What’s the difference between an MVHR Heat Exchanger and an Enthalpy that recovers moisture?

If you’re researching installing a Mechanical Ventilation with Heat Recovery (MVHR) system for your project, you may have heard of an Enthalpy heat exchanger or ERV (Energy Recovery Ventilation).

But what is an “Enthalpy” heat exchanger? How does it change humidity in the home? And is it expensive to install?

The basics of enthalpy heat exchangers

A standard MVHR has a diamond-shaped core made up of thousands of little sections that allow air to flow next to one another. This is where the useful process of moving the heat from the stale outgoing air to the filtered supply air happens.

Standard Heat Exchanger MVHR

The stale and fresh air never mixes because the little sections are made of plastic, copper, aluminium or similar. Any water vapour in the air that condenses as it moves from warm to cold air is drained out of a condensate trap below the MVHR unit. This is why we ask for a plumber to connect the MVHR to a waste water pipe during our design process – to drain that condensation.

An enthalpy heat exchanger is set up in exactly the same way, but instead of solid plastic or copper, the material for the heat exchanger is a polymer which allows for water vapour to transfer over from the warm exhaust air to the fresh supply air. It’s important to note that air still stays separate from mixing: it is only the water vapour that transfers. Water vapour is smaller/lighter than air (why clouds float) and the enthalpy material allows this water vapour to transfer. Smells and small particles do not transfer, according to Zehnder’s Enthalpy Exchanger Datasheet.

Image shows heat and water vapour but not smells or particles cross an enthalpy exchanger MVHR

Why would an MVHR want to recover moisture from the exhaust air?

Part of a well-designed MVHR’s specification is to monitor and control indoor relative humidity (RH) in the home. As occupants, we are healthiest when the air is not too dry (like on airplanes) and not too damp (like in saunas). Keeping the humidity in a building stable also helps prevent condensation, mould, fabric damage and ensures the space is comfortable, particularly in summer.

The ideal RH value is between 40% and 60%, ideally 50%. With a standard MVHR, although it is removing water vapour with the condensate drain, it’s also being generated again by normal occupant behaviour (cooking, washing, drying clothes, breathing). So a healthy equilibrium is maintained.

A problem can occur in homes which are very large but under-occupied, and for this reason an enthalpy exchanger could be a better solution.

Large properties need a dedicated indoor humidity strategy 

When an MVHR system is designed it must meet Building Regulations’ Part F requirements. This calculates the whole house maximum airflow rate based on total floor area, number of wet rooms and number of bedrooms. The problem is that these calculations bear no relation to actual occupancy – you could have four people living in a 500m2 home with massive bedrooms, or eight people living in a 90m2 three bedroom home.

Fewer people in a building will naturally cook, shower and dry clothes less, but if their ventilation system is sized for the floor area, the MVHR will be caught in an increasing cycle of stripping away the little bit of water vapour generated into the condensate drain until the air in the home is overly dry.

When we design an MVHR system, we always discuss the actual expected occupancy, and although we have to abide by the Building Regulations for the maximum airflow rate, we offer control functionality to schedule the airflow rates to match the actual occupancy.

If we have calculated that there is a risk of overly dry air, we would specify an enthalpy exchanger. The good news is that there is only a very small price difference of £150 ex VAT between the enthalpy and the standard heat exchanger. Plus, money is saved not having to employ a plumber to install the condensate drain, and the unit saves space as it can be placed directly on the floor.

Are there any issues with an enthalpy heat exchanger?

Yes, an enthalpy heat exchanger should be carefully specified by a diligent designer, as it has a slightly poorer heat exchanger efficiency. A consideration of excess humidity also needs to be considered, particularly in summer, as high humidity is as bad as low humidity.

Are enthalpy exchangers easy to install?

Yes, they literally switch the diamond core heat exchanger with the enthalpy version. However, they must be ordered with the main unit, otherwise they can be more expensive as the original unit will ship with a pre-heater installed to protect against frost, whereas an enthalpy doesn’t need this pre-heater.

Enthalpy exchangers are only supplied by one company in the UK. If you are interested in specifying an enthalpy heat exchanger for your MVHR project please contact me at Patrick [@] Heatspaceandlight.com, or complete the MVHR design form here.

Enthalpy Heat Exchanger ERV MVHR

 

Patrick

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