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I have a house which has a central heating and cooling system which I assume uses a heat pump. This central heating system is not zoned so when I turn it on, I need to heat the entire house.

I live alone so I generally occupy my bedroom or my living room and I have a small electric heater which can adequately heat up this room.

I'm wondering, how do I figure out at what point is it cost effective to use the central heating unit vs the electric heater?

Is it possible to work out roughly that say with everything being equal (eg. all rooms having the same insulation, the same desired temperature) if the room I was heating took up a certain percentage of the total area of my house, would it be better to use the central heating or the electric heater?

How much more efficient is a heat pump compared to generating heat from electricity?

  • What is the energy source for your central heating system? Natural gas? Electrical? Something else? Considering you are asking here, I assume you mean better from a sustainability point of view. How is the electricity generated? – gerrit Apr 7 '15 at 17:23
  • @gerrit - electricity is from the grid so likely coal. Even if we got solar it is mainly used at night. The central heating uses gas I'm pretty sure but for the sake of simplicity and like for like comparison, might be best to assume that they both from the same source and are both electric. – dizzy Apr 7 '15 at 17:40
  • @dizzy - please stop changing your question. It makes my answer look stupid, which makes me look stupid. – Sophit Apr 13 '15 at 18:49
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    @sophit I don't see any history of changes to this question – EnergyNumbers Apr 13 '15 at 19:36
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From a cost or sustainability perspective, and assuming identical efficiency and identical power sources, it's always better to heat just the one room.

Yes, the room has poor insulation and easily leaks heat into the rest of the house. At the most extreme, with near-zero insulation around the room, you would effectively heat the whole house by heating the one room. Thus the worst-case scenario is identical energy use, and the best-case is less energy use.

The reality is always more complicated because you're comparing an electric heater to a gas heater. Those sources have very different costs. There's also the personal comfort issue, concern over burst pipes if the weather is freezing, increased risk of a fire, and many other issues.

In practice, your best option is to just try it out and measure your costs. Be sure to discount the transition period, as it can take days for a house to fully cool. Then you can compare your electric heating bill (factoring in cost, transmission efficiencies, pollution caused by generation, pollution caused by transportation, pollution caused by mining, etc.) with your gas bill and make a determination.

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    @Dizzy specifically mentioned a heat pump. These are almost always mains electricity powered (although they do not have to be) and, very importantly, are usually 2 to 4 times more efficient in heat out per energy in that individual room heaters. This can skew the one versus many decision. – Russell McMahon Apr 11 '15 at 11:25
  • He changed the question after I answered it, which annoys me. It used to be about central gas vs an electric room heater, where the electricity was from coal power. – Sophit Apr 13 '15 at 18:46
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There are a number of considerations, which makes the decision harder than is at first apparent. A key factor is that if your central heating system does in fact use a "heat pump" as you suspect, then it is much more efficient than any other heating system.

Unlike all other heaters, a heat pump does NOT generate heat per se, but "lifts" heat energy from a cold source to a hotter "sink". The energy used is related to the difference between hot sink and cold source. A well designed modern heat pump can achieve efficiency gains of 4+ times compared to an electric heater when the temperature difference is not too severe and maybe a factor of 2 in colder conditions.

An electric heater converts electrical energy to thermal energy with 100% efficiency. A heat pump is 2 to 4 times better. This seems to 'break the laws of physics, but doesn't. A gas heater is not 100% efficient at the point of use, but as an electric heater depends on a conversion process at some other location, and as there are transmission losses, the overall efficiency may be lower or higher than for gas.

Heat energy used can be measured by multiplying heating power x time used.
So a 1 kW (1 kiloWatt electric) heater operated for one hour uses 1 kWh = 1 kiloWatt-hour of electrical energy. An electric heater typically makes 1 kW of power per "bar". A fan heater on low or a 1 bar heater will often be rated at about 1000 Watts, and a 2 bar heater or fan heater on high ~= 2 kW. A gas heater will typically be rated at 5 - 15 kW output depending on design and the setting relative to 100%. A gas heater will often be operated at a small percentage of full output. Heat pumps usually are 1000-2000 Watt output rated for entry level units and as much heat as you wish to buy at the top end.

If you use a heat pump to heat say 3 rooms of your home and keep internal doors open so that temperatures are relatively equal then in cold but not (literally) freezing outdoor temperatures a heat pump may often use less energy that a single bar heater in one room. As outside temperatures drop below zero the 1 bar heater in a room will probably use less energy.

  • Re: "An electric heater converts electrical energy to thermal energy with 100% efficiency." I tried to explain this to someone, with the understanding that in this case 'efficient' is more like a swearword. Electric heat is in all cases 100% efficient at turning cash in to waste. If it keeps you from freezing, then it is a necessary expense. But there are ways to get far more heat for the same amount of money. The other thing to remember is that every single watt of electricity becomes heat in your building. It doesn't matter what the electricity is doing. Efficiency is relative. – user2423 Jul 28 '15 at 23:59

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