A descale is not proven by how the flow feels, it is proven by readings compared against benchmarks. Three French texts supply those benchmarks: the order of 30 November 2005 for hot water temperatures, the order of 15 September 2009 for ambient carbon monoxide and for the efficiency assessment at the annual service, and the 3CL-DPE 2021 method for the appliance's conventional efficiency, namely 91 + 3 log Pn on a net calorific value basis for a condensing gas boiler of 70 kW or less built from 2016. No text imposes a descaling interval: what justifies it is the drift recorded between two visits.
Recognising a scaled exchanger from measurements, not symptoms
What scale physically changes inside the exchanger
Calcium carbonate precipitates where the water is hottest and most turbulent, forming a layer whose thermal conductivity is far below that of the metal. The measurable consequence is twofold: the heat transferred for a given temperature difference drops, and the wall temperature on the flue side rises. On the primary circuit the deposit also reduces the flow section, so the flow rate falls at constant circulator speed. All three effects can be measured; none can be guessed.
The readings to take before opening anything
- The flow/return ΔT under stabilised conditions, at the same measuring point and the same setpoint, before and after.
- The cold fill pressure, how it moves under heating, and the flow rate or circulator speed.
- On a gas or oil boiler, the flue gas temperature and the combustion values taken with the analyser.
Add the water hardness measured on site and the hot water setpoint actually programmed. Without those two figures you can neither justify the recommendation nor size the preventive treatment.
Telling scale, sludge and corrosion apart before choosing a product
Scale is pale, hard, non-magnetic, and concentrates on the hot water side. Sludge is black, often magnetic, and points to a fouled heating circuit. Corrosion leaves brownish-red traces and signals a leak in the near term. Acid descaling, power flushing and inhibitor dosing do not address the same causes, and acid on a sludged circuit simply moves the problem. A water softener treats what comes in, not the deposit already formed.
The benchmarks to compare your readings against
The table to keep on the visit sheet
These figures come from texts in force and can be quoted as they stand on a job report or in a reply to a complaint.
| Quantity recorded | Benchmark or threshold | Source |
|---|---|---|
| DHW temperature at the outlet, rooms used for washing | 50 °C maximum | Order of 30 November 2005 |
| DHW temperature at the outlet, other rooms | 60 °C maximum | Order of 30 November 2005 |
| Temperature at every point of the distribution system | 50 °C minimum, excluding final pipes | Order of 30 November 2005 |
| Volume of the final supply pipes | 3 litres maximum | Order of 30 November 2005 |
| Storage of 400 litres or more | 55 °C minimum at the outlet, or a daily raise | Order of 30 November 2005 |
| Ambient CO concentration | below 10 ppm normal, 10 to 50 ppm abnormal, 50 ppm and above immediate shutdown | Order of 15 September 2009, article 3 |
| Reference efficiency, condensing gas boiler ≤ 70 kW, from 2016 | 91 + 3 log Pn on NCV at full load | 3CL-DPE 2021 method, § 13.2.2 |
Why the hot water setpoint cannot simply be lowered
The idea of dropping the setpoint to slow precipitation runs into a public health floor. The order of 30 November 2005 requires a temperature of 50 °C or above at every point of the distribution system, except for the final pipes feeding the outlets, whose volume must be as small as possible and in every case at or below 3 litres. For storage of 400 litres or more, the water must be permanently at 55 °C or above at the outlet of the equipment, or raised to a sufficient temperature at least once every 24 hours. The room to manoeuvre is therefore downstream, at the mixing valve, not at the appliance setpoint.
What the reference efficiency is worth in the conversation
The 3CL-DPE 2021 method tabulates a conventional efficiency by appliance type and age. It does not replace your measurement, but it gives the order of magnitude below which an appliance has clearly dropped off. Setting the combustion efficiency read on the analyser against that reference turns an observation into a quantified gap, and it is that gap, not a photograph of the deposit, that justifies the work on a quote. The measurement method is set out in our article on combustion analysis.
Choosing the method by exchanger type and access
Chemical descaling on circulation
Circulation with a dedicated pump is the answer when the exchanger cannot be removed in a reasonable time, or when the deposit is spread across the whole water path, which is the common case on the hot water side. You isolate the circuit, circulate the compatible product, rinse until the water runs clear and check the pH before refilling. On the aluminium-silicon heat exchangers of condensing boilers, the manufacturer's instructions override any site habit.
The product is chosen against the exchanger metal, never against habit or the price per drum.
| Descaler family | Where it is commonly used | What it must not touch |
|---|---|---|
| Sulfamic acid | Hot water exchangers, cylinder coils, stainless steel and copper | Galvanised steel and zinc-plated parts |
| Phosphoric acid | Plate exchangers and heat bodies, moderate aggressiveness | Light alloys without a suitable inhibitor |
| Citric or lactic acid | Light deposits, sensitive premises, constrained discharge | Nothing prohibitive, but a much longer contact time |
| Hydrochloric acid | Excluded by most boiler manuals | Stainless steel, pitting corrosion risk, and aluminium |
Three rules hold the rest together. The product is chosen from the appliance manual and the safety data sheet, not from the wholesaler's shelf. The temperature and contact time on the technical sheet are maximums, not targets. And rinsing continues until the pH is back to neutral, a value you record and put in the report, otherwise the residual acid keeps working on the metal after you leave.
Strip-down and soak descaling
When the exchanger is accessible (hot water plate exchanger, coil, cylinder heating element) and the fouling is localised, removing it and soaking it gives direct control over contact time and allows mechanical brushing. Reassemble with new gaskets. It is also the right option when you suspect a mixed scale and sludge deposit, because you can see what you are taking out.
Prevention upstream, without overtreating
Filter on the incoming main, sludge trap and magnetic filter on the heating return, bleeding and rinsing at every intervention. In hard water, upstream treatment is justified on the hardness figure you recorded, not on a general rule. Hardness is read in French degrees, and together with the hot water setpoint it is what sets the scaling rate.
| Recorded hardness | Water classification | What you recommend |
|---|---|---|
| Below 8 °f | Soft | No softening, watch corrosion and aggressiveness instead |
| 8 to 15 °f | Slightly hard | No treatment, annual review of the readings |
| 15 to 25 °f | Moderately hard | Filtration and ΔT monitoring, treatment only on drift |
| 25 to 35 °f | Hard | Upstream treatment defensible, especially above a 55 °C setpoint |
| Above 35 °f | Very hard | Upstream treatment, with residual hardness recorded at every visit |
Calcium carbonate precipitation accelerates markedly above 60 °C, which is why a cylinder set to the highest position scales far faster than one held at the minimum sanitary value. On the heating side, corrosion inhibitor is dosed to the real circuit volume and logged again after every top-up.
Running the job and keeping the site safe
Preparation and neutralising the risks
Isolate the appliance, close the flow and return valves, cut the electrical supply and the gas. Partial drain to bring down the level and the pressure, protection for the floor and any sensitive connections. Personal protective equipment is mandatory as soon as an acid is handled: gloves, goggles, ventilation of the room. Check the descaler against the metals and gaskets present, and keep neutralising agent within reach for a splash.
Carrying out the work, rinsing and refilling
Inject the product without exceeding the temperatures and durations on the technical sheet, watch the reaction and stop if the flow rate drops. Rinse until the water runs clear and the pH returns to neutral. Neutralise the effluent according to the safety data sheet before discharge. Refill, bleed, restore the pressure and restart the appliance before you begin the checks.
Checks after the intervention
Hot and then cold, run a tightness check on bleed valves, isolating valves and circulators. Check the flow rate and the absence of air noise. Then repeat exactly the same readings as before the job, under the same load conditions: ΔT, flue gas temperature, combustion values. It is the comparison under identical conditions that constitutes proof, not the absolute value measured afterwards.
Writing the job up and committing to what comes next
What goes on the report and on the invoice
The report has to carry the before and after readings with their measuring conditions, the product used and its safety data sheet, the volume rinsed, the final pH, and the make and reference of the measuring instrument used, which the order of 15 September 2009 requires to be stated on the service certificate. Two or three dated photographs of the exchanger, the filter and the bleed point complete the file. On the invoice, keep the descale separate from the other items: it is a servicing operation and does not belong inside a retrofit lump sum.
What descaling does not fix
Descaling restores heat exchange; it corrects neither a hydraulic imbalance, nor a badly set heating curve, nor an oversized appliance that cycles. If the ΔT is still out of range after rinsing, look at the flow rate and the controls before concluding the exchanger is finished. Announcing a consumption saving on the descale alone, without having checked those three points, invites a complaint you will not be able to document. On the appliance itself, oversizing is settled with a calculation rather than by eye: the output required is worked out to EN 12831-1 from the data recorded in the home.
What you propose next
The visit is the moment to set out the work plan: upstream treatment sized on the hardness you recorded, an annual check with logged readings, power flushing if the heating circuit is loaded. On treatments that work without softening, stick to the tests and published opinions rather than the sales argument, as set out in our article on magnetic anti-scale treatments.
