A winter gas bill that jumps can feel like proof that something is wrong with the furnace. Sometimes it is. But a heating bill is set by three separate things, and only one of them is the house. The other two are the price of the gas and how cold the billing period was. A bill can double with nothing wrong at home, and a bill can stay flat while the furnace quietly wastes fuel.
This guide sorts the three apart, using your own bill as the evidence. It covers where to find your rate, what a colder month does to usage, and what in the house can push usage up. It also covers what a homeowner can check safely and what is a job for a technician.
Three things set the bill
The U.S. Energy Information Administration, which publishes the federal government’s energy statistics, describes the weather’s part plainly. “Weather can affect household heating expenses in two ways,” its Winter Fuels Outlook says. “First, cold weather raises the amount of energy required to keep a house at a specific temperature. Second, because cold weather raises aggregate demand and can disrupt supply, it can cause energy prices to rise.”
That gives the three questions to ask of any high bill:
- Price. Did the cost of each unit of gas go up?
- Weather. Was the billing period colder, or longer, than the one you are comparing it with?
- The house. With price and weather accounted for, did the house use more gas than it should?
The first two are outside your walls. The third is the one a tune-up, a filter or a repair can change. Working through them in that order keeps you from paying for a fix to a problem the house does not have.
Step one: read your own bill
The bill already holds most of the answer. Before comparing totals, find four things on it:
- The usage, in therms or a similar unit. This is how much gas the house used, separate from what it cost.
- The days in the billing period. A 35-day bill in January and a 28-day bill in February are not a fair comparison.
- The charges, split between a fixed monthly amount and the charges per unit of gas.
- The same month last year, if the bill shows a usage history, or from last year’s bill if you kept it.
Compare usage with usage, not dollars with dollars. If usage is about the same as last year and the total is higher, the price moved. If usage is higher, the next question is whether the weather was colder.
EIA notes that degree-day data, the measure of how cold a period was, is sometimes printed on the bill itself: “Some electric and natural gas utilities publish degree-day data on their websites and some include it on customer utility bills.” If yours does, it is the fastest way to answer the weather question.
The price: where to find your rate
The rate is on your bill and on your utility’s published rate pages. It is worth reading from the source rather than from a news story, for two reasons.
First, the price of gas on a bill does not follow the wholesale market day to day. EIA explains that “for natural gas, changes in retail prices lag changes in wholesale prices largely due to the nature of utility regulation,” and that “some state utility commissions set the rates utilities can charge for natural gas deliveries a year or more in advance of billing.” It adds that “the timing and frequency of rate changes can vary by utility,” and that “in times of high price volatility, some utilities may adjust retail rates several times in a year.” A price change can reach your bill months after the headlines.
Second, part of every bill has nothing to do with the gas itself. EIA points out that “residential natural gas prices include charges to cover utility operating costs and the cost to transport and distribute natural gas, which are not directly linked to wholesale commodity natural gas prices.” Those charges can change through a rate case, which is a formal request by the utility to the state regulator.
A pending change for Xcel Energy gas customers
If Xcel Energy supplies your gas, one such rate case is in progress. Xcel’s own page on its 2025 Colorado natural gas rate review says that in July 2026 it “reached a settlement agreement with multiple parties”, and that “if the proposed agreement is approved, it will increase the average Colorado residential natural gas bill by about 8.6%, or $5.16 per month, starting in October 2026.” That is Xcel’s figure for an average bill, and it depends on the Colorado Public Utilities Commission approving the agreement. The proceeding is No. 25AL-0538G. Check your own bill and your utility’s rate pages for the rate that applies to you.
This guide does not quote a per-unit gas rate. Xcel’s rate summaries change with its gas cost adjustment, and a figure printed here would be out of date before most readers saw it. Your bill is the current source.
The weather: heating degree days
The weather question has a standard measure. A heating degree day is a count of how far a day’s average temperature falls below 65°F: a day that averages 50°F counts as 15 heating degree days, and a day at or above 65°F counts as none.
EIA uses exactly that example. “An average daily temperature of 50°F results in 15 HDDs (65 minus 50 equals 15). A 30-day month of those 50°F days would result in 450 HDDs for the month.” Add up the days in a billing period and you have one number for how cold that period was. EIA’s explainer puts it simply: “A heating degree day indicates a cold day and measures how much heating we need to keep a building warm.”
That is what makes it useful for a bill. If this January’s billing period had noticeably more heating degree days than last January’s, the house had more heating to do, and higher usage is what you would expect. If the degree days were about the same and usage still rose, the weather does not explain it, and the house becomes the question.
Price and weather can also move against each other. In EIA’s forecast update of April 7, 2026, the average U.S. household heating with natural gas was estimated to spend $686 over the winter of November 2025 to March 2026, 6% more than the winter before, while heating degree days for the country were estimated 3% lower. Those are national averages, not figures for any one house, but they show a bill rising in a winter that was not colder.
The house: where the gas goes
With price and weather accounted for, higher usage points to the house. ENERGY STAR puts the scale on it: “Nearly half of the energy used in your home goes to heating and cooling.”
Heating is not the only thing using gas. EIA notes that households heating with natural gas “may also use natural gas for water heating, cooking, or clothes drying.” A new household member, more laundry or a water heater problem can raise a gas bill in winter without the furnace being involved.
For the heating itself, the sourced causes of wasted gas fall into three places.
The ducts. ENERGY STAR estimates that “in a typical house, however, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.” Heat lost into an attic, crawlspace or garage is paid for and never felt. Leakage is measured with a pressure test, not guessed at.
The envelope. ENERGY STAR says that “sealing air leaks around your home and adding insulation can help your home be more comfortable and energy efficient and provide up to a 10% savings on your annual energy bills.” A house that loses heat faster than it should makes the furnace run longer for the same comfort.
The equipment. A furnace that is not moving or heating air as designed uses more fuel to deliver the same warmth. ENERGY STAR’s advice is that “a yearly tune-up of your heating and cooling system can improve efficiency and comfort.” On airflow, it is direct: “A dirty filter will slow down air flow and make the system work harder to keep you warm or cool — wasting energy.” The furnace manual adds that “improper filter maintenance is the most common cause of inadequate heating or cooling performance.”
What a homeowner can check
These checks involve nothing inside the furnace, the gas line or the wiring:
- The filter. Pull it and look. A clogged filter restricts airflow through the whole system. ENERGY STAR says to “check your filter every month, especially during heavy use months (winter and summer).”
- Clearances. Supply registers open and uncovered, return grilles clear, and nothing stored against or around the furnace.
- The thermostat. Check the setting and the schedule. A schedule that was changed, a setting nudged up on a cold day and never brought back, or a thermostat reading warm because a lamp or appliance sits next to it can all raise usage.
- The carbon monoxide alarm. Test it. It has nothing to do with the bill, but it belongs in any winter check of a gas-heated house. If it sounds, see what to do when a carbon monoxide alarm goes off.
Anything involving the burner, the gas supply, the venting or the electrical parts of the furnace is for a technician.
What a technician measures
A technician’s part is to measure what the homeowner cannot see. The furnace manual sets the targets: a furnace must be installed “with a duct system which has external static pressure within the allowable range, as specified on the furnace rating plate,” and “the air temperature rise must be within the range listed on the furnace nameplate.” Those two readings show whether the system is moving and heating air as designed. For what a full visit covers, see heating maintenance and tune-ups. A furnace that also runs in short bursts may have another problem, covered in furnace short cycling.
When the bill “doubled”
A bill that doubles is the most alarming version of this, and the three-part check applies to it the same way. Before assuming the furnace is at fault, line up the two bills:
- Were the billing periods the same length?
- Was the usage in therms double, or only the total in dollars?
- Were the heating degree days for the two periods similar?
- Did the rate or the fixed charges change between them?
A comparison of a mild autumn month against a cold midwinter month can show a large rise in usage from the weather alone. If usage doubled with similar degree days and similar billing days, that is a real change in the house, and it is worth a visit.
For help with the cost itself, Xcel’s rate review page directs customers who are struggling to pay to call 800-895-4999 or visit its Energy Assistance page. For rebates and credits on heating upgrades, see winter heating rebates and credits in Boulder. For the wider picture of heating costs and safety here, see the guide to winter heating costs and safety in Boulder, and for the equipment side, the guide to winter heating in Boulder.
Frequently asked questions
Why is my gas bill so high? A heating bill is set by three things: the price per unit of gas, how cold and how long the billing period was, and how much gas the house used. Check the usage and the billing days on your bill before the dollar total. If usage is steady, the price moved. If usage rose, compare the heating degree days before blaming the house.
What is the common mistake that doubles your heating bill? No single mistake is documented as the cause. The comparison is where readers go wrong: a short, mild billing period set against a long, cold one can look like a doubled bill with nothing wrong at home. Compare usage, billing days and heating degree days, not just the total.
Why is my gas bill so high when I hardly use it? Part of every bill is not the gas itself. EIA notes that residential natural gas prices “include charges to cover utility operating costs and the cost to transport and distribute natural gas.” A bill has fixed charges that apply whether the house uses a lot of gas or very little.
What uses the most gas in a house? In a gas-heated house, heating. EIA reports that in households heating with natural gas, “space heating accounts for most of the winter-season energy consumption and expenditures across the country.” Water heating, cooking and clothes drying use gas as well.
How much is the average heating bill in the winter? EIA’s forecast update of April 7, 2026 estimated that the average U.S. household heating with natural gas would spend $686 over November 2025 to March 2026. That is a national average for the whole winter, not a figure for Boulder or any one house.
How can I lower my heating bill this winter? Start with the homeowner checks: the filter, clear registers and grilles, and the thermostat setting and schedule. ENERGY STAR points to a yearly tune-up, sealing ducts, and air sealing and insulation, which it says can “provide up to a 10% savings on your annual energy bills.” Check your utility’s assistance and rebate programmes as well.
Sources
- U.S. Energy Information Administration, Winter Fuels Outlook 2025-26 (weather effects; wholesale and retail prices; space heating share; forecast update of April 7, 2026) – https://www.eia.gov/outlooks/steo/report/perspectives/2025/10-winterfuels/article.php
- U.S. Energy Information Administration, Degree days (definition; degree-day data on utility bills) – https://www.eia.gov/energyexplained/units-and-calculators/degree-days.php
- Xcel Energy, 2025 Colorado Natural Gas Rate Review (settlement agreement July 2026; proposed average residential increase; Proceeding No. 25AL-0538G; energy assistance) – https://www.xcelenergy.com/company/rates_and_regulations/rates/rate_cases/2025cogas
- ENERGY STAR, Heating and Cooling (nearly half of home energy; filter; yearly tune-up) – https://www.energystar.gov/saveathome/heating-cooling
- ENERGY STAR, Duct Sealing (20 to 30 percent of air lost in a typical house) – https://www.energystar.gov/saveathome/heating-cooling/duct-sealing
- ENERGY STAR, Seal and Insulate with ENERGY STAR (up to 10% savings) – https://www.energystar.gov/saveathome/seal_insulate
- Goodman, Installation Instructions for *M9S80 & *C9S80 gas furnaces (static pressure, temperature rise, filter maintenance) – https://documents.alpinehomeair.com/product/goodmangm9s8gc9s8installationmanual.pdf
