Rinnai RE180iN review: the flow rate is real, the venting is the bill

Every other heater in this comparison is sold on a flow rate that needs a footnote. This one is not.

Rinnai advertises 8.5 GPM. Work it out independently: 180,000 BTU input at roughly 82% thermal efficiency is about 147,600 BTU of usable output, which at a 35 °F rise gives 8.4 GPM. The marketing figure and the arithmetic land within a rounding error of each other, which after four electric units quoting summer numbers is worth pausing on.

More importantly, the number that matters is also good. At a 70 °F rise — a genuine northern winter — this unit delivers 4.2 GPM. That is two simultaneous showers with margin. The largest electric heater here, at 28.8 kW and 120 amps, manages 2.8.

What non-condensing actually costs you#

This is where the price of the box stops being the price of the heater.

A condensing gas heater extracts so much heat from the exhaust that the flue gas comes out cool and wet — cool enough to vent through PVC, which is cheap, light and easy to run. A non-condensing unit like this one sends exhaust out at around 300 °F, which PVC cannot carry. It requires Category III stainless steel, sealed.

The practical consequences:

• Parts: $300 to $600 for a straightforward short run with a wall termination.
• A long horizontal run, a roof termination, or an awkward path through finished space can double that.
• Labour is higher too, because stainless vent assembly is more exacting than solvent-welding plastic.

Rinnai's own condensing RU series avoids all of this and vents in PVC, which claws back much of its higher purchase price immediately, then keeps saving through 94–96% efficiency against roughly 82%. If your household uses a lot of hot water, run that comparison before committing to the RE.

The RE makes sense when the vent run is short and simple, and when the lower purchase price outweighs a modest efficiency gap over the life of the unit.

The 90-day warranty trap#

The warranty on this unit is genuinely excellent, and part of it is easy to lose.

Registered within 90 days of installation:

• 15 years — heat exchanger, limited
• 5 years — parts
• 5 years — labour

Not registered within 90 days: the labour cover drops to one year. Everything else stands.

That sounds administrative until you consider what warranty service on a gas appliance involves. You cannot fix this yourself; any covered repair requires a qualified technician, and a call-out plus labour on a tankless heater runs $200 to $500 before parts. Losing years two through five of labour cover on an appliance expected to last fifteen years is a real financial exposure created by not filling in a form.

Do it the day the installer leaves. Keep the invoice — registration requires the installation date and usually the installer's details.

Where the gas advantage really comes from#

It is worth being precise about why gas wins here, because it is not simply "gas is better."

Electric resistance heating is 100% efficient at the point of use, and this unit is only about 82%. On efficiency alone electric wins. What gas has is energy density in the supply: a 3/4 inch gas line can deliver 180,000 BTU per hour without anyone rewiring anything, whereas delivering the electrical equivalent means 100 to 120 amps, three double-pole breakers, and frequently a service upgrade.

So the comparison is not really about the heaters. It is:

• Electric — cheaper box, sometimes far more expensive install, hard ceiling on output.
• Gas — dearer box, venting bill, but the supply can carry three times the power.

In a warm climate where 24 kW is enough, electric is usually the cheaper total. In a cold climate where you need 4 GPM, gas is the only route that does not involve rebuilding your electrical service. APUS against EcoSmart works through this decision in detail, and the electrical requirements for electric tankless units covers what the electric alternative demands.

Sizing it honestly#

The arithmetic, as ever:

GPM = BTU output ÷ (temperature rise × 8.34 × 60)

At 147,600 BTU of usable output:

• 35 °F rise — southern summer — 8.4 GPM
• 50 °F rise — mid-Atlantic — 5.9 GPM
• 70 °F rise — northern winter — 4.2 GPM

Against 2.0 GPM shower heads, the winter figure is two showers running together with capacity to spare for a sink. That is the definition of whole-home service, and it is why this unit is in a different conversation from the electric options. Our the GPM calculator runs the numbers for your own region, and how to size a tankless water heater explains the method.

One caution on gas supply: 180,000 BTU per hour is a large draw. A house already running a furnace, a range and a gas dryer may need the meter capacity and the pipe run re-evaluated. That is a gas fitter's calculation and it should happen before the unit is ordered, not after.

Gas supply is the other constraint people miss#

Venting is the cost that surprises people. Gas supply is the one that occasionally stops the project altogether, and it is worth checking before the unit is ordered rather than after.

180,000 BTU per hour is a large continuous draw — larger than most furnaces. The question is not whether you have gas, but whether the meter and the pipe run can deliver that much on top of everything else that might be running at the same time.

A rough audit of a typical house:

• Furnace — 60,000 to 100,000 BTU/hr
• Range — 40,000 to 65,000 BTU/hr
• Gas dryer — 20,000 to 25,000 BTU/hr
• This heater — 180,000 BTU/hr

Add those and you are somewhere between 300,000 and 370,000 BTU/hr of connected load. A standard residential meter rated at 250,000 BTU/hr cannot serve that simultaneously, and the utility has to uprate it — usually free, but on their schedule rather than yours.

Pipe sizing is the second half. Capacity falls with distance, and a 3/4 inch line that is adequate at 20 feet may not be at 60. This is a gas fitter's calculation against a sizing table, not a judgement call, and getting it wrong produces a heater that ignites and then drops out under load — a fault that is very frustrating to diagnose after the fact because everything looks correctly installed.

Ask your installer to confirm both the meter capacity and the pipe sizing during the quote. It is ten minutes of work at that stage and a second visit if it is skipped.

Who should not buy this#

• Anyone without a gas line. Running one for a water heater alone rarely pays back.
• Anyone with a long or complex vent path. Price the RU condensing version instead and vent it in PVC.
• Heavy hot water users. The efficiency gap compounds; the condensing unit is the better fifteen-year purchase.
• Anyone hoping to install it themselves. Gas, sealed venting, and a labour warranty contingent on correct fitting.

The verdict#

This is the best whole-home tankless heater in this comparison, and the one whose true cost is furthest from its listed price.

The hardware deserves the score. Honest specifications, 4.2 GPM in a real winter, a 15-year exchanger warranty and the deepest service network in the category are all serious advantages, and none of the electric units here can match the output at any price.

Just budget for the whole thing rather than the box: roughly $1,200 for the unit, $300 to $600 for stainless venting, and $800 to $2,000 for a licensed installer. Then register it within 90 days. Do those three things and this is a fifteen-year appliance that will not disappoint you in February.