Airflow | United Kingdom

Cooling Delta-T and Split Temperature Sanity Check

A quick field reference for using return-to-supply temperature difference without over-reading one number.

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 return and supply locations so the next tech knows whether the split was measured at the cabinet or at the duct runs.
Note indoor and outdoor conditions when the split was taken, especially after pull-down or high humidity complaints.
Record what follow-up measurement proved the diagnosis: airflow, charge, compressor performance, or latent-load explanation.

Comeback prevention

Confirm the system is stabilized before comparing the final split with the first reading.
Make sure airflow was verified if the split improved after filter or blower corrections.
Call out any high latent-load conditions so a future revisit does not misread the same number.

How to use the number correctly

Take return and supply readings as close to the equipment as practical, not at the farthest register.
Let the system stabilize before trusting the number, especially after recent door openings or thermostat changes.
Use wet conditions and airflow context; delta-T alone is not a charging method.

Quick interpretation

Observed split	What it can mean	First checks
Below normal	High latent load, low charge, bypass, or weak compressor	Airflow, filter, blower speed, charge method
In normal band	System may be fine	Confirm airflow and customer comfort complaint
Above normal	Low airflow or restriction	Filter, coil, blower wheel, supply/return pressures

Do not miss these traps

A perfect split with terrible airflow can still leave rooms uncomfortable.
A low split during pull-down after a hot start may be normal for the moment.
Furnace temperature rise belongs to the nameplate range, not to cooling split rules.

Use This Sheet For

Use it for triage, then confirm with airflow and charge readings before you call the job diagnosed.

Quick Hits

Typical comfort-cooling split often lands around 16°F to 22°F once the system is stable.
Low split can be airflow, charge, compressor, latent load, or simply a system not fully loaded yet.
High split often points toward low airflow, dirty filters, or dirty indoor coils before it points to perfect performance.

United Kingdom airflow lens

Use Pa and fan performance data confidently, especially where short duct runs or ventilation systems make the airflow picture less obvious than in large forced-air homes.
Compact returns, tight plant spaces, and retrofit ductwork can make static issues show up as noise and comfort complaints before they show up as obvious low airflow.
When airflow notes are borrowed from US material, translate the units and the system type before acting on them.

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.