My baxi combi 100 he plus keeps losing pressure cant find a leak anywhere inside but some water coming outside from expansion pipe?

there is in your sistem an expansion basin where it must to be air inside .when the basin loose the air it heapend what you say.with a pump air (like for the whell car)you put back the air on 1,2 atm pressure

I would go with the valve first cheaper, then see if it still doing the same thing

Hi the pipe out side is the pressure relief valve change the valve or it has something stuck on the seating , changing it will stop the pressure drop

There is a 15mm pipe from the pressure relief valve which should so through to the outside. These valves are prone to leak after time. Look at the end [outside] of this pipe. Wet = leak from valve

I have the same boiler with a similar issue. The water is more than likley leaving the system from the pressure relief valve, which will be routed outside. It sounds like you have lost the pressure in your expansion tank, which is inside the boiler. With the boiler pressure at zero and the power isolated, the expansion tank can be accessed from a panel at the top of the boiler on the right hand side. You will see a valve on the red expansion vessel like the one on a car tyre and can be pressurised with a car tyre air pump. I think from the manual the pressure should be about 1 bar, but it depends on the length of pipework on your system. A competent boiler engineer should be able to sort this out, but it may be that your valve is leaking, or the expansion vessel has failed and the problem will occur again.

Check the pressure relief valve & the pipe to the outside

water will stop coming out of the once pressure drops . it will act the same and water will flow out of pipe the same .

rads not heating could be down to old system ie pipework etc and sludge building up. r there any cold spots on rads? when boiler turning off do u mean going to reset? if so could be over heat prob at boiler . there should be a pressure pipe coming from boiler going to outside if its a combi. if this is leaking your expansion vessel might need re charged as this is where the water will be going.

The first thing you need to do is to put some more water into your heating system. This is usually done by allowing water from your (pressurised) domestic cold water supply to flow into the system: On older boilers, this is done with a connection (often made with a removable hose) on the pipework just outside the boiler. Newer boilers may have a built-in connection with some sort of removable link (sometimes looks like a plastic key).

Once you've got the boiler up to working pressure (typically about 1 bar - 15 psi), you should be able to bleed your upstairs radiator properly.

This may be all it takes, though the pressure excursions you're getting when you run the hot tap suggest there may be another problem. See what happens to the pressure as the boiler heats up and cools down. There really ought to be very little pressure difference between the two conditions. If you see significant changes, this usually means that the "pressure vessel" (or in hydraulic terms, system accumulaor) has lost its air charge.

The pressure vessel is a metal container inside the boiler, usually painted in red enamel. Somewhere near the top, you will find a schrader valve (like the one on a car tyre), and you can use this along with a tyre pump to replenish the air in the top of the vessel. This will reduce your pressure transients for a while, but you need to keep an eye on it. If the problem returns, you probably need to replace the pressure vessel (or clag another one into the plumbing alongside the boiler).

A.

generally preesure problems are caused by the following- obviously a leak, bleeding of rads without topping up the pressure, pressure relief valve passing( probably an integral part to the boiler), or potentially a problem with the expansion vesell.
the expansion vessel is a tank which
holds water and air- it should be pressurised with air by about 10 psi or up to 1bar, if the diaphragm has split this ill cause pressure problems all day long

hope this info helps