Expansion tanks, air elimination and system pressure

On her first service call with Grace, Maya watched a homeowner point at a puddle under a boiler. The relief valve had been dripping for weeks. Grace tapped the expansion tank: it rang hollow at the top and dull at the bottom, a sign that it was full of water. "The relief valve is not the problem," she said. "It is doing its job. The tank has failed, so every time the water heats up it has nowhere to expand, and the pressure climbs until the valve opens." This lesson explains why water expansion must be managed, how expansion tanks are sized and set, how a hydronic system is filled to the right pressure, and how air is removed and kept out.

Water expands when it is heated

Water's density falls as it warms. A closed system filled with 50 °F water and heated to 180 °F grows in volume by about 3 percent. In a system holding 87 gallons, that is about 2.6 gallons of extra water. Water barely compresses, so if that 2.6 gallons has nowhere to go, the pressure rises very quickly, enough to open the relief valve in a few minutes of firing. The expansion tank gives the extra volume a place to go.

Modern expansion tanks are diaphragm or bladder tanks: a steel shell divided by a flexible membrane, with system water on one side and a cushion of compressed air or nitrogen on the other. As the water expands, it pushes against the membrane and compresses the air, and the pressure rises only gradually. Older systems used a plain steel compression tank, a tank hung near the ceiling with air in direct contact with the water. Those tanks work, but the air slowly dissolves into the water, so they must not be paired with automatic air vents, which would remove the air the tank depends on. Diaphragm tanks keep the air separate and are standard in new work.

Fill pressure and the static height

A hydronic system is filled to a pressure high enough to push water to the highest point with some pressure left over, so that air vents at the top work and the water near the top of the system does not drop below atmospheric pressure. Every foot of height takes about 0.433 psi. A common rule is to fill to the static height times 0.433 psi plus about 4 to 5 psi at the top. The bungalow's highest radiator is about 16 ft above the boiler, so it needs 16 × 0.433 + 5 ≈ 12 psi; Grace set the fill at 12 psi.

The expansion tank's air side must be precharged to the same pressure as the cold fill pressure at the tank's location before the tank is installed or with the system drained. If the precharge is lower than the fill pressure, the tank partly fills with water at start-up and has less room left for expansion. Tanks usually ship with a factory precharge of 12 psi, which suits many two-story houses but must be checked with a tire gauge and adjusted for each job.

Tall buildings show how static height dominates. Kingsway's boilers are in the penthouse, so the system's highest point is close to the plant and the fill pressure at the expansion tank is modest. But the fan coils on the lowest floors sit about 150 ft below, where the static pressure is over 64 psi higher. Every valve, coil and fitting low in the building must be rated for that pressure, which is one reason engineers like to put plants at the top of tall buildings rather than in the basement.

Sizing an expansion tank

An expansion tank must be large enough to accept the expansion volume without the pressure rising past a safe maximum below the relief valve setting. The sizing equation uses absolute pressures, the gauge reading plus atmospheric pressure:

Here V_exp is the system volume times the expansion fraction, P_fill is the fill pressure and P_max is the highest pressure allowed at the tank, usually set about 3 psi below the relief valve setting so the relief valve does not weep as the system approaches full temperature.

# Expansion tank size: V_tank = V_exp / (1 - Pfill_abs / Pmax_abs)
def density_c(t):      # water density in kg/m3 from temperature in C
    return 1000 * (1 - (t + 288.9414) / (508929.2 * (t + 68.12963)) * (t - 3.9863) ** 2)
def tank_gal(volume_gal, t_fill_f, t_max_f, p_fill_psi, relief_psi, margin_psi=3, atm=14.7):
    c = lambda f: (f - 32) / 1.8
    v_exp = volume_gal * (density_c(c(t_fill_f)) / density_c(c(t_max_f)) - 1)
    p1, p2 = p_fill_psi + atm, relief_psi - margin_psi + atm
    return v_exp, v_exp / (1 - p1 / p2)

for t_max in (140, 160, 180):
    v_exp, v_tank = tank_gal(87, 50, t_max, 12, 30)
    print(f"max {t_max} F: expansion {v_exp:4.2f} gal, tank at least {v_tank:4.1f} gal")

max 140 F: expansion 1.46 gal, tank at least  4.1 gal
max 160 F: expansion 2.01 gal, tank at least  5.6 gal
max 180 F: expansion 2.64 gal, tank at least  7.3 gal

The script shows how strongly the maximum temperature drives tank size. A heat pump system that never exceeds 140 °F needs a much smaller tank than the same system with a boiler that can reach 180 °F. Systems with glycol need larger tanks again, because glycol mixtures expand more than water; manufacturers publish correction factors.

Where the expansion tank connects

The point where the expansion tank connects to the system is the point of no pressure change. Because the tank holds a cushion of air at a set pressure, and the system's water volume does not change when the circulator starts, the pressure at the tank connection stays the same whether the circulator is running or not. Everywhere else, pressure rises or falls when the circulator starts: it rises downstream of the circulator outlet and falls upstream of its inlet.

That is why the circulator should pump away from the expansion tank. With the tank at the circulator's inlet, the circulator adds its head to the system pressure everywhere in the loop, which keeps air vents pushing air out and keeps pressure well above the boiling point. If the circulator pumps toward the tank instead, its inlet side drops below the fill pressure, and in a system with a low fill pressure the high points can fall below atmospheric pressure, drawing air in through vents and valve packing. The connection to the tank should be a short, unrestricted pipe with no valve that could be closed by accident, or a lockable service valve where the code and manufacturer allow one.

The fill connection

Makeup water enters through a pressure-reducing valve, often called a fill valve, set to the cold fill pressure. The plumbing code requires a backflow preventer on the connection from the potable water system, because boiler water can contain treatment chemicals and corrosion products. A backflow preventer with an intermediate atmospheric vent, listed to the relevant ASSE standard, is common on plain water systems; systems with antifreeze or chemical treatment need a reduced pressure principle assembly, as the Ohio plumbing code and the water purveyor require.

Many installers now leave the fill valve isolated after commissioning, or install a water meter on the fill line, so that a leak shows up as falling pressure or a meter reading instead of being silently made up with fresh water that brings in oxygen and minerals. A system that needs regular makeup has a leak, and the leak should be found.

Air in hydronic systems

Air causes noise, poor heat transfer, corrosion and circulator damage. It enters a system three ways: as pockets trapped during filling, as gases dissolved in the fill water, and through leaks or vents when part of the system falls below atmospheric pressure. Cold water holds more dissolved gas than hot water, so when fresh fill water is heated, the dissolved air comes out of solution as tiny bubbles, especially in the boiler where the water is hottest and the pressure is lowest relative to its temperature.

Air removal works in stages. During filling, the system is purged: each zone is flushed with fast-flowing water through purge valves until the water runs out with no bubbles. Manual bleed valves at high points release trapped pockets. In operation, a microbubble air separator, installed where the water is hottest and the pressure lowest, usually on the supply leaving the heat source, slows the water and gives the tiny bubbles a surface to collect on; they rise to an automatic float vent at the top and escape. Over the first few weeks, the separator removes the dissolved air until the water is nearly air-free.

Velocity also matters. Water moving at least about 2 ft/s carries small bubbles along to the separator; slower water in large horizontal pipes lets them collect at high points. Automatic air vents are useful at high points that cannot be reached easily, but they must be installed upright and are a common source of small leaks, so many installers prefer a good central separator and manual vents at the emitters.

Dirt separators, often combined with air separators and fitted with magnets, catch magnetite and other particles. In retrofits like the bungalow, where decades of sludge come loose when a new system starts, a dirt separator protects the new heat exchangers and is cleaned out several times in the first season.

Filling and purging the bungalow, step by step

Maya filled the bungalow's new system herself, with Grace watching. First she checked the expansion tank's precharge with a tire gauge and adjusted it to the planned fill pressure while the tank was still empty of water. She closed the isolation valves on the boiler and the heat pump so that dirty flushing water could not reach their heat exchangers, and opened every thermostatic radiator valve fully, because a closed valve leaves an unpurged radiator.

She connected a hose from a purge valve on the return to a floor drain, opened the fill valve's fast-fill lever and let water rush through one circuit at a time until it ran clear and free of bubbles, then closed that circuit and moved to the next. Fast flow is the point: slow trickling leaves air in the high spots. With the circuits purged, she opened the heat source valves, purged them the same way, closed the purge valve and let the system come up to the fill pressure.

Next she bled each radiator at its manual vent, starting on the lowest floor and working up, and checked the pressure gauge after each one, topping up as the air came out. She started the circulator, listened for gurgling, and bled again after a few minutes of running. Grace had her record the fill pressure, the time and the water meter reading on the commissioning sheet. Over the next weeks the microbubble separator would remove the rest of the dissolved air, and the first follow-up visit would include another round of bleeding and a check that the pressure had settled where it should.