Expansion tanks and system pressurization

Water expands when it is heated and contracts when it cools. In a closed hydronic loop, that change in volume has to go somewhere, or system pressure will swing between levels high enough to lift relief valves and low enough to draw air in at high points. Expansion tanks and the makeup water system manage that volume change and set the pressure baseline the whole system operates around.

Expansion control is often treated as a minor accessory, but a wrongly sized or wrongly located tank is behind many recurring complaints: air in terminal units, relief valves that weep, pumps that cavitate and glycol systems that lose fluid. This guide explains the principles and the decisions that matter.

Why pressure control matters

A closed loop needs three pressure conditions to be satisfied at all times:

• Minimum pressure at the highest point must be above atmospheric, with margin, so air is not drawn in and automatic air vents can work. A common rule is to maintain at least 30 to 35 kPa (4 to 5 psi) above atmospheric at the top of the system, though designers vary.
• Pressure at the pump suction must exceed the pump's net positive suction head requirement, especially in hot water systems where vapour pressure is higher.
• Maximum pressure anywhere must stay below the relief valve setting and the rating of the weakest component, typically the boiler, chiller barrel, or terminal equipment.

The expansion tank is the one point in the system where pressure is fixed. Everything else floats relative to it as pumps start and stop.

The point of no pressure change

Wherever the expansion tank connects is the point where pump operation does not change pressure. When a pump runs, it adds pressure on its discharge side and reduces pressure on its suction side relative to that point.

For this reason, the tank should normally connect at or near the suction side of the main system pump. The pump then adds pressure to the system when it runs, which helps keep high points positive. If the tank connects on the discharge side, starting the pump lowers pressure throughout much of the system, which can pull air in at high points and cause cavitation.

The air separator is usually located at the same point, upstream of the pump, with the tank connected to it or near it. See air and dirt separation in hydronic systems.

Types of expansion tanks

Diaphragm and bladder tanks dominate in new commercial work because they keep air out of the water. Plain steel tanks still exist in many older Canadian buildings and work if the air control fittings are maintained; converting them to bladder tanks without understanding the system can cause problems.

Sizing an expansion tank

Sizing depends on three things: how much water expands, the pressure range allowed, and the type of tank.

1. Calculate system volume. Add the water content of piping, boilers or chillers, coils, terminal units and buffer tanks. Pipe volume tables and manufacturer data are needed; estimates are a frequent source of error.
2. Determine the temperature range. For heating, use fill temperature (often around 4 to 10 C) to maximum operating temperature. For chilled water, use the full range from minimum to maximum likely temperature, including summer shutdown.
3. Calculate expansion volume. Multiply system volume by the net expansion of the fluid between those temperatures, less the expansion of the piping. Glycol solutions expand more than water, so use fluid-specific data. See glycol systems.
4. Set the fill (minimum) pressure at the tank location: static height above the tank plus the required positive pressure at the top.
5. Set the maximum pressure at the tank location: typically a margin below the relief valve setting, adjusted for elevation and pump effects between the tank and the relief valve.
6. Apply the tank acceptance formula for the tank type. For a diaphragm or bladder tank, the required total volume equals expansion volume divided by an acceptance factor that depends on the ratio of fill to maximum absolute pressure.

The tank's air precharge must be set to match the fill pressure at the tank location before the tank is filled. Precharge left at the factory value is one of the most common commissioning errors.

Tall buildings

In high-rise buildings, static pressure at the bottom of the system may approach equipment pressure ratings. Designers may use pressure-break heat exchangers to split the building into zones, or use active pressurization units that hold a tight pressure band.

Makeup water and fill systems

A pressure-reducing fill valve supplies makeup water to keep system pressure at the fill setpoint. Plumbing codes and the authority having jurisdiction require backflow prevention on the makeup connection; the device type depends on whether the system contains chemicals or glycol, and requirements vary by province and municipality.

For glycol systems, a glycol feed unit with its own reservoir is normally used instead of a direct water connection, so leaks do not dilute the mixture. A makeup water meter is valuable on any closed loop: steady makeup indicates a leak and brings fresh oxygen and minerals into the system.

Relief valves

The relief valve protects the boiler or other pressure vessel. Its setting and capacity are governed by the boiler and pressure vessel code requirements, which are administered provincially (for example, by ABSA in Alberta, Technical Safety BC in British Columbia and TSSA in Ontario), referencing CSA B51. The expansion tank sizing should make sure normal operation never comes close to the relief setting.

Common problems

• Waterlogged tank: failed bladder or lost precharge. Pressure swings widely with temperature and the relief valve weeps on heat-up.
• Tank on pump discharge: air problems at high points and pump noise.
• Precharge not matched to fill pressure: reduces effective acceptance volume.
• Isolation valve closed: tank cut off from the system, often after maintenance. Lockable valves help.
• Undersized tank after expansion: additions to the system without resizing expansion capacity.
• Continuous makeup: hidden leaks bring in oxygen and cause corrosion; see water treatment for closed and open loops.

Frequently asked questions

How do I check if my expansion tank is working?

With the system isolated from the tank and the water side drained, check air precharge with a tire gauge at the tank valve. A tank that has lost precharge or shows water at the air valve likely has a failed bladder.

Do chilled water systems need expansion tanks?

Yes. The temperature range is smaller than in heating systems, but chilled water still expands as it warms during shutdowns, and the tank still sets system pressure.

Key takeaways

• The expansion tank sets the point of no pressure change, so connect it at the pump suction in most systems.
• Size the tank on calculated system volume, fluid-specific expansion and the allowable pressure range.
• Set the air precharge to the fill pressure at the tank location before filling.
• Monitor makeup water: continuous makeup means a leak and accelerated corrosion.
• Verifying pressurization is part of the building commissioning process for any new or modified hydronic system.