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Member questions: Underfloor heating and carpets, rebalancing air flows and weather louvre air flowAugust 2018

Compiled by Steve Samsom, Information & Knowledge Manager, BSRIA

Q: If carpet and underlay are fitted in a house with underfloor heating, will it still be possible to obtain the full heat output of the underfloor heating system?

It all depends on the thermal resistance of the carpet and underlay, and whether the underfloor heating system was designed for it. Although generally underfloor heating systems are most effective when used with hard floors, they can be designed to work with carpets and underlays with a maximum TOG value of 1.5. If the thermal resistance is too high, the output from the underfloor heating system will be restricted, reducing the output. If the system flow temperature is increased to provide a greater output to overcome the higher thermal resistance, this may result damage to the floor structure. More information can be found in BSRIA guide BG 4/2011 Underfloor Heating and Cooling, which is available from the BSRIA bookshop.

Q: Is it a requirement to rebalance air flows in ventilation systems in a healthcare building?

For critical systems in healthcare buildings, HTM 03-01 Part B requires annual verification of supply and extract airflow rates. This isn’t required for non-critical systems, however, it would be good practice to do so every few years, and it should certainly be done if there have been modifications to the system or if the system is known to be performing incorrectly. HTM 03-01 Part B can be downloaded for free from

Q: I have a building where we are looking to replace the air intake weather louvres with a desire to increase the quantity of air flow through the louvre. We have analysed at using a louvre with a lesser rain ingress classification and the risk of ingress is not acceptable. A colleague suggested a chevron arrangement of weather louvres to increase the available area. This is an unusual arrangement as I am wondering if we need to apply an additional factor to the louvre co-efficient due to the circumstance that the air will be changing direction.

We are not sure if the air will move in that way as it is more likely to flow in a diagonal direction so the effective frontal area is not dissimilar to that being replaced, and the longer path length through the louvre may increase resistance only slightly.

Two possibilities can be suggested:

  • Room for a plenum type arrangement spaced away from the wall. Thus giving more time for water carried over through the louvres to drop out of the airstream before it reaches the dampers.
  • Increasing the depth of the louvre to possibly give improved water rejection.

Observations demonstrate that most resistance seems related to entry and exit from the louvres and a longer straight path does not add greatly to air resistance. In turn this may influence your product choice.

Q: Do you have the new BSI ISO 17772-1 2017 Energy Performance of Buildings. Indoor Environmental Quality Part 1. Indoor Environmental Input Parameters for the design and assessment of energy performance of buildings for loan?

Our Information Service has purchased hard copies for members to order online and borrow for one month. Find out more about BSRIA's Information Centre services.

The publications mentioned in the questions above are all available for members to borrow from the BSRIA library, open 8.30 - 5.00 pm Monday to Thursday and 8.30 - 4.30 pm on Fridays.
E: T: +44(0)1344 465 571


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