Measurements in Heating and Air Conditioning

Posted on 2026-03-08 | Guides

DN, PN, inches, BTU, Watt, … So many terms we see on the specifications of our devices without fully understanding them. Today, add another string to your bow to make the most of your heating or air conditioning system.
Avoid buying an undersized hydraulic part, a spare part of the wrong size, or even a burner that is unsuitable in terms of energy output.

Inch Connections and DN

In the heating field, it is common to have the same measurement in two different units: inches and DN.
For example, when you select a thermostatic head, the connections may be DN 10 with a diameter of 3/8". DN stands for the Nominal Diameter of a pipe, flange, or fitting. It is measured in millimeters. The EN ISO 6708 standard mandates that the initials "DN" be specified before the nominal diameter expressed.
When the measurement is in inches, a " is placed after the value. It is a measurement used in many fields.

Diameter Nominal Diameter (DN) Inches
5 mm X 10 mm DN 5 1/8"
8 mm X 13 mm DN 8 1/4"
12 mm X 17 mm DN 10 3/8"
15 mm X 21 mm DN 15 1/2"
20 mm X 27 mm DN 20 3/4"
26 mm X 34 mm DN 25 1"
33 mm X 42 mm DN 32 1"1/4
40 mm X 49 mm DN 40 1"1/2
50 mm X 60 mm DN 50 2"
66 mm X 76 mm DN 65 2"1/2
80 mm X 90 mm DN 80 3"
102 mm X 114 mm DN 100 4"
127 mm X 140 mm DN 125 5"
152 mm X 165 mm DN 150 6"

Measurements Indicated for Hydraulic Parts

Pressure reducer, flow detector, sanitary temperature sensor, … Many parts for boilers, water heaters, or even irrigation pumps have specifications related to specific measurement units.
Sometimes, the technical details of a product are expressed with very specific measurements. Let’s list them together:

  • The bar (bar): When we talk about Pressure, it is generally expressed in bars. It is a universal measurement determined by atmospheric pressure. For example, a 7 bar valve can withstand 7 times the pressure similar to that of the atmosphere.
  • The KiloPascal (kPa): 1 bar equals 100 kPa. Thus, a butterfly valve with a maximum pressure of 1000 kPa would withstand a pressure of 10 bar.
  • Water Column Meters (mCE): This particular measurement often appears when determining pumps and circulators. It is considered that 10 mCE equals 1 bar, although to be precise it is 10.2 mCE.
    To understand this unit, imagine a 70-meter high tube. Connect a 7 bar valve at the bottom. Now, fill this long tube with water. The valve holds.
    Add another 10 meters of tube and water, and then disaster strikes, an apocalypse, a real tsunami unleashed by a valve that no longer holds! If I may give you a piece of advice, avoid doing this experiment with a sanitary valve.

1 bar = 10 mCE = 100 kPa

Flow rate is different. It is the amount of liquid that flows over a determined period. Depending on the hydraulic part, the quantity is expressed in liters or cubic meters, over an hour, minute, or second.
This may be the case for a lifting pump , for example, which is generally measured in m³/h.


1 m³/h = 1000 L/h = 16.7 L/min = 0.28 L/sec

Converting Energy Powers

When it comes to an air conditioner or a heat pump, for example, the power of the device can be found in both Watts and BTU (British Thermal Unit) over a unit of time.
This information gives an idea of the thermal value needed when using your heating or air conditioning device.
An air conditioner or boiler with up to about 40 watts is generally used in residential settings, while a thermal installation requiring more energy will be found in commercial or industrial settings.
To convert Watts to BTU/h, simply multiply by 3.41. To convert BTU/h to Watts, multiply by 0.29.
For example, a boiler of 35 W produces 10.15 BTU/h, and an air conditioner rated at 9 BTU/h corresponds to 30 W.


1 W = 3.41 BTU/h and 1 BTU/h = 0.29 W

Now that you have all this information, measurements on our products will hold no secrets for you. To better understand spare parts on the Expert By Net website, check out our Tips section !

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