65,000 or 80,000 BTU? The Inlet-Temperature Math Behind Winter RV Showers

Buying an RV water heater on BTU alone is like buying a truck on horsepower alone: the headline number tells you what the machine can burn, not what it can carry up a hill. Two SEFFY models sit in the same 15×15 inch cutout, share the same plumbing spec and the same 32–52°C set range, and differ mainly in rated heat input — 65,000 BTU against 80,000 BTU. When those two quotes land on the same desk, the question is not which is stronger. It is whether your coldest month needs the extra headroom. Here is what the difference actually buys you, with the arithmetic shown.

The Physics: BTU Is Input, Rise Is What You Feel

Heating water costs energy in proportion to volume and temperature rise. The trade rule of thumb in North American units is 500 BTU per hour, per gallon per minute, per degree Fahrenheit — a figure that falls straight out of the specific heat of water and converts to the metric form without any change in physics.

Converted to the way the rest of this article reads it:

• 80,000 BTU ≈ 23.4 kW
• 65,000 BTU ≈ 19.0 kW

And the flow that a given heat input can sustain is:

Flow (L/min) = P ÷ (4,186 × ΔT) × 60

where P is in watts and ΔT is the rise in kelvin. Because 1 liter of water weighs 1 kg, that is the complete calculation.

Two cautions before the numbers. Rated heat input is input, not delivered heat — the arithmetic below ignores exchanger losses, so real outlet temperatures run slightly under what it predicts. And the rated flow on a spec sheet is quoted at a 25K rise, which assumes an inlet around 20°C. Late-autumn groundwater and an unheated fresh tank are nowhere near 20°C, and that is precisely why winter performance differs from the brochure.

Engineering: Where the Headroom Goes

On paper both models carry the same up-to-12 L/min rating at a 25K rise. The heat-load arithmetic shows where each model runs out of room:

• 10 L/min at a 25K rise needs 17.4 kW. The CZ-TGZ covers it with 6 kW spare; the CZ-B covers it with about 1.6 kW spare.
• 12 L/min at a 25K rise needs 20.9 kW. The CZ-TGZ covers it; the CZ-B's 19.0 kW sits about 1.9 kW short.

That shortfall is not a defect — it is what "65,000 BTU" means. In practice the CZ-B delivers 12 L/min at a slightly smaller rise, or holds the full 25K rise at slightly less flow. If you want 12 L/min and a full 25K rise at the same time, the 80,000 BTU unit is the one that does it without compromise.

The gap widens exactly when it matters most. Cold inlet water demands a larger rise, the rise consumes your heat budget, and flow is what is left over.

Both units also refuse to fire below a minimum start-up flow of 2.5 L/min (shut-off at 2.0 L/min), so there is a floor on how far you can turn the tap down before the burner drops out. That floor is independent of BTU.

The CZ-TGZ additionally carries a lead-free stainless steel heat exchanger, a circular rainproof anti-backflow flue, a keyed access door lock, and smart auto-drainage for No-Winterize Freedom. The CZ-B shares the exchanger, flue, and lock; at 11.8 kg it is marginally lighter than the CZ-TGZ's 12 kg.

This is where the BTU difference is actually spent. At a 4 °C inlet the two models separate by roughly 1.7 L/min — one shower running, or one shower with the galley tap open.

Comparison: CZ-TGZ vs CZ-B Across Inlet Temperatures

Flow figures are theoretical maxima for the rated heat input, ignoring exchanger losses.

The practical reading: in summer, both models are effectively identical. In a January cold snap with 4°C inlet water, the 80,000 BTU unit gives you about 9.3 L/min at 40°C and the 65,000 BTU unit about 7.6 L/min. Both still make a shower. Only one makes a shower you can run alongside a second draw.

Choosing Between Them Without Overbuying

The gap only matters under one condition: simultaneous draws in cold weather. That condition is easy to test before you order.

Read the third row carefully, because it is the decision point for most buyers. A shower at 6 L/min and a galley tap at 3 L/min together demand 9 L/min, and at 4°C inlet that is exactly the line where the two models separate. If your customers routinely run a tap while someone showers — and most families do — the 80,000 BTU unit is not overbuying. It is the unit that avoids the complaint.

If your market is warm, or your customers shower one at a time on a low-flow head, the 65,000 BTU unit does the same job for less money and less weight. That is a legitimate answer, not a compromise.

Three questions to settle it with your supplier before you order:

1. "What is the outlet flow at 4°C inlet, at a 40°C outlet, for each model?" Not the 25K rating — the winter figure. If the answer is only the rated number, you are being sold a summer spec.

2. "What is the minimum start-up flow, and the shut-off flow?" Both units start at 2.5 L/min and cut out at 2.0 L/min. This sets how far a tap can be opened before the burner drops out, and it is independent of BTU.

3. "Do both models use the same cutout and the same connections?" If they do, you can stock one opening and two price points. If they do not, you are stocking two installation kits.

Practical Pitfalls

• Do not compare BTU without the rise. A higher number at a bigger rise can deliver less water than a lower number at a smaller rise.
• Size for your coldest month, not your average. The brochure rating assumes warm inlet water; a northern winter does not.
• The 25K rating is a benchmark, not a promise. At 4°C inlet you are working against a 36K rise, and flow drops accordingly on either model.
• Auto-drainage is not a substitute for the procedure. It triggers automatically below 3°C on the CZ-TGZ, but you still open the valves in the documented four-step sequence at the end of the season.
• Measure the existing door before ordering. Both use the 15×15 inch standard panel, with an 18×18 inch oversized panel available for larger openings.
• These are exterior wall-mounted RV appliances vented to the outside. They are not portable units and are not for use in tents or cabins. Combustion produces carbon monoxide, which is colorless, odorless, and toxic.
• SEFFY RV heaters are designed to international gas-safety standards; we are actively preparing for EU CE certification, expected soon.