Wet underfloor heating (UFH) systems offer a highly efficient and comfortable way to heat modern and traditional homes alike. Especially in colder climates like Edinburgh, they provide even heat distribution, reduced energy consumption, and a clean, radiator-free aesthetic. However, one of the most common operational problems faced by homeowners and installers is airlocks and flow issues.
If your underfloor heating system isn’t warming up properly, or if only certain zones are affected, there’s a good chance that air or circulation problems are to blame. In this post, we’ll explain how wet UFH systems work, the causes and symptoms of airlocks and flow issues, and how to deal with them—whether you’re a homeowner, landlord, or property manager.
How Wet Underfloor Heating Systems Work
A wet UFH system circulates warm water through a series of flexible pipes embedded in the floor, connected to a central manifold. This manifold is typically split into zones, each with its own control valve and thermostat. Water is heated via a boiler or heat pump and pumped through these loops at a low temperature, delivering radiant heat across the entire floor surface.
The system relies on consistent water flow throughout all pipe circuits. Any disruption—whether from trapped air, sludge, or imbalance—can result in cold patches, reduced efficiency, or complete loss of heat in one or more zones.
What Are Airlocks in UFH Systems?
An airlock occurs when air becomes trapped in the pipework or manifold, preventing water from flowing freely. Because air is lighter than water, it collects in high points of the system and disrupts the loop’s circulation.
Why airlocks are a problem:
- Water cannot circulate through the affected circuit
- Heating in that area becomes patchy or stops altogether
- Pumps may work harder, increasing energy use and reducing lifespan
- Over time, the system may become noisy or over-pressurised
Airlocks are especially common in UFH systems that have recently been installed, drained, or refilled. They’re also more likely to develop in older systems that haven’t been properly serviced or bled.
Symptoms of Airlocks or Flow Problems
If your underfloor heating system isn’t functioning as it should, here are key signs that you may be dealing with an airlock or circulation issue:
- Uneven heating across rooms or specific zones staying cold
- Cold spots on the floor where heat used to be consistent
- Gurgling or bubbling noises in the manifold or pipework
- Low or fluctuating pressure in the UFH circuit
- The pump running constantly without reaching target temperatures
- A single loop not warming up while others are working fine
In multi-zone homes (common in larger Edinburgh properties), airlocks may only affect one section of the house. For instance, your ground floor may heat up perfectly while the kitchen extension remains cold.
What Causes Airlocks and Flow Issues?
There are several reasons why your wet UFH system may develop an airlock or poor circulation:
1. Initial Filling or Commissioning Errors
Air can remain trapped in the system if it wasn’t properly flushed and filled during installation. This is especially common in DIY installations or systems fitted without using proper filling and pressure tools.
2. System Draining or Repairs
Any work involving boiler maintenance, pipework replacement, or pump upgrades can introduce air into the system unless bled thoroughly.
3. Incorrect Pump Settings
If the pump speed or pressure is set too low, it may be unable to push water through long or complex loops—leading to partial flow or stagnation in some circuits.
4. Manifold Imbalance
If flow rates are not balanced correctly between zones, one loop may be starved of circulation while another receives too much.
5. Sludge and Debris
Over time, sludge, scale, or corrosion can restrict water flow, especially in older or poorly maintained systems.
How to Fix Airlocks in Wet UFH Systems
1. Bleed the Affected Circuits
Each UFH manifold typically has manual or automatic air vents to release trapped air. Follow these steps:
- Turn off the heating system and allow it to cool.
- Open the affected loop’s isolation valve.
- Use the bleed valve (or manual vent) to release trapped air until water flows steadily.
- Repeat for each loop, one at a time.
If you are unsure which circuit is affected, start with the coldest zone or check flow meters on the manifold.
2. Use the Filling Loop (Pressure Flushing)
If bleeding doesn’t resolve the issue, a stronger method is to flush individual loops using the filling loop:
- Close all heating zones except the one you want to flush.
- Connect a hose to the filling loop and force clean water through the affected pipe.
- Air and debris will be pushed out through the return side.
- Continue until water runs clear and free of bubbles.
- Repeat for other zones as necessary.
This is best done by a professional, particularly if your boiler setup is complex or your home is fitted with multiple heating circuits.
3. Check and Adjust Flow Meters
Manifolds often include flow meters that regulate how much water is delivered to each circuit. If a flow meter is stuck or set too low, the loop may not heat up properly.
- Locate the manifold and inspect each flow meter’s reading.
- Clean or replace any blocked or faulty meters.
- Adjust the flow settings to ensure balanced distribution.
Flow meters should typically read between 1–3 litres per minute depending on loop length and design.
4. Inspect the Pump
In some cases, the circulation pump may be to blame. Check:
- That the pump is switched on and working
- The speed setting is appropriate for your system size
- The pump isn’t blocked with air or debris
Air-locked pumps may need to be manually vented or replaced if faulty.
How to Prevent Future Airlocks and Flow Issues
1. Schedule Regular Maintenance
Have your underfloor heating system inspected annually by a qualified engineer. This helps prevent sludge build-up, balance the system, and spot emerging problems early.
2. Use an Inhibitor and Magnetic Filter
Adding a corrosion inhibitor to the water and fitting a magnetic filter protects the system from internal degradation that can lead to flow restriction and air pockets.
3. Install Automatic Air Vents
For systems that frequently develop airlocks, automatic air vents at high points in the pipework or manifold can remove air without the need for manual bleeding.
4. Keep System Pressure Steady
Check your boiler or manifold pressure regularly. A drop in pressure can indicate an airlock, while over-pressurising the system may lead to stress and leaks.
When to Call a Professional
If your attempts to bleed the system and adjust the manifold don’t resolve the issue—or if you’re unsure of what to do next—it’s wise to contact a qualified underfloor heating specialist.
In Edinburgh, many homes have older or bespoke heating systems, including those retrofitted into Georgian townhouses or Victorian flats. These may have non-standard configurations, shared boilers, or unusual pipe layouts. A heating engineer familiar with local property types can diagnose and repair the issue quickly and safely.
Estimated Repair Costs (Edinburgh Rates)
| Service | Estimated Cost (incl. VAT) |
| Airlock diagnosis and bleed (1 zone) | £90–£150 |
| Full system flush and rebalance | £250–£450 |
| Pump replacement | £300–£500 |
| Manifold servicing or flow meter repair | £150–£300 |
Prices may vary depending on property size, floor access, and system complexity.
Final Thoughts
Airlocks and flow issues in wet underfloor heating systems are common but often easy to resolve when caught early. Regular maintenance, proper balancing, and system flushing can prevent the majority of problems—saving you money and discomfort in the colder months.
For Edinburgh homeowners, especially those living in older or listed buildings, it’s crucial to understand how your heating system works and when to seek help. With a bit of care and the right support, your UFH system can continue to provide efficient, silent, and comfortable warmth for many years to come.