Boiler Air Separation, Pressure & Circulation | Mountain Man Plumbing
Source: https://mtnmanplumbing.com/boiler-repair/air-separation-pressure/
Title: Boiler Air Separation, Pressure & Circulation \| Mountain Man Plumbing
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## On this page
- Separate pressure from movement
- Get trapped air out.Keep collecting the bubbles.
- Pressure, circulation and room to expand.
- “Pumping away”means away from the tank connection.
- Match the flowto the work.
- What does a pump curve actually tell us?
- Can warm floors and radiators share a boiler?
- Why might one small room make the boiler keep restarting?
- One cold room.Several possible reasons.
- The safety valveshouldn’t keep dripping.
- Can you explain it?
- Which water reaches your house?
- Keep following the plumbing.
EXPLORE THE BOILER GUIDE Choose a page below.
- Boiler basics ↗ How it works & why it fails Open page →
- Water & antifreeze ↗ Water care & freeze protection Open page →
- Air & pressure ↗ Noisy pipes & cold rooms You’re here
- Calculators ↗ Heat, flow & antifreeze math Open page →
← BOILER REPAIR GUIDE THE FLOW SIDE / MOUNTAIN MAN PLUMBING
# 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.
- Removing trapped air
- What the valves and tank do
- Pump placement
- Flow & controls
- Why one room stays cold
LESSON 3 OF 4 · NO PREVIOUS TRAINING NEEDED
## Separate pressure from movement
**By the end:** Explain how the loop stays full, circulates and absorbs expanding water.
The fill valve keeps the loop full, the pump circulates water, and the expansion tank accepts the extra volume as water warms.
See the whole learning path ↗
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.
Caleffi: flow, head loss and pump curves ↗
Link to this answer
### Can warm floors and radiators share a boiler?
Yes, with a design that gives each circuit the water temperature and flow it needs. Tubing under a floor may need cooler water than radiators. A mixing valve blends hot water leaving the boiler with cooler water coming back, to give the floor the right temperature. Outdoor reset is a control that adjusts the heating-water target with the weather. Temperature controls and hydraulic separation are selected together for those circuits.
A thick concrete floor stores a lot of heat and takes time to warm. A lightweight floor panel responds faster. The controls need to account for that response and keep the finished floor within its temperature limits.
Link to this answer
### 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.
Caleffi: temperature controls and buffer tanks ↗
Link to this answer
## 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.
Compare other boiler symptoms ↗
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.
- GO DEEPER **Boiler repair ↗** How heating works, why it fails & what service should explain
- GO DEEPER **Water quality & maintenance ↗** Mineral buildup, metal damage, antifreeze & care
**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.
- ← Lesson 2 · Water & antifreeze
- Lesson 4 · Explained calculators →
WARM ROOMS. A SYSTEM YOU UNDERSTAND.
## Let’s find the cause.
Tell us what changed, which rooms are affected, and the boiler’s make and model. A photo of the display and the overall piping helps.
(385) 766-8935 ↗
### Sources & the manuals behind this guide
- CPSC: Triangle Tube / AERCO recall and remedy update
- Triangle Tube: closure, recall and claims notices
- Utah R156-55a: S410 scope (301y) and natural-gas certification (308b)
- Caleffi idronics 18: hydronic water quality
- Caleffi: recurring air, corrosion and troubleshooting
- Caleffi: circulation and piping mistakes
- Amtrol: heating expansion tanks
- Lochinvar: model-specific installation and service manuals
- CPSC: what to do about a CO alarm
Residential hot-water heating guide. Safety and licensing notices checked September 8, 2026. The linked manuals provide the model-specific measurements and service procedures.
From our local guides
## Which water reaches your house?
Provider boundaries and published water-quality records, connected to our softener and filtration guides.
- **Holladay: three water providers ↗**
- **Cottonwood Heights: canyon water and minerals ↗**
- **Highland: hard water at home ↗**
- Find plumbing help in your city ↗
- Explore this topic across our service area ↗
Connected in your home
## Keep following the plumbing.
- 01 **Boiler repair & hydronic heating →** Return to the complete heating-system cutaway and repair guide.
- 02 **Hydronic calculators & learning tools →** Explore how flow, heat and resistance affect circulation.
- 03 **Backflow protection explained →** Understand protection where a drinking-water supply connects to another system.
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