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What is the Constant Flow (CF) module?

Heat recovery unit with constant flow

What is the Constant Flow (CF) module?

The Constant Flow is a system based on the principle of maintaining a constant airflow rate in the installation. It detects the difference between the dynamic pressure around the fan and the static pressure in the duct upstream of the fan. Different values of the airflow rate are configured to operate with the respective values of these two pressure differences.

This enables the system to adjust the airflow rate according to the pressure it detects. The CF monitors the pressure in the ducts on an ongoing basis and in the event of a resistance rise, it increases the fan speed to maintain a constant flow rate, thus unchanged since the first day when the MVHR unit was installed.

Why choose an MVHR with the CF?

Ventilation systems will go out of adjustment during use, which is a natural process. It is perfectly normal for ventilation systems to work inconsistently, although most users are unaware of this. In fact, what they don't know is that they will never obtain the air volume that they need in their systems without the CF.

The three following factors decrease airflow rate in a heat recovery system set initially:

  • Air filter contamination
  • Condensation of water in the heat exchanger between the fins
  • High-temperature difference causing changes in the mass airflow rate

Pressure resistance in the system becomes greater in each of the above cases. 

Buying an MVHR with the CF module means you can be sure that you will have a constant volume of air supplied and extracted from the building all year round to match the demand predicted in the heat recovery design. The improvement is highly evident, as resistance can rise even five times from 20 Pa to over 100 Pa on a dirty filter. In many ventilation systems, instability in supply and extract airflow rates is as high as 30%.

This directly increases the costs of heating a building. With the CF module in place, customers are sure that the designed volume of ventilation air is used for heat recovery, which maximises energy efficiency and reduces heating costs.

What are the effects of an unbalanced MVHR system in the house?

There are two outcomes of an unbalanced system in the house.

Positive pressure 

Increasing pressure resistance in the exhaust duct line due to the ongoing contamination of the extract filter, as well as increasing pressure in the exchanger caused by condensation result in the extracted air volume decrease.

This leads to positive pressure building up inside, which drives air out of the building through all possible leaks. The leaking air does not flow through the heat exchanger in the MVHR unit and is not used for heat recovery. This clearly means higher energy loss in the building and increased bills for heating it. What is more, the leaking air does not flow through the filter, thus reducing air quality in the house.

Negative pressure

Increasing pressure resistance on the supply duct line causes the supply air volume decrease. This leads to negative pressure inside, which drives air out of the building through leaks.

The leaking air does not flow through the heat exchanger in the MVHR unit and is not used for heat recovery. This clearly means higher energy loss in the building and increased bills for heating it. What is more, the leaking air does not flow through the filter, thus reducing air quality in the house.

What is balanced ventilation?

When a house is fitted with a balanced ventilation system, the air volumes at the supply and exhaust are equal. The pressure in the building is equal to or very close to the pressure outside. No air escapes through leaks in the building.

The heat exchange unit uses 100% of the set airflow rate, which directly boosts the heat and energy recovery potential, as well as reduces heating costs.

Who do we advise to buy an MVHR unit with the CF?

The MVHR unit fitted with the Constant Flow module is recommended to anyone who wants their systems to work properly according to the assumptions set out in the ventilation design. Our range boasts a series of PremAIR MVHRs with the Constant Flow module. 

Check MVHRs with CF module

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