Radiant Floors

Radiant Floors

Underfloor heating.
Radiant heating loop prepared for finished floor installation. Photo courtesy of Wikimedia Commons

Radiant floors use hot water from the boiler to heat a coil hidden below the finished floor.  This heat emitter configuration has a few advantages.  First, it is wall to wall so there are no baseboard units to inhibit furniture placement again the wall.  Second, because heat rises, it keeps the room barefoot comfortable—this is especially important in rooms with cathedral ceilings. Finally, radiant floors use low temperature water.  Fin-style baseboard and other hot water heat emitters require a water temperature of 160-180ºF. Radiant floors work best at a water temperature of 120-140ºF.  That means that in addition to boilers that are fired on oil, gas, or wood pellets, radiant floors are also well-suited to work with air-to-water or geothermal heat pumps.  In homes that already use high water temperatures, the radiant zone can be mixed down to the lower water temperature needed for radiant floors.

These two excellent videos explain how radiant loops can be embedded in concrete, or run under a wood sub-floor.

When evaluating radiant floor heating, consider the insulating value of the finished floor layers that will cover the radiant loops.  In this case, low insulating values are better.  You want surfaces that conduct the heat from the radiant loop through the sub and finished floors into the room.

Heat Loss

The two variables of of heat loss are a) geographic location, and b) building performance. Together they will determine the quantity of heat needed to maintain comfort.

Geographic location drives the building's “design day." Your design day is the difference in temperature between the inside and outside of your house on the theoretical coldest day of the year; so in the northeast we calculate a day that is -10ºf outside and comfortable 68ºf inside. This is referred to as the “delta-T” value. A typical delta-T for the northeast is 78º.

Building performance is the big one because there’s nothing we can do about geographic location. Today we can build a really tight thermal envelope, but most of our heating pollution comes from the millions of homes built over the last 100-150 years. We are getting better at buttoning-up those old buildings with insulation and weather sealing but weatherization is still  never going to be enough to overcome the heat loss from a 78º delta-T. That means we need heating systems that can quickly put a whole lot of heat back into those old houses on the days that are really cold.