pressure rising after hot water use

[ilewis]

Hi

After using hot water a few times today (taps and shower) while the central heating was off, we noticed the pressure on the boiler rising. it was at 1.2 initially. When hot water turned on, burner fired up and pressure dropped (as it normally does to 1.1 ish). On turning hot water off, needle went back up to 1.2 ish and then continued to rise to 1.3.

The 22mm heating pipe next to the 15mm hot water supply pipe was slightly warm at the top near where it is attached to the boiler. The rest of the pipe was cool. Is it normal for the heating pipe to warm up slightly at the top. is this what's causing the pressure rise? do i have a problem?


Thanks in advance for any advice.

 

[oldplumber]

if you heat water the pressure will rise, as it cools it drops, simplees as the meercat says

 

[ilewis]

usually when we use hot water (without central heating) the pressure drops slightly as the burner fires up and then goes back to the level it was at when the hot water is turned off. It doesnt continue rising.

 

[tackleburger]

The pressure drops slightly as the pump starts then stablises. When in hot water mode your combi is working at ful pelt the same water that is in your heating is being pumped round the boiler and circulated to the hot water heat exchanger. The CH pipe on the boiler may well get hot near the boiler but this is just conducted heat as the diverter valve is confining the heating water to the boiler as stated. Your pressure will rise as the central heating water in the boiler is being heated and therefore will expand. It may rise slightly more when you turn off the tap as by doing so you stop the circulation of cold mains water therefore the latent heat left in the boiler will cause further expansion it will then cool and return to it's nominal pressure. I think your boiler is working normally.