how do I determine which heater is more efficient?

A forced air electric furnace in our apartment cycles on an off via thermostat. In cold weather, set at a low daytime temp, it runs for about ten to fifteen minutes in an hour, I’d say. We also occasionally use, in closed rooms, small ceramic space heaters. The one in my office runs almost continuously. The furnace reads 3.0/3.7 KW. The heater is 1500 Watts. Is the furnace listing the amount it uses per hour? Does that mean that the furnace uses about twice as much electricity as the space heater if they’re both running? I’m wondering how to determine the best way to heat the house during the day. Am I better off running the furnace at a slightly more comfortable temp and not using the space heaters, or leaving the house relatively cold, but warming a single room with a space heater. Is there more information I need?

To my knowledge, electric heat efficiency doesn’t really vary much between heaters, since they all do the same thing - send electricity through a coil or other item with a relatively low resistance. In all other appliances, maximizing efficiency means eliminating waste heat, but since heat is the end product of a heater, there is no waste (which is why electric heat is considered very efficient). So it seems that using the smaller heaters to heat only the rooms you need heated is the way to go.

No “per hour” about it; that’s the amount it uses over any amount of time. Watts are power units, which means that they already have the “per time” included in them. So your small heater uses 1500 Joules per second, and your furnace uses 3000 (or 3700) Joules per second.

And I’ll agree with Running with Scissors: All electric heaters are essentially 100% efficient (except for heat pumps, which are effectively more than 100%), and it’s always more efficient to just have the heat where you are, and leave everywhere else cold.

Even if my furnace is electric, I’m sure it’s not particularly efficient. None-the-less, if it runs for only 10 minutes every hour and the space heater that uses 1/2 the watts every second runs all hour, aren’t I going to spend more money heating the little room than I will heating the whole house?

The thinking is that if you ran the electric furnace enough so that you felt as warm in your office as you do with the small space heater running, it would have to run much more than 10 minutes per hour, because it would probably also be putting more heat into spaces where you didn’t need it all the time.

Unless the small space heater is making your office warmer than your want, it should be more efficient to leave the furnace set at a lower temp, and use the space heater where you want it warmer than that.

If your furnace is electric, it’s pretty much 100% efficient. Tell me, what other form of energy would the “waste” be, if it wasn’t 100%?

Since the energy-cost-per-unit-of-heat is the same between the two, then all that matters is where the heat is applied. The more localized the heat, the better.

“Put the heat where it’s needed,” and “Localize the heat,” sound reasonable. I’m having trouble with the numbers, however, although I may not have enough data to work with, or it may be innacurate.
But - I run the furnace for maybe up to 15 minutes per hour, and the forced air heats the whole house to a moderately comfortable 72 degrees, using 3 kw per second, and I heat my office running the little ceramic heater 60 minutes per hour, using 1.5 kw per second. And it warms the room to around the same temp. Isn’t it cheaper to run the furnace and heat the house?

Right. If it’s only running 10 minutes, that’s probably because it’s warmer where the thermostat is, but the room is still cooler than if the space heater was used.

Also, am I thinking wonky, or is 3kW really small for a whole house (or single apartment apartment) heater? That’s only about 10,000 BTU (per hour), as seen here.

No, using 3 kW, and using 1.5 kW. If you insist on putting in the “per second” part yourself, then the unit you want to be using is Joules, not Watts.

Nitpick: 1 Watt = 1 Joule/sec. Watt is a measure of the instantaneous energy exerted. Saying “3 kw per second” or “1.5 kw per second” is redundant and misleading.

Just looking at your numbers, your space heater uses half the energy of your furnace. If it runs for 4 times the duration of your furnace, it will use twice the energy. Are you sure your heater is running the entire time? It seems to me that your office would be uncomfortably hot if this were the case. Perhaps you are hearing a fan and the heating element is only active for certain periods?

Yes, given those numbers it would be cheaper to run the furnace.

But looking at those numbers, something looks suspect, as ApexRogers points out. If your entire house, including the office, can be maintained at 72 degrees by 3 kw for 15 minutes per hour, it should not take 1.5 kw constantly to maintain only the office at the same 72 degrees.

It may be that the forced air furnace spreads the heat out more so it feels warm at a lower temp than you need to feel comfortable using the space heater. It could be that the space heater is not using the full 1.5 kw all the time (perhaps the heating element cycles while the fan runs constantly as pointed out above). It could be the although the space heater is rated for 1.5 kw, it’s actually only using something .8 kw, either because it’s defective in some way or was optimistically rated. Or perhaps the furnace is using more than 3 kw, because it’s been modified or the rating is wrong.

But something looks odd about those numbers. The only way to tell is to actually measure the current used by each device over a period of time while carefully monitoring the temperature, then compare the actual power used to the temperature achieved.

But generally speaking for electric heaters, watts in equals heat out at pretty much 100% efficiency.

The electric company charges you for each Kwh you consume.

Your furnace is rated at 3 or 3.7 Kw, correct? So if it runs continually for 1 hour, it consumes 3 or 3.7 Kwh (Kilowatt hours) of electricity.

Your space heater is rated at 1500 watts, or 1.5 Kw. If it runs continually for 1 hour, it consumes 1.5 Kwh of electricity.

If your electric company charges you \$0.20 per Kwh, the furnace costs you about \$0.60 to \$0.80 to run for an hour, while the space heater costs you about \$0.30 to run for an hour.

Slight tangent - I’m assuming that the 3.7 Kw for the furnace is some sort of peak. I would think normal running mode is closer to 3 Kw - but I dont know much about electic furnaces.

BTW - I did a similar thing last year when calculating using the central A/C or a window A/C at night. The central A/C runs at about 2800 watts, whereas the window unit runs at about 700 watts. The numbers are +/- a little, but the general ratio was 4:1. Basically it came out to be that if the central A/C would need to run for more than about 15 minutes per hour in order to keep the bedroom cool and comfortable - I’d be better off having the window unit run constantly from sundown to sun up (when I’d go to work). Since, in reality, the window unit would be cycling and running maybe 20-30 minutes out of every hour, it was cheaper to run the window unit.

So what others said - more efficient to deal with smaller spaces.

The ducts between the furnace and the rooms might be going through the attic or crawlspace, and might be uninsulated and/or leaky, which would cause waste.

That’s pretty much my calculations. And the furnace doesn’t run for an hour straight. It runs for a few minutes and then cuts off for several minutes. So it seems to me that there’s a balance point where letting the furnace heat the house costs as much as letting the heater heat the room.

A watt is the rate energy is delievered. Energy is sold by the Killowatt hour. A 3kw furnace running for 15 minutes in a hour consumes .75 KWH. A 1.5 kw or 1500 heater consumes 1.5 KWH every hour.

This assumes the heating element on and consuming max rate during the entire time.

Just wanted to bump this post, I’m quite curious about this. Is there a chance the decimal in the 3.0 and 3.7 is a speck of paint or chipped off the engraved plate or something? It seems to me that a furnace would have to be at least ten times that big, or else it’d be cheaper to just install baseboard heating.

But that is heat loss in the distribution system, not waste as YamatoTwinkie is using the term.

Electric resistance heating is 100% efficient.

Rather than looking at this problem from the supply side, the correct way to look at it is from a heat loss calculation for the space. Set up the heat transfer equations and the answer is simple - heat only the space you need heated. Additional energy needed to heat spaces you do not need heated is wasted (in the way Key Lime Guy is using the term). Using the furnace heats spaces you do not need heated. Using the space heater does not.

Yeah, this has me wondering also. Usually when two ratings are given for electric heat, it is for two different voltages, usually 208V and 230V (or maybe 240V). But the ratios don’t seem to match up here.

I know an electric heating element is %100 efficient but what about the inefficiency of its heat transfer and the wattage it takes to power the circulating fan?
Could that be considered waste?

What inefficiency of its heat transfer? All of the energy you put into the element is converted to heat. You will have distribution losses though, as **Key Lime Guy **pointed out. The space heater is already the best choice, and these losses only make it more so. Of course, it also depends where the ducts are located, if they run through a conditioned space, the heat loss through the ducts goes into the space you are trying to heat, so no loss. If the ducts are in a cold attic, then that heat is lost.

Likewise, the fan energy is converted to heat and ends up heating the air. If you are designing an air conditioning system, you have to take this fan heat into account when determining the total load. In a heating system, this fan heat works in your favor.