The result
A plot using a System 3 MEV strategy failed its ventilation flow‑rate test because extract rates at the terminals did not meet Part F requirements under the correct operating condition.
ATSPACE traced the failure to commissioning settings and installation factors commonly seen on active sites, adjusted the system in a controlled sequence, and achieved a Part F pass on retest.
The fix avoided repeated trial‑and‑error adjustments and prevented wasted retest slots.
Project snapshot
Service: System 3 MEV flow‑rate testing + remedial support
Client: Main contractor + M&E subcontractor
Site: Fieldstone Rise, Plot 27, 9 Copperleaf Avenue, Peterborough PE2 6SP
Building type: Three‑bed new build home
Ventilation strategy: System 3 MEV (continuous extract)
Programme stage: Post‑fail remedial window before handover
Compliance driver: Approved Document F
ATSPACE delivery: Test review, flow measurement, adjustments, valve balancing, verification test + reporting
Team: ATSPACE accredited ventilation engineer + coordinator
Why System 3 MEV plots fail Part F testing
Most System 3 failures are not caused by the fan being wrong.
They are caused by setup, configuration and airflow‑path issues such as:
- incorrect trickle/boost settings
- valves left at factory or random positions
- crushed or extended ducting
- resistance from tight bends or long flexi runs
- terminals installed incorrectly or swapped
- insufficient door undercuts or transfer paths
- closed or missing background ventilators
- testing conducted in the wrong system mode
This plot had a typical combination of these factors.
What ATSPACE was asked to do
- identify the practical reason for the failed flow‑rate test
- adjust the installation so extract rates met Part F
- avoid intrusive rework and programme disruption
- provide clear, retest‑ready evidence for handover
What ATSPACE did
Step 1: Confirm test context
Before touching any settings, we checked:
- the system operating mode during the original test
- whether the fan was in commissioning condition
- whether valves had already been altered
- whether doors and background ventilators were positioned correctly
- whether ducting showed obvious restrictions
This avoided adjusting a system based on incomplete information.
Step 2: Measure flows at each terminal
We:
- measured extract rates in each wet room
- compared them to required rates
- listened for high‑resistance noise
- identified terminals starved by duct routing
Step 3: Identify dominant causes
The failure came from:
- incorrect trickle + boost configuration
- valves not balanced to the duct network
- high resistance in one branch due to routing + compression
Individually minor — collectively enough to fail.
Step 4: Adjust settings and balance valves correctly
A precise sequence matters:
- correct the fan operating condition
- set system to commissioning mode
- adjust valves to meet per‑room requirements
- recheck total extract rates
- confirm one room wasn’t fixed by starving another
- confirm boost operation
- recheck stability after adjustments
Step 5: Verify air‑paths
System 3 depends on replacement air.
We checked:
- door undercuts
- internal transfer paths
- background ventilator positions
Restricted air movement can cause terminal under‑performance even when the fan is strong.
Step 6: Retest and reporting
We retested under correct conditions and issued clear evidence showing the rectified flow rates.
What we adjusted to go from fail to pass
Adjustment 1: Correct operating mode
The system was not in correct commissioning mode.
Impact: Incorrect testing mode can make a compliant system appear non‑compliant.
Adjustment 2: Valve balancing
Valves were not balanced relative to duct resistance.
Impact: One room can draw too much flow while another is starved.
Adjustment 3: Branch resistance correction
One branch had routing compression increasing resistance.
Impact: Even capable fans cannot overcome high‑resistance branches.
Adjustment 4: Transfer‑path confirmation
Restricted internal air paths were impacting airflow.
Impact: Extract is ineffective without matching replacement air.
Outcome
The plot achieved a Part F pass on retest without repeated cycles or wasted testing appointments.
The project gained
- a controlled fix instead of guesswork
- a repeatable method for similar plots
- reduced retest‑risk across the house‑type run
- clearer understanding of settings, balancing and transfer paths
- compliance‑ready evidence for handover
Common mistakes this project avoided
- increasing fan speed without balancing valves
- adjusting valves before confirming system mode
- ignoring duct resistance and blaming terminals
- retesting without verifying transfer paths
- using fixes that cannot be repeated across plots
CTA
If your System 3 MEV plot has failed Part F testing, ATSPACE can identify the real cause, adjust the system properly and help you achieve a pass without delay or unnecessary rework.
Ask for
- System 3 MEV flow‑rate testing + remedial support
- commissioning checks + valve balancing
- duct‑resistance investigation
- retest + reporting for compliance packs
Frequently asked questions
Can a System 3 MEV fail even if the fan is fine?
Yes — duct resistance, poor routing and crushed flexi often cause failures.
Should I adjust valves or fan settings first?
Correct operating mode first, then balance the valves.
How do we avoid failing multiple plots?
Fix the repeat cause and apply the same configuration across all similar plots.
