Understanding and measuring energy transfer
By Gilles LegaultCooling Heating HVAC Systems Hydronics Building Mechanical Electrical HVAC Systems
It is crucial to calculate the load when designing a heating system.
To design an energy efficient and comfortable heating system, it is important to understand and measure the energy transfer. In short, we have to replace the energy leaking through walls, ceilings, floors and openings in the building envelop. Since the energy always seeks a balance, it will move from a high-temperature area to a low-temperature area. This transfer will occur by conduction, convection or radiation.
If the energy input exceeds or does not meet the system’s energy loss, its efficiency will decrease, as will the comfort (and satisfaction) of the occupants. We must calculate the amount of energy that will find its way through the energy shell using the following formula:
Q = A (k/x) (T)
Q = heat transfer rate through a given material
k = conductivity of the material (Btu/F·hr·ft)
x = material thickness
T = temperature differential on both sides of the material (F)
A = surface of the material (sq. ft.)
POINTS TO CONSIDER
These calculations are essential when designing a heating system to ensure it will not be over or undersized, which means it will be efficient and will provide room comfort. In addition, these calculations are required by the CSA B214 standard, which has been part of the National Building Code since 2010. The 4.6.2 (a) point of the standard says that the design load heating calculation – in kWh (Btuh) – must be performed in each single room.
For contractors who say that they do not have time to perform load calculations, there is great software available, some of which use the CSA F280 Standard Form Heat Loss Gain Calculation standard.
Think about it. How can we determine the supply temperature and the required flow rate of each zone if we do not know what the load is? While some contractors choose the oversizing “strategy” to avoid complaints, the lack of comfort and excessive operating costs would generate even more unpleasant and expensive complaints, not to mention the damage to your business’s reputation.
We should also be aware that identical house projects, such as those we saw in the 1970s, no longer exist. Thus, the heating/cooling system design of a particular house does not apply to the design of another similar (in appearance) house. Today, every home is different and the slightest change in the building envelope and occupant behaviour can make a big difference in its heating load.
Why does CSA B214 require room-by-room load calculations? Just as the load of each house is different, so is the load of each room. For example, consider a one-storey slab on grade house with two identical 100 sq. ft. bedrooms. One of these rooms has two outer walls, while the other one has only one. Assuming that the thermal resistance is: R20 walls, R40 ceilings, R10 slab, and that each room has a 4 ft. x 4 ft. window, the room with two outer walls will need 27 Btuh/sq. ft. while the one with one outer wall will only need 20 Btuh/sq. ft.
If the contractor has chosen the 25 Btuh rule of the thumb value, it is easy to figure out that the heating system for one room will be undersized and the other one will be oversized. In both cases the customer will not be happy.
There is no good reason to undertake the installation of any type of heating system without a prior calculation of energy loss. These calculations ensure our customers will be comfortable and happy. In addition, they allow us to meet CSA B214 and comply with the National Building Code.
Gilles Legault is a certified hydronic designer. He is a technical consultant at Agence Jacques Desjardins Inc. and is on the CIPH – Quebec Chapter board of directors. He is a regular contributor to Plomberie, Chauffage & Climatisation (PCC). Readers can reach him at email@example.com.