Knowledge · Guide

Heat recovery in ventilation: efficiency and economics

How much heating energy ventilation heat recovery really saves: recovery against fan power, humidity, operating hours, and the limits of the calculation.

Heat recovery is the point at which a ventilation system either saves energy or gives it away. The calculation behind it is manageable — it is just often done incompletely, because the offsetting items are missing.

How much heat actually comes back?

The share transferred corresponds, to a good approximation, to the temperature recovery. An example: outdoor air −10 °C, exhaust air 22 °C, so a difference of 32 K. At a recovery of 75 % the supply air reaches around 14 °C without a heating coil having intervened. The remaining kelvin up to the set temperature stay the job of the heating system — heat recovery does not replace it, it relieves it.

It gets interesting over the year: the gain is greatest when the temperature difference is large, which is precisely when heating happens. For the economics, therefore, it is not the design point alone that counts, but the distribution of operating hours across the heating season.

What do you subtract again?

Two items that are readily missing from rough calculations:

• Fan electricity. Every pascal of pressure drop costs electrical power — in every operating hour, including summer, when nothing is being recovered at all. At 10,000 m³/h, an extra 100 Pa of resistance corresponds roughly to 0.4 kW of shaft power. Anyone driving the recovery up with narrower gaps pays for it here. Details in the article on pressure drop and fan power.
• Frost protection. With humid exhaust air and hard frost the unit needs a defrost or bypass strategy. Both reduce recovery in exactly those hours when it would be worth most — condensate and the risk of freezing covers the measures.

Why humidity counts

If water vapour from the exhaust air condenses on the plates, latent heat is released; the energy actually recovered is then above the dry value. That is a real gain, but not a free pass: the condensate has to be drained away, and it has a say in the choice of material.

Conversely, humidity can also be the goal. An enthalpy plate heat exchanger transfers moisture back into the supply air through a vapour-permeable membrane — in winter the indoor air stays less dry, and in summer the same membrane keeps part of the outdoor humidity outside and relieves the cooling plant. Whether that suits your system is settled by the design comparison.

What this calculation does not do

It is an estimate, not a sizing. What it does not include: the actual characteristic of the chosen unit across the load range, the control strategy, fouling in operation, and the question of whether the existing fan has the reserve at all. That is precisely why our configurator calculates your specific point against the design catalogue instead of offering a rule of thumb — and our team comes back to you, with reasons, when your requirements cannot be met at the same time.

Frequently asked questions

How much energy does heat recovery in ventilation save?

The share of heat recovered corresponds, to a good approximation, to the temperature recovery: at 75 % and a design difference of 32 K the unit transfers 24 K that no longer have to be heated. From that result you have to subtract the additional fan electricity for the pressure drop — it is incurred in every operating hour, even when there is no heating load outside.

Is humidity recovery worth it?

It is worth it when dry supply air is a problem in winter, or when cooling and dehumidifying in summer. The membrane of an enthalpy plate heat exchanger transfers water vapour back and thereby relieves both humidification and cooling. It is not suitable for loaded process exhaust air — there the cross-flow unit remains the choice.