How to calculate a heat pump balance point
Find the outdoor temperature where heat pump output meets building heat loss, using a load line, capacity at temperature, and a worked crossing.
Open the referenceEngineering library
Engineering guides
The design side of the library, written for the reader who is specifying or verifying a system rather than diagnosing one. A page belongs here when its primary purpose is engineering, design, calculation, sizing, specification, modelling, system selection, performance analysis, or commissioning.
What this section is for
What you can expect here
The method, not a number
An engineering page states the calculation it uses, the inputs it needs, and the design conditions the result is only valid under. It does not stand in for a load calculation, a manufacturer capacity table, or a stamped design.
Assumptions in the open
Every figure carries the condition it was measured or derived at. Where a standard, a rating method, and a manufacturer's own table disagree, the page says so rather than picking the convenient one.
US and UK practice
Capacity in BTU per hour and kilowatts, pressures and flows in both conventions, and the standards each market actually works to, so a method reads correctly wherever the system is being designed.
Published
Engineering guides
How to choose HVAC design temperatures
Why load calculations use annual percentile temperatures rather than record extremes, what each percentile costs in hours of exceedance, and when to move one.
Open the referenceHow to size for sensible and latent cooling load
How to split a cooling load into its sensible and latent parts, read a coil's sensible heat ratio, and avoid oversizing the part that removes moisture.
Open the referenceHow to calculate duct friction rate
How to derive a duct system's design friction rate from equipment static pressure and total effective length, with a worked example and the errors that undersize a system.
Open the referenceHow to size a geothermal ground loop
The inputs that set vertical bore length for a closed-loop ground source heat pump, why soil thermal conductivity has to be measured, and what a rule of thumb per ton leaves out.
Open the referenceHow to calculate VRF piping capacity derate
How equivalent piping length and vertical separation reduce VRF system capacity, why altitude derating is separate, and where selection software gets the inputs.
Open the referenceHow to size refrigerant line sets
How suction and liquid line diameter are selected against velocity limits for oil return and pressure drop, and why longer runs change the diameter that works.
Open the referenceHow to check A2L refrigerant minimum room area
How refrigerant charge, mounting height, and minimum room area interact under EN 378 and ASHRAE 15 for mildly flammable A2L systems, and where the check applies.
Open the referenceHow to read a psychrometric chart
How to trace a cooling, heating, humidifying, or dehumidifying process on a psychrometric chart, and what each axis and curve actually represents.
Open the referenceHow to correct radiator output for flow temperature
How to convert a radiator's EN 442 catalogue rating to its actual output at a lower heat pump flow temperature, using the standard's exponential correction.
Open the referenceHow to size a circulator pump
How to find a circulator's duty point from required flow, system head loss, and the pump curve, and why matching either figure alone is not selection.
Open the referenceHow to verify HVAC commissioning tolerances
The tolerance bands testing, adjusting, and balancing work to for air and water flow, what functional performance testing checks beyond flow, and how to document it.
Open the referenceHow to size a heat pump hot water cylinder
Why heat pump domestic hot water sizing depends on recovery time and coil surface area, not stored volume alone, and how a slow reheat rate changes the calculation.
Open the referenceHow to estimate seasonal HVAC energy use
How degree-day and bin methods estimate seasonal heating or cooling energy, the assumption each one makes, and where a heat pump's efficiency curve breaks that assumption.
Open the referenceHow to calculate heat pump ROI
The calculation structure a defensible heat pump ROI comparison needs: bin-level energy, both fuel prices, installed cost difference, and a stated sensitivity range.
Open the referenceHow to choose a duct sizing method
What equal friction, static regain, and constant velocity duct sizing methods each optimise for, and which system scale and duct type each one actually suits.
Open the referenceHow to calculate residential ventilation rate
How ASHRAE 62.2 derives a whole-house mechanical ventilation rate from floor area and occupancy, and how much credit measured envelope leakage actually earns.
Open the referenceHow to calculate the cost of oversizing HVAC equipment
What a safety factor added on top of a load calculation costs in cycling frequency, latent cooling performance, and part-load efficiency, worked through with numbers.
Open the referenceHow to select between VRF and hydronic systems
The load shape, zoning pattern, and distribution constraints that favour a VRF system over a hydronic one for commercial and multi-residential buildings, or the reverse.
Open the referenceHow to size a rooftop unit economizer
How to size an economizer's damper and duct path for minimum ventilation position and full free-cooling position, and why the two conditions need separate checks.
Open the referenceHow to calculate apparatus dew point and bypass factor
How a cooling coil's apparatus dew point and bypass factor predict leaving air condition, and how to construct both on a psychrometric chart from entering conditions.
Open the referenceHow to select a buffer tank for a heat pump
How to size a hydronic buffer tank from a heat pump's minimum run time and the system's minimum circulated volume, rather than from a litres-per-kilowatt rule.
Open the referenceHow to calculate total effective length for ductwork
How to add straight duct length and fitting equivalent lengths into the total effective length figure that sets a duct system's friction rate, with a worked run.
Open the referenceHow to calculate water heater recovery rate
How recovery rate and tank volume combine into first hour rating, and how to calculate a tank or heat pump water heater's recovery rate from its input and efficiency.
Open the referenceHow to size heat trace for condensate lines
How to calculate the heat loss a condensate line presents at design temperature and match it against heat trace output per foot, rather than selecting by habit.
Open the referenceHow to calculate static pressure drop across a coil
How coil pressure drop scales with face velocity and how to read manufacturer coil data at the actual design airflow rather than at the tested reference condition.
Open the referenceHow to calculate pipe insulation thickness
How to check insulation thickness against the surface temperature needed to avoid condensation on a chilled or refrigerant line, not only against an energy-loss target.
Open the referenceHow to write a sequence of operations
How to write a control sequence as explicit states, triggers, and setpoints rather than narrative prose, so it can actually be verified during commissioning.
Open the referenceHow to read expanded performance data
How to interpolate a manufacturer's expanded performance table for the exact design condition, rather than reading the nearest published row as the answer.
Open the referenceHow to calculate duct loss in unconditioned space
How leakage loss and conduction loss combine to reduce delivered capacity for ductwork routed through an attic or crawl space, and why a leakage test alone misses half the loss.
Open the reference