Refrigeration | United Kingdom

Superheat and Subcooling Field Notes

A charging reference for deciding which number matters, what the pattern suggests, and when airflow must be solved first.

Built for United Kingdom field work where boilers, hydronic controls, ventilation, and air-source heat pumps sit beside lighter-duty ducted systems.

Ticket note prompts

Write down indoor and outdoor conditions, metering device type, and the manufacturer charging target used on the call.
Capture both superheat and subcooling even if only one number controls the final charge.
Document what airflow or condenser condition issue had to be corrected before charge was finalized.

Comeback prevention

Verify the final charge after the system is fully stabilized, not immediately after adding or recovering refrigerant.
Recheck condenser cleanliness, fan speed, and line-set restrictions if the numbers drift again.
Leave leak-search notes whenever the correction depended on adding refrigerant.

How to read the pair

Look at superheat and subcooling together instead of treating one number like a verdict.
Use stable indoor and outdoor conditions before calling a charge final.
Verify condenser cleanliness and fan performance before chasing impossible charging numbers.

Pattern guide

Pattern	Possible direction	Next check
High superheat, low subcooling	Often undercharge or starved evaporator	Leak search, airflow, metering device
Low superheat, high subcooling	Possible overcharge or liquid stacking	Weigh-in history, condenser airflow, line restrictions
Low superheat, low subcooling	Overfeeding TXV or compressor issue	Bulb placement, compressor performance

Charge after these checks

Filter, coil, blower wheel, and fan speed are clean and reasonable.
Indoor load is real enough to stabilize the evaporator.
Condenser coil is clean, outdoor fan is healthy, and line-set restrictions are not obvious.

Use This Sheet For

Use manufacturer charging data first, but keep this nearby so the system pattern makes sense fast.

Quick Hits

Fixed metering devices are usually charged by target superheat; TXV systems are usually charged by target subcooling.
Abnormal airflow will corrupt charge interpretation, so static pressure and blower setup come before final charging.
Low charge, restriction, overfeeding metering devices, and condenser airflow problems all leave different fingerprints when you slow down enough to compare both numbers.

United Kingdom refrigeration lens

Heat pumps and splits still need the same airflow-first discipline before final charge interpretation.
Shorter pipe runs do not remove the need to document installation history, commissioning settings, and condensate routing.
F-Gas requirements and manufacturer commissioning procedures should shape the final service record.

Country operating lens

Units and measurements

Expect Pa and L/s to appear more often than in. w.c. and CFM on airflow and fan checks.
Temperatures are generally discussed in C, so split and rise expectations need to be translated mentally if a manual uses US data.
Gas and combustion calls demand tighter commissioning and documentation discipline than quick rule-of-thumb service.

Code and handoff reminders

Gas appliance work must follow the competency and legal requirements that apply locally before any fault-finding becomes a sales pitch.
Electrical isolation, F-Gas obligations, and manufacturer commissioning records matter more than generic field shortcuts.
When you change settings, document them clearly so the next visit understands what was altered and why.

Safety Note

Treat these as field notes, not manufacturer data. If the equipment label, service manual, combustion analysis, or local code disagrees, those sources win.