Expansion Tank
A vessel connected to a closed hydronic loop that absorbs the increase in water volume as it heats, so system pressure stays within its working range.
What an expansion tank does
Water in a heating loop does not occupy the same volume cold as it does at operating temperature: it expands by a few percent as it heats. In a closed circuit that difference matters, because water is essentially incompressible. With nowhere to put the extra volume, every temperature rise would push pressure up to the safety relief valve set point and force a discharge — followed by a shortfall on cooldown, made up by fresh water, and then the same cycle again.
The expansion tank solves this by giving the loop a compressible gas volume to work against. The gas takes up the expansion by compressing slightly, then expands again as the water cools and returns to its original volume. System pressure swings through a narrow band instead of running into the protection.
Compression tanks and bladder tanks
Two designs are in service. A compression tank is a plain vessel where air and water sit in direct contact with no separation. It works, but the air gradually dissolves into the water and migrates into the system: the tank slowly waterlogs, and that dissolved air comes back out somewhere less convenient — high points, circulators, terminal units.
A bladder or diaphragm tank puts a flexible membrane between a precharged air cushion and the system water. The air no longer dissolves, the charge can be checked and corrected, and the usable volume stays predictable over time. This is the usual choice on commercial systems, particularly around cascaded hot water boilers where stable loop pressure shapes how every appliance behaves.
Precharge and sizing
A bladder tank is not interchangeable between systems: its air charge is set to match the static pressure at its connection point, so the membrane begins working as soon as the water expands rather than well after. An over-precharged tank absorbs nothing until pressure has climbed significantly; an under-precharged one fills with water too early and gives up part of its usable volume.
Sizing depends on the water volume of the loop, the span between fill temperature and operating temperature, and the pressure window allowed between fill pressure and the relief valve set point. A loop protected with glycol deserves particular attention: glycol solutions expand more than pure water over the same temperature rise, so a tank sized for water becomes undersized the moment the loop is converted.
What a failed tank reveals
The classic failure is quiet: a relief valve discharging on every heating cycle while cold-fill pressure looks perfectly normal. Replacing the valve in that situation fixes nothing — the valve is doing exactly what it was built to do. The check belongs on the tank’s air charge, with the tank isolated and its water side depressurised; if water comes out of the charging connection, the membrane is punctured and the tank is replaced.
A waterlogged tank costs more than a leaking valve. Repeated discharge-and-makeup cycles continuously introduce fresh water into what should be a closed loop, and with it dissolved oxygen and minerals, with the predictable consequences for corrosion and for scale on heat transfer surfaces. A seemingly minor pressure symptom turns into a water chemistry problem — which is why unusually frequent makeup is always worth explaining rather than topping up.