Why Air Release Valves and Deaerators are Non-Negotiable

Mars · 2025-09-29T20:21:15Z

For as long as we have had closed-loop heating systems, air has been their enemy. It slips in during installation, it finds its way in through microscopic leaks, it emerges from solution whenever temperatures and pressures fluctuate. Left unmanaged, it corrodes, blocks, howls and undermines the very purpose of a heating system. This is why, traditionally, every system has included an air release valve. Recently, during pipework alterations on our buffer tank and central heating system, our installer removed ours. We asked him to reinstate it. He refused, insisting we did not need one. Against our better judgement, we let it slide. The consequences were immediate and, frankly, disastrous. The system had been drained, flushed and refilled, and we strongly suspect that the underfloor heating circuits had also been emptied. A new pump was installed. Once everything was re-pressurised and switched on, the system was awash with air. The sound was intolerable. Each time the pump started, the noise of bubbles and air being whipped through the pipework was enough to stop conversation. It sounded like an industrial kettle boiling itself inside-out. Watch the short video below.For days this continued. The installer said this was normal and that we should just keep bleeding radiators repeatedly, and only on the very first attempt did any significant air escape. Every effort thereafter produced nothing, yet the system is filled with air. The installer had no solutions. He fiddled with unions, sprayed water around the utility cupboard in the process, and then shrugged. His advice was to keep bleeding radiators for a few weeks and wait for the problem to resolve itself. It did not. This is the crux of the matter: in any heating system, but especially in modern low-temperature heat pump systems, air is not an inconvenience, it is a systemic threat. It reduces efficiency because circulation is impeded. It reduces heat transfer because radiators and underfloor loops cannot emit what they are meant to emit when pockets of gas stand in the way. It accelerates corrosion because oxygen is now present in the very medium that is supposed to be protecting the steel and copper of your system. Most dangerously, it damages pumps through cavitation. Cavitation is not a word most homeowners know, but it ought to worry them. When a pump operates in the presence of air and fluctuating pressure, gases come out of solution, forming vapour bubbles. When these collapse, they do so violently enough to pit and scar metal. Over time, cavitation will destroy a pump, and with it the very heart of the system. This is not abstract theory: it is visible, measurable damage and once it starts it cannot be undone. What makes this particularly unforgivable is that the solution is neither exotic nor expensive. Automatic air vents have long been the standard defence, positioned at high points to allow accumulated air to escape quietly and continuously. They are small, cheap (around £10) and their absence is noticeable only when it becomes a problem. Deaerators, meanwhile, represent the next level of protection. Devices such as the IMI Zeparo ZTVI do not merely deal with the large bubbles that eventually collect in predictable places. They strip out micro-bubbles, the dissolved gases that appear under low pressure and high velocity, which cause the fizzing and hammering that homeowners hear and that engineers know will one day destroy a pump. A good deaerator functions regardless of orientation, can be rotated to suit cramped plant rooms and will operate for the lifetime of the system. This is where the argument so often collapses into frustration. Why do some installers continue to resist? The excuses vary. Some genuinely believe that bleeding radiators periodically is sufficient, as though we were still in the era of cast iron boilers thundering away at 80C and masking inefficiencies with brute force. Others are cutting costs, shaving off every component that is not absolutely necessary to get the system running on the day of commissioning. Some simply do not understand the hydronic dynamics of low-temperature systems, where small disruptions have disproportionate consequences. And some, frankly, are arrogant: convinced that they know better than manufacturers, better than decades of practice, better than physics itself. But there is no way around it: low-temperature heating systems are especially vulnerable. The very qualities that make them efficient (constant circulation, low flow temperatures, finely balanced emitter sizing) also make them intolerant of error. Air that might have been masked in an old boiler system will wreak havoc in a heat pump circuit. Radiators and underfloor loops designed for low delta-T cannot afford to lose surface area to air pockets. Pumps that are already working near the lower end of their headroom cannot endure the added burden of cavitation. Efficiency gains of a few percentage points, which matter enormously to the home...

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02/10/2025 12:00 pm

Posted by: @allyfish

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I installed them at the top of a vertical riser pipe, so that they would be out of the flow stream and so air could settle out.

I think that's a key point.

An air-release valve should not only be at a high-point in the pipework, but also positioned clear from any flow.
You want the air to rise up through a bit of pipe before it meets the valve.

I have automatic air-release valves at the far ends of my UFH manifolds.

In my experience they do still purge air into the atmosphere with the cap still in place, provided it isn't screwed down hard.
The plastic cap isn't meant to be 'air-tight'.
Air escaping under pressure is still possible.

Yes, the automatic type can potentially jam.
I've had that happen one in the last 20-years.
And I now keep a spare on-the-shelf.

FWIW I'm in a soft-water area, with no limescale issues.

Save energy... recycle electrons!

👍

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