Leak Detection and Isolation on Direct District Heating

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The DIGI HIU provides direct central heating, with optional leak protection
Schematic of HIU controls

Direct Systems

Direct heating on heat networks is commonplace across Europe , but there is pressure to move in the same direction now in the UK, where we do love our indirect systems. The key reasons are as follows:

  • Direct heating is more efficient - there is a saving of between 5C and 10C on the heat required from plant rooms as one saves an entire heat exchange process.
  • Water quality - possibly the main cause of system inefficiency - is far easier to maintain as one single system maintained centrally rather than potentially hundreds of individual systems each needing to be maintained (testing, flushing, filling, and dosing) locally.
  • It is easier to control, with only one circuit to manage flow through.
  • Simpler and cheaper - no need for pumps, expansion vessels, filling loops, pressure relief valves or discharge pipes.

Why then with so many advantages are direct systems not used more ? The following are the perceived disadvantages of direct heating:

  • Direct systems expose occupants to potentially higher pressures, where a heat network may be up to 10 bar pressure compared to the typical 1.5 bar pressure of a traditional heating system. This has safety implications when it comes to bleeding or servicing radiators, and also impacts on radiator and valve selection.
  • Direct systems expose the heat network to contamination from individual heating systems, and potentially a wider array of materials.
  • Burst or vandalised pipes or radiators can effect the entire network, and not just one property.
  • The volumes and temperatures of water discharged under failure are potentially far greater, as a heat network may contain thousands of litres of water up to 90C.

Overcoming these disadvantages, and enabling greater use of direct systems, is one of our aims in the design of the latest generation of Heat Interface Units. Our goals are as follows:

  • To reliably detect leaks and isolate individual systems from a heat network when there is a problem
  • To automatically isolate system when power is removed, making the servicing of radiators safer.

Modes of Failure

It is helpful to run through the types of system flood and the potential damage, with and without flood protection.

Type of system

Flow Rate Volume
Mains water 30 lpm Unlimited (see note 1)
Closed Loop Heating System 20 lpm (see note 3) 12 litres
Direct Heating 40 lpm (see note 3) 1000 litres (see note 2)
Direct Heating with Limited Flow 2 lpm (see note 3) 1000 litres (see note 2)
Direct Heating with Limited Flow and 10 Minute Detection 2 lpm (see note 3) 20 litres

Note 1: Over 40,000 litres would be discharged in 24 hours at 30 lpm.

Note 2: 1000 litres represents the system volume and expansion volumes.

Note 3: The initial flow rate from any burst or open connection may well be very high as a result of trapped air within radiators. Until such air reaches atmospheric it will push water out of the system. After a few seconds the flow will drop to the match the rate water is supplied into the system.

Electronic HIUs

The range of electronic HIUs for direct heating from Thermal Integration make it possible to detect leaks in systems and thereby shut-off the primary supplies preventing extensive damage, or loss of services to other properties.

Key to the detection of leaks is the on-board pressure and temperature sensing that provides second by second data for rapid intervention in the case of catastrophic failure, as well as the ability to detect the smallest weeks.

The standard spring-return isolating valve used for pre-pay isolation is utilised for protection.

Flood Limitation

Flood conditions can be limited by a fixed orifice on the flow to the heating system sized to peak load conditions and a known minimum pressure drop. A hole of diameter 5mm will loose 15kPa at 4 litres per minute. (Calculator)

A number of other factors will limit the flow rate under failure, such as the size of expansion vessels, pipe sizing, and the presence of air vents. See Further Considerations below.

It is important to understand that, by following a few basic principles, the potential for flooding can be far lower than at first expected.

That said, it is fairly straightforward to limit the flow into a direct heating circuit using a Danfoss RA-DV pressure independent radiator valve, with a peak flow of 125 litres/minute, which equates to 5kW roughly. This flow is enough to heat a modern property, but little more. The flow will remain limited over the full pressure ranges, and is not forced open by a high differential pressure.

Leak Detection using Temperature Data

The following graph shows an HIU cycling on central heating overnight.

It can be seen that when the flow rate drops, the temperature of the primary flow also drops. This is due to heat loss from flow pipework leading up to and inside the HIU, which has more effect the slower the flow.


Flow rates well below 1 litre per minute will still result in a hot primary supply, unless pipework insulation is lacking. Flow rates as low as 5 litres per hour should be detectable.

It should also be able to determine from the rate of temperature drop, when flow rates are know, levels of pipe insulation - or lack of.

Even a small trickle, potentially caused by a leak, will maintain an elevated temperature of the flow, and this can be used for leak detection.

As the position of stepper motors in the HIU are known, a condition where the primary flow temperature remains higher than it should when stepper valves are shut indicated a leak. A flow of heat into the system, without any demand.

This form of leak detection only works under no load conditions, so would not detect a burst radiator when the heating is on, for example. It is also fairly slow, requiring 2 - 5 minutes to monitor temperatures. It does however provide effective leak protection under no-load conditions when the supply is connected - i.e. normal standby conditions.

See also, Flow and return temperatures, and what they can tell us.

Leak Detection using Static Pressure Data

The HIU also provides a reading of static pressure that can be used to identify significant leaks, by a drop in pressure. The pressure sensor is fitted into the central heating flow. Typically a Grundfos RPS combined pressure and temperature sensor is used.

With central heating in flats typically drawing less than 3 litres per minute, any leaks greater than 2 or 3 litres per minute will introduce higher than normal pressure drops across a fixed orifice for regulating flow. Catastrophic leaks - burst pipes or radiators - show up as a significant drop in pressure and can raise an alarm within seconds.

Hydraulic Leak Detection

To avoid nuisance leak detection (exceptional load rather than a leak) it is normal when an abnormal pressure drop is detected under load, or elevated primary flow temperatures under no load, to initiate a hydraulic leak test, isolating the primary supply for a few seconds and watching pressures. Any leaks will show up as a sudden drop in pressure to atmospheric. If pressure is maintained then operation can resume.

Non-Electronic Leak Protection

With a spring return isolating valve it is possible to simply cut power to it when there is low pressure in the system. This can be achieved by a pressure switch, rather than a pressure sensor (requiring electronics). A pressure switch therefore provides a simple alternative or backup method of protection at a fixed pressure point.

Such an approach only protects systems against small leaks when the flow is isolated and pressure can drop to the setting on the switch. Care must be taken that the pressure is not set too sensitively to interfere with normal operation.

Resetting such protection typically requires either a momentary switch to over-ride, or a filling loop to re-pressurise above the switch setting.

This form of protection should not required when full electronic monitoring is deployed, as it only protects against a floods at the point of simultaneous failure of electronics, which is never likely to happen where electronics are part of a monitored and alarmed eco-system. Where there are however systems not connected to remote monitoring, it becomes advisable.

The DIGI Heat Interface Unit

Direct heating with leak protection is enabled using the DIGI HIU in combination with the Data Monitor v1 interface.


Isolation Valve Selection

The Belimo Spring Return Valve provides the perfect solution for the application. The key features that it provides are:

  • Isolation against 16 bar pressure
  • Reliable spring return operation up to 10,000 operations
  • Full bore flow

Closing time is under 70s. With an example peak flood rate of 25 litres per minute this would result in a similar volume discharged before isolation. Such loss will have negligible effect on network operation, and is not far above the volumes that would be discharged from an indirect system under failure.

TRF230 Actuator
R2S Valve Body

Remote Connection

The Data Monitor connects to a remote server by one of the following means:

  • Ethernet
  • WiFi
  • Bluetooth connection to a billing system to act as relay
  • RS245/232 connection to a billing system to act as relay


The dashboard below (live and updating every 30 seconds) provides a working example of the information at our disposal as standard from a Thermal Integration HIU. As well as leak detection, the system is capable of identifying a host of other network related issues that can be tied in through the same automated (open-source) alarm structure. These include:

  • Tamper alarms
  • Component faults
  • Low and high temperature and pressure alarms
  • Differential pressure monitoring to control central pumps
  • Leak detection
  • Pre-pay isolation
  • Confirmation of DHW response times
  • Identification of poorly balanced systems
  • Remote commissioning

Works best in Mozilla Firefox, then Chrome.

See also article on Data Protection.


On detection of a leak:

  • All isolation valves are closed.
  • An email is sent to list of alarm recipients (typically once per day).
  • HIU put into shut-down mode, with slow blinking red LED to user.

Following a shut-down, the system can only be returned to operation by either:

  • Remote command from network operator. Once 1 bar pressure is reached, the system will restart protection. If 1 bar is not achieved within 3 minutes then the system will shut down again.
  • Manual refilling of system to a minimum of 1 bar pressure.

Additional Considerations


Consider the following when selecting radiators:

  • The pressures in the network, including any allowance for hammer. Some feel leaving a bit of air in the top of a radiator helps provide local hammer resilience.
  • Sizing to obtain the lowest achievable return temperatures, within reason.

TRVs and DP valve selection

Consider the following when selecting TRVs:

  • The TRV may be the only point at which compression plumbing fittings are introduced into the system. Be warned - most bursts in pipework happen at a poorly made compression joint. They are avoided by experienced manufacturers in district water lines for a reason.
  • Sizing to the correct differential pressure, temperature drop, flow rate, and radiator output. ALWAYS do the maths.
  • Selection and setting of the DP valve across the heating circuit. Typical ranges are 5-20 kPa (less common but preferable) and 20-60 kPa. Running at lower differential pressures improves TRV control, makes balancing easier, and increases the range available for any given flow.

Decent bleed valves

One of the most dangerous things you can do on a direct system is to use radiator bleed valves where the plug can be fully removed, or it requires insertion into the flow of water. A bleed valve must be fail-safe when opening, limit flow, and must direct the discharge away from the user and minimising splash - i.e down and towards wall.

The discharge point should not be easily movable by the user as we wish to protect against children using the things as a water pistol.

Hydroscopic air vents can manually or automatically remove any air trapped within a radiator or heat emitter both during filling the system and in normal operation. A fibre washer provides the seal when water reaches the vent, but they may need replacement every 3 years due to scale build-up. The cap can however be removed to isolate and prevent tampering or weeping from a scale up washer seal, and the discharge point is fixed in the body so cannot be tampered with.

Once a system has been commissioned, bled, and run for a bit, the simplest way to cut down on venting requirements on radiators is to remove gasses from the system when they are returned to plant, and not to recirculate them out to the network to end up in radiators.

No automatic air vents on flow pipework

Water is in-compressible, and nature 'abhors a vacuum', which is why water only drains properly from a system proved there is a route for air to replace it. Draining domestic hot water or heating systems, for example, is greatly assisted by opening up a high level tap to let air in, or opening the bleed point on a radiator. Its obvious to one when holding your finger on the end of a drink straw, but confidence in the principle often waivers when there is a dangerous volume of hot water at play.

The same principles hold true in a heat network. Water in pipework higher than a point of burst will not automatically drop out of the system - provided there is no path for air entry - i.e. no automatic air vents - the water will stay put, although the static pressure will become negative (once any expansion is system has been discharged).

With the return pipework into properties inherently protected by non-return valves, automatic air vents on the return pipes are not a problem. Air will be cleared through to either radiators and or back int the return pipework, where it can then be vented.

Note that a central circulating pump will however continue to push water into the network, maintaining pressure - and potentially flooding - however the interaction will depend on other factors.

This thinking, however, may also not apply to buildings over 10m in height, where it is possible to obtain negative pressures low enough to start pulling a near vacuum by a leak on the ground floor. Water would in theory boil at the lower pressures and temperatures.

Expansion within the system

Although water is in-compressible, air is not. Indeed, we deliberately build regions of trapped air into systems in the form of expansion vessels to take up expansion of water through the compression of air, or in the form of hammer arresters. The volume of water that floods from a system is closely related to the volumes of compressed gas in a system. The following sources apply:

  • Expansion vessels
  • Air in radiators
  • Bubbles within water flow

Expansion vessels can produce the biggest risk of all, containing possibly a few hundred litres of water with compressed air behind it. Like a capacitor, an expansion vessel can release its stored water in a flash if given the chance, so a useful trick is to limit the size of the connection and pipework to the vessel. Expansion and contraction happens very slowly, so there is never a need for high flow rates in or out of a vessel, and reducing the size of the connection has no adverse effect. When there is a burst elsewhere in the system, the reduced connection simply limits the rate the vessel can supply water. See centralised leak protection below.

Modern expansion systems make less use of air volume, and more use of actively feeding water into and out from a system in response to pressure sensing. They can also include equipment such as vacuum de-gassers to help extract dissolved gasses from the system.

Air in radiators can be negated in relation to flooding network wide by the addition of a non-return valve into the flow pipe also. This will prevent the back-feed of pressure (stored in trapped air in radiators) into the common flow and hence towards a burst. Other measures can also be taken, such as the possible use of hydroscopic air vents (a washer based vent where the washer swells up and seals in the presence of water) but reliability and need for maintenance needs to be considered. Taking the return from the top of the radiator to vent air should also be avoided as it increases return temperatures and effect system efficiency.

Centralised leak detection

This fact that a leak requires a supply of water highlights another form of leak protection. On the assumption there are no automatic air vents or anti-vacuum valves on the network, so water cannot drop towards a leak, and there is little in the way of suspended gases, the only real source of pressure to feed a leak or burst is the expansion vessel.

Under normal conditions, the only water entering or exiting an expansion vessel is down to expansion and contraction of system water. 100kW of NET boiler input results in approximately 0.5 litres per minute of expansion per minute, and the same in reverse with 100kW NET load and 0.5 litres/minute flowing out the vessel. So, it is possible to measure the flow rate exiting an expansion vessel to estimate system load for starters, but also, a significant leak on the network is easily detectable.

Bleeding radiators will also result in water from the vessel entering the system. Such events can be ignored as it is a sustained and steady flow of water we are looking to alarm.

The most suitable method for detecting flow is through the use of a fixed orifice, sized to the peak load of the system, or the boiler input - whichever is the larger. A differential pressure sensor across the orifice provides an indication of flow, while the orifice also serves to limit flood potential.

One must consider however what action to take when a leak is detected. This method just provides a signal that a leak is present. Actions that can then be taken include:

  • Turning off circulating pumps
  • Isolating expansion vessel as the source of further water

The location of the leak is not known however, unless electronic HIUs are used, or additional isolating valves and pressure sensors are introduced around the network to help identify where pressure is lost.

Respect for water quality

The importance of maintaining decent water quality can never be understated.

Where direct heating systems are used, the importance is profound. Every pipe, radiator, and TRV's long-term function is reliant on maintaining water quality at source.

The overall costs of systems are greatly reduced when going direct, both in capital costs (no pumps, discharge pipes etc) and in maintenance (visiting hundreds of systems on a regular basis to check them), however no expense should be spared when it comes to maintaining quality in the plant-room, as well as ongoing monitoring. A good set of liver and kidneys is paramount, but so too is a regular checkup, and preferably with one of those watches that phones the doctor in an emergency.

Direct Heating Checklist

  • Non-return valves on flow into and return out of properties
  • No automatic air vents on flow risers
  • Electronic HIUs with pressure sensing reporting leaks through BMS/billing infrastructure
  • Decent quality radiators, bleed valves, and TRVs
  • Decent quality spring return isolating valves that can shut off against high pressures when leaks are detected
  • A fixed orifice limiting flow into properties heating circuits
  • A fixed orifice limiting the rate of water supply from expansion vessels
  • Smaller expansion vessels, preferably using alternative expansion methods
  • Maintaining decent water quality