixes things. It allows the engineer to get ahead of things that may be starting to show themselves.
If the EV is down on pressure then he obviously didn't check a serviceable item and if it continues to drop then the boiler pressure may eventually get to the point where it lifts the PRV. If it does that, the valve may also need replaced. Adding to your costs. Caught early during a service would avoid all of that.
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