Water Quality in Heat Networks

Protecting System Performance, Asset Life & HIU Reliability – Water is the lifeblood of any heat network. Yet, when poorly managed, it becomes the single greatest threat to system performance, efficiency, and longevity.

From central plant to the smallest Heat Interface Unit (HIU), water quality directly determines reliability, energy efficiency, and lifecycle cost. Failures in water treatment are not isolated issues — they cascade across the entire network.

Why Water Quality Matters in Heat Networks

Heat networks operate as closed-loop systems, where the same water continuously circulates through pipework, plant, and HIUs. While this design improves efficiency, it also means:

  • Contaminants accumulate over time
  • Corrosion products remain trapped in the system
  • Biological activity can develop unchecked
  • Poor commissioning practices persist throughout the lifecycle

BSRIA BG50/2021 identifies water quality as fundamental to achieving “a well-maintained, functional closed water system” .

Without proper control, even a newly commissioned system can begin to deteriorate within months.

The Critical Risks of Poor Water Quality

1. Corrosion – The Primary Threat

Corrosion is the leading cause of failure in heat networks. Modern guidance highlights that:

  • Dissolved oxygen is the most critical driver of corrosion
  • Oxygen ingress often occurs due to:
    • Poor pressurisation control
    • System leaks
    • Inadequate deaeration

Corrosion leads to:

  • Formation of magnetite sludge (black iron oxide)
  • Blocked strainers and HIU plate heat exchangers
  • Premature failure of pumps, valves, and heat exchangers

Once corrosion products form, they cannot be easily removed and continue to circulate indefinitely

2. Sludge & Particulate Contamination

Corrosion and debris form suspended solids that:

  • Accumulate in low-flow areas and HIUs
  • Reduce heat transfer efficiency
  • Increase pumping energy
  • Cause balancing issues across the network

In HIUs specifically, this leads to:

  • Blocked plate heat exchangers
  • Sticking control valves
  • Reduced domestic hot water performance

3. Scale Formation

Untreated fill water introduces hardness minerals such as calcium and magnesium. These:

  • Precipitate under heat
  • Form insulating scale on heat exchange surfaces
  • Significantly reduce thermal efficiency

Even a thin layer of scale can dramatically increase energy consumption.

4. Microbiological Growth & Biofouling

Closed systems are not sterile environments. Bacteria such as:

  • Pseudomonas
  • Sulphate-reducing bacteria (SRB)
  • Nitrite-reducing bacteria (NRB)

The Impact on Heat Interface Units (HIUs)

HIUs are particularly vulnerable because they represent the final point of heat delivery and contain precision components.

Poor Water Quality Causes:

  • Blocked plate heat exchangers → reduced heat transfer
  • Fouled control valves → unstable temperature control
  • Sensor inaccuracies → poor system performance
  • Increased maintenance and call-outs

Ultimately, this results in:

  • Customer complaints
  • Higher operational costs
  • Reduced system efficiency
  • Reputational damage

BG50 & UK Best Practice Guidance

What is BG50?

BSRIA BG50/2021 is the UK’s leading guidance document for water treatment in closed systems. It provides:

  • Scientific principles of corrosion, scale, and biofouling
  • Best practice for design, commissioning, and maintenance
  • Guidance on monitoring and treatment technologies

Although BG50 is not a statutory standard, it is widely regarded as industry best practice for chemical dosing. For chemical free solutions, VDI 2035 should be followed.