Heating or Cooling Bill Doubled: What to Check

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Heating or Cooling Bill Doubled: What to Check

Short answer

Compare the same month a year ago rather than last month, because most shock bills are the weather plus a longer billing period. If the gap is still there, switch the thermostat right off, wait five minutes, and go and listen. Anything still running after that is costing you money every hour of every day.

A bill that doubles is frightening, and many of them turn out to be arithmetic rather than a fault. So this page does the arithmetic first, because it costs nothing and it stops you paying somebody to look at a machine that is fine.

If the numbers still do not add up after that, there is a five minute test that finds most of the rest.

Before you start

What to carry: the last two bills, the same month from last year, and a torch. Twenty minutes.

Gas or electric? It changes where to look. A gas bill jumping in winter is the furnace or the water heater. An electric bill jumping in winter, in a house with a heat pump, is usually the backup heat strips. An electric bill jumping in summer is the cooling side.

Sudden or gradual? A jump in one month is a switch, a stuck part or the weather. A slow creep over several years is age and dirt, and it is a completely different conversation.

One thing that is not a billing question, and one to be careful of. A gas bill that jumps with no weather to explain it, and any smell of gas. Do not treat that as a meter reading problem. Get everybody out and call the gas company from outside: what each smell means and which ones mean leaving.

A high bill is a sales lead. A free energy check offered straight after one, by somebody who also sells systems, tends to end in a quote for a new system. Do the free checks on this page first. Then you know whether anything is actually wrong before anybody measures your house.

Rule out the bill itself

Comparing January with October only tells you that January is colder. The honest comparison is the same month, a year apart.

Check 1

Four things on the bill before the machine

Compare the same month last year. Not last month. Heating in January against heating in October is not a comparison, it is a weather report.

Count the days. Billing periods vary. A thirty five day period against a twenty eight day one is a quarter more on its own, and it happens every year without anybody noticing.

Look at the rate. Utilities move their rates and their time of use bands, sometimes mid winter. The same energy at a new price looks exactly like using more of it.

Check for an estimate. An estimated reading followed by a real one produces a catch up bill that looks like a disaster and is really two months in one. The bill usually says which readings were estimated.

And what else changed in the house. A second freezer, a hot tub, somebody at home all day now, a car charging overnight. None of those are the heating, and all of them show up on the same bill.

The off test

Five minutes and a walk round the house. Anything still going once the thermostat is off is running around the clock.

Check 2

Switch it right off and go and listen

Set the thermostat to Off, not just down. Wait five minutes so that any normal shutdown delay has finished. Then walk round.

The outdoor unit still humming. The contactor has welded itself shut, so the compressor runs whatever the thermostat says. This is the single most expensive fault on the list and it is invisible from indoors: what keeps running and why.

The indoor blower still going. Either the fan is set to ON at the thermostat, which runs it every hour of every day, or a heat strip or relay has stuck.

The water heater. Harder to judge on gas. On an electric one, a tank that is scalding hot or a relief valve that weeps means an element or its thermostat is stuck on: how to test the thermostats.

If you can read your meter, take a reading with everything switched off, wait an hour, and read it again. A house with nothing running still uses something, but not much.

The things that genuinely double it

Check 3

In rough order of how much they cost

Emergency heat left on. On a heat pump this locks out the compressor and runs the electric strips instead, at two to three times the running cost, and the house still feels fine: where that setting is.

The fan set to ON. A blower is a motor running constantly, and it is one of the few things on this list you can fix from the sofa.

A lost schedule. After a power cut, a programmable thermostat can come back on a default schedule or on hold, and heat the house all night to the daytime temperature.

A frozen coil. The system runs continuously and delivers almost nothing, which is the worst possible combination: why a coil ices up.

A branch that has come off in the loft. You are paying to heat or cool the roof space, and the rooms never quite get there: how to find it.

A dirty coil, a clogged filter or low refrigerant. None of these doubles a bill on its own, but each one makes every single run longer, and together they add up to a real number.

Somebody changed the setpoint. Two degrees is worth more than most people expect, and it is worth asking before assuming a fault.

What it usually is not

It is rarely the age of the system. An old furnace or air conditioner costs more to run than a new one, but it gets there slowly over years rather than in a single billing period. Age does not explain a jump.

It is rarely the filter on its own, although a bad one makes everything else worse.

And it is rarely a fault you cannot hear. Nearly everything that costs serious money is something running when it should be stopped, which is exactly what the off test looks for.

If the bill has been creeping up for three or four years rather than jumping, then dirt, refrigerant charge and general wear are the right place to look, and the honest way to judge a system is on how long it has to run rather than on how it feels: how to tell a tired system from a busy one.

Questions people ask

The bill doubled and the house is no warmer.

That is the strongest sign of something stuck on, because paying twice as much for the same comfort means energy is going somewhere other than into the rooms. Do the off test first.

Can a smart thermostat actually save money?

It can, mostly by not heating an empty house. What it cannot do is fix a stuck contactor or a leaking duct, so it is worth doing this page before buying one.

My neighbour’s bill is half mine.

Different house, different insulation, different number of people and often a different tariff. It is not a useful comparison, and your own bill a year ago is.

Is it worth reading the meter myself?

Yes, and it is the fastest way to settle an argument with a utility. A reading now and another in a week gives you real numbers rather than an estimate.

The jump happened the month after a repair.

Worth a phone call to whoever did it. A fan speed left on high, a thermostat wired for the wrong changeover, or a strip left enabled are all easy to do and easy to undo.

Everything checks out and the bill is still high.

Then it is probably the house rather than the machine, and that is insulation, draughts and duct losses: where the heat actually goes.

Sources: U.S. Department of Energy Energy Saver Guide (2017) (heat pumps against electric resistance heat), Goodman service instructions RS6200006 (Comfort Alert code 8, welded contactor: compressor always runs), Honeywell T6 Pro installation instructions (fan On: fan is always on), and the blower off delays in Carrier 58SC0A/58SC1A and Lennox SL280UHV 508245-01 furnace instructions