A mechanical ventilation heat recovery unit for ventilating an enclosed space to an external environment, the unit comprising: a first flow path through which a first, outfeed air flow of air drawn from within the enclosed space is delivered; a second air flow path through which a second, infeed air flow of air drawn from the external environment is delivered; a heat exchanger which is fluidly connected to the first and second flow paths (5, 7) such that the first and second air flows (F1, F2) pass through the heat exchanger and heat from the first air flow is transferred to the second air flow, thereby raising the temperature of the second air flow; wherein the first flow path has an inlet chamber which is upstream of the heat exchanger, and an outlet chamber which is downstream of the heat exchanger; wherein the second flow path has an inlet chamber which is upstream of the heat exchanger, and an outlet chamber which is downstream of the heat exchanger; and a flow measurement system for measuring the flow rates of the first and second air flows (F1, F2), wherein the flow measurement system comprises a first pressure tap in the inlet chamber of the first flow path, a second pressure tap in the outlet chamber of the first flow path, a third pressure tap in the inlet chamber of the second flow path, a fourth pressure tap in the outlet chamber of the second flow path, and at least one pressure detector for detecting the pressures (Pa, Pb, Pc, Pd) at the first to fourth pressure taps (33a-d), with the flow rates of the first and second air flows (F1, F2) being determined by the pressure differentials (ΔP1, ΔP2) between the pressures (Pa, Pb, Pc, Pd) at respective ones of the first and second pressure taps (33a, b) and the third and fourth pressure taps (33c, d); characterized in that the pressure taps each include a micro plenum chamber for smoothing the sensed pressure, and the micro plenum chamber comprises an elongate channel having a distal region which projects into the respective inlet or outlet chamber (15, 25; 17; 27), and a proximal region to which the at least one pressure detector is fluidly connected. a first flow path through which a first, outfeed air flow of air drawn from within the enclosed space is delivered; a second air flow path through which a second, infeed air flow of air drawn from the external environment is delivered; a heat exchanger which is fluidly connected to the first and second flow paths (5, 7) such that the first and second air flows (F1, F2) pass through the heat exchanger and heat from the first air flow is transferred to the second air flow, thereby raising the temperature of the second air flow; wherein the first flow path has an inlet chamber which is upstream of the heat exchanger, and an outlet chamber which is downstream of the heat exchanger; wherein the second flow path has an inlet chamber which is upstream of the heat exchanger, and an outlet chamber which is downstream of the heat exchanger; and a flow measurement system for measuring the flow rates of the first and second air flows (F1, F2), wherein the flow measurement system comprises a first pressure tap in the inlet chamber of the first flow path, a second pressure tap in the outlet chamber of the first flow path, a third pressure tap in the inlet chamber of the second flow path, a fourth pressure tap in the outlet chamber of the second flow path, and at least one pressure detector for detecting the pressures (Pa, Pb, Pc, Pd) at the first to fourth pressure taps (33a-d), with the flow rates of the first and second air flows (F1, F2) being determined by the pressure differentials (ΔP1, ΔP2) between the pressures (Pa, Pb, Pc, Pd) at respective ones of the first and second pressure taps (33a, b) and the third and fourth pressure taps (33c, d); characterized in that the pressure taps each include a micro plenum chamber for smoothing the sensed pressure, and the micro plenum chamber comprises an elongate channel having a distal region which projects into the respective inlet or outlet chamber (15, 25; 17; 27), and a proximal region to which the at least one pressure detector is fluidly connected.