Your gas or electric bill arrives as one number, but the house spends energy on at least three jobs: heating the rooms, heating water, and standby losses from pilots, controls, and idling equipment. If you want to know what winter heat actually costs, pull those jobs apart. You do not need a data logger or a clamp meter for a first pass. You need twelve months of bills, a calendar, and an hour with a spreadsheet or a notebook.
Find the summer floor
Summer is the cleanest window in the year. With no heating call and no cooling, the usage is mostly water heating plus standby. Choose a summer billing period that looked normal to the household, with the usual number of showers and no houseguests or vacations. Divide the therms or kilowatt-hours by the days in that period. That daily figure is your floor.
Suppose July shows 14 therms over a 30-day period. That works out to about 0.47 therms per day. Treat that as the baseline for water heating and standby until a better number replaces it. If you have a second summer month, repeat the check, because a single bill can hide an unusual week.
Subtract the floor from a cold month
Now take a January billing period, say 90 therms over 31 days, or about 2.9 therms per day. Subtract the summer floor and you get roughly 2.4 therms per day that the cold weather appears to add. That is your heating share, with two honest caveats. Incoming water is colder in winter, so the water heater works harder than it did in July. Families also tend to take longer showers when it is cold. Both push some water-heating energy into the heat column.
To make the result useful, pair each billing period with the heating degree days for the same dates. Published degree-day totals are available for many weather stations. Divide the heating share by the degree days to get a rough therms-per-degree-day figure, then compare that figure across winters. If it climbs without a change in occupancy, something in the house has changed, such as a thermostat schedule, a duct leak, or a worn door seal. Those are separate jobs for a separate day.
Where a combi breaks the method
The floor method assumes you can subtract a stable baseline from a heating season. A wall-hung combi complicates that. ACHR News covered Navien's NCB700, describing it as "providing heating and hot water performance in a single wall-hung unit." When one burner serves both loads, a hot-water draw during a heating call runs through the same burner cycle. Your subtraction then assigns part of the water-heating energy to space heat, and the heating figure comes out too high.
The summer floor still works for a combi. With no heating call, the burner serves only hot water and standby, so the July reading remains a fair baseline. What changes is the winter interpretation. Treat the cold-month heating share as an upper bound on space heat, not a clean measurement. Keep a simple diary of showers, dishwasher runs, and laundry during the cold weeks. That diary lets you estimate how much hot-water demand overlapped with heating without touching the appliance.
Keep the result honest
Write the method down on the bill, not in memory. Note the meter read dates, the degree-day totals, occupancy changes, and any appliance swaps. If your calculated heating use per degree day does not match what the equipment should be doing, check the basics before you blame the equipment. Review the thermostat setpoint history, confirm the filter is clean, and compare how often the burner or compressor ran against the schedule you programmed. A bill can tell you that the house is using more energy. It cannot tell you why without the notes.