Are you using the 40/70 rule? | Blauberg

Are you using the 40/70 rule? | Blauberg

When specifying a heat recovery system it is vitally important that the guidance of approved document F is followed closely, as Blauberg’s Rob Holton explains.

Heat recovery units are generally sized to meet the needs of a given property; the number of bedrooms, the number and nature of wet rooms, the overall size in metres squared, the number of occupants in permanent residence and the air tightness of the property are all factored into the equation. But what of the day-to-day reality of living in a heat recovered house?

Heat recovery (or MVHR) is one of the three continual ventilation solutions as recognised by approved document F (the other two being MEV and PIV) and is the only solution that has a fully balanced extract and supply of air that is mechanically controlled. Being continuous, MVHR airflows are calculated at a lower rate than intermittent airflows and, as such, the only time that this airflow is adjusted is when it is increased. The unit is never switched off.

Approved document F had a major revision in 2010 (2011) based on a survey of properties built to the 2006 edition. The survey showed that numerous examples of MVHR systems were failing due to the occupants switching them off. The reason that these units were being switched off was found to be noise.

Occupants complained that the units were too noisy at night and would switch them off before going to bed and then back on again (if they remembered) in the morning, thus rendering the system intermittent and no longer able to maintain the required airflows. This led to poor air quality, sub-standard ventilation, condensation build-up, black mould and, in some cases, health issues.

In many examples, the survey revealed that the noise was down to poor installation and a lack of understanding by the end user but it was also noted that the specification of some of the units had been compromised in order to keep costs to a minimum.

So, what is the 40/70 rule and what does it have to do with this?

This is simply a failsafe method to ensure that the unit that has been specified will always meet the requirements of the end user, both in terms of Part F and customer satisfaction.

When a trickle and boost rate have been calculated, make sure that the unit is capable of these requirements at no more than 40% of their total capacity for the trickle rate and 70% of their total capacity for the boost rate. For example, if there is a total trickle requirement of 38l/s and a boost requirement calculated at 62l/s you might assume that a unit that is capable of 80l/s, at the relevant resistance to your ductwork system, would be adequate.

The problems that you could encounter with this are that the unit would be running on trickle (all day and all night) at 47.5% of its total capacity and would boost to 77% of its total capacity. If you have ever had the chance to experiment with the engineering mode of a heat recovery unit, you will know that the difference between an acceptable trickle rate of 37.5% and a seemingly acceptable rate of 47.5% is significant in terms of noise and, let’s not forget, energy consumption.

With modern house designs favouring ever increasingly spacious homes that have a disproportionate ratio of individual rooms to overall floor space, I often find that the calculations for the trickle rate actually exceed the calculated boost rate and, as such, a single rate is used. With this in mind it becomes even more important to ensure that the specified unit is not going to ever be switched off and, by following the simple rule of no more than 40% of total capacity for the trickle rate and 70% for the boost rate, you’ll know that you’ll not be getting a call back from an unhappy customer.

Find out more about Blaurberg’s BPEC Domestic Ventilation training course here.[1]


  1. ^ here. (

Leave A Reply

Your email address will not be published.