Replacing Radiators in a Retrofit Project: How to Size Steel Panel Radiators When the Building Is Already Built

Replacing radiators in an existing building is a fundamentally different engineering problem from specifying them in a new build. The shell is fixed, the pipework is already in the walls, the occupants may still be living there, and — most importantly — the heating system may be running at flow temperatures that bear no relation to the conditions under which the original radiators were rated. Project buyers procuring steel panel radiators for renovation and retrofit schemes across Europe, Russia, and Central Asia regularly encounter all of these constraints at once. This guide addresses the sizing logic, the pipe-centre and connection realities, and the on-site sequencing that experienced project teams work through before a single new emitter is cut in.

Why you cannot size by physical dimensions alone The instinct to replace a radiator like-for-like — same height, same width, same number of panels — is understandable but often wrong. The original emitter was sized for a system running at a specific design flow temperature, typically ΔT50 in older high-temperature systems (mean water temperature around 75 °C, room at 20 °C). If the refurbished building now runs a condensing boiler or district heat connection at lower flow temperatures — say 55/45 °C flow/return — the same physical radiator delivers measurably less heat than it did under its original design conditions. The relationship is non-linear: EN 442 defines a characteristic exponent that governs how output falls as ΔT drops. A Type 22 panel that delivers 1 000 W at ΔT50 might deliver only around 650 W at ΔT30. The correct starting point is the room's actual heat demand at the real design outdoor temperature — calculated from the building's current envelope, not from the old system's installed capacity — and then back-calculating which panel type and size achieves that output at the system's actual operating temperatures. See [EN 442 ΔT50/ΔT30](/en/blog/understanding-en442-dt50-dt30) for the correction factor methodology.

Pipe-centre and connection constraints in existing walls New-build projects enjoy the luxury of specifying pipe centres to suit the chosen radiator. Retrofit projects do not. Existing pipework is embedded in plaster, tiled over, or chased into concrete, and the centres are what they are. Standard steel panel radiators are available with bottom-centre, bottom-left, bottom-right, and side connection configurations, and pipe-centre distances vary across manufacturers. Before selecting panel height, the project team must measure the actual pipe-centre distance (the horizontal distance between the flow and return connections as they emerge from the wall) and confirm that a readily available panel configuration matches it — or budget for pipework modification. Taller panels of the same width occupy more wall height without changing the footprint; where pipe centres are wide or connection stubs are low, a taller Type 22 or 33 panel may be the only way to achieve the required output without altering the distribution pipework. Type 33 panels (three water-carrying panels, three convector fins) offer the highest output per unit of wall width, which matters when the window reveal or furniture layout constrains the available width. Type 22 panels are the standard workhorse for most mid-range retrofit outputs and are the most reliably stocked across the supply chain.

Working around occupants and phased replacement Large retrofit projects — apartment blocks, hotels, institutional buildings — are rarely vacated for the duration of the works. Phased replacement introduces sequencing decisions that affect both the procurement plan and the on-site logistics. The practical approach is to complete replacement room-by-room or zone-by-zone, restoring heat to each zone before moving to the next, and to schedule the most critical rooms (bedrooms, bathrooms) outside the coldest weeks of the heating season where the schedule permits. When existing radiators are drained and removed from a live system, the primary circuit loses water; a filling and pressurization plan must be in place for each phase. If the building's existing radiators include a mix of old cast iron or aluminium sections alongside the steel panels being replaced, the hydraulic balancing of the whole system should be revisited after each phase — differing flow resistances can cause uneven distribution across the newly mixed emitter population.

Pre-season flushing before cutting in new emitters Flushing is non-negotiable when new steel panel radiators are being added to an existing system. Older systems accumulate magnetite sludge, scale, and suspended corrosion products in the pipework and in the old emitters that are staying in the system. Connecting a new steel panel into a dirty circuit without flushing first accelerates internal corrosion of the new emitter — the combination of particulate contamination and oxygen ingress through open connections during replacement work creates exactly the conditions that degrade internal coatings. The correct sequence is: power-flush the existing circuit (or at minimum, drain, flush, and refill with clean inhibited water) before any new emitters are connected; check and adjust inhibitor concentration to the manufacturer's specification; install the new panels; bleed and re-balance. For the technical background on why water chemistry matters for steel radiator longevity, see [protecting steel radiators from corrosion](/en/blog/protecting-steel-radiators-from-corrosion-water-quality). Pre-season — ideally August or September in Northern European climates — is the right window: the system can be fully drained and pressure-tested before the heating season begins, and any connection issues discovered during commissioning can be fixed without disrupting occupants in cold weather.

JD Radiator (Tianjin Jiuding Yangguang HVAC Co., Ltd.) is a family-run manufacturer since 2002 supplying steel panel radiators to project buyers in Europe, Russia, and Central Asia. Our panels are EN 442 compliant with Declarations of Performance available for specification purposes, and GOST compliance is held for Russian-market orders. We supply Type 11, 21, 22, and 33 panels across a full height and width range, with multiple connection configurations to suit retrofit pipe-centre constraints. If you are sizing for a renovation project and want to work through the output-at-actual-temperature calculation against a specific panel range, write to lunan@jdradiator.com / kevin@jdradiator.com or call +86-22-69189950; the full product range is at /en/products and credentials at /en/credentials.