Measurement Methodology: The Domestic Ventilation Compliance Guide, a supporting document to Part F, shows a vane anemometer and hood assembly as a type of equipment that could be used to demonstrate compliance. It was found in BSRIA’s tests that this type of equipment (being the most widely utilised test equipment) actually creates a resistance which then affects fan performance in exactly the same way as if you were to partially block the inlet or outlet of the fan. The degree of change in performance is dependent on the type, make and model of fan and the volume of air moving through the fan. The higher the flow rate, the more the fan performance is adversely affected. This means that a perfectly installed system in a well-designed application could still be seen to be apparently underperforming when it is actually fully compliant.
BSRIA found that the potential error could be in excess of 40% for 60l/s applications and 20% for 15l/s and 30l/s installations. The result of these findings meant that more acceptable measurement methods needed to be developed.
After much consultation the agreed solution was to deem three measurement methods acceptable:
1. Unconditional Method
This method covers measurement equipment that does not apply a resistance to the system, or automatically compensate for it, and therefore the reading displayed can be assumed to reflect the true performance of the installation.
2. Conditional Method.
The method relies on having specific correction factors for both the fan and the measurement equipment combination. For every type/make/model of fan in combination with every variation of hooded anemometer used, there is a unique correction factor. Only when the correction factors are used can the reading obtained be assumed to reflect the true performance of the installation. It should be noted that these correction factors do not currently exist.
3. Minimum Benchmark Method
This method uses the typical errors observed to set benchmark levels which factor in the generic impact of hooded vane anemometer test equipment. This is a crude approximation and can lead to errors.
This method does not guarantee compliance. As a consequence of using these benchmarks, compliant systems will be deemed non-compliant and non-compliant systems will be deemed compliant.
It should be noted that it is a requirement for all test equipment to be UKAS calibrated on an annual basis. Calibrations should be for the system (the instrument and the hood assembly used) and must be for volumetric flow not air velocity. As standard, instruments are provided with calibrations for velocity only and only generic coefficients are provided for the accompanying hood. Failure to have and use the correct calibration coefficients will result generally in measurement inaccuracies.
Application: A number of installations use fans in unsuitable applications. This is as a result of both poor design, but also installers deviating from design, e.g. changing the duct materials, increasing run lengths or using inappropriate terminals. These unsuitable applications usually increase the system resistance and therefore affect fan performance.
Installation: The widespread use of flexible ducting, and its poor installation, results in squashed or restricted ducts, connections to fan spigots or exhaust terminals, where the duct is required to turn through 90°, or have a bend where the radius is less than the diameter of the ducting, again applying additional resistance.
In-situ measurements demonstrating non-compliance may not be as a result of a single failure, but a combination of the aforementioned issues. To ensure compliance, numerous factors need to be taken into consideration and will require checks at design, installation and verification stages.
- Does the design limit system resistance?
- Have the lengths of ducts been kept to a minimum (Flexible ducting maximum lengths, are 1.5m for axial fans and 6.0m for centrifugal fans)?
- Have the number of bends been kept to the minimum?
- Has the right terminal been selected?
- Considering the manufacturers’ fan performance data, does the selected fan have the capacity to work in the designed installation?
- Can the design be implemented or does placement of other services or structural elements obstruct duct routes?
- Does the design take into consideration the design air permeability of the dwelling?
- Have ducts outside the thermal envelope been specified to be insulated?
- Have vertical ducts been designed to incorporate a condensate trap?
- Has adequate air transfer (door under-cuts) been allowed for?
- Has the design been followed?
- Does the installation limit uncalculated losses?
- Has flexible ducting been installed such that it is extended to 90% of its total length?
- Do any flexible bends have a radius at least equal to the diameter of the duct?
- Is ducting supported to prevent sagging?
- Have any seals around ducting penetrating the structure been sealed in such a way as not to restrict the ducting?
- Are horizontal penetrations through the building envelope angled downward to prevent water ingress?
- Are any high noise levels noticed at either start-up or during operation?
- Are the controls compliant with requirements (e.g. 15 minute over-runs for intermittent fans in rooms without opening windows or operated by the light switch for rooms with no natural light, and manual controls clearly identified)?
- Has the test equipment used for verification been UKAS calibrated within the last year?
- Have the correct calibration coefficients been used for the measurements?
- Have Test results been provided to Building Control within 7 days of the test?
The amnesty for testing ended on the 1st January 2014, and future compliance can only be achieved by good design, installation and commissioning with accurate measurements. Engaging designers, installers and testers who understand the importance of each stage of the process will be key to delivering compliant systems.
If any of the following three key points is not met, the installation is likely to be non-compliant and warrants further checks:
- On first fix inspection, does the ducting look professionally installed? No sharp bends, no excessive use for flexible ducting and no sagging where it is used.
- Does the actual installation mimic the design information? Does it follow the ductwork routes on the design drawings; is the use of rigid ducting as indicated in the drawings; are the terminal grills etc. as specified in the design?
At final inspection, with system in operation, do the controls work, are they correctly labelled and does it sound noisy?
Further information can be found in the free to download guide Domestic Ventilation Systems - a guide to measuring airflow rates (BG46/2013) (simple registration required).