Can you compare the “old” induction units with the Dadanco induction terminal units?
 Yes. The major differences between the old and new units are that the new have:
a)       Lower fan operating pressures, by a significant value.
b)       Lower operating noise levels
c)       Designs for in-ceiling units that allow maximum lettable floor area to the client are the norm.
 
How do you compare an induction system with an all-air system?
 This question arises when a comparison of the cost to install and operate air conditioning systems is addressed at the design stage of a new project. The answer is that the comparison must be on a total system basis. This possibly sounds rather simplistic, but it is wrong to concentrate on one aspect of either system and make a comparison on that feature alone. For instance, the smaller ducts required in a Dadanco induction system is a reality, and a cost saving, but this does not mean the duct systems should be the only basis for comparison.
 
Both systems will have the same refrigeration or cooling capacity and, as a result, common refrigeration plant. The differences, and basis for comparison, are in the air handling and the plant and riser spaces that accommodate the air systems, as well as energy savings. A case can also be made for lower maintenance costs for the induction based system. While the air side is the obvious and major difference, there are also differences in the water systems, with water required to be distributed to the air-water induction terminal units.
 
Considering the air system as a whole, the Dadanco system has smaller air handling units, smaller risers, smaller ducts in the false ceilings and the potential for lower false ceiling heights. Economy cycles are applicable to induction systems. The advantage of a constant volume air supply to maintain air movement rates is an important element of indoor air quality considerations.
 
Are the pressure losses comparable in all-air and induction systems?
Yes. The pressure loss through the duct is what is important. It is therefore necessary to assess the two systems as complete installations, taking into account the AHU losses, filters, OA/RA path losses, the SA duct loss and then the VAV box and its duct and outlets or the induction terminal unit. No outlet loss is considered for the induction terminal unit as the pressure loss across it is supplied from the pressure at the nozzles. The losses between the VAV box, downstream duct and supply outlet will be very close to or higher than the induction terminal unit selected at 200Pa.
 
The VAV boxes vary in their pressure losses depending on the type of box and what added features it has such as noise attenuation and reheat banks. Some makes with these two features, and the downstream duct and outlets, have considerably higher losses than a terminal unit.
 
On the duct system, the route length will be very similar as will the Pa/m static loss. Therefore the total duct static loss will be similar for the VAV or terminal system. In a like manner, the losses across the cooling coil, filters and OA/RA system will be close to the same.

Is a filter necessary in an induction unit?
No. The secondary air is drawn from the room and induced into the induction unit. A considerable amount of return air goes back to the primary air AHU where, after mixing with the outdoor air introduced to meet the building code ventilation requirements, it is filtered and cooled or heated. It is the same as an all-air system in that the supply air from the outlet entrains room air as it leaves the outlet, to circulate in the space and satisfy the air distribution criteria.
 
The vast majority of early induction installations are with units located beneath windows, in a decorative enclosure, with a low level return air path to the unit. With the carpets of that time and foot traffic, coils were fitted with lint screens to catch carpet fluff and other dust. To this end, the lint screen was successful. The perception developed that all units should have lint screens. With in-ceiling units, this is not necessary. A review of European makers’ catalogues shows that they do not offer lint screens as standard, and they have supplied induction units continuously to the market from the early 1950’s. Experience has shown them to be unnecessary.
 
Does the ventilation rate in an induction system vary as the load varies?
No. The induction system is a constant volume system, maintaining the same mix proportions of outdoor air and return air. No matter how the load varies with the seasons, the outdoor air rate remains constant. The outdoor air quantity is at least equal to the ventilation rate required by the building regulations. The primary air can be all outdoor air if inter-connected variable dampers are fitted to the return and outdoor air intakes at the primary air AHU, with the upper limit for the outdoor air component being the full primary air flow.