How Often Should an HIU Be Serviced?

The Recommended HIU Service Interval

All major HIU manufacturers — including Danfoss, SAV, Switch2, Caleffi and Alfa Laval — recommend that their units are serviced once every 12 months. This is not a commercial recommendation designed to generate work for engineers; it reflects the mechanical and thermal demands placed on an HIU in normal residential use.

An HIU runs continuously throughout the heating season and — because it also provides domestic hot water on demand — is effectively always on standby. The plate heat exchanger, strainer, valves and actuator all experience wear and accumulation over the course of a year, and in London's hard water areas, limescale build-up can degrade performance significantly within 12 months.

The annual service interval is also the standard required to maintain manufacturer warranties on most HIU brands. Allowing a service to lapse can void the warranty, leaving you or your client liable for the full cost of any subsequent component failures.

What's Included in an Annual HIU Service

A professional HIU service from Collide Group follows a 30-point checklist. Here is what is included:

The service visit typically takes between 45 minutes and 1.5 hours depending on the condition of the unit. You will receive a written service report detailing all readings, any remedial work carried out, and recommendations for any components that should be monitored or replaced in the coming year.

What Happens If You Don't Service Your HIU?

Skipping the annual HIU service may seem like a cost saving in the short term, but it consistently leads to higher costs over time:

• Reduced efficiency and higher heat charges. A limescaled plate heat exchanger transfers heat less efficiently. The HIU needs to draw more primary energy to deliver the same hot water temperature, which means higher heat consumption — and higher bills for the resident.
• Increased breakdown risk. The strainer, if not cleaned, can become blocked and starve the heat exchanger of primary flow — causing sudden hot water loss. Actuators that are not tested regularly may seize. Problems that could be caught at a service become expensive emergency repairs.
• Shorter unit lifespan. Data from the heat network industry consistently shows that regularly serviced HIUs last significantly longer than neglected ones. A well-maintained unit can be expected to operate reliably for 15–20 years; an unserviced one may fail within 8–10 years.
• Voided manufacturer warranty. Most HIU manufacturers require annual servicing to be carried out and documented as a condition of the warranty. An unserviced unit that fails within the warranty period may not be covered.
• Regulatory non-compliance. For heat network operators, failure to maintain HIUs may constitute non-compliance with the Heat Network (Metering and Billing) Regulations, which require accurate metering and efficient heat delivery.

HIU Service Cost in London

A single annual HIU service from Collide Group starts from £120 per unit. The price may vary depending on:

• The brand and model of HIU (some require more labour-intensive servicing)
• The condition of the unit — if significant descaling is required, this adds time
• Access — units in confined cupboards or requiring removal of cladding take longer

For blocks of flats or properties with multiple HIUs, we offer bulk servicing packages from £95 per unit (minimum 10 units). See our full HIU service cost guide for detailed pricing, or request a bulk servicing proposal for your building.

Who Is Responsible for HIU Servicing?

Responsibility for HIU servicing depends on the tenure and building type:

• Social housing / local authority tenants: The housing association or local authority is responsible for maintaining the HIU and communal heating system. Tenants should report faults and service requirements to their housing provider.
• Private tenants (assured shorthold tenancy): Under the Landlord and Tenant Act 1985, landlords are responsible for maintaining heating and hot water installations. In a property with an HIU, this includes the HIU itself. The landlord should arrange annual servicing.
• Leasehold owner-occupiers: The position varies. In most leasehold blocks, the building owner or managing agent is responsible for the communal system, but the HIU in each flat may be the leaseholder's responsibility — check your lease. Many managing agents arrange block-wide HIU servicing programmes for convenience and compliance.
• Property managers and managing agents: If you manage a block with communal heating, you are likely responsible for arranging annual HIU servicing for all units. A bulk servicing contract with a specialist company like Collide Group is the most efficient and cost-effective way to meet this obligation.