If your home uses an oil boiler with a domestic hot water coil, you’re not alone. These setups were common for decades — but when it comes to energy efficiency, they don’t hold up against modern water heating systems.
In this post, we’ll break down what UEF means, how domestic coils compare, and what your options are if you’re thinking about upgrading.
What Is UEF?
UEF (Uniform Energy Factor) is a standardized measure of water heater efficiency. It tells you how much hot water you get per unit of energy used, factoring in standby losses, cycling, and real-world performance.
- Higher UEF = better efficiency
- UEF replaced older metrics like EF (Energy Factor), and it’s now required for all new water heaters sold in the U.S.
But What About Domestic Coils?
A domestic coil uses your boiler (usually oil-fired) to heat domestic hot water through an internal heat exchanger — no separate water heater tank is required.
The catch? These systems are not rated with an UEF, because they’re not standalone water heaters. But that doesn’t mean we can’t estimate how they compare.
Comparing UEF Values: Domestic Coils vs Modern Water Heaters
| Water Heater Type | Approx. UEF | Efficiency Notes |
| Domestic Coil on Oil Boiler | ~0.40 – 0.50 | Low efficiency, constant standby losses |
| Standard Electric Tank | 0.90 – 0.95 | Reliable but electric rates may be higher |
| Standard Gas Tank (non-condensing) | 0.60 – 0.70 | Middle of the road |
| Tankless Gas | 0.82 – 0.95 | Efficient, endless hot water |
| High-Efficiency Gas Tank | 0.80 – 0.86 | Good mix of performance & value |
| Indirect Tank w/ High-Eff Boiler | 0.80 – 0.90 | Solid option if your boiler is modern |
| Heat Pump Water Heater | 2.5 – 3.5 | Top efficiency, great for long-term savings |
Bottom line: A domestic coil system is among the least efficient ways to heat water — especially if your boiler runs year-round just for hot water.
What Does That Mean for Your Wallet?
Let’s say you use 50 gallons of hot water per day, and your oil boiler runs at 85% AFUE (average). That could mean:
- 100–150+ gallons of oil per year just for hot water
- At $4/gallon, that’s $400–$600/year, even before space heating
In contrast, a heat pump water heater might cost under $150/year in electricity, depending on your rates.
Should You Upgrade?
If you’re dealing with:
- Inconsistent hot water
- High oil bills in the summer
- An aging boiler or hot water setup
…it might be time to consider switching. Popular upgrade options include:
- Heat pump water heaters (super-efficient, best savings long-term)
- Indirect tanks (if your boiler is high-efficiency and runs year-round)
- High-efficiency gas or electric tanks (better UEF, lower maintenance)
Final Thoughts
Domestic coils were a clever solution in their time, but today’s systems leave them in the dust when it comes to efficiency, comfort, and savings.
If you’re ready to lower your oil usage — and your utility bills — it’s worth exploring the new generation of water heating options.