Home Heating Systems & Heat Transfer

Understanding the differences between conduction, radiation and convection heat transfer is relatively easy.  Understanding how they relate to each other in a radiant home heating situation is much more complex.  To some degree conduction, radiation and convection exist in all types of home heating systems; however the proportions of each can be quite different from system to system.  This difference in proportions will affect the comfort within the home and the efficiency (cost) for heating the home.

Home heating systems employ some type of heated surface which is used to transfer heat into the home.  The heated surface may take different forms and be in different locations.

For example:

1)      Forced Air Furnace – located outside the primary living area with a heat exchanger that heats air which is then circulated via ducts to rooms in the primary living area.  The air enters the primary living area from the duct work under pressure and is forced to return to the furnace through another set of ducts.

2)      Hot Water Baseboard Furnace – located outside the primary living area with a heat exchanger that heats water which is then circulated via pipes to rooms in the primary living area. In the primary living area the heated water runs through a baseboard heat exchanger warming room air which then circulates by natural convection.

3)      Electric Baseboard Heat – located in the primary living area with a heat exchanger that heats air which then circulates by natural convection.

4)      Hot Water Radiator Furnace – located outside the primary living area with a heat exchanger that heats water which is then circulated via pipes to rooms in the primary living area.  In the primary living area the heated water runs through a radiator.  The radiator must have enough mass to store the heat from the incoming water. The heat in the radiator dissipates into the room through a combination of natural radiation and convection. The proportion of radiation versus convection is dependant on the size, design and location of the radiator in the room.

5)      Hot Water Radiant Floor Furnace – located outside the primary living area with a heat exchanger that heats water which is then circulated via pipes to rooms in the primary living area.  In the living area the heated water runs through a network of pipes imbedded in the floor giving up its heat to the mass of the floor.  The floor gives up its heat to the room largely through natural radiation and some conduction to the objects in direct contact with the floor.  Convection from the floor is minimal in comparison to heat transfer by radiation and conduction.

6)      Electric Radiant Floor Elements – located in the primary living area with heating elements embedded in the mass of the floor. The floor gives up its heat to the room largely through natural radiation and some conduction to the objects in direct contact with the floor.  Convection from the floor is minimal in comparison to heat transfer by radiation and conduction.

7)      Wood Stove – located in the primary living area with a heat exchanger (firebox) heating relatively little thermal mass. The heat dissipates into the room mostly through natural convection and some through radiation.  The proportion of radiation versus convection is dependant largely on the temperature which the stove is operated.

8)      Masonry Heater – located in the primary living area with a heat exchanger (firebox) that heats its substantial thermal mass.  The heat stored in the thermal mass dissipates into the room mostly through natural radiation and some through convection.  The proportion of radiation versus convection is dependant on the size, design and location of the masonry heater in the room.

In the descriptions of eight home heating systems listed I have given some general indication as to the proportions of the various heat transfer types for each system.  I would now like to group these systems in some general proportional categories.

1)      Convection – Forced Air Furnace and Electric Baseboard heat predominately by convection.

2)      Convection/Radiation – Wood Stoves and Hot Water Baseboard heat predominately by convection with radiation accounting for a smaller amount.

3)      Radiation – Hot Water Radiant Floors and Electric Radiant Floor Elements heat predominately by radiation with a small amount by conduction and very little by convection.

4)      Radiation/Convection – Hot Water Radiators and Masonry Heaters heat predominately by radiation with convection accounting for a smaller amount.

The common element that is present in category 3 and 4 heat systems and is missing in those of category 1 and 2 is the presence of significant thermal mass for heat storage.  The thermal mass present in large radiators, floors and masonry heaters significantly lowers the temperature of the heated surface area that transfers heat into the home.  When these surface area temperatures stay in the 75 to 150 °F range heat transfer by radiation will predominate.  Between 150 to 300 °F range heat transfer by radiation and convection will even out.  Above 300 °F heat transfer by convection will predominate.

© Mid-Atlantic Masonry Heat Inc. 2011
Revision 10/04/2011

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