| Some of the links on my blog are affiliate links. This means that if you click on the link and buy the item, I will receive a commission at no extra cost to you. My opinion remains unaffected. |
In this small article I will introduce you to the heat flow rate, also called heat performance, as another basic and important quantity of heating technology. The heat flow rate is given as 
(Q point).
Table of Contents
Performance P
In order to understand the heat flow, the performance is used as a basis. The performance 
indicates how much work 
was converted in a certain time 
. The unit for the performance is Watt ![[W]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-a09e3af8e250a3ef49f6f1cc633c12e3_l3.png "Rendered by QuickLaTeX.com")
.
-> 
Heat Flow Rate 
In heating technology the heat performance is given as heat flow rate . This means that the heat flow rate indicates how much quantity of heat 
was converted in a certain time . In other words, if 1 joules of heat is converted per second, a heat flow rate of one watt is produced.
The heat flow rate can be calculated using the following formulas:
![\[\dot{Q} = \frac{Q}{t}}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-61ab54f36b5d3966875ed6bd737baab3_l3.png "Rendered by QuickLaTeX.com")
![\[\dot Q=\dot V \cdot \rho \cdot c \cdot \Delta\vartheta}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-f4556d8b6fc37dc2aec74c09a2278632_l3.png "Rendered by QuickLaTeX.com")
Example calculation – Calculate heat flow rate for a radiator
To better understand the topic of heat flow rate, I will show you in the following example calculation how you can apply the formulas. In this example we calculate the output of a radiator.
Task:
In a room with two radiators, a radiator is to be replaced, but the craftsman cannot determine the performance of the old radiators. In old hydraulic balancing documents, he finds the system temperatures of 75/55/20, a room heating load of 900 W and the calculated volumetric flow rates for the radiators of: Radiator 1 = 14.4 l/h and radiator 2 = 26.2 l/h
- To what extent can the room heating load help to determine the radiator output?
- Radiator 1 is to be replaced. What is the radiator output at the given volumetric flow rate?
- Is the calculated radiator output appropriate for radiator 1 and should the craftsman choose a radiator of this size? If not, what is the appropriate radiator output?
Important: For the calculation of the radiator output we need the density of water 
, the specific heat capacity of water 
and the temperature spread 
.
Given:
- Room heating load
= 
- Flow rate radiator 1:
= 
at 75/55/20 - Flow rate radiator 2: =
at 75/55/20 - Temperature spread Delta Theta:
- Specific heat capacity:
- Density:
= 
= 
at 75/55/20
at 75/55/20
Wanted:
- Radiator output for radiators 1 and 2
Solution:
To what extent can the room heating load help to determine the radiator output?
After the calculation, the room heating load helps the craftsman to estimate whether the installed radiators are oversized or undersized. He can then adapt the radiator to be replaced to the real demand.
The reason for this is as follows: many radiators in old and renovated buildings are far oversized. If we now replaced the radiator with the oversized radiator output, we would supply more energy to the room than it needs according to the calculated room heating load.
Calculation of radiator output (heat flow rate) :
To calculate the radiator output we use the following formula:
My book recommendations:
* Affiliate Link - Last updated prices on 2024-09-14 / Picture source: Amazon affiliate program
To understand how the unit Watt results, you can shorten the units of the given sizes below.
![\[\dot Q=\frac{l \cdot kg \cdot Wh \cdot K}{h \cdot l \cdot kg \cdot K} = W}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-0c890a9f3e5965514c20615e5ab26494_l3.png "Rendered by QuickLaTeX.com")
With the given formula we can now calculate the heat flow rate and use the given values.
![\[\dot Q=14,1 \frac{l}{h} \cdot 1\frac{kg}{l} \cdot 1,16\frac{Wh}{kg\cdot K} \cdot 20 K = \underline{\underline{327,12 W}}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-03591f3a1e46d734446cd0eb637346a2_l3.png "Rendered by QuickLaTeX.com")
The radiator output 
for radiator 1 is thus 327.12 W.
Is the calculated radiator output appropriate for radiator 1 and should the craftsman choose a radiator of this size?
To answer this question, we have to calculate the radiator output for radiator 2 because by adding up the calculated output values, we can compare the result with the room heating load. To do this, we insert the given values into the formula:
![\[\dot Q=26,2 \frac{l}{h} \cdot 1\frac{kg}{l} \cdot 1,16\frac{Wh}{kg\cdot K} \cdot 20 K = \underline{\underline{607,84 W}}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-ec8216c47a06ce3462d4d8ca94eec7a0_l3.png "Rendered by QuickLaTeX.com")
The radiator output for radiator 2 is thus 607.84 W.
In total, we would have a total radiator output of:
![\[\dot Q_{ges} = 327.12 W + 607.84 W = \underline{\underline{934.96 W}}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-bd4842fddea3def5d05d5204fdbe0330_l3.png "Rendered by QuickLaTeX.com")
This results in a total radiator output of 934.96 W. The calculated room heating load is 900 W. It is therefore sufficient for the craftsman to install a radiator with an output of 300 W for radiator 1.
Conclusion
I hope I could bring you a little closer to the heat flow rate with these example calculations. If you have questions, suggestions or criticism to this contribution, I’m looking forward to read your comments.
Greetings! Martin
Related links and sources:
Energy and work in heating technology
Wikipedia – Power (physics)
