Unvented Combi: Pressure loss when hot tap on!


5 Mar 2017
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United Kingdom
Hi all,

I have an unvented electric combi boiler, and I'm losing pressure in the closed-loop central heating when a hot tap is turned on in the house. With the water-jacket cold filled to 1.2bar, I find I lose all pressure after about 5 minutes of having the hot bath tap running. This is with the system cold, electric off, and the boiler isolated from the household radiator pipework. I'm also getting a slower loss of pressure when all household taps are turned off, losing about 0.5bar over 8 hours.

The PRV is not leaking (tested by taping a freezer bag over it and leaving for a few hours). The external drain plug is not leaking either. The expansion cylinder is new, and I've checked the precharge (set to 1.5bar). The AAV is dry too, so no problems there. I've also physically disconnected the household radiator out/return pipes from the boiler because I had a niggling feeling one of the isolation valves might have been seeping, but they are holding water fine!

The mains water is approx 3bar, and the system cold fill pressure is 1.2bar. My understanding is that a leak in the internal heat exchanger would cause the water jacket to increase to mains pressure, but I am finding the opposite! What could possibly be causing a pressure loss at the filling loop pressure gauge when a hot water tap is running?! Where can I be losing pressure from when the boiler is isolated from the household radiators? The boiler isn't externally leaking, but the water is going somewhere!

Thanks in advance for your help.

Last edited:
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Hi Mike make and model of your boiler would help as , what you have described does not make much sense

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