The room-by-room heat load determines the heat demand of each room and is the basis for sizing the heat generator and heating surfaces. Hydraulic balancing sets the water flow and temperature with which each heating surface transfers this output.
Both calculations concern the same system and are commissioned for different reasons.
Determines the heat demand of each individual room under design conditions. The basis for sizing the heat generator and the heating surfaces, in existing buildings as well as for renovation and new-build planning.
Sets, for each heating surface, the water flow and the temperature with which it can deliver the required output to the room. Required as a mandatory part of the Hamburg programme Wärmeschutz im Gebäudebestand (thermal insulation of existing buildings), when balancing the radiators after a heating replacement, and as part of heating optimisation.
The two services build on each other: the heat load specifies the output to be transferred in each room; the balancing determines the water flow and temperature with which the existing heating surface can actually deliver this output. If the heat load is only roughly estimated, the balancing is only rough as well.
DIN EN 12831
The heat load states the heat output a room needs at the local design temperature. Two things are sized on this basis: the heat generator and the heating surfaces.
With older gas boilers, oversizing was common and had few consequences. A heat pump that is too large, by contrast, cycles on and off, which reduces efficiency and service life. The rated output of the old boiler is therefore no guide.
The second variable is the flow temperature, and it is determined by the least favourable room: a single heating surface that cannot transfer its room's heat load at the planned temperature raises the flow temperature of the whole system. Only the heat load and the balancing together show which surfaces these are. Once they are identified and enlarged, the flow temperature can be lowered. The seasonal performance factor (SPF, German: JAZ) of a heat pump depends on this, as does the return temperature required by the network operator for a district heating connection.
The reason for the calculation is not limited to replacing the heating: in a renovation, the calculation follows the planned condition of the building envelope; in a new build, it is the basis of the entire heat supply. For subsidised heat generators, a room-by-room calculation to DIN EN 12831 is mandatory; simplifications are permitted within the VdZ form.
Primary source: funding guideline for BEG individual measures (BEG EM) of 17 July 2026, Technical Minimum Requirements (Technische Mindestanforderungen) No. 3.1 – Federal Ministry for Economic Affairs and Energy (in German).
VdZ method B
In an unbalanced system, the heating water takes the path of least resistance. Heating surfaces close to the pump receive more water than necessary, distant ones too little, and these cannot deliver their output. This is usually compensated for by a higher flow temperature, with correspondingly higher consumption.
Balancing to method B starts from the room-by-room heat load and determines, for each heating surface, the required water flow and the associated system temperatures. The decisive factor is the output that the existing heating surface can transfer under these conditions. The valve settings follow from this. Method B is the more precise of the two variants and the one required in the funding programmes.
In the Hamburg programme Wärmeschutz im Gebäudebestand run by IFB Hamburg (Hamburg's state investment and development bank), balancing to method B must be carried out and documented, even if it is not funded in the individual case.
The design temperatures change with the new heat generator. The radiators must be rebalanced accordingly so that each room receives its heat load even at a lowered flow temperature.
If the heat generator stays in place, consumption can be reduced through balancing, the heating curve, a pump replacement and lower system temperatures. Balancing is the core of this measure.
If the balancing is owed because of a legal obligation under §§ 60b and 60c of the Building Modernisation Act (GModG, formerly GEG), it is excluded from funding in the Hamburg programme. Whether such an obligation exists must be clarified before the work is commissioned.
Both services are eligible for funding, but through different routes and under different conditions.
| Route | Subject | Conditions |
|---|---|---|
| Condition for heating funding | Balancing as part of the subsidised heating replacement | Not funded separately, but a condition: without balancing and without a room-by-room heat load, the heating funding is not granted. |
| Heating optimisation under the Federal Funding for Efficient Buildings (BEG) | Balancing, heating curve, pump replacement, lowering of system temperatures | Funding rate 15 per cent. Limited to residential buildings with no more than five residential units. The system must be more than two years old, and no more than twenty years old if it uses fossil fuels. |
| IFB Wärmeschutz im Gebäudebestand | Balancing as a mandatory part of the programme | Proportional funding of 75 per cent of eligible expenditure, capped at €3,750 per residential unit. Cannot be combined with other funding and not eligible where a legal obligation exists. |
Primary sources: BEG EM funding guideline of 17 July 2026, Nos. 5.4 and 8.4.1, published by the Federal Ministry for Economic Affairs and Energy (in German); IFB Hamburg: Wärmeschutz im Gebäudebestand (in German). Forms: VdZ: confirmation forms for hydraulic balancing (in German).
Survey of the rooms, building component layers, heating surfaces and the existing building services, including pumps and valves.
Calculation to DIN EN 12831 for each heated room, for renovations based on the planned condition of the building envelope. The sizing of the heat generator and heating surfaces follows from this.
Determination of the required volume flows and the settings for each heating surface, sizing of the pump and heating curve.
Handover of the calculation and the confirmation form to VdZ method B as evidence for the funding body.
The calculation and the adjustment of the system are two separate steps. I provide the calculation and the settings; the adjustment itself is carried out by the heating contractor doing the work.
The heat load answers the question of demand: how much heat output does each room need. The balancing answers the question of transfer: with what water flow and what temperature can the existing heating surface deliver this output to the room. The first calculation is the basis of the second.
If a new heat generator is to be sized, the room-by-room heat load is the starting point and the balancing follows after installation. If the system stays in place and the aim is consumption and comfort, the balancing comes first. In the Hamburg programme Wärmeschutz, balancing is mandatory in any case.
For funding, a room-by-room calculation is specified, although the VdZ form allows simplifications. For a heat pump I advise against it: the sizing depends on the least favourable room, and this room cannot be reliably identified by a rough estimate. The same applies to sizing the heating surfaces, which is done room by room and therefore requires a room-by-room calculation.
For the survey, yes. For rented units, access must be announced in good time; in apartment buildings, arranging appointments often determines the schedule.
The adjustment of the system is carried out by the contractor in any case. The calculation behind it, however, is often shortened, for example by using rule-of-thumb values per square metre. That is not sufficient for funding, and for sizing a heat pump it is technically risky.
For a quote I need details of the building type, number of rooms and residential units, the existing heating system and the reason for the calculation. You will receive a reply within two working days.
