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electric radiant heat 120 vs 240 need advice

Posted by rileysmom17 (My Page) on
Wed, Feb 10, 10 at 21:05

I am retrofitting an electric radiant heat system under carpet. It can be configured for 120 in series or 240 in parallel. The 120 volt system would draw 14 amps and the 240 system 15 amps. I am getting this from the bid documents and I hope I'm using the right terminology. I don't understand enough about electricity to know whether one system is significantly more efficient than the other in terms of electricity use to get the job done, ie raise the room temperature to 68 degrees. The 240 system is about 15% more expensive. Your thoughts would be appreciated.


Follow-Up Postings:

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RE: electric radiant heat 120 vs 240 need advice

Appears to be some confusion. If this is-one-and-the-same system, the amperes at 240 volts would be just half that of 120 volts.


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RE: electric radiant 120 vs 240

The original post, after some reflection, might be a spoof. The system as described would be connected in series for 240 volts and in parallel for 120 volts. If the documents really are as stated, have absolutely no further dealings with the purveyor of the documents or equipment. And if you are spoofing us, we prefer our fun in other ways.


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RE: electric radiant heat 120 vs 240 need advice

I'm not knowledgable enough on the actual specifics of design, but I've owned a home with still wired (off at the breaker) and potentially active electric radiant heat for 15 years. Our blower motor on our forced air furnace went out and had to be replaced & we had to rely on the radiant that was still wired as our heat source for a coupe of weeks. Electric radiant HEAT is very EXPENSIVE to use. Far more than forced air furnace type. Over carpet, it will also be very ineffective. You'd be throwing your money away. The owner previous to us installed gas fired forced air heating and it reduced the heating bills by 2/3. Our neighbor next door was just able to replace his electric radiant heat this past year due to the government incentives on energy efficient HVAC offered. He chose an electric heat pump system, and his bills have also gone down by at least half. Now, electric radiant is an extremely people comfortable system. But it's not cheap to run.

And, to avoid any confusion here, there is a difference in the floor warming systems used to warm floor tiles and electric radiant HEAT. Warm floor systems are designed to just take the chill off the floor, not as whole room or whole home heating systems. The warming systems typically cost pennies a day, and can be a nice (but expensive to install) luxury in a bath or kitchen.


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RE: electric radiant heat 120 vs 240 need advice

busman you should rename yourself Cynical. I'm just a regular person trying to get a better understanding of a product. Think of me as the ordinary homeowner trying not to get ripped off.

This is a system that uses 1/16th inch thick fabric pads under carpet with electrical wires running through them. It was explained to me that with the 240 system there must be an even number of paired pads. With the 120 system you can have an odd number of pads. This is the reason for the parallel wiring with 240 vs the series wiring with 120. Is this a spoof?

Obviously energy efficiency is important to me. I have a heat pump for the second floor and 50% of the heated space does not actually need to be heated, it is either laundry / storage or is warm enough from the gas heat rising from the first floor. I have baffled the ducts to the max, when I cut everything else off but the master suite the ducts sound like monsters live in them - the system is sized for the entire upstairs, not just my little part of it. Thus the radiant floor.

So my original question stands: would you install the 120 or the 240, and why?


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RE: electric radiant heat 120 vs 240 need advice

I think you need to talk to this company and confirm your numbers. Power = voltage x current. For the 240V option, if the current is more and the voltage is more, then the power(heat) is going to be more.

As for choosing a system, the guideline is about 12 watts/sq ft. If your numbers are correct, a 240V running at 15 amps = 3600 watts or enough to comfortably heat a 300 sq ft area. If the room is insulated etc and you only want to get to 68 degrees, you can probably heat more than that.

If you want comparisons to other heating systems, I suggest you try posting on the heating forum. They have experience with a variety of different systems.


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RE: electric radiant heat 120 vs 240 need advice

OK sorry for the confusion, one of the numerical values was given without a unit, and the designer kept talking about not having too many amps so I thought it was amps. I consulted someone who could look at the diagram and he tells me that what I said was amps was really watts.

So for the same wattage, is there an advantage of 240 over 120?

I appreciate your opinions.


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RE: electric radiant heat 120 vs 240 need advice

In terms of operation, systems of the same wattage will cost the same to run and provide the same amount of heat. The advantage of the 240V system is that will draw less current. That may or may not be important to you depending on the rest of your electrical needs.

Frankly though, you really need to have someone do calculations about what the cheapest way to warm your house is. A modern heat pump might be cheaper to operate in 100% of the space than a radiant system is for 50% of the space. Running electricity through a wire is typically the most expensive possible way to make heat.


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RE: electric radiant heat 120 vs 240 need advice

I found a cost calculator on the web page of the manufacturer which did ask specifically for information from my recent electric bill. At current rates they calculated an annual running cost of $395. I'm going to do my best to estimate KWH on the heat pump by comparing Jan and May bills (usually very little heat or cool in that month).


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RE: electric radiant heat 120 vs 240 need advice

I found this interesting.

Here is a link that might be useful: Look here


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RE: electric radiant heat 120 vs 240 need advice

I'm live_wire's neighbor who suffered with the radiant electric heat for the last 10 years we owned the house. I don't know how the previous owner, a retiree, managed. Well, we didn't suffer. We were toasty warm. But our wallets sure did! $700 electric bills for a 2000 square foot moderately insulated house--and in the SOUTH! The new heat pump hasn't even been through a complete winter, so I don't have an average, but this December was a lot colder than last year. The bill was down to $300! I was doing cartwheels!

We did decide to keep the radiant heat in the master bathroom. I LOVE it there. But, when we renovate it, we're looking to go with one of the newer tile warming systems instead. Even though I love it, I know it's too costly to run on a regular basis, so I've only turned it on when the temps were gonna be under freezing. Unfortunately, that was most of January, so our $400 bill for January probably has at least $50 worth of bathroom heat on the bill.


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RE: electric radiant heat 120 vs 240 need advice

I'll take a stab at this.

Each of these heater pads is a resistive heater element capable of handling 15 amps (or thereabouts). A single element can be connected to 120v and 'process' the 15amps. This makes it a 1800 watt heater. (This makes its resistance around 8 ohms.)

You can then connect each one of these in PARALLEL to 120 volts one at a time and the current requirement will add together. That is, 3 units would require a single 45 amp circuit.

For 240v service, you would connect two of these units in SERIES to keep the current at 15 amps. As you were told, you would have to add units in pairs in order to keep the current through any single unit to 15 amps.

To state whether 120v or 240v is better, from a pure wattage standpoint neither has an advantage. It would depend on how many units you were going to install and how you intend to control them. Let's say that 2 units are enough for your room. Then you would to either run 2 15-amp 120v circuits, one 30 amp 120v circuit, or 1 15 amp 240v circuit. From an installation standpoint, the single 15-amp 240v circuit would probably be the cheapest to install.

From a 'running it every day' standpoint, there is no difference.

From an efficiency standpoint, resistence heaters are probably the most expensive. The heat pump should be 2-3 time less expensive to operate than electrical resistance heat. I think putting these heaters under carpeting will just make it worse.

I see you've closed off all the vents upstairs. This is not good for the heat pump.

My suggestion would be (depending on the cost of these carpet heaters) to purchase some electric radiator-style heaters and put them in the room. OR an in-the-wall electric heater. My parents had this in a room at the far end of the house. It would get hot enough to run you out of the room.


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RE: electric radiant heat 120 vs 240 need advice

Thanks to everyone who got me untangled. I did some energy use calculations. I looked at my May 09 KWH = 491, compared to my Jan 10 KWH = 1310, the difference being 819 KWH. I decided to assign 90% of the difference to the heat pump (I have gas heat downstairs) = 737 KWH. This is 24 KWH per day. The manufacturer calculates 12 KWH per day to run the pads based on watts and 8 hour run time. So unless their math, or my math, is way off I don't see myself at risk for a major increase in electric use - if anything the opposite.

weeidmeister I haven' closed off all the vents, I just have baffles in them already to try to redirect air to the master suite and this has not been sufficient because in order to get an adequate effect, I DO have to shut off too many vents and clearly the system is working against higher airflow resistance and making noises about it. So I open the vents.


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RE: electric radiant heat 120 vs 240 need advice

120v x 14 amp = 1680w for 8 hours, that is 13kwh.

However, I assume that is "per pad." How many pads are you going to use? More than 2 and suddenly you are paying MORE to heat plus the installation costs.


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RE: electric radiant heat 120 vs 240 need advice

I have just received the "official" calculation of energy costs which is quite specifically based on watts, amps, hours of operation, including factoring in ramp up time, with 4 different temp set points throughout the day. The daily KWH came to 12.16, ie pretty much exactly what I got off the easier web site calculator. This is for the entire system, not per pad. At 20 cents per KWH and 4 months of heating the operating cost would be $291.60. Right now I'm paying 9 cents per KWH. Also I heat a little longer than 4 months but would probably reduce the floor temp in early Spring. If rates stay the same, it looks like < $200 per winter.


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RE: electric radiant heat 120 vs 240 need advice

Good luck, I'm highly suspicious of your/their calculations though. 1680w is the same as a room sized electric space heater. Unless you are heating only 1 small room, you are going to use more power than that.


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RE: electric radiant heat 120 vs 240 need advice

Dear Bill, I'd like to understand your concern. The energy use was calculated from watts and run time. This seems to me to be not open to question. That is, unless the run time data point is in error, you will have your total energy usage.

In the formal calculation, total heating time is calculated to be 8.3 hours per day. The pads are expected to cycle on and off and only draw power 50% of the time (within the 'on' cycle of the thermostat). So I guess the "mistake" would be that the pads are on for much more than 50% of the time, therefore much longer than 8.3 hours per day. The calcs are based on floor temp, not room temp. Thus the fundamental thought error is that a floor temp of 80 degrees would be sufficient to make me comfortable with the room temp.

Is this what you're getting at?


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RE: electric radiant heat 120 vs 240 need advice

Basically, yes. Electric heat is electric heat - whether it is laying on the floor as sitting up as a space heater. It doesn't become more efficient just because you lay the wires horizontally. 1680 watts will produce a fixed amount of heat - equivalent to a space heater. If you are only heating one smallish room, then a space heater would be fine. If you are trying to heat a large area, I think the company is greatly exaggerating the effectiveness of their product.


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RE: electric radiant heat 120 vs 240 need advice

The only advantage to using 240 volts is if the current is large enough on 120 V to require larger wire.

A 15 amp 240 V can use #14 wire and can deliver 3600 watts, but that must be derated to 80%, or 2880 watts of load.

A 120 V circuit could only deliver 1440 watts.

If you move up to #12 wire, the load size increases to 3840 watts on a 240 V circuit, and 1920 watts on a 120 V circuit.

The next wire size is #10, and it is rated for 30 amps.
This can be loaded with 5760 watts at 240 V and 2880 W at 120 V.

Now you can really see were the 240 V comes in handy for larger loads.
You can use the smallest wire gauge allowed, #14 to deliver the same 2880 W at 240 v that would require the much larger #10 wire at 120 V.


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RE: electric radiant heat 120 vs 240 need advice

The average wattage of our radiant heat is between 3000 to 4500 per room, depending on room size. (yes, we have a 800 amp service!) That wimpy 1600 watts will barely keep the floor warm. Especially under carpet, which will insulate the heat and won't allow radiant transfer, which is the whole point of using radiant heat.


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RE: electric radiant heat 120 vs 240 need advice

You say that energy efficiency is important to you. Resistive electric heat of any kind is about as INefficient as heat gets.

Doesn't matter whether it's radiant panels or baseboard units or a portable space heater or an oil filled radiator or your carpet pads. They all produce heat exactly the same way. They all will cost exactly the same amount to heat a room to the same measured temperature. They all are equally inefficient.

Now, the heater manufacturers will tell you that their heaters are 100% efficient. And they are. They turn 100% of the electricity coming into them into heat.

However, the process that makes the electricity is not 100% efficient.

Many power plants burn coal or natural gas. A coal plant typically converts about 35% of the coal's energy into electricity. Usually the remaining 65% goes up the stack as waste heat - heating the outdoors.

A natural gas plant is a bit more efficient, typically 50% IIRC (I'm sure someone will correct me if these numbers are outdated or I'm remembering them wrong).

Newer power plants better these numbers, especially cogeneration plants where they make use of the waste heat, but they still don't get to 100% or even 90%. And then there is further loss in the transmission lines to your home.

So which would you rather do? Heat your house with half of the energy in a cubic foot of natural gas, or heat it with nine-tenths of that energy? A good forced air gas furnace can easily be 95-96% efficient. Even a cheap one is 80% efficient.

No contest.

A natural gas fired hydronic heating system is just as cozy on the feet as your carpet pads. It may not be quite as efficient as a forced air system (depends on installation) but will still easily be in the 80% range or better.

Circumstances can alter the equation, of course. Maybe your electricity is generated not from coal or gas, but from falling water (hydropower) or even wind or solar sources. (We won't discuss nuclear energy so as to not "generate" controversy over that.)

Or maybe you don't have natural gas service where you live. That leaves LP gas, coal, wood, and a few other fuels. In a sunny region, solar heating may even be viable for part of your heating needs.

And of course there's electric heat. The most efficient electric heat choice might depend on where you live.

In cold climates, the most efficient electric heat system is a ground source heat pump ("geothermal" heat). It draws heat from the earth. By the "100% efficient" standard of a baseboard heater maker, a GSHP is typically 350% efficient! That is, 1 kWh of electricity gives you as much heat as 3.5 kWh in a carpet pad like you are considering. The downside of a GSHP is usually a relatively high cost of installation.

In a very mild climate, an air exchange heat pump will have almost as good a coefficient of performance (COP) as the GSHP. The colder it gets, the less efficient a conventional heat pump is. As you get close to 0 deg F an air heat pump drops to about the efficiency of baseboard heat or your carpet pads. A conventional heat pump is more costly to install than baseboards or carpet pads, but less than a GSHP.

This is really just a start, and we're getting off topic here. I suggest you take this discussion over to the HVAC forum.


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RE: electric radiant heat 120 vs 240 need advice

davidr thank you for your cogent and detailed explanation. Maybe I should add some details. Currently I have a 13 SEER heat pump for the second floor. The problem is that the "second floor" consists of 3 distinct functional zones - the laundry and a large storage area which do not need to be heated at all; the two guest bedrooms which are more than adequately warmed by heat rising from the second floor; and the master suite which exists in near total thermal isolation because the interior wall between the master and everywhere else is insulated. To warm the master suite with my 13 SEER heat pump I am also warming a lot of space that doesn't need it and I don't get any passive heat transfer through the walls thanks to the insulation. YES I have baffles where the vent runs leave the plenum and YES I adjust them but as per previous comments the system is sized for the whole square footage and is not happy when I restrict air flow too much to the other areas.

So to shorten the story, my intention is to use less electricity, not more, by turning the heat pump OFF and utilizing radiant heat to heat the only area that actually lacks for heat. I have tracked my KWH usage for several years. The system is getting installed Friday. So in pretty short order I should be able to judge the affect of the switchover.

I have cross posted on HVAC but gotten no replies.


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RE: electric radiant heat 120 vs 240 need advice

A $30 1600 plug in space heater would have provided the same amount of heat to your room as the very expensive (and much more difficult to install) heating system you have just chosen. If the room is insulated enough, the expensive radiant might even keep it warm enough without any other heat source.What R value are you dealing with for your build? Also, since you are installing this as radiant heat, are you also insulating under the wiring? That will help to keep the heat generated in the space you intended. But, the carpet will also act as insulation, stopping the radiation of the heat into the room. It may have been cheaper to reconfigure your heat pump. The air handler could potentially have been dialed down in speed to coincide with damping off the areas that you don't want to heat. I'd get a consult from the company that installed your heat pump about modifying it before I'd pull the trigger on the expensive proposition you're about to embark on.

Really, if you can back out of the contract, I'd suggest you do so and rexamine your options. You're not spending your money wisely in this instance.


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RE: electric radiant heat 120 vs 240 need advice

This is a retrofit in an existing house which I did not build. The heat pump is 3 years old. I have had my very patient HVAC contractor out many times and he has not offered "dialing down the system". It already has a variable speed fan, but I think you are talking about something different. The "inside" part of the heat pump is also located in the attic over the master bedroom so noise is a problem. I'm a light sleeper, that's just the way it is. I was advised for best efficiency to keep the thermostat within a 2 degree range, so that the auxiliary heat wouldn't come on, which means the pump runs at night.

I have tried radiant heat with deLonghi oil-filled radiators which created warm pockets but did not disperse the heat through the room. Most recently I tried a sun cloud space heater with infrared bulbs, which was noisy and could not be effectively utilized to preheat the bedroom for the morning (couldn't handle the noise).

I did think about this, and I realized that the money I had spent trying to work within my current system (including the $500 for the sun cloud, which fortunately I was able to return) was money that had not solved the problem. I need quiet even heat and radiant floor heating is the best way to get it. Once I have some energy use stats I will post here again, and I will be up front about the costs as well, which I think are lower than you might suppose.


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RE: electric radiant heat 120 vs 240 need advice

On this forum, some seek advice, some seek approbation.


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RE: electric radiant heat 120 vs 240 need advice

And others of us need to seek a dictionary now.....

for those interested - approbation = official approval


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