MIS 3005-D vs MIS 3005-I: who designs, who installs, who signs

MIS 3005 - the Microgeneration Installation Standard for heat pumps in the UK - was split into two documents on 1 April 2022. Before that, it was a single standard covering design and installation together. Today MIS 3005-D handles the design side (heat loss, emitter sizing, DHW, cylinder, performance), and MIS 3005-I handles the installation side (commissioning, testing, sign-off, handover). The current versions in force are the 2025 issues of each.

Most growing UK heat pump installers run their teams as if the split never happened. The same person does the design in one tool, walks out, fits the kit, comes back to the office to fill in the commissioning paperwork - and the data captured on each side has to be reconciled by hand at the end. That works at 5 installs a month. It doesn't work at 20.

This post is for installers in the scaling band - heat pump volume in the 15-50/month range - where the design-to-install data leak is the operational problem that the MIS 3005 split was supposed to make explicit, and where the cost of getting it wrong is starting to show up in MCS audit findings, BUS voucher delays, and customer service load.

What changed in 2022 - and why it matters at scale

Before 1 April 2022, MIS 3005 was one document. The same standard covered designing the system and installing it. The 2022 split into 3005-D and 3005-I formalised what was already practice at the bigger installers: design and install are separate disciplines, with separate evidence requirements, separate sign-off, and (often) separate people doing them.

The operational change for installers running through the split sat in a few specific places:

• Design evidence becomes a distinct deliverable. Under the single-doc standard, the heat-loss calculation, emitter sizing, DHW sizing, weather compensation strategy, and noise assessment lived in a designer's working files. Under 3005-D those become a named design output - the MCS design certificate - that has to be produced, signed, and retained.
• Install sign-off is against the design, not against general best practice. 3005-I requires the commissioning data to match the design assumptions. If the design specified 45°C flow at -2°C outside and the install commissions at 55°C flow, that's not just "the system needs balancing" - it's a deviation from the design that has to be reconciled before sign-off.
• The MCS Installation Database (MID) record stitches them together. Every MCS install registered on the MID after 1 April 2022 references the 3005-D design output and the 3005-I install evidence. Audits draw from both sides.

If your team only thinks of the install side - kit going in, system running, commissioning paperwork at the end - you're missing half of what MCS audits will look at. The single-doc mental model leaks data exactly because it doesn't distinguish what has to be captured upstream versus what has to be captured at install.

Most of the heat pump installers I work with at 15-25 jobs a month are running a 2025 install operation on a pre-2022 paperwork mental model. The split was the regulator naming the operational problem - the installers running tight commissioning are the ones who actually changed the workflow when the standard changed.

MIS 3005-D: what design has to evidence

MIS 3005-D (Design) covers the work that has to happen before the engineer arrives on site. The current 2025 issue specifies the technical evidence required for an MCS design certificate:

• Heat loss calculation to a recognised methodology (BS EN 12831 or MIS 3005 D Appendix), capturing fabric U-values, ventilation losses, and room-by-room peak heat demand
• Hot water demand calculation including occupancy, draw-off patterns, and cylinder sizing (with reference to MCS 026 where applicable)
• System type selection (monobloc vs split, air-source vs ground-source) with justification
• Heat emitter design - radiators sized to design flow temperature, underfloor heating sized to floor area and pipe layout, or a mixed-emitter system with the appropriate sizing logic
• Cylinder selection including coil sizing for the heat pump output and integration with the controls
• Weather compensation strategy - the design flow temperature curve against outside ambient
• Estimated Seasonal Performance Factor (SPF) based on the property, climate zone, and emitter design
• MCS 020 (a) sound assessment at the proposed unit location, against the ≤37 dB(A) Permitted Development noise threshold at the assessment position
• System schematic with flow paths, controls, isolation, and safety devices

The output is the MCS design certificate. The certificate has to be produced, signed by an MCS-certified designer, and retained for audit. Under the single-doc mental model these data points existed - but they often lived as scribbles, spreadsheet tabs, and "in the designer's head". Under 3005-D they have to be a documented, retrievable artefact.

MIS 3005-I: what install has to evidence

MIS 3005-I (Installation) covers everything from the engineer arriving on site through to commissioning, sign-off, and handover. The current 2025 issue specifies the technical evidence required for an MCS installation certificate:

• Installation against the design - kit selection, hydraulic layout, controls, electricals, and emitters as specified
• Refrigerant work sign-off (split installs only) by an F-Gas Category I or II qualified person, with the F-Gas certificate flowing into the install record
• Pressure testing and system filling to spec, with water quality samples recorded
• Commissioning data captured against the design - actual flow temperatures, return temperatures, system flow rate, DHW cylinder performance, legionella pasteurisation cycle, weather compensation settings as commissioned
• Electrical commissioning - isolation, RCD, earth bonding, MD calculation verified against the design assumption
• Sound assessment validated - the install matches the design assumption used for MCS 020 (a), and the engineer signs that off on site
• Homeowner handover - system operation, controls, warranty, maintenance schedule, MCS documentation explained and signed
• Commissioning certificate issued, MID registered within 10 working days

The thing that ties 3005-I to 3005-D is the principle that install evidence is matched against design assumptions. The MCS 020 (a) sound assessment is the cleanest example: the design picks a location and a noise scenario, the install has to validate that the actual installed unit matches those assumptions, and the audit can look at either side. Engineers signing off the install without re-checking against the design assumption is one of the more common audit findings I see in onboarding conversations.

The handoff: where data leaks at 15-50 installs a month

The MIS 3005 split formalised what happens at scale: design and install are different disciplines, often different people. The operational question is how the design output gets into the install record without re-keying.

Across the heat pump installer onboardings I've seen this year, the same three leakage points come up:

1. Heat-loss output to install pack. Designer runs the heat-loss calc in a dedicated tool (often Spruce, sometimes a spreadsheet, occasionally a manufacturer-provided sizing tool). The output is a PDF or printout. Install pack is assembled by admin from the design output - manually. Errors here are silent until commissioning fails to match design. By that point the engineer is on site.

2. Emitter sizing to install instructions. Designer specifies radiator sizes and underfloor heating zone layouts. Install instructions are written or assembled from the design. If the designer changes a radiator spec late and the install pack isn't regenerated, the engineer fits the original spec. Customer call comes in three weeks later: "the bedroom is colder than the lounge."

3. Commissioning data back to design baseline. Engineer commissions the system, captures flow temperatures, weather compensation settings, and runtime data. Without a reference to the design assumption, the engineer has no way to validate "this matches the design" - they're just capturing what they did. The check happens (or doesn't) back at the office when admin assembles the MCS install certificate.

The cost of each leak compounds at volume. At 5 installs a month, the office can sanity-check every job manually. At 20, that's a full-time admin role and rework cost is 2-4 hours per job. The 25+/month installers I work with have solved this not by hiring more admin, but by making the design output a structured input to the install record - so the install pack, commissioning checklist, and MCS certificate are all generated from one underlying data set.

Spruce, in-app heat loss, and the MCS-grade design output

For UK heat pump installers, the realistic design-to-install handoff has two stops:

Sales-stage indicative heat loss. At the sales-and-quote stage, you don't need an MCS-grade heat-loss certificate - you need an indicative number that's good enough to put a credible price and system size in front of the customer. Payaca's in-app heat-loss estimate does that: archetype-based or EPC-derived peak heat load, regional climate data, sCOP-aware. It's not MCS-grade and doesn't claim to be. It's what lets a sales-stage estimate be quoted without booking a designer's time on a job that hasn't been signed yet.

MCS-grade design output via Spruce. Once the job is signed and design work begins, the Project pushes to Spruce. Spruce produces the MCS-grade heat-loss calculation, emitter sizing, schematic, and MCS 020 (a) sound assessment - all to MIS 3005-D, all signed off by an MCS-certified designer. The output syncs back to the Payaca Project as a structured record + PDF artefact, so the design data is available to the install team without anyone re-keying.

McInnes Group, a new heat-pump install business currently moving through Payaca onboarding, is exploring exactly this combination - design done in Spruce, the install side carried by Payaca, with the project record stitching the two together against the 2025 standards. Eaasy Heat - a Growth-tier Payaca customer since April - had "MCS Compliance" as a keyword in scope at their first onboarding session and the heat-pump commissioning workflow front and centre of the onboarding agenda. Both are exactly the design-to-install handoff problem this post is about.

What the Payaca + Spruce combination doesn't do is the MCS-grade design itself. Spruce is the design tool; Payaca is the project, install, and commissioning record. The integration is what makes the design data flow without re-keying, but the design work is still a separate, MCS-certified-designer activity. The blog Spruce sometimes gets paired with "solar design tools" - it isn't one. Spruce is heat-pump only, UK-only, and the MCS standard it serves is MIS 3005-D.

Carrying design data through to commissioning, MID, and handover

Once the design output is in the Payaca Project, the install pack, commissioning checklist, MID registration data, and homeowner handover pack all draw from the same data set. What that looks like in practice:

Install pack - kit list, hydraulic layout, controls schematic, electrical schematic, customer notes, location-specific install instructions. Generated from the design output rather than re-typed by admin.

Mobile app on site - engineer has the install pack on a phone or tablet. Commissioning checklist is part of the same record. Required fields are enforced: flow temperatures, return temperatures, system flow rate, F-Gas certificate (split only), MCS 020 (a) sound check, electrical isolation, pressure test, water quality, weather compensation settings, homeowner handover signatures. Engineer can't sign the install off without each field captured.

Commissioning certificate - generated from the captured fields. Validates against the design assumption (e.g. commissioning flow temp matches design flow temp ±2°C; weather compensation curve matches design curve).

MID registration data - same data set. Admin reviews and submits to the MCS Installation Database, doesn't re-key.

BUS voucher redemption - the MID record is the upstream evidence for redemption submission. Tight design-to-MID flow drops the install-to-redemption window from the 2-3 week typical to under 48 hours.

Homeowner handover pack - MCS commissioning certificate, MID confirmation, O&M manual, warranty docs, weather compensation explainer. Assembled from the captured data and emailed to the customer before the engineer leaves site.

The pattern is one design output flowing into seven downstream artefacts. The operational lift comes from cutting six re-key steps (one per downstream artefact) out of the post-install workflow.

A vocabulary note: "commissioning" means three different things

One thing that consistently confuses scaling install teams: the word "commissioning" gets used for three different things in heat pump operations, and conflating them is a source of audit risk.

Term	What it actually means	Who signs
System commissioning	The engineer's set-up of the heat pump system on site - flow rates, temperatures, weather compensation, DHW cylinder, controls. Captured during install.	MCS-certified install engineer
MIS 3005-I commissioning sign-off	The MCS-recognised statement that the system was installed and commissioned per the design, captured on the MCS commissioning certificate.	MCS-certified install engineer, retained on MID
Customer / contractual commissioning	The customer-facing handover and acceptance - paperwork explained, system demonstrated, warranty activated, contract closed out.	Engineer + customer signatures

All three happen at the end of the install. They share evidence but they're not the same artefact. If your team uses "commissioning" loosely, the symptom is admin asking "did we send the MID certificate?" and getting "yes, the system was commissioned" back - two different questions, two different answers.

Payaca-side, "commissioning" in our product surface refers to the structured data capture step at install (system commissioning + MIS 3005-I sign-off, both rolled into the on-site commissioning checklist). Customer handover is a separate step in the project pipeline. Keeping those two steps distinct in the project record is what stops the three meanings of "commissioning" collapsing back into one ambiguous tick-box.

Three questions to audit against this month

If you're running 15+ heat pump installs a month and haven't worked the design-to-install handoff through, these three questions will surface the leaks:

1. Can your install engineer see the MIS 3005-D design output on their phone on site? If the answer is "they get a PDF emailed" or "the designer told them what to do verbally", that's a re-key risk waiting to bite. Engineers should have the design as structured data they can validate against, not an artefact they re-interpret.
2. What's your average time from commissioning complete to MID registered? If it's more than 5 working days, the design-to-MID data flow isn't tight enough. Under V5 that's a working-capital cost as well as an audit risk - MID gates BUS voucher redemption, and BUS pays you only after redemption is approved (typically 10-15 business days from approval).
3. How many MID entries in the last 3 months had to be edited after submission? Every edit is a sign the data didn't flow cleanly from design through install to MID record. At scale these become MCS audit flags.

If any of these answers are worse than you're comfortable with, the fix isn't more discipline at install. It's making the design output a structured input to the install record, so the right data flows through every downstream document without anyone typing it twice.

The broader playbook for heat pump commissioning under BUS V5 - covering MID registration, voucher redemption sequencing, DNO notification via the ENA HP/EV form, Part L/F and homeowner handover - sits in Heat pump commissioning under BUS V5: a 2026 compliance playbook. The V5 upfront-discount mechanic specifically is covered in BUS V5 and the upfront discount: what changed on 28 April 2026. This post is the design-side companion to both.

Sources: MCS - Microgeneration Installation Standards; MIS 3005-D 2025 issue (Design); MIS 3005-I 2025 issue (Installation); MCS 020 sound assessment standard (September 2025 update).

Related reading: Heat pump commissioning under BUS V5 | BUS V5 and the upfront discount | Heat pump installer economics: what actually drives margin | Heat pump installer solutions