Lower heat loads make more design options available.

Preparing an accurate heat loss or a peak design heating load calculation is the foundation of an efficient and effective radiant heating system. I say “efficient and effective” because before the mid 1980s, when the modern wave of radiant heating found its roots, sizing a heating system was more an art than a science. It was hard to find any boiler with a Btu/hr output of less than 125,000, and the controls that regulated many hydronic systems were limited to a single (line voltage) thermostat, operating high-limit aquastats and a BIG pump.

Figure 1.

Selection of appliances and controls will maximize the energy efficiency of domestic water production while integrating space heating.

I once tried to sell a radiant heating system to a builder whose project was a showcase for green building technology. He coined one of my favorite heating-related sound bites: “The best heating system is the one that never comes on.”

I was puzzled and asked how this was possible. He answered that his 3,500-square-foot home had a high-performance building envelope. As part of determining a net heat loss, every internal source of heat gain had been factored in, and a high-efficiency heat recovery ventilator would recover the majority of the exiting Btu out of the ventilation air.

The total heat load had been calculated meticulously to be a mere 18,500 Btu/hr. From this example it is easy to see that we have moved to an era when sizing a system is now a science, and over-sizing the heating system would have been very easy to do.

Understanding how heat is lost from a building is essential to capturing all sources of heat loss and correctly determining the load requirements of our system. According to the Engineering ToolBox (www.engineeringtoolbox.com), the overall heat loss from a building can be calculated as:

H = Ht + Hv + Hi  where:
H = overall heat loss (W)
Ht = heat loss due to transmission through walls, windows, doors, floors and more (W)
Hv = heat loss caused by ventilation (W)
Hi = heat loss caused by infiltration (W)

Figure 2.

According to the U.S. Department of Energy, 30.4% of our residential energy consumption is spent on space heating. This amount is estimated to be $76 billion in 2010. See Figure 2 below:

The exterior of an auto shop, which makes an excellent application for radiant heating.