Why Air Release Valves and Deaerators are Non-Negotiable

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 homeowner’s bills and carbon footprint, are simply sacrificed.

This is why it is impossible to accept the suggestion that air release valves or deaerators are optional. They are not in my opinion. And when installers tell homeowners that they do not need them, they are not only wrong, they are negligent. The cost of retrofitting is always higher, the damage already underway by the time the oversight is corrected. Worse still, the homeowner has often lost confidence in the system entirely, and in the technology that powers it.

In our case, the lesson was hard learnt. We currently live with a system that growls and fizzes, that never truly clears, that reminds us daily of one moment when we failed to insist. It is not fine. It will not fix itself. And it should never have been allowed to happen. So now we’ve had to hire another installer to add an air release valve, which is another disruption and another labour cost.

Demand an air release valve at the very least. Better still, insist on a deaerator. Do not let your installer dismiss the need for one, or wave you away with talk of bleeding radiators. Do not allow them to take something so fundamental and treat it as an optional extra, because the day will come when your system has to be drained and refilled, and without proper air removal you will be right back in the same mess. Air in a heating system is not a minor inconvenience; it is a slow-motion disaster. And it is entirely preventable.