Combi boiler pressure

Hi I'm slowly renovating a house and had a new combi central heating system installed about 8 months ago. Every week when I visit the property, the towel rad (highest point in the system) needed a little bleeding. About 4 weeks ago, the boiler's display said "bleed radiators". So I did. Then the display read "call engineer" and the pressure was 0. I called the installer and he said to top it up. I did this to a pressure of 1 bar. The pressure varies between 1 and 2 bar when cold and hot. Is this normal? Since topping up the system hasn't need bleeding. In my uneducated state, I'm thinking that maybe the expansion vessel had a leak and was letting its air into the system. This being the air that needed to be bled every week. And now the vessel is just full of water. What's the likelihood of that being an explanation for the requirement for frequent bleeding? Without a working expansion vessel, would the hot pressure be more than 2 bar? Ta.
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On 18/03/2017 14:07, Grumps wrote:

Without a working expansion vessel the pressure would go way over 2 bar when the system is hot, causing the pressure relief valve to open and discharge water to the outside world. Then, when it cooled, the pressure would fall to zero. Very unlikely that this is happening.
Much more likely that there was dissolved air in the water when the system was initially filled, and this collected in the towel rail. Hopefully you've now got rid of it all, and shouldn't have any further problems.
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On 18/03/17 15:42, Roger Mills wrote:

However a pressure vessel that hasn't *failed*, but has lost a lot of its pressure, behaves exactly like the OP describes. Essentially it needs pumping up with a car or bike tyre inflator.

Pressure should not vary between 1 and 2 bar. That's a fpressure vessel that is barely working.
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On 19/03/2017 09:17, The Natural Philosopher wrote:

You can indeed revive a pressure vessel in that way if it has simply lost its charge pressure and not failed completely.
But the OP's symptoms didn't indicate a pressure vessel problem, anyway!

Not at all! It depends on the volume of water in the system vs the size of the pressure vessel. A cold/hot rise from 1 bar to 2 bar is perfectly acceptable. It's still a long way from it's blow-off pressure of 3.5 bar.
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On 19/03/2017 09:17, The Natural Philosopher wrote:

How often would you expect to have to top up a sealed system?
(or what rate of loss would you consider normal?)
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On 18/03/2017 14:07, Grumps wrote:

Yup that's not surprising, there is a fair amount of dissolved air in the water that will come out over time.

Every time you bleed the air out you reduce the overall system volume a bit. Eventually the pressure will be too low for the boiler to operate.

Yup that perfectly normal.

Not on an 8 month old boiler. I expect you have just seen the system water loosing its dissolved air. There may also be a tiny weep / leak that (in a hard water area anyway) will tend to self seal.
Make sure you add some inhibitor about once a year - that prevents any corrosion and generation of gas in the system from that.

Yes - massively... The pressure would exceed that of the safety release valve (usually about 3.5 bar) and that would vent water outside. Once the boiler cools you would have low pressure. So the loss of pressure would be a daily (or more frequent) cycle.
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On 18/03/2017 16:17, John Rumm wrote:

Thanks both. I'm reassured that all is OK now.
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Grumps formulated on Saturday :

If you think the pressure release valve might be opening or leaking, a simple way to check, is to add a plastic sandwich bag over the outlet fixed with a rubber band. It will catch any released water.
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