Sensible & latent heat

Enthalpy

In heat-recovery ventilation (HRV) systems, cool fresh air from outside the house is heated up, as warm waste air from inside the house passes out of the building. When the air contains moisture, the heat in that air is of two types: sensible heat (the heat needed to heat air from 0 °C to the actual temperature) and latent heat (the heat needed to moisturize the airflow from 0% relative humidity to the actual humidity). Together, these two types of heat comprise the enthalpy of the air – the total amount of energy per unit of mass contained in it. In many HRV systems on the market today, only sensible heat is transferred from one airflow to the other. Latent heat is not used, and as the airflow cools (i.e., as it approaches the end of the duct), the humidity in the airflow (the latent heat) condenses to water, rolls out of the system – and is lost.

The humidity of the indoor air

The fact that the humidity in the waste air leaves the house and does not return via the fresh airflow means that in cold climates the air in the home gradually dries out, unless additional measures (such as installing a humidifier) are taken. If the air becomes too dry, the discomfort (e.g., dry eyes, headaches, etc.) may result. For maximum comfort, we need to maintain a certain level of humidity in the air in our rooms (40%-60% RH). To ensure this, we would ideally like to be able to transfer not only all the heat (sensible and latent) to the fresh airflow, but also the humidity – at least on those occasions when the humidity of the outside air is lower than ideal for comfort.

Blockage by frozen condensate

In cold climates, where the temperature of the duct walls at the outer end may fall below freezing point, the condensation on the duct wall rolling out of a traditional HRV system may freeze. In severe conditions, this frost formation can even lead to the duct outlets becoming blocked. However, if the latent heat could be transferred to the other airflow before the condensate freezes inside the HRV system, this problem of blockage would be resolved.

Feasible? Worthwhile?

But is it possible to recover the latent heat contained in the airflow? And even if it were, would it be worth it? Our answer at Recair on both counts is most definitely ‘Yes’. Recovering latent heat is certainly worthwhile because latent heat often constitutes a substantial proportion of the total heat energy in the airflow. For example, in 1 kg of waste air at 20 °C with a relative humidity (RH) of 60%, the total energy is 42.5 kJ, more than half of which (22.5 kJ) is latent heat. This shows clearly that HRV systems that only recover sensible heat are often not recovering as much heat as they could. Now, however, a better alternative is available – thanks to Recair.

Recair’s solution – Recair Enthalpy

Our latest product – the Recair Enthalpy core – recovers both the sensible heat and the latent heat, resolving all these problems. It does this by periodically alternating the flow in the ducts from waste air to fresh air, and back again. The moisture that condenses during the ‘waste-out’ phase is evaporated when the direction of the airflow is reversed and fresh air enters the duct containing the moisture. Similarly, in freezing exterior conditions, frost created in the waste-out phase sublimates in the incoming fresh air when the direction of the airflow is reversed.

Conditions for the optimal recovery of enthalpy

For the optimal recovery of enthalpy (i.e., both sensible and latent heat), certain conditions need to be met. The chart below shows these conditions, and how the Recair Enthalpy satisfies them.

Extra benefits offered by the Recair Enthalpy:

• You can control the humidity of the air in your home. The Recair Enthalpy enables you to vary the timing of the in- and outflows of air. In this way, providing the building is well insulated, you can control the humidity of the indoor atmosphere with considerable accuracy.
• Outlets never become blocked by ice. Both conventional cross-flow and conventional counter-flow heat exchangers suffer from the same problem: in colder climates, ice starts forming at the outlet of a duct, as condensation starts to freeze. By reversing the direction of the airflows at the right moment, the Recair Enthalpy prevents any significant formation of ice in the waste airflow.
• The air entering the home can be cooled. In hot climates or in summer, we may want the indoor temperature to be lower than the outdoor temperature. The Recair Enthalpy, when installed in your ventilation system, can cool the incoming fresh air, by allowing it (in a reversal of the normal situation) to transfer its sensible heat to the outgoing waste air.
• The air entering the home can be dehumidified. In hot, humid climates, the Recair Enthalpy will remove the humidity in the incoming air, as the outgoing cooler indoor air causes that humidity to condense. The condensate evaporates and leaves the building when the direction of the flows is reversed. This considerably reduces the amount of energy needed to cool the air.
• Cool night air during summer. On hot summer nights, you want the air entering the home to be cool. This means it must not enter through the exchanger, or it will take up heat from the warmer air leaving the home. The Recair Enthalpy solves this easily and cost-effectively. You simply close off the incoming route. All the cool air then enters the home directly via a separate route, bypassing the heat exchanger.

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