✅ Hydraulic resistance in a water installation - e.g., heating or DHW
Hydraulic resistance is a phenomenon that occurs in hydraulic (i.e., water-based) installations, involving the loss of pressure as fluid flows through various system components. Hydraulic resistance is caused by the "friction" of the fluid against pipe walls and changes in flow direction, constrictions, valves, elbows, heat exchangers, tanks, filters, manifolds, heating couplings, and other devices. In short—most equipment or installation components contribute to a gradual pressure drop due to hydraulic resistance.
In practice, hydraulic resistance means that a circulation pump must operate with sufficient power to overcome this resistance and ensure the flow of the medium (most commonly water or a water-glycol mixture) throughout the system. Otherwise, our heating system will stop working efficiently, leading to underheated areas. This is caused by the pressure drop.
Every element, even a simple pipe, can cause some hydraulic resistance, but the primary sources include constrictions or specific devices. They cannot be eliminated if our heating system is to remain functional—hydraulic resistance is an inherent part of fluid flow. The key sources are:
Shortly, practically every element of HVAC system causes a specific pressure drop value.
Of course, installations should be designed to avoid unnecessary constrictions where they are not needed, but also to prevent excessively large, energy-consuming water volumes. Improperly designed installations with too many elbows, directional changes, and constrictions - can result in too high pressure drops. And this can even result in:
A separate issue is the expected pressure drop when selecting certain heat exchangers, especially plate heat exchangers. This involves predicting hydraulic resistance for a given heat exchanger (always at a specific flow rate—since resistance increases with higher fluid flow). In more advanced systems, pressure drop calculations are used to select pumps, but in simpler setups (e.g., single-family homes), such precision is often unnecessary. The need arises in larger facilities.
As a rule of thumb, it's worth knowing that higher hydraulic resistance is generated by undersized heat exchangers relative to the installation. Therefore, oversizing heat exchangers is a good idea to avoid excessive pump load.
BLOG:
Undersizing of a heat exchanger - what does it mean in practice?
Oversizing the heat exchanger - what is it for?
What is the production process of brazed plate heat exchangers?
How to choose a heat exchanger for a gas boiler?
How to properly connect a Nordic Tec plate heat exchanger? Can I reverse the flow directions?
History of plate heat exchangers
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