Quiet pipes.
Heat that gets there.
Water carries heat from the boiler to your rooms, so a pocket of trapped air can leave a room cold by interrupting that flow. The system needs enough pressure to keep its highest pipes full. Once it is filled, the pump keeps the water moving around the loop.
TWO DIFFERENT AIR-REMOVAL JOBS
Get trapped air out.
Keep collecting the bubbles.
Removing big air pockets
Opening a pipe for repair lets air inside. During refilling, the technician directs water through one branch at a time and out through a purge valve, carrying the trapped air with it. That process is called purging.
Collecting small bubbles during use
An air separator catches tiny bubbles as water flows through it and releases them through a vent. It is often near the boiler’s hot outlet: warmer water releases dissolved gas more readily. Separate vents at high points release air collected at those locations.
Recurring air needs investigation. If the same noise or cold area returns, check pressure, leaks, repeated filling and other ways air or oxygen enters the system.
Caleffi: air elimination in hydronic systems ↗HOW THE SYSTEM FILLS AND CIRCULATES
Pressure, circulation and room to expand.
Fill valve: adds water
Adds enough water and pressure to keep the pipes filled, including the highest ones. Static pressure is the pressure present even when the pump is off. A backflow-prevention device protects the drinking supply from heating water flowing backward into it.
Water pump: creates movement
A circulator is the pump that moves heating water around the loop. Water returning down the pipes balances the water going up, so the pump’s work is overcoming resistance in the pipes, valves and heating equipment.
Expansion tank: gives warming water room
Water takes up more space as it warms. The tank accepts that extra volume against a cushion of air. A flexible sheet called a diaphragm separates the air from the water. Tank size depends on total liquid volume, mixture, temperatures and allowed pressures; its starting air pressure must also be set correctly.
How the house affects water pressure
The fill pressure has to keep water at the highest point in the house. A taller system therefore needs more starting pressure. As the water heats, the expansion tank absorbs its extra volume. Comparing the cold and hot gauge readings helps the technician find a fill-valve or expansion-tank problem.
Amtrol: expansion-tank setup · Caleffi: closed-loop circulation and pressure
WHY THE PIPING RELATIONSHIP MATTERS
“Pumping away”
means away from the tank connection.
The pump raises pressure from its inlet to its outlet. The expansion tank’s connection acts as a steady pressure reference. When that connection is near the pump inlet, the pump’s pressure increase helps keep the pipes beyond it properly filled. Installers call this “pumping away” from the tank connection.
What that helps
Keep enough pressure at high points so the pipes stay filled and air removal works properly. A poor arrangement can make an air problem worse when a pump starts. The tank, fill connection, separator and pumps need to be designed together.
How the layout is planned
The layout starts with the flow each boiler and heating branch needs. Where separate pumps serve those branches, their connections must let each pump do its job without starving another branch. The arrangements below show how that is achieved.
Why one pump can affect another
If two pumps share restrictive piping, switching one on can change the other circuit’s flow. A hydraulic separator is a chamber that lets the boiler’s pump and the room-heating pump move water at different rates with less interference. Both sides still share water. A plate heat exchanger has separate water passages divided by metal plates: it passes heat across while keeping the liquids apart. That is useful for an outdoor antifreeze mixture.
Caleffi: tank connection and hydraulic detailing · Common piping errors
WHAT A PROPER PUMP SELECTION STARTS WITH
Match the flow
to the work.
How much water?
First find how much heat the room loses to outdoors. Then choose how much the heating water should cool as it gives that heat to the room. Together, those values tell us the water volume needed each minute. Too little can limit heat delivery; adding more does not increase room heat in the same proportion.
Through how much resistance?
Long or narrow pipes, bends, valves and equipment resist water flow. The pump must deliver the needed gallons each minute while overcoming their combined resistance. Pump charts often express that pressure requirement as “feet of head.” Head expresses pressure as the height of a column of the pumped liquid.
What does a pump curve actually tell us?
A pump curve shows how much water a pump can move against different amounts of resistance. The system’s resistance rises as more water moves through it. Where those two curves meet is the expected working flow, called the operating point. Changing pump speed, closing a zone valve or adding antifreeze moves that point.
Why might one small room make the boiler keep restarting?
Sometimes one thermostat-controlled area, called a zone, needs less heat than the boiler can produce even at its lowest setting. The boiler quickly hits its target, stops, then starts again. A properly sized and controlled buffer tank adds water volume to absorb that surplus and lengthen the cycles.
A buffer tank can be useful during mild weather, when only a small part of the house needs heat. Its added water absorbs the boiler’s output and continues supplying the room between burner cycles.
One cold room.
Several possible reasons.
Follow the heating request from the thermostat to the room. Each step gives the technician a useful place to check.
The thermostat does not request heat
The thermostat, wiring or zone control may not be requesting heat. Check that request before looking for a water-flow problem.
Heat is requested, but water does not move
A closed zone valve, failed pump, trapped air or a blockage can stop the water. The technician checks valve movement and water flow to find where the path is interrupted.
The water moves, but output is weak
Water must be warm enough, enough of it must reach the room, and the floor, radiator or baseboard must be able to release the heat. Covered baseboard heaters can leave a room cold even when the boiler is hot.
The problem changes with other zones
Another area may be taking too much flow, its pump may affect this one, or a control may temporarily give another job priority. Test areas both separately and together to see which change causes the problem.
DON’T NORMALIZE REPEATED WATER LOSS
The safety valve
shouldn’t keep dripping.
The relief valve releases water if pressure becomes excessive. Repeated discharge could come from an expansion tank problem, a fill valve that keeps admitting water, or an internal leak from a separate tap-water system into the heating water. The safety valve itself may also need repair or replacement after the cause is corrected.
Never cap, plug or isolate the relief path. Keep away from hot discharge. A technician should find the pressure source and verify safe operation.
Why the expansion tank needs a specific pressure check
The tank starts with a set air pressure, called its precharge. A technician safely removes water-side pressure for the specified precharge check, then assesses tank size, the flexible internal divider and the piping connection.
See what happens when one number changes. Explore how water carries heat, how antifreeze gets diluted and why valves need pressure. Start with guided examples or enter your own measurements, with explanations in both views. Open the calculators ↗
CHECK YOUR UNDERSTANDING
Can you explain it?
What does “10 feet of head” on a pump chart mean?
Try answering in your own words, then compare your reasoning.
Show the answer & why
Head expresses the pump’s pressure difference as the height of a column of the liquid being pumped. In a filled heating loop, water coming down balances water going up. The pump uses that pressure difference to overcome resistance around the loop.