Entering water temperature
The temperature of fluid arriving at a heat pump from its ground loop, hydronic circuit, or buffer tank, distinct from the flow temperature the heat pump then delivers outward. Equipment selection sets a target range for it, and capacity and efficiency both depend on where the actual figure falls within that range.
Also called EWT
What is entering water temperature on a heat pump?
Entering water temperature is the temperature of fluid arriving at a heat pump before it does its work, distinct from the flow temperature it delivers outward. Ground loop, buffer tank, and circulator sizing are all built around keeping this figure inside the equipment's rated range.
Entering water temperature: how it works
A ground source heat pump's capacity and efficiency are both published against entering water temperature, because that figure sets how much heat the refrigerant circuit has to extract from, or reject to, the fluid arriving at it. A loop or buffer sized too small lets entering water temperature drift outside the equipment's rated range during peak load, cutting capacity or efficiency exactly when the system needs it most.
Where you meet the term
- In manufacturer expanded performance data, where capacity and coefficient of performance are tabulated against entering water temperature.
- In ground loop sizing, where bore length is calculated to hold entering water temperature within range at peak heating and cooling hours.
- In buffer tank sizing, where stored volume protects entering water temperature during a call the primary loop's own volume could not sustain alone.
How to check it
- Read capacity from the manufacturer's table at the actual entering temperature. A nameplate or nominal capacity figure applies at one reference entering water temperature. The equipment's real output at a colder or warmer entering condition has to come from the manufacturer's own performance table. Technician
- Check whether entering water temperature drifts across a season. A ground loop with a heating-dominated or cooling-dominated annual load imbalance shifts its own entering water temperature year over year, which a single-season design figure does not capture. Technician
Entering water temperature: what it is not
- Entering water temperature is not the same figure as flow temperature. One is what arrives at the heat pump, the other is what it sends onward, and confusing the two misreads a performance table.
- A colder entering water temperature is not free efficiency room. Reaching it costs more bore length or a larger buffer, which is a design trade rather than a given.
Entering water temperature questions
Does entering water temperature matter for an air source heat pump?
The equivalent figure for an air source heat pump is entering air temperature at the outdoor coil, which serves the same role in its own performance tables, though the term entering water temperature specifically applies to water or ground source systems.
Why does a colder entering water temperature reduce capacity?
The refrigerant circuit has less temperature difference to work with when extracting heat from colder water, which reduces the heat it can move per unit of compressor work, lowering both capacity and coefficient of performance together.
Can entering water temperature be corrected after installation?
Within limits. Adding buffer volume or correcting a hydraulic separation problem can help, but a ground loop undersized for the building's actual peak load needs additional bore length, which is a substantial retrofit rather than a control adjustment.
Where this definition comes from
Written from government guidance, standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Geothermal Heat PumpsDepartment of Energy guidance on ground source heat pump loop configurations, describing vertical, horizontal, pond, and open loop arrangements and the site characteristics, land area, soil condition, and drilling access that decide between them.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, and the bin and degree-day methods used for seasonal energy estimation.