Stub Stack with Air Admittance Valve: Solving Mystery Bathroom Drain Smells
When a Hampshire customer called about an intermittent drain smell that came and went without explanation, we diagnosed trap loss caused by an unvented waste system. Here's how we fixed it with a simple, cost-effective solution.
Background video: AAV installation by Will
"There's a smell, but I can't work out where it's coming from"
Sound familiar? This Hampshire homeowner had tried everything - bleach, drain unblockers, even called a drain company to check for blockages. Nothing worked because the problem wasn't in the drain itself. When we investigated, we discovered their drainage system had no existing vent pipe at all - a common issue in older properties and extensions where ventilation was overlooked.
Intermittent drain smell
Smell comes and goes, often after heavy water usage elsewhere in the house
Gurgling sounds
Bubbling or gurgling noises from drains when other fixtures are used
Slow drainage
Water drains slowly, sometimes with air bubbles rising
Empty U-bend smell
Classic sewer smell that indicates the water seal has been lost
The Key Clue
The customer noticed the smell got worse after using the washing machine or running a bath. This pattern told us immediately: we were dealing with trap siphonage caused by negative pressure in the waste system.
What is Trap Loss (Siphonage)?
Every drain in your home has a water trap (also called a U-bend or P-trap). This curved section holds a small amount of water that acts as a seal, preventing sewer gases from entering your home.
✓ Normal Operation
Water in the trap creates an airtight seal. Sewer gases can't get through. Your bathroom smells fresh.
✗ After Siphonage
Water is sucked out of the trap by negative pressure. The seal is broken. Sewer gases flow directly into your bathroom.
Why Does Siphonage Happen?
When water flows down your waste pipes, it creates negative pressure (a partial vacuum) behind it - like when you put your thumb over a straw and lift it out of a drink. If there's not enough air entering the system to equalize the pressure, this vacuum will suck water out of nearby traps.
This is why the smell comes and goes - it only happens when enough water flows through the system to create significant negative pressure. Running a bath, flushing multiple toilets, or using the washing machine can all trigger it.
Why First-Floor Bathrooms Are More Vulnerable
First-floor bathrooms typically have longer waste pipe runs to reach the main soil stack. Longer pipes = more water movement = more negative pressure. Plus, the added height means gravity has more effect when siphonage occurs. This is why trap loss is most common in first-floor en-suites, loft conversions, and bathroom additions.
Stub Stack with Air Admittance Valve (AAV)
After assessing the customer's system, we recommended installing a stub stack with an AAV - the most cost-effective and least disruptive solution for their situation.
1What is a Stub Stack?
A stub stack is simply a short section of vertical pipe that extends up from your waste system. It provides a route for air to enter the system, equalizing pressure and preventing the vacuum that causes siphonage.
- •Typically 110mm diameter (same as soil pipe)
- •Extends vertically from waste branch
- •Must terminate above highest flood level
2What is an AAV (Durgo Valve)?
An Air Admittance Valve sits on top of the stub stack. It's a one-way valve with a simple mechanical diaphragm that opens automatically when negative pressure develops, letting air in to prevent siphonage.
- •Opens: When negative pressure pulls the diaphragm up
- •Closes: When pressure equalizes (gravity + light spring)
- •No batteries, no electricity, no maintenance (mostly)
How This Combination Solves the Problem
Water Flows
When you flush or drain water, it creates negative pressure in the waste pipes
AAV Opens
The negative pressure pulls the AAV's diaphragm open, allowing air to rush in
Traps Protected
Air equalizes the pressure, protecting your U-bends from siphonage. No more smells!
How We Fixed This Bathroom
Here's exactly what the installation involved - a half-day job that transformed this bathroom from smelly to fresh
Remove Vanity Unit to Access Pipework
First, we carefully removed the vanity unit to get clear access to the waste pipework underneath. This revealed the extent of the problem - and an unexpected discovery (see below).
Discovered WC Incorrectly Fitted
When we removed the pan to investigate, we found the WC pan connector had been fitted directly into a pushfit tee without a soil pipe collar. This is incorrect installation - the missing collar could have been contributing to the smell problem. We fixed this while we had access.
Fitted Soil Pipe Collar on Pan Connector
We properly fitted a soil pipe collar on the pan connector to ensure a correct, airtight seal. Even if this wasn't the primary cause, it was important to fix while we had everything apart - belt and braces approach.
Install T-Piece for AAV Connection
With access to the waste pipework, we installed a new T-piece fitting that would allow us to connect the stub stack and AAV. This needs to be positioned correctly - above the flood level of the highest appliance.
Fit Air Admittance Valve
The AAV was fitted to the top of the stub stack. We use quality branded valves (in this case McAlpine) that are Building Regs compliant and will last 20-30 years. The valve was positioned in a ventilated, accessible location.
Box In Pipework for Neat Finish
We don't just leave exposed pipework - that's not how professionals work. The new pipework was neatly boxed in, maintaining access to the AAV for future maintenance while giving a clean, finished appearance.
Test System - Smell Eliminated
After reassembly, we thoroughly tested the system. Running water, flushing the toilet, checking for any signs of the original problem. Result: smell completely eliminated.The customer was delighted.
What Else We Found: The WC Installation Problem
A Common DIY Mistake
When we removed the vanity unit and WC pan to access the pipework, we discovered the toilet had been fitted incorrectly. The pan connector had been pushed directly into a pushfit tee without a soil pipe collar.
✗ What We Found
- • Pan connector fitted directly into pushfit tee
- • No soil pipe collar present
- • Potentially leaky/incomplete seal
- • Could allow sewer gases to escape
✓ How We Fixed It
- • Fitted proper soil pipe collar
- • Ensured airtight seal throughout
- • Reassembled to correct standard
- • Belt and braces approach to the fix
Why this matters: Even if the missing AAV was the primary cause of the smell, the incorrectly fitted WC could have been a contributing factor. By fixing both issues, we eliminated any doubt and ensured the customer wouldn't have ongoing problems. This is why proper diagnosis and thorough workmanship matters.
While We Were There: Heating System Issues
Sometimes one problem reveals others - that's the value of having a professional who looks at the whole picture
The Customer Mentioned: "While you're here..."
During the visit, the customer mentioned they'd been having issues with their Nest heating system - the boiler was activating randomly, and some radiators weren't heating properly. Classic symptoms that pointed to a pressure problem.
What We Diagnosed
- •Filling loop was not connected to mains (unusual)
- •System pressure was at zero
- •Radiators had air trapped in them
- •Towel rail needed adjusting
Quick Fix Applied
- ✓Reconnected filling loop to mains
- ✓Repressurised system to correct level
- ✓Bled all radiators
- ✓Adjusted towel rail valves
Our recommendation: While we got the heating working again, we recommended a full heating service. Zero pressure systems often indicate a slow leak somewhere, and the disconnected filling loop was unusual - worth investigating properly. This shows why it pays to use a plumber who understands the whole house, not just the immediate problem.
The Lesson: Thorough Inspection Pays Off
What started as a "mystery drain smell" visit uncovered three separate issues: the missing AAV (primary cause), an incorrectly fitted WC (contributing factor), and a heating system problem (unrelated but important). This is why we take time to understand the whole picture - multiple problems often coexist, and fixing just one might not solve everything.
Why We Recommended an AAV Over Other Solutions
We always explain all available options so customers can make informed decisions
Full External Vent Stack
£1,500-3,000+
Pros:
- ✓Maximum air supply
- ✓Building regs gold standard
- ✓Works for any system
Cons:
- ✗Expensive (£1,500-3,000+)
- ✗Major disruption
- ✗May need planning permission
- ✗External pipe visible
Best for: Large properties, major renovations
Stub Stack with AAV
£285-450
Pros:
- ✓Cost-effective (£285-450)
- ✓Minimal disruption
- ✓No external work
- ✓Quick installation
Cons:
- ✗AAV needs replacing every 20-30 years
- ✗Must be accessible
- ✗Slightly less airflow than open vent
Best for: Most first-floor bathrooms, retrofits
Larger Diameter Waste Pipes
£500-1,000+
Pros:
- ✓No moving parts
- ✓Permanent solution
Cons:
- ✗May not solve pressure issues
- ✗Expensive retrofit
- ✗Doesn't add air to system
Best for: New installations only
Signs You Might Have the Same Problem
🚨 Warning Signs
- •Intermittent drain smell that comes and goes
- •Gurgling sounds after flushing or draining
- •Smell gets worse after using washing machine
- •Smell appears after running a bath elsewhere
- •Air bubbles rise when other drains are used
🏠 Common Property Types
- •First-floor en-suite bathrooms
- •Loft conversions with new bathrooms
- •Extensions with added bathrooms
- •Properties with long waste pipe runs
- •Older properties with modified plumbing
Important: Not all drain smells are caused by trap loss. Blocked drains, cracked pipes, and failed seals can also cause odours. A proper assessment is needed to diagnose the exact cause before recommending a solution.
Building Regulations & AAV Installation Requirements
Air Admittance Valves have been accepted under Building Regulations Part H in England since 1999. However, they must be installed correctly to be compliant:
✓ Installation Requirements
- 1.Must be accessible for inspection and replacement
- 2.Must be above the flood level of the highest fitting
- 3.Must be in a ventilated space (not in sealed boxes)
- 4.Must be correctly sized for the waste pipe diameter
- 5.Every property still needs at least one open vent to atmosphere
✓ Quality Brands We Use
- •McAlpine - British-made, excellent reliability
- •Floplast - Good value, widely available
- •Marley - Premium quality, long lifespan
- •Durgo - The original AAV brand (now Studor)
We never use unbranded or cheap AAVs - a failed valve is worse than no valve at all.
Frequently Asked Questions
What is an Air Admittance Valve (AAV)?
An AAV, also known as a Durgo valve, is a one-way valve that opens automatically when negative pressure develops in your drainage system (like when you flush a toilet). It lets air into the pipes to prevent siphonage, but closes when pressure equalizes to prevent sewer gases escaping into your home.
How long does an AAV last?
Quality AAVs from brands like McAlpine or Floplast typically last 20-30 years. They have no batteries or electrical components - just a simple mechanical diaphragm. We recommend checking them every few years and replacing if they stick open or closed.
Is an AAV compliant with Building Regulations?
Yes, AAVs are fully compliant with Building Regulations Part H in England when installed correctly. They must be accessible for maintenance, fitted above the flood level of the highest appliance, and in a ventilated space. They've been accepted since 1999 as an alternative to external vent stacks.
Why does trap siphonage happen more in first-floor bathrooms?
First-floor bathrooms often have longer waste pipe runs to reach the soil stack. Longer pipes create more negative pressure when water flows, making siphonage more likely. The height also means gravity has more effect on the water in your U-bend when siphonage occurs.
Can I install an AAV myself?
While AAVs are available at DIY stores, we recommend professional installation. Incorrect positioning (too low, not accessible, or in a non-ventilated space) can cause the valve to fail or not meet Building Regulations. Plus, you need to correctly size the stub stack and ensure all connections are properly sealed.
Mystery Drain Smell in Your Bathroom?
If you're experiencing intermittent drain odours, gurgling sounds, or suspect trap loss, let us diagnose the problem properly and recommend the right solution for your property.
Proper Diagnosis
We identify the actual cause, not just treat symptoms
Clear Pricing
AAV installation from £285, fully explained upfront
Building Regs Compliant
All work meets Part H requirements
Serving Andover (SP10/SP11), Marlborough (SN8) & Hungerford (RG17)
Related Plumbing Guides
More helpful information for Hampshire & Wiltshire homeowners
