For overnight heating, is it more efficient to use an electric space heater in a bedroom or a whole house natural gas furnace?

Question

Is it more efficient to heat a bedroom with whole house's forced-air natural-gas burning furnace, or only an electric space heater in the bedroom?

For a very location-specific application, electric space heaters seem like an efficient option just by virtue of focusing the energy of heating on an easier area to heat. Especially with an older, draftier house that probably loses heat throughout the forced air delivery system, as compared to a warming space heater right at the toe of the bed or something.

Adding to this, electricity can be sourced from renewable sources, which can't be said for the fracked natural gas fueled furnace. So shifting energy use onto electric serves as a divestment from fossil fuels and an investment in renewables.

Answer 1

In terms of energy-efficiency within your own home (i.e. when we ignore energy losses outside the home), it will almost certainly be more efficient to use the electric space heater. It will be close to 100% efficient, and all the heat will get delivered where it's needed. So there will be reduced heat loss from the rest of the house. Fossil-gas furnaces tend to be inefficient (it could be in the range 60-80%, though a modern modulating condensing one could get up to 90%).

In terms of cost-efficiency, there's a good chance that using fossil-gas will be cheaper. Particularly if the electricity network you are on uses fossil-gas to generate electricity.

In terms of resource-efficiency and sustainability, the better option will depend on your electricity contract and supplier. If your buying of more electricity increases investment in renewables, or if your supplier is already 100% renewables, then the electric heater is definitely better. If your electricity supplier burns fossil gas, then both options are problematic, and the balance will depend on all the combined inefficiencies in each system: a CCGT gas plant will throw away 50% of the energy from the gas, and the transmission and distribution system might throw away another 5%. Your furnace might throw away 15-40%, and the internal losses within your house might be another 10-40%. If your electricity supplier burns coal, then the gas furnace will be better than the electric space heater.

Ultimately, the best resource efficiency will almost certainly come from properly insulating and ventilating the property so that very little artificial heating is needed in winter, and very little artificial cooling is needed in summer. The EnerPHit standard and accompanying guides are probably the best way to go about that.

Answer 2

EnergyNumbers has a good answer (+1), but why don't you measure it and let us know?

Of course "efficiency" is a little ambiguous. Green house gas production? Just your $$? The latter is easier, the former changes over time.

Get one of those power meters for the plug and see what it uses overnight. Most space heaters are ~1500 W, which is a lot. Your power bill has the rate and distribution costs.

The natural gas for the furnace is a little tougher. But you should have a monthly history. You can see when you turned your furnace on for the winter. If you have other gas appliances, then subtract the previous month and call the rest your furnace. E.g. turn on furnace in October, so subtract what you used in September and call the rest your furnace use. Take your winter monthly bill, subtract this base-use, divide by 30 (days/month) and that gives your daily cost of your furnace. You probably turn your heat down at night for sleep but also while your out during the day. It might balance out so call it 1/3rd at night (8 of 24 hours). (Maybe it's 6 hours of sleeping (1/4th), but with the furnace down you use it less, but it's colder at night - it probably all washes out).

So 1/3rd (or 1/4th) of 1/30th of your monthly winter gas bill (minus base use) vs. average (multiple measurements, please :) of 1 night of powering a space heater.

Again, if you're measuring $$, don't forget distribution charges, etc.

Normally I'd just say turn the furnace down and use a heavier blanket with thicker pyjamas and call it a day. But old houses can be leaky, and they leak faster when they're warmer (rate of heat transfer is directly proportionate to the difference in temperature). I have an old house and until I finish renovations, it's leaking heat in the winter. But you can seal windows in the winter (plastic cover) for rooms you don't use, etc. Plus I notice my furnace goes on more if I turn my furnace fan off at night because all the heat migrates upstairs and thermostat is on the main floor; it's more efficient to leave the fan on.

Mostly I want you to measure it and let us know. :)

Answer 3

If you shut off the heat to the rest of the house, the rest of the house gets cold. And while this reduces the gas bill, it doesn't reduce it as much as you think. You have cooled off the mass of the house, so it takes some extra gas burning time to make up for this.

To put numbers on this you would need to do the following:

• Record the amount of gas used every day at the end of a day's cycle. Say supper time.
• Record the day's high and low. You may be able to use the local met station's hourly recordings.
• In a spread sheet calculate the degree hours of heat you used. So for example when it's 40 outside and you have your thermostat set on 70, then from 8 to 9 you used 30 degree hours of heat. If you only use 2 temperature measures, assume that the high was at 2 p.m. and the low was 1 hour past dawn. Linear interpolation between these two times is sufficient.
• Do this for several weeks, alternating weeks where you did a thermostat set back, and used the electric heater, or just left the thermstat set at normal temp.
• On weeks you used the elecltric heater record the power it used overnight.

Now for each week, calculate the gas used. Use the degree heating hour figures to correct for warm weeks versus cold weeks.

For each week you used the electric heater (plugged into a Kill-o-Watt meter) Do a similar correct.

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Confounding factors. Since the bedroom isn't insulated on all walls heat will leak into the adjacent house rooms.