What you need to Know About Radiant Heating

In-floor radiant heating systems are popular in my very cold climate.  Many new homes are being constructed as slab on grade with only one level, a perfect place to use an in-floor heating system.  (I’m a fan of pairing radiant in-floor heat with an air source heat pump.  The heat pump will supply any needed cooling along with the heating load during spring and fall, more on that in a bit.)

Hot water tubing ready to be cast into a concrete slab.

Another radiant heating system that used to be much more popular was hot water and electric baseboard heat.  Both these are much less common in new construction than they used to be.  One additional, but much less common radiant system is an electric panel system installed in a wall or ceiling.  I’ve run into a few of these systems in older homes over the years.

A radiant heating system the ceiling of a 1970’s home.

Before we get into these radiant heat systems, let’s review how heat moves, heat moves from someplace hotter to someplace cooler in three ways.

Convection heat is heat that moves by way of air currents.  A forced air heating system is an example of convection heat.

Conduction heat moves by direct contact between two surfaces.  If you’ve ever touched something hot (or something cold), you’ve experienced conduction heat.

Radiant heat is the transfer of electromagnetic radiation.  I’ve used this example before, stand in front of a large picture window on a cold winter day and you feel the cold of the window.  What you are actually feeling is heat leaving your body and moving to the cold surface of the window.  With a radiant heating system, the heat from a hot surface, (the heated floor, wall, ceiling, or baseboard heater) moves to the colder surrounding surfaces.  The radiant heat is warming surfaces within sight of the heat source.  (Or surfaces in direct contact of the heated surface, this would be conductive heat).

Hot water radiant heat in a slab on grade home.

What radiant heating does not do is directly heat the air.  The air temperature will increase due to air currents moving across the surfaces warmed by the radiant heat, (furniture, walls, and other objects either in a line of site or in direct contact with the heat source).  If there is no air movement, the temperature of the air can be lower than that of the surrounding surfaces.

I’ve heard complaints from homeowners about this over the years.  When they wake up in the morning, their home feels cool.  Once they get up and move, they create air currents which brushes across the warm object, in turn, warming the air.

How can you keep a home with radiant heat more comfortable?  By installing and using ceiling fans.  Just a small amount of air moving will keep air temperatures more comfortable.  (With a ceiling fan, the air should be pulled up during the heating season.  A downward airflow can make a person feel cooler.)

What about spring and fall and in-floor heat?  These times may result in cool overnight temperatures which require some heat, but then the temperature warms during the day to the point where no heat is needed (and maybe a little air conditioning will be used).  In-floor heating systems are not good in these instances.  In-floor heat in concrete slabs will heat the mass of concrete.  This heat transfer is very slow, both in heating the mass and in the cooling of the mass.  Often this results in overheating of the home during these “shoulder” months of spring and fall.  This is where an air source heat pump can shine, supplying just a little heat and cooling if necessary.

Radiant in-floor heating systems are very popular because of the comfort they can provide.  They can take a little getting used to if you are not familiar with the nuances of the system.  You just need to remember, they are slow to heat, slow to cool, and require a little air movement to optimize comfort.

2 Replies to “What you need to Know About Radiant Heating”

  1. Thanks for your article. What is the optimum water temperature your looking for with radiant piping system ?

    1. That’s a great question. What I was taught is that in-floor systems will be around 120°F and baseboard systems much hotter, closer to 180°F. How in-floor systems are designed, spacing and size of tubing, flow rates, etc… have a lot to do with the overall performance of the system. Of course, the best system won’t help a poorly performing envelope.

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