Product images: Amazon listings

MIZUDO 120,000 BTU Natural Gas Review

Name: MIZUDO 120,000 BTU Natural Gas
Brand: MIZUDO
SKU: B0F99S3Z14

7.6Very good · out of 10Rated from the listing's specs, no buyer reviews yet

Written from the listing's specs, not from our own use · How we rate

The MIZUDO 120,000 BTU Natural Gas ticks most of the boxes we care about: up to 5.1 GPM, app control and built-in freeze protection. It suits a cabin, a garage apartment or an in-law suite, and its sibling listing, the MIZUDO 199,000 BTU Natural Gas Indoor, holds #1 in our indoor natural gas tankless water heater ranking.

Variants of this model

These are sold as variants of one listing family. Each variant has its own page.

MIZUDO 199,000 BTU Natural Gas Indoor · 10.3GPM, 199,000BTU, Natural Gas · 9.2/10
MIZUDO 120,000 BTU Propane · 5.1GPM, 120,000BTU, Propane · 7.6/10

Price$539.99Price on Sep 30, 2026 · may have changed

Check the price on Amazon ↗Affiliate link · price and stock on Amazon

Compare side by side ↗

Specifications

Brand	MIZUDO
Category	Gas tankless water heaters
Fuel	Natural gas
Stated maximum flow	5.1 GPM
Stated gas input	120,000 BTU/h
Vent size	3"
Freeze protection	Stated in the listing
Wi-Fi	Built in, per the listing
Stated warranty	2 years
ASIN	B0F99S3Z14

Full listing title: MIZUDO 120,000 BTU Natural Gas Tankless Water Heater,Max 5.1 GPM,WiFi&APP

Listing evidence

Fuel	Natural Gas Full listing title
Stated maximum flow	5.1 GPM Full listing title
Stated gas input	120,000 BTU Full listing title

Updated October 2026

Exact listing, ASIN B0F99S3Z14

Rating from a published formula

Compared with its closest rivals

Two gas tankless heaters on natural gas too, with about the same flow, closest to it on our rating, each with a dated price.

Compare side by side ↗

MIZUDO 120,000 BTU Natural Gas

7.6Very good · out of 10$539.99 Price on Sep 30, 2026 · may have changed

Stated: 5.1 GPM · 120,000 BTU/h
Gas tankless water heaters

This product ↑

ThermoMate Condensing Natural Gas

7.7Very good · out of 10$559.99 Price on Sep 30, 2026 · may have changed

Stated: 5.28 GPM · 114,000 BTU/h · 120 V
Gas tankless water heaters

See the review ↗

Ranein Condensing Natural Gas, 5.1 GPM

7.4Good · out of 10$559 Price on Sep 30, 2026 · may have changed

Stated: 5.1 GPM · 120,000 BTU/h · 120 V
Gas tankless water heaters

See the review ↗

Up to 5.1 GPM from a 120,000 BTU burner

That's the maximum flow the listing states, and headline flows are usually quoted at a mild temperature rise. A cold January asks for a 63 °F rise (42 °F in, 105 °F out), and at that rise every tankless gives less than its headline; the maker's chart says how much. Counting 2 GPM per shower, as our calculator does, that's one shower and a sink at a time, maybe two showers where the water comes in warm.

What it has going for it

What changes in the morning shower rush

Wi-Fi app. Set the temperature, see the flow and get error codes on your phone, which beats reading a blinking code in the crawl space.

Freeze protection. Stated in the listing. Useful in a garage, a crawl space or a cabin that sits empty in winter, as long as the unit keeps power.

Natural gas or propane?

Natural gas: that's what the listing names, so it runs off the gas line, which may need upsizing for a tankless burner.

Propane: it's a different set of gas jets and pressures. Only the maker's conversion kit, if there is one, and a licensed gas fitter can switch it; buy the propane version instead if you have a tank.

Great for you if…

You like to see what your appliances are doing
It will live in a garage, a cabin or another cold spot

Skip it if…

You want a tight temperature hold: the listing claims no ±1 °F control
You'd rather wait for buyer reviews: none are captured for this listing yet
You want the high-efficiency kind: the listing doesn't call it condensing

Gas, venting and what's in the box

What the MIZUDO 120,000 BTU Natural Gas listing names:

Up to 5.1 GPM
120,000 BTU/h input
Natural gas
3" vent
Freeze protection named
Wi-Fi app control
CSA, ETL or UL mark named

Warranty & support: 2-year warranty, as stated in the listing · CSA, ETL or UL certification named in the listing.

That is the maker's promise, not ours. Keep the invoice, register the MIZUDO 120,000 BTU Natural Gas if the maker asks, and read the install conditions: many tankless warranties need a licensed install and regular flushing.

For the MIZUDO 120,000 BTU Natural Gas, the installation manual settles the venting, the gas line and the clearances.

Price and cost per GPM of stated flow

$539.99 Price on Sep 30, 2026 · may have changed

At the price on Sep 30, 2026, each GPM of stated maximum flow works out at about $105.88 (price ÷ stated GPM, our arithmetic). That puts it in the cheaper third of the 10 gas tankless heaters we have a price and a stated flow for.

The heater is often the smaller half of the bill: the venting, a gas line that may need upsizing and a licensed installer's day come on top.

The button opens the exact listing (ASIN B0F99S3Z14), where today's price, stock and seller apply. Sizes and variants of the same family often sit at different prices.

A better fit for your house?

Two showers and the dishwasher at once? The Rheem RTG 8.4 GPM Propane Outdoor (7.5/10) states up to 8.4 GPM.

Want the high-efficiency kind? The ThermoMate Condensing Natural Gas (7.7/10) is a condensing unit rated 0.86 UEF, per its listing.

On propane, not natural gas? The ThermoMate Condensing Propane (7.6/10) runs on propane.

Our take

The verdict

We like the MIZUDO 120,000 BTU Natural Gas for up to 5.1 GPM and app control. No buyer has reviewed it yet, so our 7.6/10 comes from the spec sheet alone. The one thing that bugs us: no stated temperature hold, while most picks here claim ±1 °F. We'd pick it if you like to see what your appliances are doing; otherwise, put it next to the ThermoMate Condensing Natural Gas first.

Check the price on Amazon ↗Affiliate link · price and stock on AmazonAll gas tankless water heaters →

Living with it

Install day

Old tank out, unit on the wall, vent through the wall or the roof (3" pipe, per the listing). Plan the gas line for 120,000 BTU/h; a licensed pro handles it, and often a permit.

The first shower

Hot water arrives after the cold already in the pipes, then stays. A trickle from a bathroom tap may not light the burner; open it a bit more.

The first winter

January water comes in colder, so the same unit gives fewer GPM. Stagger the showers if you sized it tight.

Once a year

Flush the heat exchanger with a pump and a descaler (the service valves make it an hour's job), and clean the inlet filter. Our flushing and descaling guide has the checklist.

The one thing we'd check first

Before the MIZUDO 120,000 BTU Natural Gas goes in your cart, check your gas line: 120,000 BTU/h is a lot of gas at once, so ask your plumber whether the existing line can feed it.

Before you buy

Your winter flow

5.1 GPM is the headline. In a northern January the water comes in near 42 °F, so a 105 °F shower needs a 63 °F rise and the real flow drops. Our flow-rate calculator runs the numbers for your house.

Vent and gas line

Ask the plumber to check the gas line size and the meter against the heater's input, and to quote the vent route at the same visit. Our install cost guide shows what that usually adds.

The warranty terms

2 years are stated. Read what it requires: a licensed install, the gas type and regular flushing are the usual conditions.

Flushing

Hard water scales the heat exchanger. Service valves on the water lines turn the yearly flush into an hour with a bucket and a pump; our flushing and descaling guide shows the routine.

How it compares

How the MIZUDO 120,000 BTU Natural Gas stacks up against the closest alternative, the ThermoMate Condensing Natural Gas.

Criterion	MIZUDO 120,000 BTU Natural Gas	ThermoMate Condensing Natural Gas
Our rating	7.6/10	7.7/10
Max flow	5.1 GPM	5.28 GPM
Gas input	120,000 BTU/h	114,000 BTU/h
Efficiency	Not stated	Condensing, 0.86 UEF
Temperature hold	Not stated	±1 °F stated

Pick the ThermoMate Condensing Natural Gas if you want our higher-rated option (7.7/10), with more flow (5.28 GPM) and a condensing design; stay with the MIZUDO 120,000 BTU Natural Gas for a bigger burner (120,000 BTU/h).

Compare side by side ↗See the ThermoMate Condensing Natural Gas review →

The score in detail

How we rated it

Spec-based rating on a 4.5–8.5 scale: 4.5 + average of the 4 criteria × 0.4. Capped at 8.5 because no customer evidence is included.

Key specifications

8 / 10

Build, warranty & certifications

8 / 10

Listing documentation

8 / 10

Fit & compatibility

7 / 10

This product has no captured buyer-review record yet, so its rating comes from the documented listing alone: four criteria scored out of 10 (key specifications; build, warranty and certifications; listing documentation; fit and compatibility), placed on a 4.5–8.5 scale. Read the full methodology.

Read the methodology →

Frequently asked questions

Is the MIZUDO 120,000 BTU Natural Gas any good?

We rate it 7.6/10 (very good). The rating comes from its documented listing alone, as no customer-review record is captured yet.

How much hot water can the MIZUDO 120,000 BTU Natural Gas deliver?

The listing states a maximum flow of 5.1 GPM and a gas input of 120,000 BTU/h. The headline flow is a mild-weather number: with cold winter water coming in, the same unit gives you less. Read the maker's flow chart at your winter rise, or let our flow-rate calculator do the sums.

What do you get with the MIZUDO 120,000 BTU Natural Gas?

Its listing states up to 5.1 GPM, Wi-Fi control from your phone and freeze protection. These are the seller's figures, not our measurements: how steady the temperature is in your shower and how it copes with your winter water can't be read from a listing.

What should I check before ordering the MIZUDO 120,000 BTU Natural Gas?

Your winter flow: 5.1 GPM is the headline. In a northern January the water comes in near 42 °F, so a 105 °F shower needs a 63 °F rise and the real flow drops. Our flow-rate calculator runs the numbers for your house. Vent and gas line: ask the plumber to check the gas line size and the meter against the heater's input, and to quote the vent route at the same visit. Our install cost guide shows what that usually adds. The warranty terms: 2 years are stated. Read what it requires: a licensed install, the gas type and regular flushing are the usual conditions. Flushing: hard water scales the heat exchanger. Service valves on the water lines turn the yearly flush into an hour with a bucket and a pump; our flushing and descaling guide shows the routine.

Where can I buy the MIZUDO 120,000 BTU Natural Gas?

We link to the exact listing on Amazon, where today's price, stock and seller apply. It is an affiliate link, and it has no effect on the rating.

Have you used the MIZUDO 120,000 BTU Natural Gas yourselves?

No, we haven't used it ourselves. Everything on this page comes from its listing, read line by line and weighed with our published formula.

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