Volume 28, number 2, September 2014

Heating via minimalist ducts: An innovative solution for condo units

For many years, a large number of condo units have been heated by warm air through diffusers positioned as close as possible to the windows, either on the floor or on the ceiling near glass surfaces.

Recent tests carried out at the Natural Gas Technologies Centre (NGTC) have shown that it is equally possible to diffuse warm air from high up on the wall opposite glass surfaces without reducing comfort when in heating mode, or to diffuse cool air when in air-conditioning mode. A warm air diffusion system can thus be made more compact by reducing the length of the air ducts. This concept of heating is called “heating via minimalist ducts.” Faster to install, this solution is ideal for heating condo units.

Integrating minimalist ducts into a building

In a traditional installation, where warm air is diffused from the ceiling, the unit has to have false ceilings throughout in order to hide the air ducts, which run right up to the windows at the extremities of a room. The heat-generation system (boiler or water heater) and the non-compact fan coil unit are usually housed in a cupboard, which takes up several square feet of the unit’s overall space.

Figure 1: Warm air heating – traditional solution

In an installation of heating via minimalist ducts, not having to run right up to glass surfaces to diffuse the warm air reduces the length of the ducts. Thus, depending on the layout of the unit, it may be possible to decrease the surface area of false ceilings needed to hide the air ducts.

In standard Floor plans, such as those shown in Figures 1 & 2, the area covered by a false ceiling (in blue) is limited to the bathroom, the hallway, and part (or all) of the kitchen in Figure 2. Also, the compact fan coil units developed by some manufacturers can be installed in the ceiling space. And adding an individual combo-type water heater frees up the cupboard space.

In this solution, reducing the number of diffusion vents needed to heat the unit is also quite possible, provided that the heating concept is appropriately sized.

Figure 2: Warm air heating – minimalist duct solution

Figure 3: View from below the fan coil unit

Savings in labour and material costs

The reduction in the number of diffusion vents, the decrease in the surface area of false ceilings and in the length of the ducts suggest that savings in the costs of labour and material can be achieved compared with the costs of a traditional warm air heating system. Also, this type of system facilitates the distribution of the fresh air needed to ensure satisfactory air quality in the unit from just one system!

One key element of the system: the diffusion vent

To guarantee that the system is functioning well, it is important that it be sized by a ventilation professional, who must ensure that the air flow is sufficient to reach the opposite wall (wall/window). As well as sizing, one key element of the system is the diffusion vent.

Appropriately siting the diffusion vent for each room is essential so that the air flow reaches the opposite wall. A curtain of warm air will thus cover any glass surface and the movement of air will be sufficient to keep the room at the temperature desired. Also, since it is essential to position each diffusion vent so that the air flow covers any large area of glass, it is important to regulate the direction of the vent deflectors during the installation.

Thermal comfort validated by laboratory tests

To validate this new solution of heating via minimalist ducts, Gaz Métro asked the NGTC to perform some tests and analyses.

The first step in the project was to show that a room can be heated by wall diffusers not mounted on the ceiling. The next step was to confirm that electric baseboard heaters, installed near patio doors or below windows, are not needed to augment the warm air system; and, lastly, to show that the minimalist duct solution delivers the same comfort as an installation with electric baseboard heaters.

The various tests carried out in the NGTC laboratory helped study, under controlled and reproducible conditions, a configuration of an air distribution system with minimalist ducts, as well as a heating system with electric baseboard heaters.

Test protocol

A test protocol was prepared before the start of work in the laboratory to ensure that the results obtained would respond to the needs defined for the project.

The tests were carried out in a room conditioned and instrumented for measuring thermal comfort. The layout simulated a room in a unit measuring 3.3 m (11 ft) by 5.4 m (18 ft) by 2.4 m (8 ft) in height. One of the walls (3.3 m) could be exposed to the cold in order to simulate winter conditions (design temperature for Montréal = -23°C). A patio door was in the middle of this wall.

Figure 4: NGTC Test Room

To represent current, i.e., traditional design practice with over-sized fan coils, two diffusion vents were used in the tests. In fact, heating a 74-93 m2 (800–1,000 ft2) unit requires about 6-7 kW of power (20-25 kBtu/h) under the design conditions, while the smallest fan coils available on the market provide 12 kW of power (40 kBtu/h).

Also, the higher the power, the faster the air flow rate, which, according to market standards, requires the use of 7-9 air diffusion vents per unit. This therefore means that the air flow rate has to be managed to ensure that it does not exceed the normal maximum speed recommended for the air ducts, i.e., 270 m/min (900 ft/min).1

To test the minimalist duct solution, the air diffusion vents were positioned high on the wall opposite the glass surface rather than on the ceiling, which configuration turned out to be viable. A wall-type installation has fewer constraints when it comes to the diameter of the air diffusion vents. In practice, a ceiling installation usually requires the addition of a firestop spacer, which increases costs.

Analysis of comfort

The comfort measured in the room during each test is illustrated using the different criteria for local comfort proposed by ANSI/ASHRAE standard 55-2010 and is presented in the following table.

Table 1: Criteria and pictograms of comfort used in the analysis

Local comfort is evaluated at various points in the room in the different positions. Also, the analysis is done when a person is either standing or sitting. (Figure 4)

The results in terms of comfort are presented in three forms:

  • local average value obtained during the test;
  • percentage of time criterion was exceeded;
  • maximum percentage criterion was exceeded.

The colours help in quickly visualizing the quality of local comfort in the room:

  • Green: criterion respected for 95% of occupants;
  • Orange: maximum variance ≤10 % of comfort value satisfying 95% of occupants;
  • Red: maximum variance >10 % of comfort value satisfying 95% of occupants.

Figure 5: Analysis of comfort — 15 cm (6 in) axial projection vents vs electric baseboard heaters, seated position

Analysis of results

The results show that only one criterion —temperature at floor level near the patio door —was not respected (maximum variance of ≤10% of comfort value satisfying 95% of occupants), whether by electric baseboard heaters or the minimalist duct concept.

It is important to note that the small variance in one comfort criterion will have for effect the temporarily slight reduction in the percentage of people who say they are satisfied (e.g., 90% versus 95%).

We can thus confirm that the concept of warm air heating via minimalist ducts without electrical back-up is viable and that it delivers as much comfort as electric baseboard heaters.

According to ANSI/ASHRAE standard 55-2010, which indicates the test method and the siting of the measurement points, one of the points located near the patio door measures the temperature as well as the air flow rate at 0.6 cm (2 in) from the floor.

With the aim of exchanging opinions and points of view on these tests, the NGTC invited a ventilation expert, Patrice Lévesque, an engineer with NovaMech and co-editor of the « Guide des bonnes pratiques en ventilation mécanique » published by CETAF (Corporation des entreprises de l’air et du froid) and the CMMTQ (Corporation des Maîtres mécaniciens en Tuyauterie du Québec). Mr. Lévesque wondered about the air flow rate and temperature felt on the floor near the patio door. In fact, despite respecting the measurement protocol defined by ANSI/ASHRAE standard 55-2010, certain occupants may sometimes experience discomfort when the air flow rate at floor level is too fast or the temperature too low due to the exchange of cold air from the patio door and the air circulating nearby. This source of discomfort could, in some cases, lead to complaints on the part of the occupants.

In the NGTC test room, Mr. Lévesque made several observations and measurements. Among these, he noted that the air flow rate is higher in air-conditioning mode; however, in his experience, this remains acceptable. When in heating mode, the air flow rate stays below 0.25 m/s (50 in/min) with a temperature that respects comfort levels, even at floor level, at the measurement point required by the standard.

Acoustic comfort

As a follow-up to the various tests of comfort carried out in the laboratory, a major real estate developer was asked to install an air distribution system (fan coil unit) in the false ceiling space at the entrance to a unit.

The objectives of this test was to validate the ease of installing and integrating this type of system into the distribution ducts, as well as to check that the noise level was similar to other frequently installed systems. Air distribution systems are usually installed in a storage area.

The tests carried out by an acoustics expert show that the measurements taken of the noise generated by the heating system are in compliance — day, evening and night.


At the end of the project with the NGTC, it was shown that this solution for heating units with a warm air system using minimalist ducts satisfied the comfort criteria of ANSI/ASHRAE standard 55-2010. It was also proven that the comfort delivered by this type of heating is equivalent to that from heating by electric baseboard heaters. It has therefore been demonstrated that a unit can be heated by warm air throughout the habitable surface without any loss of heat when the air diffusion vents are positioned near or high on the central walls rather than on the periphery. Furthermore, this solution is just as quiet as traditional warm air heating systems.

Received wisdom: hot air rises

The natural phenomenon of air convection makes warm air rise in a room. However, with radial air projection, given an adapted flow and sufficient speed, not only does the warm air not stay on the ceiling, it also guarantees sufficient movement of air in the room. The air flow rate also means that, once the air touches the opposite wall, it descends back to earth and warms the fresh air at floor level.

Marc Francoeur, Eng., CEM, LEED AP
Marc Beauchemin, Eng., CEM
Alice Hamel. Jr. Eng.

1 https://www.acca.org/industry/system-design/speedsheets