Build a Retained Heater Interface Register for Solar Hot Water Retrofits
A retained heater interface register turns a solar hot water retrofit into a set of explicit plant-room decisions. Each retained component receives one status: Release, Hold, nebo Redesign. Procurement starts only after every mandatory interface reaches Release.
Published by the SOLETKS Technical Content Team, which prepares solar thermal product and service documentation for the SOLETKS service site. Technical content review: Active AI model. Last updated: September 17, 2026.
This worksheet serves commercial domestic hot-water projects that retain part of the existing heating plant. It does not replace the project engineer, the retained equipment manual, or local safety requirements.
Map the retained plant before selecting an interface
Draw one verified circuit from the cold-water entry to the delivery loop. Give every tank, heater, exchanger, pump, valve, sensor, and recirculation branch a unique equipment tag. Record the model and document revision beside each retained item.
The existing plant normally presents one of two planning boundaries: a heater in the domestic-water path, or a separate primary heating circuit that transfers heat through a coil or exchanger. SOLETKS already explains direct and indirect solar heating loops in its project library. This register uses that distinction only to organize the survey; the exact connection still comes from the retained equipment documentation and the project design.
For general interface context, see the CIBSE solar thermal CPD a PNNL solar water-heating overview. Neither establishes compatibility for an unidentified retained heater. The SOLETKS commercial hot-water system page provides product context for the proposed solar side.
Use this first table as the survey index:
| Register field | Evidence entered by the project team | Release condition |
|---|---|---|
| Existing circuit | Marked hydraulic drawing | Every retained component has a tag and flow direction |
| Retained heater | Nameplate and current manual | Model and document revision are legible |
| Solar interface | Proposed connection points | Both sides of the interface are named |
| Operating limits | Manufacturer or design record | Required inlet, pressure, flow, and control conditions are stated |
| Safety boundary | Project safety schedule | Isolation and discharge paths are identified |
| Service continuity | Cutover method statement | Temporary service and rollback owner are named |
If a row lacks its release condition, assign Hold. If the proposed circuit conflicts with an identified limit, assign Redesign.
Classify every interface with one decision rule
An interface row needs a specific object, evidence owner, evidence location, and decision. General notes such as “boiler compatible” or “installer to verify” are incomplete because they do not identify the exact model or the responsible record.
| Interface | Evidence owner | Release | Hold | Redesign |
|---|---|---|---|---|
| Solar store to retained heater | Equipment supplier | Written acceptance for the exact model and arrangement | Acceptance record missing | Arrangement conflicts with a stated limit |
| Collector loop to exchanger | Project engineer | Circuit, fluid, and pressure boundary defined | One input remains unknown | Boundary requires a different exchanger or circuit |
| Recirculation return | Controls designer | Return location and operating sequence documented | Sequence not documented | Return path defeats the intended storage layers |
| Backup heat | Controls designer | Start, stop, priority, and fault response documented | One state has no response | Logic prevents the intended solar-first sequence |
| Cutover isolation | Site manager | Isolation points and rollback path labeled | Label or owner missing | Safe isolation requires a different sequence |
“Not applicable” is a valid result only when the row contains a reason and a named decision owner. It is not a shortcut for missing evidence.
Calculation: is the register ready?
Use a simple release calculation:
Open evidence items = mandatory rows − released rows
Release completion = released rows ÷ mandatory rows × 100%
Illustrative substitution: 8 mandatory rows − 6 released rows = 2 open evidence items; 6 ÷ 8 × 100% = 75% release completion. Source: assumed register counts used only to demonstrate the worksheet arithmetic.
The result is greater than zero, so the procurement decision is Hold. Close both items or redesign the affected interface before equipment release. This is an editorial workflow example, not a field result or equipment limit.
Repeat the calculation after each design revision. The register reaches procurement readiness only when the result equals zero and no row carries Redesign.
Use a second calculation to test the proposed interruption:
Available interface-work time = scheduled shutdown window − isolation allowance − restart allowance
Illustrative substitution: 8 hours − 1 hour − 2 hours = 5 hours available for interface work. Source: assumed schedule inputs used only to demonstrate the method.
If the planned interface work requires 6 hours, the recommendation is Hold because the work is longer than the available window. Source: assumed schedule input used only to demonstrate the release decision.
Define operating states before the shutdown window
Write the expected plant state for four situations:
- Solar input is available and the store is below its target.
- Solar input is unavailable and backup heat serves the load.
- The store reaches its high-limit response.
- A sensor, pump, or communication fault occurs.
For each state, name the active heat source, pump command, valve position, alarm route, and recovery owner. If any state lacks an observable response, assign Hold to the controls row.
Use the site’s commercial solar thermal commissioning checklist to organize the later settings and handover record; the interface register remains the decision record for the retained plant.
The cutover method then becomes a controlled change between two known states. It should include the last safe stop point, the rollback action, and the person who releases the next step.
Build the occupied-building cutover plan
Divide the work into release points instead of one continuous shutdown:
| Release point | Povinný záznam | Release owner | Hold condition |
|---|---|---|---|
| Before procurement | Completed interface register | Project engineer | Any mandatory row remains open |
| Before site work | Method statement and isolation map | Site manager | Isolation or access route is unclear |
| Before interruption | Temporary service and rollback plan | Building operator | Required service cannot be maintained |
| Before refill or restart | Inspection and signed test record | Commissioning lead | Leak, pressure, or control record is incomplete |
| Before handover | Updated drawing, settings, and open-item log | Building operator | Operating documents do not match the installed arrangement |
Schedule the interruption only after the building operator accepts the service plan. If the job stops at a release point, the rollback instruction should return the plant to the last documented operating state.
Prepare the retrofit enquiry package
Send one coordinated package rather than separate photographs and quotations. Include:
- the marked hydraulic drawing;
- retained equipment nameplates and current documents;
- the proposed solar connection points;
- the interface register with Release, Hold, and Redesign decisions;
- available hot-water operating records, labeled as measured or estimated;
- plant-room access, roof route, and interruption constraints;
- the required service-continuity window;
- the decision owner and due date for every open item.
The supplier response should identify the proposed interface, the equipment retained, the equipment changed, the exclusions, and the remaining project inputs. Submit the package through the Kontaktní stránka SOLETKS.
Completion condition
The article’s task is complete when the project has one traceable register, zero open mandatory rows, no unresolved Redesign decision, and an accepted cutover sequence with rollback ownership. Until then, the correct status is Hold.