VDI 2035: water quality in heat pumps and heating systems
ES
ENInformative and educational content, prepared by Q-Tech from public sources and from the data declared by manufacturers. It is not technical or legal advice. Legal notice and sources at the end of the article.
On 18 June 2025, Q-Tech attended a dedicated training session on VDI 2035 in Granada, at the invitation of Domusa Teknik and Sertegra. It was given by Andoni Catalán of Aquateknika Ingeniería Aplicada (Donostia). VDI 2035 is the German guideline on water in heating systems. This article brings together its values and what they mean for a heat-pump system.
1. What VDI 2035 is
| Key point | What it says |
|---|---|
| Publisher | VDI (Verein Deutscher Ingenieure), the Association of German Engineers |
| Current edition | Part 1, March 2021. It incorporates the former Part 2 (corrosion): scale and corrosion are now covered in a single document |
| Scope | Water-based heating systems to EN 12828, with flow temperatures up to 100 °C |
| Outside its scope | Circuits with antifreeze (glycol) and district heating networks (in Germany, AGFW FW 510). Chilled-water and cooling circuits have their own guideline, VDI 6044 |
| Status | A technical guideline, not a law. Many manufacturers of heat generators and pumps cite it in their manuals and warranty terms |
| In Spain | There is no equivalent standard with mandatory values. There is the report UNE 112076 IN, "Prevention of corrosion in water circuits" |
2. The four water values
The guideline distinguishes two modes of operation: low-salt, the recommended one, and saline, which allows higher conductivity in exchange for much less oxygen.
| Parameter | Low-salt (recommended) | Saline |
|---|---|---|
| Electrical conductivity at 25 °C | Above 10 and up to 100 µS/cm | Above 100 and up to 1,500 µS/cm |
| Dissolved oxygen | Below 0.1 mg/l | Below 0.02 mg/l |
| pH without aluminium | 8.2 to 10.0 | 8.2 to 10.0 |
| pH with aluminium alloys | 8.2 to 9.0 | 8.2 to 9.0 |
| Appearance | Clear, free of sediment | Clear, free of sediment |
| Total hardness | Depends on the system output and litres per kW (section 3) | |
| Item | Reading |
|---|---|
| Fill water | To run low-salt, the system is filled with demineralised water, normally below 10 µS/cm at the unit outlet |
| When to measure pH | 8–12 weeks after filling: the water self-alkalises during that time. Measuring tolerance: ± 0.2 |
| pH below 7.5 | A sign of higher corrosion risk: find the cause |
| Oxygen in tap water | About 9 mg/l at 20 °C in contact with air: some 90 times the low-salt limit |
| Documents older than 2021 | Still in circulation with pH 8.2–8.5 for aluminium and the old hardness table. Check which edition each source cites |
3. How hard the water may be
The maximum hardness depends on two figures: the total output and the specific system volume. This is the total water volume of the system divided by the output of the smallest heat generator, in litres per kW.
| Total heating output | Up to 20 l/kW | Above 20 and up to 40 l/kW | Above 40 l/kW |
|---|---|---|---|
| Up to 50 kW, generator with 0.3 l/kW or more | No requirement | 16.8 °dH (30 °fH) maximum | Below 0.3 °dH |
| Up to 50 kW, generator with less than 0.3 l/kW or with electric heating elements | 16.8 °dH (30 °fH) maximum | 8.4 °dH (15 °fH) maximum | Below 0.3 °dH |
| Above 50 and up to 200 kW | 11.2 °dH (20 °fH) maximum | 5.6 °dH (10 °fH) maximum | Below 0.3 °dH |
| Above 200 and up to 600 kW | 8.4 °dH (15 °fH) maximum | Below 0.3 °dH | Below 0.3 °dH |
| Above 600 kW | Below 0.3 °dH | Below 0.3 °dH | Below 0.3 °dH |
Conversions: 1 mol/m³ = 5.6 °dH = 10 °fH; 0.3 °dH is about 0.5 °fH. In Spain hardness is given in French degrees or in mg/l of CaCO₃ (1 °fH = 10 mg/l).
| Heat-pump example | Volume | Litres per kW | Maximum hardness |
|---|---|---|---|
| 8 kW heat pump, 200 l buffer and 100 l of circuit | 300 l | 37.5 l/kW | 16.8 °dH (30 °fH); with an electric backup heater, 8.4 °dH (15 °fH) |
| The same heat pump with 400 l in total | 400 l | 50 l/kW | Below 0.3 °dH: demineralised water |
| Rule | What it means |
|---|---|
| Changes in 2021 | Full softening moves from 0.11 to 0.3 °dH, and applies from 40 l/kW instead of 50. Requirements become stricter up to 40 l/kW |
| Total water added | The table applies as long as fill and top-up water do not exceed 3 times the system volume |
| Top-up | If more than 10 % of the volume is topped up, find the cause. Hence the water meter on the fill line |
| Hardness times litres | The risk depends on hardness and on the litres added: soft water topped up many times brings in as much lime as hard water added once |
Reading: with a buffer tank and underfloor heating, a domestic heat-pump system easily exceeds 40 l/kW. In that case the guideline calls for demineralised water.
4. Why heat pumps are more sensitive to water
A current system combines narrow water passages and mixed materials: steel, copper, brass, stainless steel, aluminium and plastics.
| Factor | What happens | Reference figure |
|---|---|---|
| Oxygen | It enters with the fill water, with every top-up and by diffusion through plastic pipes, even when the circuit is under pressure | DIN 4726: a barrier pipe lets through no more than 0.32 mg/(m²·day) at 40 °C. A pipe without a barrier, about 50 times more: over 200 days, like changing the water about 100 times |
| Magnetite sludge | It forms only if oxygen gets in (Grundfos). The rotor of permanent-magnet pumps attracts it until they block | First step: find the air inlet or the leak |
| Scale | It insulates heat-exchange surfaces and promotes under-deposit corrosion | Rule of thumb from heat-pump manufacturers (Alpha Innotec): 1 mm of scale, about 10 % less efficiency. Calcium carbonate conducts about 2.2 W/(m·K); copper, around 400 |
| Galvanic corrosion | The less noble metal corrodes when it touches a nobler one and the water carries current | Small anode and large cathode: fast attack. Lower conductivity weakens the "cable", which is the water. A dielectric union breaks the contact |
| Components | Plate heat exchangers, short-stroke valves and flow meters tolerate little dirt | Grundfos recommends water that meets standards such as VDI 2035 |
| Glycol (outside VDI 2035) | As it degrades through heat, oxygen or contact with copper or aluminium, it forms acids and lowers the pH | Industry references: unused inhibited glycol, pH 9.0–9.5; below 8, inhibitor running out; below 7, drain, clean and refill |
Indicative galvanic series, in seawater and in volts against the calomel electrode: magnesium −1.60; zinc −1.03; aluminium alloys −0.79; carbon steel −0.61; copper −0.36; passive 316 stainless steel −0.05. It ranks the metals; it is not for calculation.
5. Softening is not demineralising
| Water softener | Demineralisation (mixed bed) | |
|---|---|---|
| What it removes | Calcium and magnesium, exchanged for sodium | Practically all salts |
| Conductivity | Stays the same or rises | Falls: allows low-salt operation |
| pH | Can exceed 10 in service and damage seals (Alpha Innotec manuals) | Some resins keep it within the guideline range |
| With aluminium | The guideline does not recommend full softening | Compatible, with pH between 8.2 and 9.0 |
| Reading | Solves scale and leaves the system in saline mode, the most demanding on oxygen | Solves scale and conductivity at the same time |
6. Heating water to VDI 2035 in five steps
| Step | What to do |
|---|---|
| 1. Measure and calculate | Hardness and conductivity of the tap water, total volume, litres per kW and circuit materials, especially if there is aluminium |
| 2. Clean | In existing systems: fine filtration and a magnetic separator in bypass, with the heating running, until the water runs clear |
| 3. Fill correctly | Demineralised water to stay below 100 µS/cm, with the pH within the guideline range |
| 4. Keep oxygen out | Barrier pipes, a correctly sized expansion vessel and no uncontrolled automatic filling. A microbubble air vent does not remove dissolved oxygen |
| 5. Monitor and record | Water meter on the fill line and a system logbook. Measure after 8–12 weeks and every year |
Chemical additives remain an exception and are always documented. One example of equipment for these steps is the range from magnetic (Germany), distributed in Spain by Aquateknika. According to the manufacturer, its mobile filter combines a 1 µm filter with a neodymium separator and works in bypass without stopping the heating. Its HWR plus regulator combines a degasser, a filter and a magnesium anode that consumes dissolved oxygen.
7. The manufacturer's manual prevails
VDI 2035 is the reference, but each manufacturer sets its own values. If its manual asks for more, the manual prevails.
| Manufacturer and unit | Hardness | Conductivity | pH | Other |
|---|---|---|---|---|
| Daikin Altherma EHS(X/H) (2021 manual) | Up to 3 mmol/l | Up to 1,500 µS/cm (ideally up to 100) | 6.5 to 8.5 | Chlorides and sulphates up to 250 mg/l |
| LG Therma V (commissioning) | 4 to 8.5 °dH | 10 to 500 µS/cm | 7.5 to 9.0 | Iron up to 0.2 mg/l |
| Wolf (boilers with aluminium) | As per VDI 2035 | Below 100 or 800 µS/cm | 8.2 to 9.0 in mixed systems | No antifreeze or inhibitors |
| Grundfos (pumps) | Refers to standards such as VDI 2035 | |||
Data from each manufacturer's manuals. The ranges do not match each other or the guideline: Daikin's pH of 6.5 to 8.5 falls outside the 8.2–10.0 of VDI 2035.
8. And in Spain
| Situation | Reading |
|---|---|
| Soft tap water | Not enough. Tap water in Málaga city leaves the plant at 95–100 mg/l of CaCO₃ (EMASA, 2021), about 10 °fH, but with hundreds of µS/cm: the system would run in saline mode |
| Hard tap water | Above 30 °fH, a heat pump of up to 50 kW fails the table from 20 l/kW; with electric heating elements, at any volume |
| Top-ups | Every litre of tap water adds lime and oxygen. Without a meter, nobody knows how much water has gone in |
| References | Report UNE 112076 IN and the informative annex on corrosion of UNE-EN 1264-4 (underfloor heating) |
What to do and what to avoid
| Situation | Option | Avoid |
|---|---|---|
| New heat-pump system with buffer tank | Calculate the litres per kW and fill with demineralised water | Filling with tap water without measuring it |
| Existing system with sludge | Clean and filter in bypass before changing the water | Refilling without removing the magnetite |
| Circuit with aluminium | pH between 8.2 and 9.0 | Full softening without pH control |
| Automatic filling | Water meter and a record of top-ups | Topping up without knowing how much or why |
| Circuit with glycol | Follow the glycol manufacturer and measure the pH | Applying VDI 2035, which does not cover it |
| Guideline and manual disagree | Meet the stricter one, normally the manual | A can of "anti-corrosion" added by eye |
Heat pumps with Q-Tech
Q-Tech advises on and integrates the heat pump with the controls, domestic hot water and the other systems in the home. It is also supplied as a pre-assembled plant room. More models in heat-pump systems.
To discuss a project: Q-Tech on WhatsApp or info@q-tech.es.
Legal notice on this content
Purpose. This article summarises, for purely informative, educational and outreach purposes, the requirements of the VDI 2035 guideline on water in heating systems and how they apply to heat pumps. It is based on the VDI 2035 training given by Aquateknika in Granada on 18 June 2025, which Q-Tech attended at the invitation of Domusa Teknik and Sertegra. Values as per the March 2021 edition of VDI 2035.
Origin of the information. The content has been prepared from the public sources listed below, from published summaries of the guideline and from the data declared by manufacturers in their manuals. Q-Tech's calculation examples are indicative.
Scope. It is not technical, legal or professional advice, and it does not replace the official text of VDI 2035, the manufacturer's documentation or the assessment of each system by a competent engineer. Data may change and the content may contain errors or omissions. It does not guarantee the condition or service life of any specific system.
Liability. Before filling, treating or maintaining a system, the requirements must be checked against the current text of the guideline, the manufacturer's manual and a professional. Q-TECH HOUSES DIVISION S.L.UNIP. accepts no liability for decisions taken solely on the basis of this content.
Trademarks and images. The trademarks and trade names mentioned belong to their owners and are used only to identify products and sources. Texts and infographics: © Q-TECH HOUSES DIVISION S.L.UNIP. (q-tech.es). They may be quoted with a link to this page; full reproduction or modification requires prior permission.
Sources consulted
- VDI 2035 Part 1 (March 2021), as summarised by BWT, Grünbeck, SYR, Reflex, Elysator and UWS ("Heizwasseraufbereitung nach VDI 2035").
- Aquateknika Ingeniería Aplicada: VDI 2035 training, Granada, 18 June 2025, at the invitation of Domusa Teknik and Sertegra. magnetic GmbH & Co. KG: "Heating Water Treatment" brochure (2020) and mobile filter and HWR plus data sheets.
- DIN 4726: oxygen-tight plastic pipes. Oxygen solubility tables for water.
- Alpha Innotec: heat-pump manuals (water quality and effect of scale). Grundfos: water-quality recommendations for circulator pumps.
- Daikin: Altherma EHS(X/H) manual (2021). LG: Therma V water requirements. Wolf: heating-water requirements.
- Hydratech and ChemAqua: glycol degradation and pH control.
- EMASA: Málaga water quality (2021). AENOR: UNE 112076 IN and UNE-EN 1264-4.
Deutsche Version lesen
Die deutsche Fassung dieses Artikels finden Sie in der spanischen Ausgabe, Abschnitt „Deutsche Version lesen“.
Leave a comment