Fred

Electric Radiant Floor Heat | Efficiency & Power Consumption

April 10, 2009 | by Fred (email) |

electric-radiant-heating-matRadiant heated floors are one of the most luxurious additions to any home, especially in bathrooms, kitchens, and anywhere else a cold floor is the source of discomfort.

Radiant heat adds an element of upper-class living to an otherwise middle-class home. And, while not necessarily appraised into the value of a home, it certainly can serve as a closing feature — a feature that pushes buyers over the edge to place an offer on your home quicker and for more money.

Of course, all that luxury comes at a price. Typical DIY bathroom electric radiant heat installations will run $300-500 over their unheated counterparts. Add in up to $750 more for professional electrical and material installation. And after the installation, the floor requires power to keep your investment producing a warm, comfortable atmostphere.  So the question is: how much power will your investment consume?

Power Consumption is Driven by Heat Loss

We all know intuitively that heat loss drives our energy bills.  Leave the window open in the Winter, and your electric bill is sure to show the effects. So answer #1 to how much energy an electric floor will use is: it depends.  How much heat is the space you’re heating losing on its own? To find out that answer, you’ll need to estimate it using a heat loss calculator like that one from Alternate Heating Systems.  Heat loss is measured in British Thermal Units (BTU) / hour, often abbreviated (incorrectly) as just BTU.

Remember when using the calculator that you may need to predict multiple numbers and take an average.  The calculator relies on an average exterior temperature.  In the milder months of Spring and Autumn, the outdoor temperature will be lower, and the heat loss less.

Armed with a BTU number that represents heat loss per hour, estimating electrical usage is easy.

Radiant Floor Electricity Usage

Electric radiant floors produce heat resistively & predictably. While different heating vendor’s systems differ in how many watts / square foot they produce, this is irrelevant for the heat loss and energy calculation.  (The only difference will be the speed with which a system can heat the room, but not its overall efficiency).

One killowatt-hour of electricity produces about 3400 BTUs of heat.  If your heat loss calculator predicts a 10,000 BTU/hr. heat loss, you’ll need about 3 killowatts of power each hour to keep the space warm with only a resistive heating source.  At $0.10/KwH, that’s 30 cents per hour, $7.20 / day, and $216.00 / month.

What if Radiant Floors Aren’t the Only Heating Source?

In most electric radiant heat installations, the radiant heat is not the only source of heat in the room.  More often than not, radiant heat is installed to augment a forced air system already installed in the house.  In these cases, the radiant heat source’s only job is to take the cold edge off the floor.  In these cases, the cost of radiant heat can be substantially reduced (by a factor of 60% or more).

Is Radiant Heat Efficient?  Pros and Cons:

This is a tough question to answer, because it really depends on your heating goals, alternative heating options, and installation characteristics. It is better to discuss in terms of pros and cons…

Pros:

  • Radiant heating systems don’t require forced air circulation in a room.  Air circulation creates heat loss when it contacts exterior walls and windows; so, theoretically, a radiant system is a more effecient delivery mechanism.
  • Radiant systems produce heat from the floor-up, which means the floor is the warmest point in the room and the ceiling the coolest. Since heat rises, the overall temperature target for the room can be reduced to achieve the same comfort level as a forced air system.

Cons:

  • Electric radiant heat systems produce heat resistively, which is the least efficient way to get heat from an electrical system.  (The most efficient is a traditional heat pump system, which extracts heat from the outside air).  Running a radiant heat system is the electrical equivalent of running the back-up (emergency) furnace in most traditional forced-air systems.
  • When coupled with a forced-air system, the advantage of no air circulation (the first pro listed above) is removed.

Is Electric Radiant Heat Worth It?

This is a decision for each homeowner.  Radiant heating systems add a wonderful comfort to a room. Is this added comfort worth the potential additional cost of the radiant system?  We think in many scenarios it is, but generally wouldn’t rely on a system to heat an entire house.

What do you think? Anything our analysis misses?  Would you install an electric radiant heating system?

4 Responses
  1. Corey says:

    I thought about this for the bathroom but decided against it. I felt I wouldn’t get the return on my investment. It would have meant wiring an extra break, plus all the radiant supplies. If I did radiant I also would have wanted a programmable thermostat to have the floor already be hot when I use the bathroom in the morning. I don’t think most folks think about the time 5-10 minute lag it takes for the radiant coils to heat the floor, and then in turn heat the room.

  2. Josh says:

    Great article, Fred. I haven’t seen a cost estimate for electric radiant flooring before and I had not considered how expensive/inefficient it can be.

    That said, I think these setups have real appeal in bathroom additions for older homes with hot water or steam heat. It can be expensive and disruptive to try to add a new radiator to a project like an add-on-dormer bathroom. A project like this will already require additional wiring, so it would be pretty easy to factor in the requirements of a radiant floor. No precious floor space lost to a radiator and no heat supply and return pipes to run through finished walls below.

  3. Fred says:

    Corey – You’re absolutely right on the warm-up time… the programmable thermostat is a must for this. We decided to install a 12 watt/sq. ft. system in the basement…. you can also get a 15 watt / sq. ft. system. The numbers indicate ours will warm up about 25% slower than the 15 watt system, but I got a better deal on it, so we went with it…

    Josh – very good point. That’s one of the reasons we decided to go with it for the basement too.. We could have had to run the ductwork for the forced air system to get the flow we needed; and, once completely, I feared that it really wouldn’t keep the place warm. With the radiant system, we could pay through the nose during the coldest months, but at least we’ll have that option to get real comfort in the space.

  4. K.S. says:

    Heat losses are far less with a radiant system, as the effect of air losses is minimized and it has the ability to store heat in all solids (walls etc.). There have been studies on this.
    a)Case Study: Seven-System Analysis of Thermal Comfort
    and Energy Use for a Fast-Acting Radiant Heating System

    b)An Evaluation of Thermal Comfort and Energy Consumption
    for the Enerjoy Radiant Panel Heating System

    c)Radiant Floor Heating
    In Theory and Practice

    The thing is that heat transfer by radiation is more efficient than forced air because it bypasses air and heats solids directly (stefan-boltzman). Furthermore, the thermal comfort is superior.

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