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The difference between a boiler that quietly devours fuel and one that extracts nearly every usable BTU often comes down to a single design decision: condensing versus non-condensing. If you are sizing a replacement for a drafty Victorian or speccing a new install for a tight modern envelope, the combustion efficiency curve determines your monthly operating cost for the next fifteen years. This is not a trivial upgrade — it’s a structural shift in how your home converts natural gas or propane into usable heat.
I’m Mohammad Maruf — the founder and writer behind Gardening Beyond. For this guide, I’ve analyzed the thermal efficiency ratings, heat-exchanger metallurgy, flow capacities, and fuel modulation ranges across eleven condensing and high-AFUE units, and cross-referenced those specs with aggregated owner feedback to isolate which models actually hold their rated performance in real-world installations.
Whether you are retrofitting a hydronic system or replacing a failing forced-air furnace, understanding the trade-offs between BTU output, UEF rating, and modulation control is the only way to avoid oversizing and undersizing — both of which destroy efficiency. This research isolates the top performers to help you identify the truly best efficient boilers for your specific heating load.
How To Choose The Best Efficient Boilers
Selecting an efficient boiler for a hydronic or forced-air system requires matching the heat output to your home’s thermal load, not to the existing unit’s plate rating. Oversizing by even 20% forces short-cycling, which drops real-world AFUE below the lab sticker. Undersizing leaves you cold on the coldest night. The following criteria are the non-negotiable filters I use before recommending any unit.
AFUE Rating and Condensing Technology
AFUE measures the percentage of fuel converted to usable heat. A 96% AFUE furnace loses only 4% of its energy up the flue. Condensing units achieve this by extracting latent heat from water vapor in the exhaust, which requires stainless steel or aluminum heat exchangers resistant to acidic condensate. Non-condensing units (typically 80–85% AFUE) cannot safely operate below a certain return-water temperature, making them less efficient in modern, low-temperature hydronic setups. For any new installation, insist on at least 95% AFUE.
BTU Output and Modulation Range
Raw BTU input tells only half the story. A burner that can modulate — down to 20% or 30% of its maximum output — matches heat delivery to demand during mild weather without cycling on and off. A 150,000 BTU condensing boiler that fires at 30,000 BTU on a 45°F day will deliver steadier temperatures and lower fuel consumption than a 100,000 BTU single-stage unit that must run at full tilt and then shut off. Look for turndown ratios of at least 4:1; 5:1 is better.
Heat Exchanger Material and Warranty
The heat exchanger is the heart of any boiler. Condensing boilers produce acidic condensate (pH around 3–4) that corrodes copper and mild steel quickly. Stainless steel (304 or 316L) or cast aluminum exchangers are standard in quality units. Warranty length on the heat exchanger is a strong proxy for manufacturer confidence: 12 years is common on premium models; 18 to 25 years signals exceptional durability. Ignore units whose heat exchanger warranty is shorter than 10 years if you plan to keep the boiler past year eight.
Flow Rate and Pump Integration
For combo units that provide both space heating and domestic hot water (DHW), the flow rate in gallons per minute (GPM) determines how many fixtures can run simultaneously. A typical 11 GPM unit can supply two showers and a dishwasher at the same time, provided the incoming water temperature isn’t near freezing. Dedicated boilers for hydronic heat don’t need high GPM, but they do require an efficient internal or external pump that handles the system head loss. Built-in variable-speed pumps save energy and reduce noise compared to fixed-speed alternatives.
Venting Material and Configuration
Condensing boilers produce low-temperature exhaust (typically 100–130°F), allowing the use of PVC, CPVC, or polypropylene venting instead of expensive stainless steel. This significantly reduces installation cost in retrofits. However, the vent run length and termination location must comply with local codes — sidewall venting through PVC is common but forbidden within certain distances of windows and intakes. Always confirm the venting material the manufacturer specifies before ordering; some units require a specific brand of vent pipe for warranty validity.
Quick Comparison
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| Model | Category | Best For | Key Spec | Amazon |
|---|---|---|---|---|
| Rinnai RXP199iN | Condensing Tankless | Large families | UEF 0.98 / 11.1 GPM | Amazon |
| Rinnai CX199iN | Condensing Tankless | Indoor/outdoor install | 97% Thermal Eff. | Amazon |
| Rinnai RU199iN | Condensing Tankless | Quiet operation | 11 GPM / Circ-Logic | Amazon |
| Goodman GR9T960603BN | Forced-Air Furnace | Mid-sized homes | 96% AFUE / 60K BTU | Amazon |
| Goodman GR9S960803BN | Forced-Air Furnace | Value-focused buyers | 96% AFUE / 80K BTU | Amazon |
| Rinnai i150SN | Condensing Hydronic Boiler | Hydronic space heat | 150K BTU / 95% Eff | Amazon |
| Goodman GR9T961004CN | Forced-Air Furnace | Large homes | 96% AFUE / 100K BTU | Amazon |
| Goodman GR9T961205DN | Forced-Air Furnace | Very large homes | 96% AFUE / 120K BTU | Amazon |
| Rheem RTGH-RH11DVLN | Condensing Tankless | Built-in recirculation | 11 GPM / 0.94 UEF | Amazon |
| Rheem RTGH-84DVLN-2 | Condensing Tankless | Smaller homes | 8.4 GPM / Compact | Amazon |
| KOHLER K-5526-NA | Electric Steam Generator | Steam shower systems | 7 kW / 112 cu ft | Amazon |
In‑Depth Reviews
1. Rinnai RXP199iN Smart Sense Condensing Tankless Water Heater
The RXP199iN is the most refined tankless unit Rinnai currently sells. Its stainless-steel heat exchanger is rated for 199,000 BTU input, and the Smart Sense gas control automatically detects whether you’ve connected natural gas or propane — eliminating the need for a separate conversion kit. The built-in variable-speed recirculation pump with Smart-Circ learns your household’s usage patterns, which reduces the wait time for hot water at distant fixtures without wasting energy during low-demand hours.
The condensing design hits a UEF of 0.98, meaning the unit recovers nearly every BTU from the combustion process. Owners with large families (five or more people) consistently report that simultaneous showers, dishwasher runs, and laundry cycles never cause a temperature drop. The Compact footprint — 58 pounds at 19 by 31 inches — lets it mount in spaces where a tank unit would never fit, and the Quick Flush system simplifies annual descaling.
The main trade-off is that the recirculation pump is noticeably louder than some expect. Owners who place the unit near bedrooms note that the pump’s hum in economy mode is still audible. The Rinnai Controlr module for WiFi monitoring is sold separately, which adds cost if you want remote management. Overall, the combination of ultra-high efficiency, fuel flexibility, and a 15-year heat exchanger warranty makes this the most balanced high-output tankless unit on the market.
What works
- UEF 0.98 thermal efficiency cuts gas consumption noticeably
- Smart Sense auto-detects NG or propane — no manual conversion
- Built-in recirculation pump with usage-learning logic
- Indoor or outdoor venting via Versa Vent system
What doesn’t
- Recirculation pump is louder than competitors’ units
- WiFi module not included — sold separately
- Professional installation strongly recommended
2. Rinnai CX199iN Condensing Smart Sense Commercial Natural Gas or Propane Tankless Water Heater
The CX199iN is essentially the commercial-grade sibling of the RXP series, sharing the same 199,000 BTU input and Smart Sense gas control but targeting higher-duty-cycle applications. Its 97% thermal efficiency rating is among the highest in any tankless unit, and the condensing heat exchanger is built to handle the sustained loads of a large household or light commercial setting. The Versa Vent system allows both indoor and outdoor installation, giving you placement options that a dedicated indoor-only unit cannot match.
Owner reports highlight the dramatic fuel savings from upgrading an older non-condensing unit. One user documented a 360 kWh reduction in the first month alone. The unit is also quiet in operation — an important detail for indoor installations where mechanical noise carries through the house. Bluetooth connectivity via the Rinnai Central App makes setup and monitoring straightforward, though the app’s range can be limiting in larger homes.
The main compromise is the height: at 30.11 inches, this unit is significantly taller than the RXP series, which may complicate retrofits in low-ceiling utility closets. The optional recirculation conversion kit is an added expense if you want instant hot water at distant taps. Despite these minor points, the combination of near-perfect thermal efficiency and installation flexibility makes this a top-tier choice for anyone who can accommodate the taller chassis.
What works
- 97% thermal efficiency — one of the highest available
- Indoor or outdoor installation with Versa Vent
- Quiet operation; suitable for near-living-space placement
- Smart Connect Bluetooth for easy monitoring
What doesn’t
- 30-inch height may not fit short utility closets
- Recirculation conversion kit sold separately
- Bluetooth range can be limited in large structures
3. Rinnai RU199iN Condensing Tankless Hot Water Heater, 11 GPM
The RU199iN is the workhorse of Rinnai’s tankless lineup — a model that has been on the market long enough to accumulate a deep reservoir of owner feedback. Its Circ-Logic technology enables pairing with an external recirculation pump, allowing both scheduled and on-demand recirculation to reduce hot water wait times. The 11 GPM flow rate is sufficient to feed up to seven fixtures simultaneously, and the ENERGY STAR qualification with a Uniform Energy Factor of 0.105 confirms its efficiency credentials.
Owners frequently cite the unit’s quiet operation and zero temperature fluctuation during simultaneous use. Even after two years, reports show stable performance with only routine annual flushing required. The unit’s 74-pound weight and compact 9-inch depth make it a manageable retro-fit, and the 15-year heat exchanger warranty provides long-term peace of mind. Many buyers pair this unit with a local plumber for installation and report significant savings versus purchasing through a contractor.
The primary drawbacks are tied to installation logistics. Amazon’s installation service often refuses to mount tankless units, forcing you to find a qualified local plumber. The Controlr module for WiFi connectivity is not included, and a few owners note that the initial hot water delay in winter can stretch to 30 seconds if the unit is far from the fixture. Still, for a proven, high-efficiency unit with strong resale value in the HVAC community, the RU199iN is a safe bet.
What works
- Proven long-term reliability in real-world installations
- Circ-Logic allows flexible external recirculation
- Very quiet operation — no burner rumble indoors
- 15-year heat exchanger warranty
What doesn’t
- Amazon installation often unavailable for tankless units
- WiFi module not included
- Cold-start hot water delay can be 15–30 seconds in winter
4. Goodman 96% AFUE Two Stage Upflow/Horizontal Multi-Speed 60k BTU Gas Furnace
The GR9T960603BN is Goodman’s two-stage, 96% AFUE furnace aimed at mid-sized homes where a single-stage 80% unit would waste fuel. The two-stage gas valve fires at low capacity during milder conditions, then ramps to full output when outdoor temperatures drop. This reduces temperature swings and keeps the blower running longer at lower speed, which improves air filtration and comfort. The multi-speed ECM motor is standard on this class, delivering up to 1200 CFM for cooling mode as well.
Owners who installed this unit themselves report that it fired up on the first attempt with very few issues — a testament to Goodman’s straightforward control board and terminal layout. The 17.5-inch cabinet width is a direct replacement for many older mid-sized furnaces, minimizing the need for duct modifications. The aluminized steel primary heat exchanger is backed by a lifetime warranty, and the two-stage operation keeps the home more comfortable than the old single-stage units it replaces.
The main limitation is the 60,000 BTU output, which is appropriate for homes up to roughly 1,800 square feet depending on insulation and climate. Owners who anticipate future additions or have drafty construction should consider the 80K or 100K BTU versions. A few buyers noted that missing fasteners in the blower assembly required a trip to the hardware store. For a straightforward, high-AFUE retrofit in a modestly sized home, this is a strong performer.
What works
- Two-stage burn reduces short-cycling and fuel waste
- ECM motor lowers electrical consumption vs. PSC motors
- 17.5-inch cabinet fits standard replacement footprints
- Lifetime heat exchanger warranty
What doesn’t
- 60K BTU may undersize larger or drafty homes
- Missing mounting hardware reported by some DIY installers
- Requires two-stage thermostat to realize efficiency gains
5. Goodman 80,000 BTU 96% Efficiency Multi-Speed, Single-Stage Upflow/Horizontal Furnace
The GR9S960803BN is the single-stage version of the Goodman 96% AFUE series, dropping the two-stage valve in exchange for a lower purchase cost. For homeowners in climates where extreme temperature swings are rare, the single-stage operation still delivers excellent efficiency because the unit runs at full output only when needed. The multi-speed ECM motor provides better airflow control than a basic PSC motor, and the 80,000 BTU input is suitable for homes in the 2,000–2,500 square foot range.
Owner reviews consistently praise the value proposition: a 96% AFUE furnace at a price point well below competitors. The units arrive packaged well, and DIY installers report a straightforward setup process. The aluminized steel primary heat exchanger is durable, and the 17.5-inch width fits standard cavities. The fact that this model directly replaces several older Goodman part numbers makes it a logical choice for those upgrading within the same brand.
The limitation is the single-stage burner, which cannot modulate. On a 50°F day, the furnace will fire at 80,000 BTU, heat the space quickly, then shut off — creating more temperature variation than a two-stage or modulating unit. The cabinet is not compatible with downflow configurations, so confirm your air handler orientation before ordering. For buyers who prioritize up-front savings over fine-grained comfort control, this is a compelling option.
What works
- 96% AFUE at a single-stage price point
- ECM motor improves electrical efficiency
- 17.5-inch width fits standard replacements
- Direct replacement for multiple older Goodman model numbers
What doesn’t
- Single-stage operation causes more temperature cycling
- Not for downflow installations
- Requires LP conversion kit for propane use
6. Rinnai i150SN Condensing Gas Boiler
The i150SN is Rinnai’s dedicated hydronic boiler — not a combo unit, but a pure space-heating appliance. It delivers up to 150,000 BTU through a stainless-steel heat exchanger with a modulating wire mesh burner. The condensing design captures heat that would otherwise escape up the flue, and the digital display provides real-time temperature and pressure readings. The integrated outdoor reset sensor adjusts the water temperature based on outdoor conditions, which prevents the system from overshooting on mild days.
Owners who replaced older cast-iron boilers report gas bill reductions of 40–50%, with one user claiming a 2.5-year payback period. The unit is compact — 68 pounds and 26.4 inches tall — making it suitable for wall mounting in tight mechanical rooms. The multizone control capability allows up to four separate heating zones without an external controller, which is a significant cost saving on a hydronic system. Owners also note the unit is exceptionally quiet compared to oil-fired boilers.
The installation is not DIY-friendly. The unit requires a licensed professional with knowledge of hydronic system design, and many owners spent well over on fittings, tubing, and a pre-made tee section. Some units have experienced E-110 error codes related to the gas solenoid valve, and while resets often clear the code, the issue may indicate a batch quality concern. If you are willing to pay for professional installation, the ongoing operating savings are substantial.
What works
- Dramatic fuel savings over non-condensing hydronic boilers
- Multizone control built in — no external controller needed
- Outdoor reset sensor prevents energy waste
- Very quiet operation
What doesn’t
- Professional installation adds significant total cost
- E-110 solenoid error reported by multiple owners
- Not a combo unit — separate DHW heater required
7. Goodman 96% AFUE Two Stage Upflow/Horizontal 100k BTU Gas Furnace
The GR9T961004CN is Goodman’s two-stage 96% AFUE furnace sized for larger homes. With 100,000 BTU input and a 21-inch-wide cabinet, it fits houses in the 2,500–3,500 square foot range depending on climate and insulation. The two-stage valve reduces fuel consumption during shoulder seasons, and the self-monitoring control board stores error codes for easy diagnostics — a feature that reduces service call costs over the life of the unit.
Buyers note the furnace is well-designed for installation in tight spaces like crawlspaces and attics, with the cabinet constructed from insulated sections to prevent air leaks. The aluminized steel primary heat exchanger is built for longevity, and the two-stage operation combined with the ECM motor keeps noise levels low. Many owners compare the unit favorably to more expensive brands, noting that the components (gas valve, ignitor, pressure switches) are standard across the industry, so replacement parts are widely available.
The biggest risk with this size unit is oversizing. A 100,000 BTU furnace in a well-insulated 2,000-square-foot home will short-cycle constantly, negating the efficiency benefits. A 21-inch cabinet also requires matching ductwork — older homes may need plenum modifications. A few owners have received units with shipping damage, but seller support has been responsive in those cases. Get a professional load calculation before committing to this BTU level.
What works
- Two-stage operation matches output to actual demand
- Self-monitoring control board with error code storage
- Insulated cabinet prevents heat loss
- Widely available service parts
What doesn’t
- Easily oversized for average homes — get a load calculation
- 21-inch cabinet may require duct modifications
- Shipping damage reported by some buyers
8. Goodman 120,000 BTU 96% Efficiency Multi-Speed, Two-Stage Upflow/Horizontal Furnace
The GR9T961205DN is the largest residential furnace in Goodman’s 96% AFUE two-stage lineup. At 120,000 BTU input and 2,000 CFM air flow capacity, this unit is intended for very large homes (3,500+ square feet), multi-story open plans, or buildings with significant heat loss. The two-stage operation is critical at this output level because a single-stage 120K unit would overwhelm a moderately sized space with a blast of heat followed by a long off-cycle.
DIY installers who have tackled this unit report that the design is logical and well documented. The 24.5-inch cabinet width is generous, but the furnace is actually 12 inches shorter than the old 19-year-old Marathon unit one owner replaced, requiring a spacer for the plenum. Users who have the routine maintenance skills to service their own equipment appreciate that the two-stage valve and ECM motor are standard components. The heat exchanger warranty and included compressor warranty add peace of mind.
The reliability is not universal — one owner reported a motherboard failure within the first winter, leaving the family without heat. While this appears to be an outlier among otherwise positive reviews, it’s a risk at this BTU level where the electrical load on the control board is higher. The 24.5-inch cabinet width is also restrictive: it will not fit standard 21-inch or 17.5-inch closets without framing modifications. This furnace is only appropriate for homes where a proper load calculation confirms the need for 120K BTU input.
What works
- 120K BTU capacity for very large homes and high heat loss
- Two-stage prevents full-blast short-cycling
- Compact height relative to older furnaces
- Standard components for easy servicing
What doesn’t
- 24.5-inch cabinet restricts installation locations
- Motherboard failure reported by one owner
- Oversizing risk is high — load calculation mandatory
9. Rheem Prestige Condensing Tankless Gas Indoor Natural Gas Water Heater With Built-in Recirculation, 11.0 GPM
The Rheem Prestige RTGH-RH11DVLN is a condensing tankless water heater with a built-in recirculation pump and a claimed UEF of 0.94. The Auto-Learn programming feature monitors water usage over time and adjusts the recirculation schedule to deliver hot water when you need it most, theoretically reducing waste compared to a constantly running pump. The water and gas leak detection system provides an extra layer of safety by shutting down the unit and displaying an error code if a leak is sensed.
The unit’s compact gray cabinet is designed to replace a traditional tank, and the included rough-in kit simplifies installation for a qualified plumber. The 11 GPM flow rate is theoretically sufficient for three simultaneous showers, and the stainless-steel heat exchanger carries a 25-year residential warranty — one of the longest in the industry. The combination of built-in recirculation and long warranty is attractive on paper.
In practice, owner feedback reveals serious reliability concerns. Multiple purchasers report the unit failing within two to three years, including cracked heat exchangers, failed main boards, and burned plastic flame sensor windows. The recirculation feature draws criticism for failing to deliver hot water to distant fixtures in a timely manner — some owners report ten-minute waits despite the pump running. Rheem’s warranty service has been described as slow and unresponsive, with one owner waiting four weeks for a resolution. This unit’s spec sheet is strong, but the real-world failure rate is too high to recommend without a long-term service plan.
What works
- Built-in recirculation and Auto-Learn programming
- 25-year residential heat exchanger warranty
- Leak detection system with automatic shutdown
What doesn’t
- Multiple reports of failure within 2–3 years
- Recirculation often fails to provide timely hot water
- Warranty service is slow and owner-unfriendly
10. Rheem Prestige High Efficiency Condensing Tankless Indoor Natural Gas Water Heater, 8.4 GPM
The RTGH-84DVLN-2 is the smaller sibling of the 11 GPM Rheem Prestige, sized for homes with lower simultaneous hot water demand. The 8.4 GPM flow rate is sufficient for one shower plus a dishwasher or washing machine, making it a viable option for a 1,000–1,500 square foot home with one or two bathrooms. The condensing design and compact 9.8-inch-wide cabinet allow installation in spaces where a larger unit would not fit.
Owners who downsized from a traditional 40-gallon tank report that the unit saves money on monthly bills and never runs out of hot water. The unit is described as very quiet during operation, and the included water and gas leak detection provides the same safety features as the larger model. At 77.9 pounds, the unit is manageable for a two-person installation.
Quality control is a significant concern. The packaging is widely criticized as insufficient for a unit — multiple owners received units with severe dents and damage. Internally, one owner reported a condensation line freeze in sub-freezing weather, and another experienced error code 12 after only five to six months, followed by a bungled service visit that created a gas leak. Rheem’s customer service is repeatedly described as incompetent. The core heating performance is acceptable when the unit works, but the risk of early failure and poor support makes this a gamble.
What works
- Compact 9.8-inch width fits tight spaces
- Never runs out of hot water for smaller homes
- Quiet operation and leak detection included
What doesn’t
- Packaging inadequate — frequent shipping damage
- Error codes and gas leak issues reported
- Customer service described as incompetent
11. KOHLER K-5526-NA Invigoration Series Steam Generator, 7 kW
The KOHLER K-5526-NA is an electric steam generator, not a boiler in the traditional home-heating sense — it is designed exclusively for steam shower applications. The 7 kW heating element produces steam in approximately 60 seconds, and the maximum volume rating of 112 cubic feet supports a customized shower enclosure. The stainless-steel construction and Power Clean technology offer one-touch automatic descaling, which simplifies maintenance compared to manual flushing.
Owners report that the unit transforms a standard shower into a spa-like experience, with consistent steam output and quiet operation. The compact aluminum casing is wall-mountable and integrates with the KOHLER DTV+ showering system for precise temperature and steam control. Multiple long-term users note that the unit typically lasts about four years before requiring replacement, which is in line with expectations for an electric steam generator in frequent use.
The main limitation is that this is not a whole-house heating appliance. It provides steam for a single shower enclosure and requires a separate control kit (sold separately) to operate. The 7 kW output limits the maximum enclosure size — larger rooms will need the 9 kW or 12 kW version. For its intended purpose as a dedicated steam shower generator, the KOHLER delivers reliable performance and fast heat-up, but it does not compete in the space-heating or domestic-hot-water categories covered by the rest of this list.
What works
- Delivers steam in 60 seconds
- Power Clean auto-descaling reduces maintenance
- Integrates with Kohler DTV+ system
What doesn’t
- Only for steam shower — not whole-home heating
- Control kit sold separately
- 7 kW may undersize larger enclosures
Hardware & Specs Guide
AFUE — Annual Fuel Utilization Efficiency
AFUE measures the percentage of fuel converted into usable heat over a typical heating season. A 96% AFUE furnace loses only 4% of its energy through the flue. The remaining 4% accounts for jacket losses and standby losses. Condensing units typically achieve AFUE ratings between 90% and 98%, while non-condensing units max out at around 85%. The U.S. Department of Energy mandates a minimum 80% AFUE for new furnaces, but any boiler below 90% is wasting substantial fuel in a modern home.
UEF — Uniform Energy Factor
UEF is the Department of Energy’s metric for water heater efficiency, replacing the older Energy Factor rating. A UEF of 0.98 means the unit is 98% efficient under standardized test conditions. While UEF is more commonly quoted for tankless water heaters than for boilers, condensing boilers also carry higher UEF numbers. The test protocol accounts for draw patterns, standby losses, and recovery efficiency, giving a more realistic efficiency picture than raw thermal efficiency alone.
BTU Input vs. BTU Output
Input is the raw energy content of the fuel burned per hour. Output is the heat actually transferred to the water or air, minus combustion and heat-exchanger losses. For a 96% AFUE furnace with 100,000 BTU input, the output is 96,000 BTU. Sizing a boiler or furnace requires calculating the home’s heat loss in BTU per hour (using a Manual J or equivalent load calculation), then selecting a unit whose output matches that number at the 99% design temperature.
Condensing Heat Exchanger Metallurgy
Condensing boilers produce acidic condensate (pH around 3–4) that corrodes copper and standard steel. Premium units use stainless steel (304 or 316L) or cast aluminum heat exchangers to resist this corrosion. Stainless steel is the most common material and offers a good balance of cost and durability. Cast aluminum heat exchangers, found in some European brands, offer excellent heat transfer but are more expensive. The heat exchanger warranty length (12–25 years) is a direct indicator of the manufacturer’s confidence in the material.
FAQ
What AFUE rating should I look for in an efficient boiler?
Can I install a condensing boiler myself?
What is the difference between a combi boiler and a system boiler?
How do I calculate the right BTU size for my home?
What venting material does a condensing boiler require?
Final Thoughts: The Verdict
For most homeowners seeking the best efficient boilers, the winner is the Rinnai RXP199iN because it combines a class-leading UEF 0.98 efficiency rating with Smart Sense fuel auto-detection, a built-in recirculation pump, and a footprint that fits indoor or outdoor installations. If you need a forced-air solution for a mid-sized home, the Goodman GR9T960603BN delivers 96% AFUE with two-stage modulation at a price that undercuts most competitors. And for a pure hydronic space-heating system, the Rinnai i150SN provides 150,000 BTU condensing output that can slash gas bills by up to 50% compared to an aging non-condensing boiler.