Question 1

How do I calculate the refrigerant charge for field pipework?

Accepted Answer

The liquid line dominates: its internal volume (pi x (ID/2)^2 x length) multiplied by the refrigerant's liquid density gives the charge in that line. A 10 m run of 3/8 inch liquid line holds about 0.5 L, which is roughly 0.23 kg of R290 or 0.45 kg of R410A. Suction lines carry vapour, so they add far less — the same run adds only a few tens of grams. Add the receiver, condenser and evaporator volumes at their typical liquid fractions, then add the manufacturer's factory charge.

Question 2

How much oil should I add when extending refrigerant lines?

Accepted Answer

A common industry rule is 20-30 mL of oil per kilogram of refrigerant added beyond the factory charge, because oil circulates dissolved in the refrigerant. Always check the compressor manufacturer's specification first — over-charging oil reduces heat transfer and can slug the compressor, while under-charging starves the bearings on long pipe runs and vertical risers.

Question 3

What are the charge limits for R290 propane systems?

Accepted Answer

R290 is an A3 flammable refrigerant, so EN 378 and IEC 60335 set charge limits that depend on where the equipment sits. Sealed commercial cabinets are limited to 150 g per circuit under IEC 60335-2-89 (raised to 500 g in the 2019 edition), while outdoor-installed monoblock units — like Karnot iHEAT heat pumps — can carry much larger charges because the refrigerant circuit never enters an occupied space. That is why Karnot R290 units ship as sealed outdoor monoblocks with water, not refrigerant, piped into the building.

Question 4

Why is CO2 refrigerant charge harder to estimate?

Accepted Answer

CO2 (R744) has a critical point of just 31C, so on a hot day the high side of a transcritical system holds supercritical fluid whose density changes steeply with pressure and temperature — there is no fixed liquid density to multiply by. Charge estimates for CO2 systems are therefore indicative only, and the final charge is set by commissioning: charging to the receiver level sight glass at a known condition per the manufacturer's procedure.

Refrigerant & Oil Charge Calculator

1. Refrigerant & Lines

2. Components & Factory Charge

3. Charge Result

Charge Breakdown

Calculation Method

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