Mechanicals-Floor Warming

When working as an energy auditor, I often see homeowners using under tile floor warming as a supplement heat source.  When working on custom tile jobs, I often install these systems.  These all electric heat sources add comfort by warming the tile, but can they be used as a source of heat for a room in a northern climate?  What do they cost to operate?  This weeks topic, electric floor warming.

I’ve installed several different under-tile electric cables through the years.  My current favorite is Ditra Heat.  These systems consist of an electric resistant cable (this type of cable has a high resistance to the flow of electricity, the result of this resistance is heat), and some sort of thermostat and probe to control the temperature.  All the systems I have installed are placed on top of the sub-floor or concrete surface to be tiled.  Some cable systems work best if placed within a mud bed of concrete.  (A thin layer of concrete, usually around an inch thick, that is hand mixed as either a dry pack or is self leveling.)  Some are integrated into a mesh grid that can be attached to the sub-floor and simply tiled over, the system I like uses a special underlayment that the cable clicks into.

Placing and testing.  All of the systems I have worked with have the resistant heating cable spliced to what is called a cold lead, a part of the cable assembly that is fed through a wall cavity and connected to the controlling thermostat.  This cold lead part of the cable does not heat (we would end up heating the wall cavity the containing the cable without the cold lead, a fire could result).  The heating cable is placed, the cold lead and the thermostat probe is fed to a electrical junction box where the controlling thermostat will be installed.

Ditra Heat thermostat probes and cold lead.  The cold lead connection is the black object in the upper right hand corner of the photograph.  The smaller cables between the heating cables are the thermostat probes.

Schluter, the manufacturer of Ditra Heat requires the cable be tested before, during, and after installation.  Documenting this process is required for warranty.  There are different meters used to test the cabling.  An ohm meter is used to test the resistance of the cable and a megohm meter is used to detect small breaks in the cable insulation.

My Ohm meter testing a recently installed heating cable.

A system I recently installed had an resistance of 120.4 Ohms from the manufacturer.  The next photo shows the nametag for the product.

Ditra Heat data tag.

This system was hooked to 240 volts with a wattage of 475 watts.  Knowing this information, what does is cost to operate the floor warming system?  Two big factors are run time and cost of power.  For this calculation, I will use a cost of $.125 per kilowatt hour (kWh) and a run time of 25%.  The run time will vary depending on the thermostat setting, climate and heat loss of the building, especially the heat loss of the floor where the floor sensors are placed.  The formula used to calculate operating costs is:

kWh x run time x cost of power = operating cost

Because the name tag is listing power consumption in watts and we require kilowatts for the calculation, we need to divide the listed wattage by 1000, (1000 watts equals 1 kilowatt which is how electricity is billed).  475 / 1000 = .475 kWh.  Plug that number into the formula and the result is:

.475 kWh x .25 hours x $.125 = $.015

It will cost $.015 per hour to operate the in-floor heating with a run time of 25%.  Multiply that cost by 24 hours in a day, and 30 days in a month and you get $10.80 per month.  Seems affordable enough, around $10 per month to keep the tile warm.  A closer look at the name tag shows this cost is for 38 square feet of floor warming.  If you have a larger area to heat the costs of course will be more.

How much heat is this 38 square feet of electric heating cable producing?  Electric heat produces 3412 BTU’s per kWh.  In the case of the example, the heating cables are producing 1620 BTU’s (.475kWh x 3412 BTU/kWh = 1620 BTU).  Is this enough heat to heat a bathroom.  Probably, it depends on the heat loss of the room.  Is it economical to heat a room using electricity at a cost of $.125 per kWh?  I wrote a blog posting several months ago about the cost of different heating sources, which you can read here.

Thermal image of Ditra Heat.

Let’s compare this electric heat to another source, such as natural gas. Natural gas produces approximately 100,000 BTU’s per therm, which currently costs around $1.25 per therm in my area. If we factor in a 90% efficient furnace, the cost of the natural gas rises to $1.39 per 100,000 BTU’s ($1.25 / 90% efficient furnace = $1.39). To produce the equal amount of heat the electric cables are producing at the 25% run time, the cost decreases to $4.05 per month.

For those nerds out there that want to know how I did this calculation, continue reading.  If your not a nerd, you can skip this part.

Earlier I said the electric cost is calculated at a 25% run time.  Knowing how many BTU’s are being produced per hour by the electric cables, I multiplied the 1620 BTU’s by the number of hours per day and number of days per month to produce a total BTU the heating cable can produce and multiplied by 25% run time.

(1620 BTU’s x 24 hours per day x 30 days per month) x .25 run time = 291,600 total BTU’s per month produced by the electric heat cables.

Next I calculated how many therms that is by dividing the 291,600 by 100,000 BTU’s per therm.

291,600 / 100,000 = 2.916 therms

2.916 x $1.39 (Cost of natural gas after efficiency correction) = $4.05

In review, electricity costs $10.80 per month and the equivalent amount of heat produced by natural gas is $4.05 per month. The actual cost will be dependent on the cost of the electricity or heating fuel and run times.  A note about operating this type of floor heating.  These cables warm a floor fairly quickly but probably won’t reach full heat within the time it takes to shower.  Usually the system is turned on around 1 hour before it is needed.  Most use a timer based on the users schedule.  Some homeowners do not use a timer.  Using a timer will reduce costs.

Is an electric tile heating system worth installing?  They definitely add comfort and will supplement another heating source, but at an increased operating cost.  There is also the cost of the installation.  Currently, the cost for materials in the earlier example is around $500, add in the installation cost and an electrician to perform the needed electrical work and the cost is well over $1000, to warm 38 square feet.  Slippers and rugs are much cheaper, just my opinion.

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