HNTAS compliance – the Heat Network Technical Assurance Scheme explained

What HNTAS is, and where it has got to

The Heat Network Technical Assurance Scheme is the mechanism by which technical standards for heat networks stop being advisory and start being assessed. Until now, the technical quality of a network has been governed largely by voluntary guidance and by whatever the design contract happened to specify. HNTAS replaces that with a formal assurance framework backed by regulation: a defined standard, assessment at set points in a network’s life, and certification that demonstrates the network meets it.

The scheme is built around a new technical standard called TS1, which replaces CIBSE CP1 (2020) as the principal reference point. TS1 covers network design, construction, commissioning, and ongoing operation, and compliance is assessed at key lifecycle stages. The draft standard was published in November 2025. The policy consultation on HNTAS closed in April 2026, and technical work on the draft code documents has continued on published timetables.

At the time of writing the scheme has not launched. It moves the sector toward mandatory technical standards from 2027, and a separate technical feedback process on the draft HNTAS code documents, TS1, and the Metering and Monitoring Standard may or may not still be open. That distinction matters if you intend to comment: check the published DESNZ and industry pages for the current position before you rely on a deadline. BESA has been appointed as the shadow training provider to develop the assessor cohort, which is a reasonable signal of how the assessment side is expected to work in practice.

This is worth stating plainly, because there is a certain amount of noise in the market: nobody can certify your network against HNTAS today, because the scheme is not live. What you can do today is establish where you stand against the published draft, and that is genuinely useful, because the timelines below start from scheme launch and not from the point you get around to looking at it.

Who HNTAS applies to, and when

HNTAS applies to both new and existing networks, but on different timelines. The distinction the scheme draws is the age of the network.

Networks built after 2014 must demonstrate minimum metering, monitoring, and performance within three years of scheme launch, with full certification within six years. Networks built before 2015 are given longer: initial compliance within three years, and full certification within eight years. The additional time recognises that retrofitting an older network to a modern standard is a capital problem, not a paperwork problem.

There is a third category that operators tend to skip over and should not. Networks that cannot cost-effectively meet the standards may need to declare end-of-life status within three years and plan replacement. If you run an older scheme where the economics of bringing it up to standard look doubtful, that is a board-level decision with a three-year clock on it, and it needs to be taken deliberately rather than arrived at by default.

The technical standards HNTAS reaches

HNTAS sets phased requirements across network condition, metering accuracy, water treatment, and supply reliability. Three areas account for most of the work, and each one has a record behind it.

Metering and monitoring

Metering is where the scheme has the sharpest edge, because it is measurable and because a separate Metering and Monitoring Standard sits alongside TS1. This is the same territory the TR1 metering requirements occupy: whether meters are installed where they should be, whether they are accurate, whether they are calibrated, and whether anyone can show it. An operator who bills on metered consumption but cannot evidence meter accuracy has a billing exposure under the authorisation conditions as well as a technical one under HNTAS.

Water quality and TS1 Annex H

Water treatment is the area operators most often underestimate, and it is also where the draft standard is most specific. TS1 sets its water quality requirements in Annex H, and the first thing to establish is which of two system types you are running, because the parameters and their limits differ between them. TS1 distinguishes Chemically Treated Systems, which rely on inhibitor dosing, from Depleted Water Systems, which control corrosion by removing what drives it. A limit that is right for one is wrong for the other, and an operator working to a single set of figures taken from elsewhere may be measuring against the wrong table entirely.

Annex H carries the detail: fill water parameters, the water quality KPIs for each system type, and the minimum sampling frequencies. The KPIs are not one test. They reach conductivity, pH, dissolved oxygen, corrosion rate by material, microbiological counts, dissolved metals and monthly top-up volume, with some parameters carrying control limits and others recorded as reported metrics only. pH in particular moves with the materials installed, and the standard is explicit that manufacturer guidance for those components should be obtained and followed.

TS1 also names the deliverable. Requirement 2.11.6 calls for a Water Quality Recording Programme setting out which parameters are recorded, by what method, at which locations, and how often, along with what happens when a limit is exceeded. That escalation is defined rather than left to judgement: reporting frequency doubles, a competent water treatment specialist is engaged, and remedial action follows until readings are stable and inside their limits. Sampling frequencies step down as a network settles, from fortnightly through pre-commission cleaning to quarterly in steady operation.

On VDI 2035, which is where many UK operators have been working from: TS1 draws on it as a source rather than deferring to it, and the Annex H values come from VDI 2035 alongside BSRIA BG 29/2021 and BG 50/2021. Working to VDI 2035 is not wasted effort, but it is not the same as working to TS1, and the assessment will be against TS1. The draft is published free by DESNZ – read Annex H against your own system rather than assume your current limits carry across.

Whichever table applies, the evidence is a run of dosing and sampling records over time, not a single result. A water sample taken the week before an assessment tells an assessor nothing about the year before it, which is precisely why the record matters more than the reading.

Heat interface units and network condition

Heat interface units sit at the boundary between the network and the consumer, and their performance drives return temperatures, which drives network efficiency. HNTAS reaches network condition and supply reliability generally, and HIU performance is part of that picture. Where a network underperforms, the cause is often at the interface rather than in the plant room.

How to prepare for HNTAS: gap analysis and readiness review

Preparation for HNTAS regulation is not complicated, but it is sequential, and the first step is the one operators skip.

Start with an HNTAS gap analysis. This means taking the published draft standard and working through your network against it, area by area, recording what you can evidence today, what you do but cannot evidence, and what you do not do at all. The middle category is usually the largest and the most surprising. Most operators are treating water and checking meters; far fewer can produce three years of dosing records or a calibration history per meter. A gap analysis is an internal exercise before it is anything else, and you do not need an HNTAS gap analysis provider to begin one – you need the draft standard, someone who knows the network, and an honest afternoon.

Follow it with a readiness review against the timelines rather than the standard. The gap analysis tells you where you are; the readiness review asks whether you can close each gap inside three years of scheme launch, and what it costs if you cannot. This is where the end-of-life question gets asked honestly, and it is far better asked now than in year three.

Then start the record, whatever state the network is in. The three-year milestones require you to demonstrate metering, monitoring, and performance – and demonstration is a function of elapsed time. A dosing record you begin today is three years long when the milestone arrives. One you begin in 2029 is not. It is the single most valuable thing an operator can do about HNTAS before the scheme exists, and it costs nothing but discipline.

If you are also drawing on capital funding, the threads converge. Grant conditions under the Green Heat Network Fund align with authorisation and HNTAS readiness, and the Heat Network Efficiency Scheme exists to support existing networks needing performance improvements – which is the same population HNTAS puts under the most pressure.

Demonstrating compliance with HNTAS

When the scheme launches, demonstrating compliance with HNTAS will mean passing assessment at defined lifecycle stages and holding certification. Before then, and underneath it afterwards, it means holding the evidence an assessor will ask for.

The distinction worth internalising is between the network being compliant and the network being demonstrably compliant. These are not the same, and only one of them survives an assessment. A well-run network with no records fails on evidence. A modest network with a disciplined record has something to show. An assessor cannot assess what you did; they can only assess what you can show you did.

In practice the evidence falls into three records. Network condition and compliance status, tracked as an assessment and action log so that identified actions are closed and the closure is dated. Water quality and chemical dosing, recorded as they happen. Meter calibration and accuracy testing, per meter, over time. None of these is exotic. What defeats operators is not the difficulty of any one record; it is that each is worthless unless it was being kept before anyone asked for it.

How our registers support HNTAS readiness

We build three registers that map onto the areas above. The HNTAS Assessment Register carries the assessment and action log – network condition, compliance status, and the actions arising, with their closure. The VDI 2035 Water Treatment Register carries the dosing and sampling record. The TR1 Metering Compliance Register carries meter calibration and accuracy testing. Each is available on the documents page, individually or in the Complete bundle, and each is yours outright once bought.

They are structured to be ready as the requirements land, rather than claiming to meet a standard that has not launched. That is a deliberate distinction: no document can make a network HNTAS-compliant, because compliance will be assessed against the network, not the paperwork. What a register does is make sure that when the assessment comes, the history exists.

If you would rather keep that evidence as a live record than a set of files, the same registers are maintained in AXIS, where each entry is timestamped as it is made and kept for the life of the record.

Frequently asked questions

What is an HNTAS compliance review?

In the sense operators usually mean it, an HNTAS compliance review is an internal exercise: working through your network against the published draft standard and recording what you can evidence, what you do but cannot evidence, and what is not happening at all. It is not a formal assessment, because the scheme has not launched and no certification against it is available yet. Its value is that it tells you which of the staged milestones are at risk while there is still time to act.

How do you demonstrate compliance with HNTAS?

When the scheme is live, by passing assessment at the defined lifecycle stages and holding certification. Underneath that, by holding the records an assessor asks for: network condition and actions arising, water quality and dosing over time, and meter calibration and accuracy per meter. Demonstration depends on elapsed time, so the records need to exist before the assessment, not be assembled for it.

How should we prepare for HNTAS regulation?

Run a gap analysis against the draft standard, follow it with a readiness review against the three, six and eight year timelines, and start the underlying records now regardless of what the analysis finds. If bringing the network up to standard does not look cost-effective, treat the end-of-life declaration as a live board decision rather than a fallback, because it carries its own three-year clock.

What is TS1, and does HNTAS require build certification?

TS1 is the technical standard HNTAS is built around, replacing CIBSE CP1 (2020) as the principal reference point. It covers design, construction, commissioning, and ongoing operation, and compliance is assessed at key lifecycle stages rather than once at handover – so the construction and commissioning stages of a new build sit inside the assessed scope alongside operation. The draft was published in November 2025; check the published DESNZ and industry pages for the current status of the code documents before relying on any specific requirement.

Does HNTAS require VDI 2035 for water quality?

Not directly. The assessed standard is TS1, which sets its water quality requirements in Annex H. TS1 draws on VDI 2035 as one of its sources, alongside BSRIA BG 29/2021 and BG 50/2021, but it is TS1 an assessor works to. TS1 also splits systems into Chemically Treated Systems and Depleted Water Systems, with different parameters and limits for each, so a single set of figures taken from VDI 2035 will not map cleanly onto either. If you have been working to VDI 2035, the practical step is to read TS1 Annex H against your own system rather than assume the limits transfer.

What is a Water Quality Recording Programme?

It is a document TS1 requires under Requirement 2.11.6. It sets out which water quality parameters will be recorded, by what method, at which locations on the network, and at what frequency – and what happens when a parameter goes outside its control limits. TS1 defines that response rather than leaving it open: reporting frequency doubles, a competent water treatment specialist is engaged, and remedial action is taken until readings are stable and within limits. The programme is the plan; the sampling record is the evidence it was followed.

Do we need an HNTAS gap analysis provider?

Not to start. A first-pass gap analysis needs the published draft standard, someone who knows the network, and an honest look at what you could evidence today. External help is worth buying when the answer is genuinely technical – whether a network can meet the metering standard economically, or whether an older scheme is a replacement case. It is not worth buying to be told you have no water treatment records. You already know that, and the fix is to start keeping them.