Our readers keep the lights on and my morning glass full of iced black tea. As an Amazon Associate, I earn from qualifying purchases.9 Best Combi Boilers | 19″ Wide Furnace Fits Any Basement

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The Goodman GR9T960603BN pairs a 96% condensing heat exchanger with a two-stage gas valve and multi-speed ECM blower inside a compact 17.5-inch cabinet. Delivering 60,000 BTU of heating output and 1,200 CFM airflow capacity, it suits homes up to roughly 1,800 square feet in moderate climates without requiring the oversized footprint of a 21-inch unit. The two-stage operation keeps first-stage runtime longer during fall and spring, reducing the short-cycling that wastes fuel and creates temperature swings.

Buyers report that the ECM motor delivers noticeably quieter airflow than older PSC-based units, especially during the first minute after ignition when the blower ramps up gradually. Owners who replaced 80% AFUE furnaces from the 1990s saw immediate improvements in gas bills and comfort consistency. The unit supports upflow or horizontal left/right configurations, but the cabinet width demands precise measurement — it will not drop directly into a narrower 14-inch plenum without sheet metal modifications.

The 10-year parts limited warranty and lifetime compressor coverage reduce long-term ownership risk, though some buyers noted missing blower motor mounting fasteners on delivery, which required a trip to the hardware store before installation could proceed. Overall, this model offers the best balance of quiet airflow and fuel efficiency for homeowners willing to source installation labor separately rather than buying from a full-service HVAC contractor.

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The Goodman GR9S960803BN delivers 80,000 BTU of condensing heat with a single-stage gas valve and a multi-speed ECM motor inside a 17.5-inch cabinet rated at 96% AFUE. The single-stage valve provides straightforward operation — full gas flow every call for heat — which eliminates the complexity of two-stage thermostat wiring and makes this an excellent choice for homeowners replacing an older single-stage unit without upgrading their thermostat. The aluminized steel primary heat exchanger resists corrosion from normal combustion byproducts, though the condensing secondary exchanger still requires neutralization of acidic condensate.

Owners consistently praise the quiet fan operation and the ease of direct replacement when the old furnace occupied the same 17.5-inch footprint. The unit ships without a horizontal kit, so if your installation requires horizontal discharge, factor in the cost of sourcing that kit separately. The ECM motor ramps up smoothly rather than slamming on, which reduces the audible thump that single-speed PSC motors produce at ignition.

The furnace cannot be installed in downflow orientation — verify your plenum direction before purchasing. Some buyers recommend upsizing the BTU if future home additions are planned because the 80,000 BTU output is calibrated for the current square footage. For a simple, reliable condensing furnace with a strong warranty and proven Goodman parts availability, this unit delivers exactly what the spec sheet promises without surprises.

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The Rinnai i150SN is a wall-mountable condensing gas boiler rated at 150,000 BTU, designed for whole-home hydronic heating systems that distribute warmth through baseboard radiators, radiant floor loops, or in-floor tubing. Its stainless steel heat exchanger and modulating wire mesh burner vary gas input to match heating demand, avoiding the wasteful on-off cycling of traditional cast-iron boilers. The unit supports multizone control by connecting thermostat and pump wires directly to the boiler’s internal controls, simplifying installation for homes with separate temperature zones.

Owners report cutting gas bills dramatically — one buyer stated the unit repaid its cost in 2.5 years when paired with a separate tankless water heater for domestic hot water, eliminating the winter load conflict that occurs when a single boiler tries to supply both space heat and showers. The digital display shows temperature and pressure readings, and the outdoor reset sensor adjusts the water temperature based on outdoor conditions, further reducing fuel consumption during warmer winter days. Child-lock safety automatically disables central heating, preventing tampering.

Professional installation is strongly recommended because the gas piping, condensate neutralizer, and electrical connections require licensed expertise. A few units have thrown E-110 error codes related to gas solenoid valve behavior, though resetting cleared the issue temporarily. The 12-year heat exchanger warranty provides solid coverage. For homeowners building a new hydronic system or replacing an outdated boiler, the i150SN combines compact wall-hung design with modulation efficiency that older floor-standing boilers cannot match.

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The Rinnai RX199iN condensing tankless water heater delivers up to 11.1 GPM of hot water at a UEF of 0.98, making it one of the most efficient models for households with three or more simultaneous hot water demands. Its Smart Sense gas control auto-detects whether the unit is connected to natural gas or propane, eliminating the need for a manual conversion kit. The Versa Vent system allows indoor or outdoor installation—a rare flexibility that lets homeowners mount the unit on an exterior wall without running a vent through the roof.

Families with multiple teenagers report never running out of hot water even when two showers, a dishwasher, and a washing machine operate simultaneously. The compact suitcase-size body mounts flush to the wall and frees up floor space previously occupied by a 50-gallon tank. The Quick Flush feature simplifies annual descaling, reducing maintenance time compared to models that require disconnecting hoses and pumping chemicals manually.

The main restriction is the distance to the furthest fixture: buyers with long pipe runs (25+ feet) notice a delay before hot water reaches the faucet, which wastes softened water and increases water bills. Adding a recirculation loop or a Controlr WiFi module (sold separately) solves this but raises the total investment. Professional installation is recommended to size the gas line correctly and avoid nuisance lockouts. For households wanting maximum flow on a single unit with outdoor installation flexibility, the RX199iN is the benchmark.

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The Rinnai RU180iN provides up to 10 GPM of condensing hot water with Circ-Logic technology that pairs with an external circulation pump for scheduled or on-demand recirculation. This design reduces the wait time for hot water at distant faucets without running the pump 24/7, which wastes energy. The unit supports up to six simultaneous fixtures, making it suitable for mid-to-large homes where staggered usage is common. The UEF of 0.101 reflects ENERGY STAR qualification, though actual efficiency depends on inlet water temperature and flow rate.

Buyers who replaced 50-gallon tank heaters praise the space savings and the elimination of standby heat loss. The RU180iN ships with isolation valves, which simplifies future maintenance by letting the service tech isolate the unit without draining the whole system.

Some owners discovered after purchase that the built-in recirculation pump is only available on the RUR series, not the RU series, meaning the external pump purchase is not optional. Additionally, the Controlr WiFi module costs extra, and the 3-inch metal vent requirement caused cold-water bursts in one installation until the vent was replaced with PVC. For buyers who want recirculation-ready hardware with a strong warranty, the RU180iN delivers — but budget for the pump and WiFi module upfront to avoid sticker shock.

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The ThermoMate RTC68iN condensing tankless water heater achieves a 0.91 UEF through dual heat exchangers — a stainless steel condensing secondary and an oxygen-free copper primary — that capture waste heat from exhaust gases. Rated at 163,000 BTU with a maximum flow of 9.38 GPM, the unit supports three simultaneous showers at 2 GPM each when the inlet water temperature sits at 62°F. The intelligent water mixing valve automatically adjusts flow to maintain stable output temperature, eliminating the cold water sandwich effect common in non-condensing tankless units.

Buyers note the CSA safety certification and the eight integrated protections including freeze protection that activates at 41°F, making the unit viable for unheated basements in colder climates. The sealed combustion design keeps operational noise lower than open-chamber models, and the unit’s compact dimensions (14.57 x 28.44 inches) simplify wall mounting in tight mechanical rooms. The 0.91 UEF translates to roughly 17% reduction in annual hot water costs compared to standard non-condensing units, according to manufacturer estimates.

The condensing process produces mildly acidic condensate that must be discharged into a drain through a neutralizer kit, which is not included. Some buyers found the installation instructions vague on condensate routing. The unit’s flow rate drops when inlet water temperature falls below 62°F, so northern homeowners with cold ground water should confirm adequate GPM for their fixture count. For buyers seeking condensing efficiency at a mid-range investment, the ThermoMate offers strong value.

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The Rheem Mid-Efficiency 7.0 GPM indoor natural gas tankless water heater provides on-demand hot water without the condensing secondary heat exchanger, keeping the purchase price lower at the cost of reduced energy recovery. Its flow rate ranges from 0.26 GPM minimum to 7 GPM maximum, supporting up to three simultaneous fixtures in a typical home. The electronic ignition requires 200 watts of power and delivers reliable startup every time. The temperature range spans 80°F to 140°F, factory preset at 120°F for a balance of comfort and scalding prevention.

Owners report that the unit delivers consistent temperatures when multiple appliances run simultaneously, and the copper and stainless steel construction provides rugged durability. Buyers who previously owned Rheem tankless units report lifespans of 14 years or more, suggesting that the brand’s non-condensing design holds up well with basic annual descaling. The Econet compatibility allows integration with Rheem’s smart monitoring system for leak detection and temperature tracking, though the feature requires the Econet module for full functionality.

Several buyers caution that the venting requires 3-inch by 5-inch double-walled stainless steel pipe, not standard PVC — a fact buried in the manual that caused installation delays. The unit also does not include brass service valves or a condensation drain, both of which must be purchased separately. One buyer received a damaged used unit with visible water leakage, indicating inconsistent quality control at the distribution level. For buyers with a tight budget who want a brand-name tankless unit with proven longevity, the Rheem works — but factor in the unexpected venting costs.

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The MIZUDO 199,000 BTU non-condensing tankless water heater offers a 10.3 GPM maximum flow rate with WiFi and app-based control for remote temperature adjustment, scheduling, and maintenance alerts. Its six-stage gas modulation maintains water temperature within ±1°F, effectively preventing the sudden cold bursts that plague basic single-stage gas valve designs. The sealed direct vent system draws combustion air from outside, preserving indoor air quality and allowing installation in bedrooms or living spaces without backdraft risk.

Buyers highlight the straightforward installation process — the unit weighs only 55.5 pounds and measures 24.8 x 15.9 inches, making it manageable for two people to mount on a wall without special equipment. The 16-plus safety mechanisms include freeze protection rated to -13°F, which exceeds the capability of many tankless models that only guard to 32°F. The phosphorous-deoxidized copper heat exchanger resists corrosion better than standard copper, extending service life in areas with hard or acidic water.

The gas fitting uses a non-standard connection that required an unexpected trip to the hardware store for some buyers. The instructions provide unclear guidance on gas line sizing, which could lead to undersized supply lines and reduced output at high demand. Long-term reliability remains unproven because the brand has less market history than Rheem or Rinnai. For DIY-friendly homeowners who want app control and robust freeze protection without paying a premium for brand legacy, the MIZUDO delivers compelling features at a competitive tier.

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The Kohler K-5535-NA Invigoration Series steam generator produces steam within 60 seconds of activation using 15 kW of fast-response heating power, supporting enclosures up to 500 cubic feet. The stainless steel heat exchanger resists scaling and corrosion during daily steam cycles, while the Power Clean technology automates the cleaning cycle to flush mineral deposits without manual scrubbing. The unit integrates with the Kohler DTV+ showering system, allowing a single control interface for steam, temperature, and lighting.

Buyers who installed the generator report a transformed daily routine — stepping into warm, enveloping steam within one minute eliminates the long heat-up delay of older steam generators. The 50-pound unit installs remotely from the shower enclosure (typically in an attic or closet), keeping the mechanical noise out of the bathroom. The three-year limited warranty covers the heating elements and control board, though several owners note that the unit typically lasts around four years before requiring replacement under normal residential use.

The steam generator requires the K-5557 control kit or the K-5548-K1 DTV+ adapter kit for operation — neither is included, adding significantly to the total investment. The generator does not condition or soften water, so homes with hard water need a pre-treatment system to prevent scale buildup inside the steam chamber. For homeowners building a custom steam shower and prioritizing instant steam generation with Kohler’s ecosystem compatibility, the Invigoration Series delivers a premium experience that budget steam generators cannot match.