my gas boiler pumped system is creating air bubbles which are pumped around the system and collect in the same 3 rads making them cooler and inefficient. I have to continually bleed them. Is there a systemor pipework fault. Is it to do with venting the system
any ideas welcomed. The system was put in my 1930's house by the previous owner.

The message
from mitch contains these words:


my gas boiler pumped system is creating air bubbles which are pumped
around the system and collect in the same 3 rads making them cooler and
inefficient. I have to continually bleed them. Is there a systemor
pipework fault. Is it to do with venting the system
any ideas welcomed. The system was put in my 1930's house by the
previous owner.

Air isn't created, it has to come from somewhere. When you refill a
system you will take in air disolved in the water but this will bleed
off in time and any further air introduced during bleeding will
progressively reduce.

ISTM that the most likely source of significant quantities of air are
either pumping over (pump pressure expelling water into the header tank
via the vent) or sucking in air through valve glands if any part of the
circuit on the suck side of the pump is below atmospheric pressure. Both
these could be construed as design faults.

Pumping over is easy to find - just go and watch the header tank when
the system is in operation or even look for a suspiciously warm header
tank.

Sucking air in rather harder to diagnose. You need in the first instance
to know the head your pump delivers and whether this is at any point
greater than the static head anywhere on the circuit. If it is you then
need to establish where the feed from the header tank joins the
circulating system (which determines the absolute pressure at that point
and thus the reference pressure for the whole circuit) and then see
whether there is anywhere near the suck side of the pump that could be
the culprit.

Figures are available to calculate the losses in the pipework quite
accurately but for a first approximation take the ratio of distance to
pump to the length of the (shortest) circuit as a whole as a guide. The
pump head will give you the pressure difference across the pump which in
the first instance can be considered as dissipated evenly round the
circuit. The distances from the pump to the header tank feed and to
possible entry points, the length of the circuit and the direction the
pump pumps should be enough to rule any possible entry point in or out.

--
Roger

In article ,
roger writes:
Air isn't created, it has to come from somewhere. When you refill a
system you will take in air disolved in the water but this will bleed
off in time and any further air introduced during bleeding will
progressively reduce.

ISTM that the most likely source of significant quantities of air are
either pumping over (pump pressure expelling water into the header tank
via the vent) or sucking in air through valve glands if any part of the
circuit on the suck side of the pump is below atmospheric pressure. Both
these could be construed as design faults.

Air will diffuse in through any leaks even if the water is well
above atmospheric pressure. It will do this until the partial
pressure of dissolved air in the water is atmospheric pressure,
but as the water is continuously heated and cooled, the partial
pressure of air will nearly always be below atmospheric pressure
when the system cools down, allowing air to leak in even when
the water pressure is high.

The other option is that it's not air at all, but gasses produced
due to corrosion in the system, due to no/insufficient inhibitor.

--
Andrew Gabriel

In an earlier contribution to this discussion,
mitch wrote:

my gas boiler pumped system is creating air bubbles which are pumped
around the system and collect in the same 3 rads making them cooler
and inefficient. I have to continually bleed them. Is there a
systemor pipework fault. Is it to do with venting the system
any ideas welcomed. The system was put in my 1930's house by the
previous owner.

Are you sure that it *is* air?

If your system has no inhibitor in it, it could be hydrogen caused by
corrosion. You can test this by bleeding a radiator and holding a lighted
match in the stream of "air" coming out. If it's hydrogen, it will burn.

If it *is* hydrogen, your system needs a good flush out, followed by
refilling and adding a suitable quantity of corrosion inhibitor.
--
Cheers,
Set Square
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