Water is incompressible. As it heats, it expands. As it cools, it contracts. In a sealed system, that movement has to go somewhere, or pressure builds until something gives.
Expansion tanks and pressurisation systems are essential to the successful operation of any heating or cooling circuit. They exist to do four specific jobs:
- Keep pressure within safe limits everywhere in the system, high enough to avoid vacuum, cavitation, or the system liquid evaporating, and low enough that working pressure is never exceeded
- Stop negative pressure from forming at the highest points of the system, which would otherwise let air get drawn into the pipework
- Absorb the volume changes in the water as it heats and cools
- Hold a water seal so the system doesn’t lose fluid
Without a well-maintained expansion tank, you might see a pressure relief valve discharge for no obvious reason, a system that won’t hold pressure overnight, or water usage that keeps creeping up over time.
Why open tanks fell out of favour (and why closed loops replaced them)
Open-header or feed tanks were an early form of expansion tank mounted above the highest point of the system, using gravity to maintain pressure. They did the job for a while, but there were a few issues.
Among these issues was the water sitting exposed to air, which let oxygen in and drove corrosion. If the water level rose above the mains inlet, there was also a risk of backflow, contaminating the mains water supply. And because pressure was limited by the height of the tank, pumps could suffer from cavitation and fail early. Poorly controlled static pressure could also draw air into the circuit and make circulation inconsistent across a building.
Closed-loop systems solve all of this with a fully sealed heating or cooling circuit. The same water circulates through the pipework and plant, rather than draining away or topping up from the main each time it’s used. The closed seal keeps oxygen out, protecting against corrosion.
Closed loops give far more control over both pressure and water quality. An expansion tank connects to the loop to absorb the water’s expansion and contraction as temperatures fluctuate.
How Automatic Heating introduced closed-loop expansion system to the Australian market.
From the early 1980s, closed-loop expansion technology began to change how heating-water, chilled-water and condenser-water systems were designed and installed across Australia. Automatic Heating Global Pty Ltd was at the forefront of that change, introducing closed-loop expansion systems to the Australian market.
‘There was a lot of education involved,’ says Anthony Plaisted, CEO of Automatic Heating. ‘Consultants and project managers needed to understand why maintaining a sealed expansion system provided many benefits for the installations.’
The aim was to help the industry understand the relationship between:
Sealed Water Systems → Static Pressure → Elimination of Oxygen → Pump Performance → Heat Transfer → System Efficiency → Equipment Life
Terry Plaisted, Director of Automatic Heating, says, ‘The technology has evolved considerably, but the fundamental engineering principle hasn’t changed. We still need to control the pressure, maintain stable circulation, and protect the equipment.’
Today, a commercial system can contain kilometres of pipework, with pumps, boilers, heat pumps, and coils all relying on controlled pressure. That’s why the expansion tank is a critical part of the design, installation and why correct sizing matters.
Sizing your expansion tank, step by step
Expansion tank sizing comes down to several factors, including total system water volume, minimum and maximum operating temperature, and working pressure.
Use our expansion tank sizing calculator here.
Here are the (simplified) steps to calculating the required size of your expansion tank.
Step 1: Estimate your expansion coefficient.
Your expansion coefficient tells you how much water’s volume grows when it’s heated across a given temperature range.
The coefficient is determined by the difference between your system’s cold water temperature (heating off) and its maximum working temperature (see the table below). The wider the temperature swing, the higher the coefficient, and the more room your tank needs to allow for.
| °C | Coefficient |
|---|---|
| 0 | 0.00013 |
| 10 | 0.00025 |
| 20 | 0.00174 |
| 30 | 0.00426 |
| 40 | 0.00782 |
| 50 | 0.01207 |
| 60 | 0.0145 |
| 65 | 0.01704 |
| 70 | 0.0198 |
| 75 | 0.02269 |
| 80 | 0.0258 |
| 85 | 0.02899 |
| 90 | 0.0324 |
| 95 | 0.0396 |
| 100 | 0.04343 |
Left column: the difference between your cold water and maximum working temperatures. Right column: expansion coefficient.
Step 2: Calculate your tank volume using Boyle’s Law
Once we have the coefficient, we calculate the system’s required volume using Boyle’s Law. From there, we select the nearest standard tank size above (never below) the result.
For the full process, pre-charge pressure calculation, or to use our sizing calculator, head to our expansion tank sizing page.
Why water quality inside the closed loop matters
A closed-loop system relies on corrosion-inhibiting chemicals in the circulating water to protect boilers, heat exchangers, valves, and pipework. Every time fresh water gets added to top up the system, those chemicals get diluted.
System leakage is often described as a ‘silent destroyer’, since small losses can go unnoticed for a long time while steadily wearing down the water’s corrosion protection.
Once that protection drops below the level components need, boilers, heat exchangers, valves, and pipework all reach the end of their working life sooner than they should. The Duraflex Smart Refill helps to prevent this.
Where the Duraflex Smart Refill Unit fits in
The Duraflex Smart Refill Unit combines automatic system refilling and backflow prevention, as required under the AS3500 plumbing code, with continuous monitoring of water usage. It tracks litres added per day, flags unusual water loss before it turns into a major leak, and can connect to a building management system for remote visibility.
Fewer litres wasted, longer component life, and fewer surprise callouts. That’s a big win for facility managers seeking equipment longevity and sustainability.
As plant rooms move to heat pumps, variable-speed pumps, and building management systems, stable pressure and well-protected water matter even more.
For the tanks themselves, our Duraflex Joule range covers heating, chilled, and condenser closed circuit applications across a wide range of sizes and pressure ratings.
‘Automatic Heating has spent decades educating the market about the principles of closed-loop expansion,’ says Terry Plaisted. ‘Today, we believe we are in a strong position to take the next generation of expansion technology into the Australian market and continue that work.’
If you’re specifying, installing, or maintaining a system, get in touch with Automatic Heating. We’ll help you calculate the right tank size and choose the right protection for your water.