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
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
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.
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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