Heat Pump or Gas Furnace in Modesto? The Numbers on MID Power

Two bills from two utilities

Most Modesto homes pay two different utilities for heat. Electricity comes from the Modesto Irrigation District across greater Modesto north of the Tuolumne River and in Salida. Turlock and parts of Ceres are on the Turlock Irrigation District instead, and in Riverbank some streets are MID and some are PG&E. Gas, almost everywhere in the county, comes from PG&E.

That split is why the statewide heat pump argument does not transfer cleanly here. Most California comparisons assume one company sells you both fuels. A Modesto household buying power from an irrigation district and gas from PG&E has a different ratio between the two, and that ratio decides which system heats the house for less. To see yours, pull out a winter bill from each and do a few minutes of arithmetic.

Reading the per-kWh rate on an MID bill

Use a bill from December or January, when the house is using the most energy. Find the energy charge section, which lists the kilowatt-hours used and the price charged per kWh.

If more than one per-kWh price appears, use the higher one

A heat pump adds load on top of what the house already uses, so the extra kilowatt-hours land in whatever block you were already reaching at the end of the month. The lower price on the first block does not apply to them.

Leave the fixed monthly charge out

You pay it with or without a heat pump, so it does not belong in the comparison.

If you are in Riverbank or Ceres and are not sure who supplies your power, the name at the top of the bill settles it. TID and PG&E bills work the same way: find the price per kWh on the energy line, and take the higher block if there are two.

Reading the per-therm rate on a PG&E gas bill

PG&E splits gas into two per-therm charges: procurement, which is the gas itself, and transportation, which is the cost of getting it to your meter. Procurement is one price per therm. Transportation has two: a baseline price for usage up to your allowance and a higher excess price above it.

Add the procurement price to the transportation price for your per-therm rate. If the bill shows you went past baseline that month, which is common for a gas-heated home in a cold December, use the excess transportation price, because heating is the usage that pushes you over. If a public purpose surcharge is listed per therm, add that too.

Write the two numbers down: your per-kWh electricity rate and your per-therm gas rate.

The worked comparison

A therm of gas holds 100,000 BTU, which is the same heat as about 29.3 kWh of electricity. A furnace burns that therm and loses some of it up the flue: a 95 percent furnace delivers 95 percent of it, and an older 80 percent unit, the kind still running in a lot of 1980s Sylvan homes and 1990s Salida subdivisions, delivers 80 percent.

A heat pump does not make heat from electricity. It moves heat from the outdoor air into the house, and in mild weather it commonly delivers around three units of heat for each unit of electricity it uses. That ratio falls as the air outside gets colder and wetter.

Putting those together gives a short test.

Against a 95 percent furnace, multiply your per-kWh rate by ten

If the result is lower than your per-therm rate, a heat pump running at that efficiency delivers heat more cheaply than the new furnace would. If it is higher, gas wins on fuel cost.

Against an 80 percent furnace, multiply by eight instead

The old furnace wastes more of each therm, so gas has to be cheaper to beat the heat pump. The heat pump's bar is lower here than it is against a high-efficiency replacement.

For a stretch of fog weeks, multiply by eleven

Defrost cycles and near-freezing damp air pull a heat pump's real efficiency down, and this gives a fairer number for the worst part of the winter.

The multipliers are rounded and real efficiency varies with the model and the install, so treat a close result as a tie. Then the decision moves to the air conditioner, the rebate, and whether the gas line is staying.

Why the Valley winter suits a heat pump

The places where heat pumps struggle are the places with long runs of hard cold, where efficiency drops and backup heat carries the house. Modesto's heating season is the opposite: weeks of cool, gray days rather than deep freezes. That keeps a heat pump working in the range where the arithmetic above holds.

The summer matters as much as the winter. A heat pump is also the air conditioner, and Modesto runs July afternoons in the mid-90s with a recorded high of 113°F. If the outdoor AC unit is near the end of its life anyway, the real question is how much a heat pump costs over a straight AC replacement, rather than heat pump against furnace on their own. That is often the case in Village One, where the subdivisions went in from the late 1990s, and in Sylvan, where an original outdoor unit on R-22 usually ends in replacement once it leaks.

What tule fog does to defrost

Tule fog nights are close to the worst case for frost on a heat pump. The outdoor coil runs colder than the air around it, and when the air is just above freezing and saturated, the moisture freezes onto the fins faster than in a dry cold snap. The unit then switches into defrost to clear it.

Here is what a new owner will see

The outdoor fan stops, there is a whoosh as the system reverses, and a cloud of white vapor rises off the unit for a few minutes. Indoors, the air from the vents may feel cooler for a short time, or backup heat may come on to cover it. On a long fog night this can repeat several times. White vapor with no smell, clearing in a few minutes, is the system doing its job.

Some of it is worth a call

Ice that stays on the coil after a cycle ends, a burning smell, or dark smoke are not defrost. The Heat Pump and HVAC Repair page covers what to check and when to have someone look.

Leaves make it worse

In La Loma, where Dry Creek's trees drop into outdoor coils every fall, a matted coil moves less air, frosts sooner and defrosts more often. Clearing the coil before December helps a heat pump through fog season more than any thermostat setting.

Watch where the meltwater goes

Each defrost drops water under the unit. A unit set on a pad above grade, with somewhere for the water to drain, keeps it from refreezing into a sheet of ice on a side path.

Dual fuel when both systems are being replaced

If the furnace and the air conditioner are both due, there is a third option: a heat pump paired with a gas furnace, called dual fuel. The heat pump handles cooling all summer and heating on most winter days. When the outdoor temperature falls below a switchover point, the thermostat turns the heat pump off and the furnace takes over.

The bill arithmetic tells you where to put that switchover. If your multiply-by-ten result came out well under your therm rate, the heat pump should carry the house most of the winter, with gas held back for the coldest fog nights. If it was close, a higher switchover leans on gas more often and costs little to try.

Dual fuel keeps the gas line and a furnace to maintain, so it is not the cheapest system to install. It suits houses where the ducts and the gas line are already in place and in good shape. In the College Area and around Graceada Park, where central air was added to older houses decades later, it is worth having the ducts checked first. A heat pump sends out cooler air than a furnace and depends more on ducts that are sized and sealed properly. The Furnace and AC Replacement page covers what a replacement involves.

Rebates change the numbers

MID's rebate catalog pays more for a qualifying central heat pump than for a central AC, at each efficiency tier, which narrows the gap in upfront cost. TID has its own heat pump rebate for customers in Turlock and its service area. The amounts, tiers and paperwork are set out in the MID rebate guide on this site; check it before you price the two options against each other, because some conditions have to be met before the system goes in.