In short
UEF on a water heater is its efficiency rating: the fraction of the energy it uses that comes out as hot water in a standard 24-hour test. Higher is better. A standard gas tank scores about 0.60–0.70, a condensing gas tankless about 0.90–0.97 and a heat pump water heater 3.0–4.0. Only compare UEF between models in the same draw-pattern bin.
UEF on a water heater is the efficiency figure printed on the spec sheet and the rating label, and it is the number rebate lists are built around. It answers one question: of the gas or electricity this unit uses in a day, how much reaches the tap as hot water. The other water heater terms are in the glossary.
What UEF measures and typical figures by water heater type
UEF is energy delivered as hot water divided by energy consumed over a 24-hour simulated-use test. The heater is run through a set pattern of hot water draws with idle time between them, so the result captures burner or element efficiency, standby heat loss and cycling. UEF replaced the older EF rating, and because the test changed, the two cannot be compared directly.
- Standard gas tank: about 0.60–0.70
- Condensing gas tankless: about 0.90–0.97
- Electric tank: about 0.90–0.95
- Heat pump water heater: about 3.0–4.0
A heat pump water heater scores above 1 for the same reason a heat pump has a COP above 1: it moves heat from the surrounding air into the tank instead of generating it.
Only compare UEF within the same draw-pattern bin
Each water heater is tested in one of four usage bins, very small, low, medium or high, chosen by how much hot water it can deliver. The bins use different draw patterns, so a medium-bin 40-gallon tank and a high-bin 60-gallon tank were not put through the same test. The bin is printed beside the UEF on the spec sheet. Compare models inside one bin, and pick the bin that matches your household.
Comparing across fuels takes the same care. An electric tank at 0.92 is not cheaper to run than a gas tank at 0.65, because a unit of energy from electricity and a unit from gas carry different prices. UEF ranks heaters of one fuel type and bin; the utility rates decide the cost.
What UEF leaves out in an Ontario home
UEF is measured on a clean heater in a lab. Hard water in Halton and Hamilton scales tankless heat exchangers and builds sediment in tank bottoms, and efficiency drops unless the unit is maintained. For a tankless, that means an annual descale flush at $150–$300. UEF also says nothing about whether you will run out of hot water, which is a question of first-hour rating for a tank and flow rate for a tankless.
On rebates, UEF and the ENERGY STAR certification built on it are what eligibility lists reference. Ontario incentives for water heating are aimed at heat pump water heaters, and no current Ontario program pays a rebate for a gas tankless water heater. The current status of each program is on our water heater rebates page.
What a higher-UEF water heater costs to install
- Gas or electric tank water heater: $1,200–$2,500 installed, depending on size and venting
- Tankless water heater: $3,500–$6,500 installed, including gas-line and venting work
- Heat pump water heater: priced by written quote, because space, a condensate drain and electrical supply vary by house
A workable rule: do not replace a sound tank under eight years old for UEF alone, since the yearly saving is small next to the installed cost. When the tank is due anyway, compare the UEF of each option inside your usage bin and weigh it against the price difference. A heat pump water heater has the highest rating but needs the right room.
ZK Mechanical quotes water heater replacement and repair with the UEF and usage bin of each model listed. If your tank is leaking or you are comparing options, get in touch with the age and size of the current heater.
Questions people ask about UEF
What is a good UEF for a gas water heater?
For a gas tank, the mid-0.60s or higher is a solid figure, with power-vented models tending to score above atmospheric ones. For a gas tankless, condensing models reach 0.90 and up, while non-condensing models sit lower. Compare only within the same usage bin, and remember that sizing and venting decide which types will fit your house.
Why is a heat pump water heater's UEF higher than 1?
Because it does not make heat from electricity directly. A small compressor pulls heat out of the air around the unit and puts it into the tank, delivering three to four units of heat per unit of electricity. The trade-offs are practical: it needs enough air volume around it, a drain for condensate, and it cools and dries the room it sits in.
Is UEF the same as EF on an older water heater?
No. EF, or Energy Factor, was the previous rating and used a different test. The numbers look similar but are not interchangeable, so the EF on a twelve-year-old tank's label should not be set against the UEF on a new model's spec sheet. Compare new models with each other using UEF, inside one usage bin.