Is Electric Radiant Heat Effecient?

[mkgrady]

The manufacturers of electric radiant heat claim their products are efficient. Is this real or a sales pitch?

I see a lot of radiant heat wires and mats being installed under tiles. Is the real reason to have it is so you can walk comfortably barefoot on the tiles? If it really is efficient then why is electric baseboard not efficient?

I have a job coming up where they want to install it on a slab in three rooms. The total square footage is about 500. I can just picture their electric meter spinning like a top and most of the heat sinking into the slab. Any feedback would be appreciated.

 

[dfmischler]

They are efficient in that 100% of the energy that you put into them becomes heat. It seems to me that the right question is are they economical to run? Or would burning natural gas or running a heat pump be cheaper? This gets complicated when you start looking at geothermal because the upfront costs and payback may be difficult to forecast.

Reading material on radiant heat

 

[dfmischler]

Useful conversions for comparing heating technologies: http://www.physics.uci.edu/~silverma/units.html

TLDR: One thousand cubic feet of gas (Mcf) -> 1.027 million BTU = 1.083 billion J = 301 kWh

Remember to apply efficiency factors to gas burning equipment when comparing to electricity.

On my last bills I paid 102.23 for 12 Mcf of gas delivered (equivalent of 102.23/(12*301) = .028 per kWh for heat),
and I paid $74.21 for 514 kWh delivered (effectively .144 per kWh). So gas is a lot cheaper for me to heat with.

 

[mkgrady]

Is it accurate to say that heating a home with electric baseboard or electric radiant under floors would use about the same amount of energy?

 

[GoldDigger]

Is it accurate to say that heating a home with electric baseboard or electric radiant under floors would use about the same amount of energy?

Ball park, yes.
But baseboard can respond much quicker to thermostat changes and so may be more efficient overall if time of day setbacks or intermittent use of the heated area is involved.
But radiant floor heating without drafts may deliver the same comfort level with less energy (lower thermostat setting.)

 

[dfmischler]

Older natural gas equipment was around 80% efficient (i.e. 20% of the energy went out the exhaust pipe). So you would consume about 125% of the energy heating with that equipment as you would if you heated with electricity. Newer equipment is claimed to be up to 97% efficient (so you would use only 103%). But with the cost of electricity in my area at over 5 times the cost of gas even at 80% efficiency you are still only spending 1/4 as much (at 97% efficiency it would be very close to 1/5 as much).

You might be able to gain some efficiencies with electric heat by heating occupied rooms to a comfortable temperature and unoccupied rooms to a lower temperature, but that is not likely to overcome the difference in energy costs.

 

[Greg1707]

supplemental heat

supplemental heat

I have been involved with several installations of electric radiant heat. I would not consider this to be a primary heat source. The purpose is to raise the temperature of the floor about ten to fifteen degrees. They do a great job of warming the floor and bare feet on the floor is quite a luxury.

Most of the products consume 12 watts per square foot. They have thermostats that can control the heat output. I do not believe that at 12 watts per square foot anyone will be bankrupted heating their kitchen and bathroom floors.

 

[Barbqranch]

I have electric radiant heat in my bathroom, and it is a most wonderful luxury. Going in there w/ bare feet on a cold morning is beyond delightful. I wouldn't use it for general heat, however, as it is much more expensive than gas, and it is slow to react.

 

[zbang]

Since the "operating efficiency" of electric and modern gas heat are close enough and unless you're concerned with the end-to-end thermal efficiency of gas vs electric, including mining, generation, distribution, etc etc, the better questions to ask are about cost to operate, cost to maintain, and the "quality of heat". By that I mean whether warm floors are important or whether you want to feel warm air around (the radiant heat vs hot air argument). There are also some subtle factors that can play in, like whether uninsulated hot air ducts are running through a crawlspace. Baseboard heating doesn't have that problem but it has others.

 

[kwired]

I have electric radiant heat in my bathroom, and it is a most wonderful luxury. Going in there w/ bare feet on a cold morning is beyond delightful. I wouldn't use it for general heat, however, as it is much more expensive than gas, and it is slow to react.

There are radiant heaters that use gas as the energy source as well. The main thing with any radiant heat is it heats objects instead of air - air is indirectly heated as those other objects transfer heat to it.

One common non dwelling application is shop buildings - say an auto repair shop or similar. If you open a vehicle door with forced air heating systems you let out a bunch of heated air, it takes time to recover that heated air. With a radiant heat system (regardless of gas or electric energy source) the floor, tools, equipment, etc. are all warmed up and they will recover that lost heated air much quicker, plus the space will feel more comfortable with the thermostat set at a lower temperature. These systems will run for longer time if turning them on when the room is cold until everything is heated up. But once everything is heated up they run much less then a forced air system will run. And when there is a loss of operation of the heating system it will take longer time to cool before it is as noticeable as it is for a forced air system.

Cost to operate is totally dependent on cost of the energy source used. More comfort at a lower temperature however will generally mean somewhat lower operation cost for a specific energy source.

 

[StarCat]

Choices

Choices

MK, my first trade is HVACR.
Back in Texas, my home state you mainly only see forced air systems for residential.
After working around some different systems in other parts of the country I changed my preference.

ON the subject of the under floor type your are speaking about I'd say no way I'd have it because its too complicated and difficult to repair in the area for what it does. A far far better option would be Electric Cove or Ceramic panels if Electric is the desired direction.

The only underfloor I'd ever consider using is a VERY well plumbed boiler fed hydronic system.
Even then I'd rather have baseboard Hydronic because its way more servicable. Also under floor hydronic will give you a major thermal lag time once warm that can cause problems in areas that have cool evenings and warm days, or as thus large swings in temperature day to night depending on the time of year.


The underfloor electric sounds just about as problematic as the old overhead style. In my view its a bad engineering idea, and I would try to steer anyone away from it. People are also failing to pay attention to the interlaced EM field that is now all over the place with this type of arrangement, never mind that any floor can be flooded.
The whole idea of having the floor warm does not come off as very practical.

The Ceramic panels are pretty interesting and are said to be more efficient than cove. Their rule of thumb is 6-8 watts/sq. ft. whereas cove is 10. When run intelligently they are definitely more efficient than forced air as you can have unoccupied zones dialed way down. Because of the way the ceramics operate and the waves they throw off they give quick response with respect to comfort. They are very easy to service if anything ever fails.

 

[kwired]

MK, my first trade is HVACR.
Back in Texas, my home state you mainly only see forced air systems for residential.
After working around some different systems in other parts of the country I changed my preference.

ON the subject of the under floor type your are speaking about I'd say no way I'd have it because its too complicated and difficult to repair in the area for what it does. A far far better option would be Electric Cove or Ceramic panels if Electric is the desired direction.

The only underfloor I'd ever consider using is a VERY well plumbed boiler fed hydronic system.
Even then I'd rather have baseboard Hydronic because its way more servicable. Also under floor hydronic will give you a major thermal lag time once warm that can cause problems in areas that have cool evenings and warm days, or as thus large swings in temperature day to night depending on the time of year.


The underfloor electric sounds just about as problematic as the old overhead style. In my view its a bad engineering idea, and I would try to steer anyone away from it. People are also failing to pay attention to the interlaced EM field that is now all over the place with this type of arrangement, never mind that any floor can be flooded.
The whole idea of having the floor warm does not come off as very practical.

The Ceramic panels are pretty interesting and are said to be more efficient than cove. Their rule of thumb is 6-8 watts/sq. ft. whereas cove is 10. When run intelligently they are definitely more efficient than forced air as you can have unoccupied zones dialed way down. Because of the way the ceramics operate and the waves they throw off they give quick response with respect to comfort. They are very easy to service if anything ever fails.

I do agree electric elements in the floor may not be a great idea. A hydronic in the floor system can be heated by any heat source though including resistance heat elements or even heat pumps, which kind of still makes it "electric heat" in those instances. I have worked in many repair shops that have put hydronic heat in the floor. It is a little expensive up front but everyone that has it loves it. Bring in a snow/ice covered vehicle and it is cleared off in no time. Bring same vehicle in a forced air heated shop and one day later it still may have some snow or ice on it unless the overall temp is set fairly high. Working on the floor under a vehicle? The floor is not cold like it is in a forced air heated shop. Guys that work in floor heated shops or even overhead radiant heated shops wear short sleeved shirts. Guys that work in forced air heated shops wear something heavier to keep them warm. Some have had more troubles then others with a particular boiler feeding it, but that is not the fault of the in floor heating system itself.

 

[petersonra]

NG costs me about $.45/therm right now.

Electricity costs me about $.08/ kwh. That's about $2.34/therm.

 

[charlie b]

They are efficient in that 100% of the energy that you put into them becomes heat.

True. But as you also pointed out, that is not the entire story. Another factor is the heater?s ability to change the temperature of the air in the room in which it is installed. We have in-wall heaters in all three bedrooms, in the dining room, and in the family room. They are all very effective in turning cold rooms into warm rooms. I would give most of the credit to their built-in fans. We also had a pair of 4-foot wide baseboard heaters in a large sitting room in the front of the house. These did not have fans, and it took a very long time for them to bring the room to a comfortable temperature. They are gone now. I removed them after we replaced the wood-burning fireplace insert with an insert that burns propane. Now we can get that room warm in just a couple minutes.

How does cost play into this discussion? It?s hard for me to say. I suspect it costs me more to heat a bedroom with the in-wall heater than to heat the front room with propane. But I have no hard numbers to share.

 

[petersonra]

True. But as you also pointed out, that is not the entire story. Another factor is the heater?s ability to change the temperature of the air in the room in which it is installed. We have in-wall heaters in all three bedrooms, in the dining room, and in the family room. They are all very effective in turning cold rooms into warm rooms. I would give most of the credit to their built-in fans. We also had a pair of 4-foot wide baseboard heaters in a large sitting room in the front of the house. These did not have fans, and it took a very long time for them to bring the room to a comfortable temperature. They are gone now. I removed them after we replaced the wood-burning fireplace insert with an insert that burns propane. Now we can get that room warm in just a couple minutes.

How does cost play into this discussion? It?s hard for me to say. I suspect it costs me more to heat a bedroom with the in-wall heater than to heat the front room with propane. But I have no hard numbers to share.

air is fast to heat because it has low mass. however, the floor and walls can still be cold because it takes a long time for the heat in the air to transfer into the higher mass floors and walls.

if all you care about is warm air, a small electric heater can do a lot of good for a small area.

 

[mkgrady]

NG costs me about $.45/therm right now.

Electricity costs me about $.08/ kwh. That's about $2.34/therm.

WOW. 5.2 to 1. I wonder that is a typical ratio. I pay about .20 per KWH but I have no idea what I pay for NG

 

[GoldDigger]

WOW. 5.2 to 1. I wonder that is a typical ratio. I pay about .20 per KWH but I have no idea what I pay for NG

Yes it is typical. With more variation the cost of propane heating is close to the cost of resistance heating.
In some areas a high SEER heat pump may even be cheaper than natural gas.
The cost of kWh and the cost per pound of LPG can both vary significantly across the country as distribution costs can be significant.

 

[kwired]

Depending on your location electric rates may or may not be favorable to use as a heating source and in places where it is at least favorable it can still vary over time. We have near the lowest electric rates in the country around here, but I think that gas prices have dropped enough recently that it is likely less expensive to heat with then resistance heat. There have been times where electric was less cost. It also depends some on whether you get on special rates with your POCO. Unlike some parts of the country, the general load on the grid is highest in summer months and it significantly lower in winter months. Some POCO's have some electric heat rates as they would like to add more load during the low consumption times - it still costs to have the infrastructure even though it is not being pushed very hard in those months. They even have rebates for installing heat pumps, used to have rebates for resistance heat a few years back but that has ceased.

About 10 years ago corn burning stoves were very popular - corn was cheap. Past few years corn was high priced but is now getting closer to what it was 10 years ago. These are about the same thing as wood pellet stoves, and some stoves burn either corn or wood pellets. Not sure how much heat you get from a bushel of corn though, so I don't know just how much it really costs to heat with one of those. I still think it costs more then some want to think. They tend to forget that just because their gas bill went down doesn't mean they saved any money, they need to factor in how much they spent on corn, wood pellets, or whatever other heat source they added uses.

 

[readydave8]

NG costs me about $.45/therm right now.

Electricity costs me about $.08/ kwh. That's about $2.34/therm.

Sometimes its a little tricky to compare. I heated an empty house with NG furnace last month, used 20.5 therms @ .399. Base charge brought the bill to $32.85, so really I paid about $1.60 per therm.

605 kWh cost was $90=0.1487 per kWh (commercial rate--EMC)

1540 kWh cost was $177=0.1149 per kWh (personal dwelling--EMC)

78 kWh cost was $21.61=0.277 per kWh (empty house--Georgia Power)

So here natural gas costs about 1/2 resistance heat?