HVAC Design Considerations

Design considerations

From the previous article we know that a zone is a part of the building that is controlled by a single thermostat. Also, a multiple zonal building is one which has multiple zones, each having its own thermostat. A zonal controller is used to connect all these zones to the main HVAC system. This controller has a connection to each zone, its thermostat and also to its respective dampers. Thus a zonal controller mainly controls the dampers of each zone. It also has connections to all the thermostats which are controlled by the HVAC system.

Now let’s take a look as to how will be design a basic zonal HVAC system. For this we will have to recollect some points that we already discussed in the previous articles so that we understand zoning in a better way.


While zoning a HVAC system there are few things that should be firmly kept in mind.

The building that has to be zoned should be highly insulated. Each level of the zone should have identical zoning design. And the zoning should be done in such a way that in case modification or repair is needed at a later stage, it can be easily carried out. Also it should be done in the most cost effective manner. A zoning having less number of zones and fewer thermostats is considered an effective design.

Let’s take the same diagram that we took in the first article and try to understand the basic design considerations before we go further. For this we will just take only one factor i.e. solar gain under effect.

hvac HOME

Here, when the sun is in the east. NE and SE are under sun’s direct heat and thus require more of cooling. SW and NW zones are not affected by solar gain and thus don’t require additional cooling. So as the condition of an effective design says, NE and SE should be taken as one zone, including E, i.e. NE,SE and E should be provided with one thermostat and in the similar way SW and NW zones should be considered as one zone, including S and also should be provided with a common thermostat.

A normal design would have made each zone as separate zones with their own individual thermostat. But this will have been an expensive zoning with 6 zones and 6 thermostats. An expert designer will keep these things in mind and reduce the zones to increase cost effectiveness.

Therefore, the above system will be an effective system with two zones and two thermostats.

Important points

For a zonal system with multiple storeys, it is very important that each zone is of equal area and size. This is done for the reason that small zones are difficult to duct. Also if the zone is small it will be difficult to do modifications or repairs if required.

Each zone should have a separate branch duct which is connected to the main duct of the HVAC system. Each branch end will have a damper in the respective zones.

duct branches


We know that the thermostatic sensors control the dampers. Sometimes due to the sudden closing of the damper there is a back pressure created which can damage the HVAC system. To prevent this, something known as “upsizing” of the damper is done. Upsizing means that the size of the damper is made 25% more then its usual size. This is to accommodate the sudden increase of “back pressure” so that the system is not affected. More the number of zones more will be the size of the duct.

zonal branches

Last but not the least, it is important that we plan the position of damper and thermostat before the whole ducting is made so that the wiring from the thermostat to the HVAC system is done easily. This can be avoided by using remotely controlled thermostat.

controlling unit


Fundamentals of HVAC system : Robert McDowall