What are hydraulic resistances or pressure drop in a heating or DHW system?

✅ 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.

Hydraulic resistance in practice - what mean pressure drops

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.

Main sources of hydraulic resistance

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:

• pipe length and diameter,
• internal roughness of pipes,
• installation components: valves, filters, manifolds, heat exchangers,
• changes in flow direction (e.g., elbows, tees),
• flow velocity.

Shortly, practically every element of HVAC system causes a specific pressure drop value.

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Excessive pressure drops can lead to...

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:

• pump inefficiency,
• reduced performance of the heating or cooling system,
• uneven heating issues (e.g., in underfloor heating).

Hydraulic resistance - pressure drop and heat exchanger selection

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.

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