Freshen up: active cooling with radiant ceilings in a Passivhaus retrofit
The article presents an active cooling system using the supply air of the ventilation system with radiant ceiling panels, installed in a multi-residential building in the historic center of Girona, certified Passivhaus EnerPHit – Demand method.
Freshen up: active cooling with radiant ceilings in a Passivhaus retrofit
The article presents an active cooling system using the supply air of the ventilation system with radiant ceiling panels, installed in a multi-residential building in the historic center of Girona, certified Passivhaus EnerPHit – Demand method. For each apartment, the system consists of an air-source heat pump, a mechanical ventilation unit with heat and moisture recovery (MVHR), a coiling coil on the supply air stream, and radiant ceiling panels. Control is carried out with a home automation system, with temperature and humidity sensors in each room. The solution offers both heating and cooling, working quietly and at low temperature, providing high thermal comfort and efficient performance when used with a heat pump. Reliable performance depends on correct system sizing, proper commissioning of the control system and of ventilation flow rates, and user maintenance and replacement of filters in the MVHR units. Systems such as this are not a good solution in homes where windows are open a lot on the summer and are better suited to warm and dry climates with lower levels of humidity.
The building is a multi-residential 6-storey building in the historic center of Girona, certified by Passivhaus EnerPHit – Demand method [Figure 1]. This private initiative – which was the first of its kind in Catalonia – put 4 new apartments of 129 m2 and a duplex of 162 m2 on the market.
Due to local heritage building regulations, insulation had to be installed inside, with some loss of thermal inertia. Active cooling using a cooling coil on the the ventilation supply air is a relatively simple concept which can be cost effective to install. However, thermal power can be limited when temperatures peak. The system presented here combines supply air cooling with radiant ceiling panels, to provide sufficient power to cover peak cooling loads.
The project data and team are shown below:
- Certification class: Passivhaus-EnerPHit – Demand Method
- Useful / gross floor area: 678 m2 / 1.038 m2
- Developer: MBD Real Estate Group
- Builder: Busquets Sitja
- Architects: López-Pedrero-Roda Architects
- M & E Engineering: PGI Engineering
- Control/home automation: Progetic
- PHPP, Passivhaus design: Oliver Style, Bega Clavero
- Passivhaus Certification: Energiehaus Arquitectos
Description and operation of the system
Given space and floor-to-ceiling height limitations, 2 cooling systems were initially considered:
- Ventilation supply air cooling only
- Ventilation supply air cooling + radiant ceiling panels
The second option was chosen, given that operative temperatures in the summer could not be maintained at ≤ 25ºC using ventilation supply air cooling only. With 19 m2 of radiant ceiling panels (covering around 15% of the ceiling surface area), peak cooling loads could be met, calculated for an outdoor air temperature of 34.1 ºC, with an absolute humidity = 10,5 g/kg .
For each apartment, the system included the following equipment:
- Heat pump: Daikin EWYQ005ADVP air-water monobloc heat pump (5.20 kW cooling / 5.65 kW heating) [Figure 2]
- Heat & moisture recovery ventilation unit: Zehnder ComfoAir550 enthalpic [Figure 3]
- Cooling coil: Zehnder ComfoPost CW10 [Figure 4]
- Radiant ceiling panels: Zehnder NIC 150 & NIC 300 [Figure 5]
- Control system:
- 1 temperature & humidity sensor per room
- 1 Loxone mini server [Figure 7]
- Various elements providing on/off control of the heat pump, 3-stepped control of the ventilation flow rate, and on/off control of each radiant ceiling circuit and control of water supply temperature to the radiant ceiling panels with a 3-way valve
In heating mode, the heat pump generates hot water, circulating it through the radiant ceiling panels at a supply/return temperature of 45 ºC / 40 ºC. At the same time, the coil on the ventilation supply air stream heats the air to around 40ºC. The fan speed is controlled to avoid excessively high flow rates, and which can lead to low relative humidity of indoor air.
In cooling mode, the heat pump generates cold water, circulating it through the radiant ceiling panels at a supply/return temperature of 7 ºC / 12 ºC. At the same time, the coil on the ventilation supply air stream cools air to around 15 ºC. The coil also provides some dehumidification of the supply air, lowering the ambient indoor air dew point temperature and preventing condensation on the ceiling panels. In cooling mode, controlling the temperature of rooms individually is not possible given that the cooling coil only works for the entire apartment.
The heat and moisture recovery ventilation unit also helps to increase the relative humidity of the indoor air in winter and decrease it in summer, improving thermal comfort and reducing the dehumidification load that the cooling coil needs to overcome.
With the ventilation flow rate of 0.4 ach (135 m3/h), radiant ceiling panels typically cover – for both heating and cooling – approximately 65% of thermal loads. The ventilation system with the heating/cooling coil covers the remaining 35%.
Radiant cooling systems must have a robust control system, to avoid problems of surface condensation. Temperature and humidity sensors were therefore installed in each of the 5 rooms where the radiant panels were located (dining room, kitchen and 3 bedrooms). The water temperature of the panels is adjusted with a 3-way mixing valve, based on the temperature and humidity data from the sensors in each room, ensuring the panel surface temperature remains above the dewpoint, avoiding condensation.
The control system also modulates the ventilation unit’s fan power, lowering or raising the flow rate depending on the temperature setpoint and dehumidification needs. A schedule prevents the fan from operating as full flow at night, to avoid noise problems. If maximum power is required at night this can be a problem. The control allows you to set different setpoint temperatures according to specific schedules or occupancy rates, for each day of the week.
In its default setting, the ventilation system works automatically with pre-established schedules (with the possibility of manual adjustment by occupants). Figure 8 summarizes its operation:
Cooling with radiant ceilings can offer an efficient solution that adds power to ventilation supply air cooling systems in Passive Houses in the summer. As the system is predominantly radiant and running at low temperature, it provides good comfort and can be more efficient than convective systems. Ceiling panels can be sized to meet heating and cooling loads, which in residential buildings retrofitted to Passivhaus standard means a coverage of between 15% to 30% of the ceiling surface area. This replaces ducted fan-coil or split systems, which take up more space in suspended ceilings, often a limitation in retrofit.
The control system presented here offers a flexible solution at a reasonable cost, with a relatively user-friendly interface. The possibility of visualizing and monitoring data remotely and in real time, facilitates the optimization of the system and helps in terms of preventive maintenance.
Systems such as this are not a good solution in homes where windows are open a lot on the summer and are more suitable for use in hot dry areas, since, in areas of high humidity, the power of the system will be limited depending on the humidity level of the indoor air and the proximity to the dew point. Robust operation depends correct system sizing, proper commissioning of the control system and of ventilation flow rates, and user maintenance and replacement of filters in the MVHR units.