Radiant Floor Basics Levi's Energy Service LLC

Radiant Floor Basics Levi's Energy Service LLC

Levi's Energy Service LLC

Radiant Floor Basics

There are a few numbers that characterize heat transfer into and from radiant surfaces, floors and walls.

The first number is 3.  One square foot of radiant floor, or wall can deliver three Btus/hr of heat energy when that surface temperature is one degree Fahrenheit above the air temperature in the space.

In fact, this number (3)  is good for any square foot of radiant heat transfer surface.  Radiant walls and floors provide a large amount of surface area and can transfer necessary heat at relatively low surface temperatures.  Traditional radiators have less surface area and must operate at much higher temperatures to transfer the necessary heat.

The second number,  for radiant floors is 85.  People are uncomfortable if they are standing on a floor that is 85 degrees Fahrenheit or warmer at the surface.  A few minutes on a warmer floor wont be unbearable, but, in general it wont be a comfortable place to live for very long.

Those two numbers give rise to the third number which is 45.  If  the room air temperature is 70 degrees Fahrenheit, and the radiant surface temperature is 85 degrees, a maximum of 45 Btu/hr per square foot of radiant surface can be delivered to the space.  The equations are 85-70=15 &  15*3=45.  This number is most important for radiant floors.  Walls can be a little warmer, as long as they are not at the head of the bed.  Warmer walls in shower stalls are good.  Warmer bench seats around built-in tubs and showers are good.  The place on the floor where you put your feet as you visit the toilet in the middle of the night can be a little warmer also.  You can call this thermal delight in architecture.

Another good number is 20.  Given a 20 degree difference in water temperature between a supply side and a return side on a circulating loop, you will deliver 10,000 Btu/hr with a 1 gallon per minute flow rate.  So, if you need 30,000 Btu/hr to heat your house, you will need to push 3 gallons per minute through your radiant heating system and the temperature of the water will have to be 20 degrees cooler when it comes back from the radiant heat transfer surface compared to when it went to the radiant heat transfer surface.

For radiant floors, another good number is 12.  Twelve inches is a good spacing for installing radiant heat distribution tubing.  If installing radiant heat tubing in a concrete slab or in a radiant subfloor system using an aluminum heat transfer material, 12 inches on-center spacing will deliver heat evenly without causing heat banding or striping where the floor surface temperatures vary more than 2 degrees across the floor surface.  Most people wont feel a temperature difference of less than 3 degrees across a floor during normal activities.

For radiant floor tubing, another good number is 300.  It is possible to push hot water through 1/2 i.d. tubing a maximum distance of 300 feet using a reasonable size pump and pumping power.  With 3/4 i.d. tubing its possible to push water up to 500 feet.  But, 3/4 tubing and 500 foot lengths are typical only of commercial size projects.  Lengths less than 300 feet are often used.

Example: For residential systems using 1/2 i.d. tubing and 1000 square feet of heat floor surface, you might use 4-250 foot long tubes.  This would result in 1 lineal foot of tube for each square foot of floor surface area, assuming the tubing is spaced 12 on center.  If the house needs 30,000 Btu/hr under heating design conditions, this would be 30 Btu/square foot.   If the desired room temperature  is 70 degrees, the floor surface temperature will be 80 degrees.  It will be necessary to push 3 gallons per minute through the floor.  This 3 gpm will be divided between the four 250 foot long tubes.  Each tube will have to carry 3/4 gallon per minute.

Pump sizing requires looking at the pressure drop characteristics of the 250 tubes when carrying 3/4 gallon per minute of water, or water/glycol solution.  Adding the pressure drop of one tube and the pressure drop of the boiler piping and other components will result in a final number for basic hydronic system design.  That number is looked up under a pump curve diagram and will determine the right pump for pushing heating solution around your system.

Unfortunately, pressure drop charts, pump sizing charts and specifying the right pump to do the job will have to be another post.

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