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In the fifth step of the“Hydronic Balancing DIY” series, we will calculate the volumetric flow for each radiator in the example building. In the previous steps for hydraulic balancing, we recorded all the important data and can now process it.
Tip: You can find the most important basics for calculating the volumetric flow and further example calculations in my article“Volumetric flow and flow velocity“.
Table of Contents
Basics for calculating the volumetric flow
To determine the volumetric flow of a radiator, we only need two variables:
- The radiator output:
in watts (W) from the calculated output from the previous calculations. - The temperature spread at 75/55:
= 20 K
The factor 0.86 is formed from the specific heat capacity and the density for water. Shortening the units results in the unit litres per hour (l/h) for the volumetric flow. The derivation for the unit and the factor can be found in section 1.2 of the article Volumetric flow and flow velocity.
We can now calculate the volumetric flows using the following formula.
![\[\boxed{\dot V= 0.86 \cdot \frac{\dot Q} {\Delta\vartheta}}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-c4ff49c1995e6b4f198a1e26da550b32_l3.png "Rendered by QuickLaTeX.com")
Example calculation for the volumetric flow of radiator no. 1
In this example calculation, I will calculate the volumetric flow for radiator no. 1 in the vestibule of the example building.
Tip: As we also want to save energy when carrying out hydronic balancing in addition to improving the hydraulic conditions, in most cases we will calculate the volumetric flows using the calculated room load instead of the radiator output.
The reason for this is that the majority of radiators in the renovated example building are far oversized (comparison of room load and installed radiator output – see step 4). If we were to calculate with the oversized radiator outputs, we would supply more energy to the rooms than they require. We only use the calculated radiator output for radiators with a comparable output.
Note: When installing a heat pump in an existing building, it is essential to compare the installed radiator output with the room heating load, as a heat pump runs more efficiently at lower system temperatures (e.g. 55/45, 45/35, 35/30). If the room heating load is greater than the radiator output, undersized radiators must be replaced.
Given:
- Space heating load
= 630 W - Radiator output = 781 W at 75/55/22
- Temperature spread: = 20 K
= 630 WCalculation:
![\[\boxed{\dot V= 0.86 \cdot \frac{630 W} {20K} \approx \underline{\underline{27\frac{l}{h}}}}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-c18e129f2c8794c86ff17690f96d5cb5_l3.png "Rendered by QuickLaTeX.com")
For radiator no. 1, the volume flow rate in the 6 m² anteroom is 27 l/h. Table 1 provides an overview of the calculated volume flows for the individual radiators. The heat flows marked in red were used to calculate the volume flows. You can now calculate the volume flows for your individual radiators and enter them in a table.
Overview of the volume flows for the example building
| Room | HK no. | Room load | HK output (75/55/22) | Volume flow |
| Anteroom (ground floor) | 1 | 630 W | 782 W | 27 l/h |
| Toilet (ground floor) | 2 | 90 W | 391 W | 4 l/h |
| Washroom (ground floor) | 3 | 130 W | 281 W | 6 l/h |
| Kitchen (ground floor) | 4 | 610 W | 610 W | 26 l/h |
| Living room (ground floor) | 5 | 915 W* | 1502 W | 39 l/h |
| 6 | 915 W* | 1502 W | 39 l/h | |
| Corridor (ground floor) | 7 | 380 W | 434 W | 16 l/h |
| Bedroom (upper floor) | 8 | 550 W | 1092 W | 24 l/h |
| Bathroom (upper floor) | 9 | 410 W | 758 W | 18 l/h |
| Office (upper floor) | 10 | 990 W | 819 W | 35 l/h |
| Guest (upper floor) | 11 | 670 W | 1020 W | 29 l/h |
| Corridor (upper floor) – NEW | 12 | 680 W | 680 W | 29 l/h |
* In the living room, I divided the room load of 1,830 W between the two radiators (915 W each).
Conclusion
With the calculated volume flows, we can now determine the pre-setting values for the pre-settable radiator valves. I will show you how to do this for various valve manufacturers in the next step of the “Do-it-yourself hydraulic balancing” series. If you have any questions, suggestions or criticism, please use the comments function.
Below you will find an overview of the “Hydronic Balancing DIY” series:
Overview of the series:
- Hydronic Balancing DIY – Example for a detached house
- Hydronic Balancing DIY – Step 1: Fundamentals
- Hydronic Balancing DIY – Step 2: Heating Load Calculation
- Hydronic Balancing DIY – Step 3: Data Recording
- Hydronic Balancing DIY – Step 4: Calculate Radiator Output
- Hydronic Balancing DIY – Underfloor Heating and Floor heating?
- Hydronic Balancing DIY – Step 5: Calculate volumetric flow rate
- Hydronic Balancing DIY – Step 6: Presetting the radiator valves
- Hydronic Balancing DIY – Step 7: Calculate heating pump
Related articles outside the series:
- What is hydronic balancing?
- How do Thermostatic Radiator Valves work?
- Calculation of old radiators in stock
- What does a hydraulic balance cost?
Important: Before you start with the instructions for hydronic balancing, I would like to point out that the working methods described here are based on my personal experience and my personal train of thought. Trying out and implementing the procedures described is entirely at your own risk and responsibility. I do not accept any responsibility. Furthermore, I recommend that you always have the calculated values checked by a specialised company or engineering firm. Because even if the method described here seems simple, calculation errors can always creep in.
Best regards! Martin
Further links and sources:
Volume flow rate and flow velocity
Wikipedia – Volume flow rate
Wikipedia – Flow velocity
Cover picture: I created the cover picture with Midjourney AI.
