How does a radiant floor design impact the size of the GHX?

Radiant floor heating works beautifully with a GCHP system...they seem to be meant for each other! But the design of the radiant floor system affects the efficiency of the heat pump, the capacity of heat pump needed, the size (and cost) of the GHX needed, the longevity of the compressor, and even the noise made by the compressor. An article about the efficiency & capacity of the heat pump can be found in a previous blog article. In this article I'd like to talk about how it affects the size of the GHX. 

Let's take a look at two radiant floor system designs: the first with pipe embedded in concrete, the second stapled under the floor with aluminum heat dissipation plates (as shown in the photos). Both systems need to deliver about 15 Btu/hr per square foot (50 W/m2). The difference is that the pipe stapled under the floor in aluminum plates has to be supplied with water at about 130°F (55°C), while the pipe embedded in concrete or Gypcrete only requires about 100°F (40°C) water to deliver the same amount of heat. 

That makes quite a difference in the efficiency of the heat pump. A water to water heat pump delivering 100°F (40°C) water operates at a COP of about 3.5, while the heat pump producing 130°F (55°C) water operates at a COP of about 2.2. The difference in heat pump efficiency has an impact on the size of the GHX. With a less efficient heat pump, the compressor draws more electrical power and contributes more heat to the radiant floor, while the more efficient heat pump will draw a larger portion of the heat it delivers from the GHX. Consequently, the more efficient heat pump will require a larger GHX. If the same size GHX is installed on a system designed to operate at a lower temperature, the efficiency of the heat pump will be compromised.