Garage Construction Project

Boiler Selection

15 min read

Context

The 24’x40’ garage has PEX radiant tubing embedded in the slab (installed October 2025). The system needs a natural gas boiler to heat the radiant floor. The loft apartment above will have a bathroom, kitchen, and laundry. The main floor will have a shop sink. All water/sewer conduits are already stubbed through the slab.

Key question: Use a single combi boiler for both radiant heat and domestic hot water (DHW), or two separate appliances?

Constraints:

  • Natural gas (conduit in slab, gas service deferred to spring 2026)
  • DIY installation (owner-installed)
  • Mechanical closet under enclosed stair (space is limited)
  • Climate Zone 6 (Clare County, MI) — design temp approximately -5°F to 0°F
  • Sealed combustion / direct vent required (fume extraction system creates negative pressure — see Makeup Air for Fume Extraction)
  • Slab radiant is primary heat for main floor; mini-splits handle loft heating/cooling
  • Must be sized after Manual J load calculation (post-insulation)

Estimated heating load (rough): 40,000–60,000 BTU/hr for radiant floor (depends on final insulation values, garage door R-rating, and Manual J results)

DHW demand: Modest — loft bathroom (shower + sink), loft kitchen sink, main floor shop sink. Peak simultaneous draw estimated at 2–3.5 GPM.

Approach Categories

ApproachDescriptionUnitsGas LinesVent Runs
Combi boilerSingle wall-hung unit provides both space heating and DHW111
Boiler + tanklessDedicated heat-only boiler + separate tankless water heater222
Boiler + indirect tankHeat-only boiler + indirect-fired storage tank (no second burner)1 combustion + 1 tank11

Option 1: Navien NCB-E Combi Boiler (Single Unit)

Model range: NCB-E 110, 150, 180, 240 (input MBH) Likely size for this project: NCB-E 150 (150,000 BTU input / ~120,000 BTU output) Estimated cost: 500–800 (PVC venting, piping, fittings, gas connector, misc) Total DIY installed: ~$2,700–3,600

Pros

  1. Lowest total cost — Single unit, single vent run, single gas line. Cheapest path to both radiant heat and hot water.
  2. Smallest footprint — Wall-hung, roughly 17”W × 28”H × 13”D. Critical advantage in a tight mechanical closet.
  3. Best DIY ecosystem — Navien is the most popular combi in North America. Extensive YouTube installation walkthroughs, active forums (HVAC-Talk, Reddit r/HVAC), and a clear installation manual. More DIY knowledge available for this unit than any competitor.
  4. PVC/CPVC venting up to 60 ft — Condensing (95% AFUE), sealed combustion, direct vent with two-pipe PVC. No expensive stainless steel vent pipe. Easy to route through the wall.
  5. Built-in recirculation pump and stainless steel buffer tank — Reduces short-cycling on radiant loops and provides faster DHW delivery. Fewer external components to buy and pipe.

Cons

  1. DHW priority pauses radiant heating — When a hot water tap opens, the unit switches to DHW mode and radiant circulation stops (typically 5–20 minutes per call). However, the 960 sq ft concrete slab has enormous thermal mass — a 20-minute pause is imperceptible in floor temperature.
  2. DHW flow rate limited at cold inlet temps — At Michigan winter groundwater temps (~40–45°F), expect ~3.4 GPM at a 77°F rise. Adequate for one shower OR one sink, tight for simultaneous fixtures. The loft’s modest fixture count mitigates this.
  3. Single point of failure — If the unit goes down, you lose both heat and hot water until repaired. Navien parts are widely available, but downtime affects two systems.
  4. More complex internals than a heat-only boiler — The DHW heat exchanger, flow sensor, diverter valve, and buffer tank add components that can fail. More to diagnose and service.
  5. Heat exchanger scaling with hard water — Combi DHW passes potable water directly through the secondary heat exchanger. If Clare County water is hard (likely — most of mid-Michigan is), annual descaling or a water softener is recommended to maintain DHW performance.

Option 2: Rinnai i120CN Combi Boiler (Single Unit)

Model range: i060CN, i090CN, i120CN, i150CN, i170CN Likely size for this project: i120CN (120,000 BTU input / ~96,000 BTU output) Estimated cost: 500–800 (venting, piping, misc) Total DIY installed: ~$3,000–4,300

Pros

  1. Rinnai reliability reputation — Rinnai has been the tankless water heater market leader for decades. Heat exchanger and burner engineering is proven across millions of installations.
  2. 10-year residential heat exchanger warranty — Best-in-class warranty coverage gives long-term confidence for a DIY installer without contractor backing.
  3. Compact wall-hung design — Similar space savings to Navien. Single unit, single vent, single gas line.
  4. Sealed combustion with PVC venting — Same easy DIY venting as Navien. Condensing efficiency (95%+ AFUE).
  5. Excellent modulation range — Modulates down to low fire effectively, reducing short-cycling on mild days. Good match for a radiant system that wants low, steady output most of the heating season.

Cons

  1. Less DIY installation content available — Rinnai’s combi line is newer and more contractor-oriented than Navien. Fewer YouTube walkthroughs and forum threads specifically about DIY combi installs. The installation manual is thorough but written for professionals.
  2. Higher price for similar capacity — The i120CN costs $300–700 more than a comparably sized Navien NCB-E with no meaningful performance advantage in this application.
  3. Same DHW priority limitation — Radiant heating pauses during hot water calls, identical to any combi. Same thermal-mass mitigation applies.
  4. No built-in buffer tank — Unlike Navien, Rinnai combis don’t include an internal buffer. For radiant systems, an external buffer tank ($200–400) is recommended to prevent short-cycling, adding cost and taking space.
  5. Single point of failure — Same risk as any combi — one unit down means both systems down.

Option 3: Bosch Greenstar 8000iF Combi (Single Unit)

Model range: Greenstar 8000iF (79, 100, 131 MBH) Likely size for this project: 8000iF 100 (100,000 BTU input / ~95,000 BTU output) Estimated cost: 500–800 (venting, piping, misc) Total DIY installed: ~$3,500–4,800

Pros

  1. Aluminum heat exchanger — More resistant to scaling than stainless steel. Better longevity with hard water, which may be significant in Clare County. Less maintenance than stainless-based competitors.
  2. Excellent build quality and quiet operation — German-engineered with tight tolerances. Reported as the quietest wall-hung boiler on the market. Matters if the mechanical closet is near living space.
  3. Integrated outdoor reset control — Built-in logic adjusts water temperature based on outdoor air temp. Maximizes condensing efficiency and comfort on the radiant floor without needing an add-on controller.
  4. Strong condensing efficiency — 95%+ AFUE. Sealed combustion, PVC venting. Same DIY-friendly venting as other condensing units.
  5. Well-regarded in the hydronic heating community — Bosch/Buderus has deep roots in European hydronic systems. Respected by radiant floor professionals for reliability in heating-dominant applications.

Cons

  1. Highest cost among combi options — $800–1,200 more than a Navien NCB-E for the same job. Hard to justify the premium in a cost-conscious DIY project.
  2. Least DIY-friendly documentation and support — Installation manual is thorough but clearly written for licensed professionals. Very little YouTube/forum content about DIY installation of Bosch combis. Troubleshooting without contractor support is harder.
  3. Parts availability and lead times — European supply chain means replacement parts can take longer to source, especially in rural Michigan. Navien and Rinnai have much deeper US parts distribution.
  4. Smaller service network — Fewer Bosch-trained technicians in the Clare/Mt. Pleasant area if professional service is ever needed. Navien and Rinnai have broader coverage.
  5. Same combi limitations — DHW priority pause, single point of failure, and limited DHW flow rate at cold inlet temperatures. The premium price doesn’t eliminate any of the inherent combi trade-offs.

Option 4: Navien NHB-80W + Navien NPE-2 Tankless (Dual Dedicated Units)

Boiler: Navien NHB-80W (80,000 BTU heat-only wall-hung boiler) Tankless: Navien NPE-2 180S or 210S (180,000–210,000 BTU tankless water heater) Estimated cost: 1,000–1,500 (tankless) + 3,600–5,100

Pros

  1. No DHW priority conflict — Radiant floor runs continuously regardless of hot water usage. Each appliance operates independently. Zero impact on floor temperature during showers or dishwashing.
  2. Each unit optimized for its job — The NHB is purpose-built for hydronic heating (better modulation, no DHW compromises). The NPE-2 is purpose-built for DHW (higher flow rate, dedicated heat exchanger). Both perform better at their respective tasks than any combi.
  3. Redundancy — If the boiler fails, you still have hot water. If the tankless fails, you still have heat. In a Michigan winter, losing heat is the critical risk — this configuration ensures a tankless failure doesn’t take down your heating.
  4. Higher DHW capacity — A dedicated NPE-2 180S delivers ~8 GPM at 77°F rise — far more than any combi. Simultaneous shower + kitchen sink + shop sink with no flow rate concerns.
  5. Same brand simplifies maintenance — Navien NHB and NPE share similar control interfaces, error code systems, and maintenance procedures. One set of knowledge covers both units. Parts cross-compatibility for some components.

Cons

  1. Double the installation complexity — Two gas lines from the meter, two PVC vent terminations through the wall (with required separation), two condensate drains. Roughly twice the piping, penetrations, and connections of a single combi.
  2. Highest total cost — $900–1,500 more than the Navien combi option. The dedicated tankless is capable of far more DHW than this project needs — you’re paying for capacity you’ll never use.
  3. More wall space required — Two wall-hung units side by side (plus clearances) may be difficult to fit in the small mechanical closet under the stair. Verify dimensions before committing.
  4. Two appliances to maintain — Two annual flushes, two filter cleanings, two sets of potential error codes. Double the maintenance burden over the life of the system.
  5. DHW capacity is overkill — A dedicated tankless sized for a full house (8+ GPM) serving a loft bathroom, kitchen sink, and shop sink is like buying a dump truck to haul groceries. The combi’s 3.4 GPM handles this application’s actual demand.

Option 5: Navien NHB-80W + Indirect Water Heater Tank (Boiler + Storage)

Boiler: Navien NHB-80W (80,000 BTU heat-only wall-hung boiler) — or NHB-110W if Manual J requires more capacity Indirect tank: SuperStor Ultra or Triangle Tube Smart (30–40 gallon indirect-fired tank) Estimated cost: 1,200–2,000 (indirect tank) + 3,600–5,400

Pros

  1. Only one combustion appliance — Single gas line, single vent run (same as a combi). The indirect tank has no burner — it’s just an insulated tank with a heat exchanger coil inside, heated by boiler water. Simpler gas and venting than the dual-unit option.
  2. Stored hot water for burst demand — 30–40 gallons of stored hot water means back-to-back showers, filling a mop bucket, and running the shop sink simultaneously with zero flow rate concerns. Better than combi for real-world burst usage patterns.
  3. Indirect tanks are nearly indestructible — No burner, no electronics, no moving parts. A stainless-steel indirect tank can last 30+ years. The simplest, most reliable DHW component available.
  4. No DHW priority issue (when properly sized) — With a boiler large enough to serve both radiant and DHW recovery simultaneously (or with piping priority logic), the slab doesn’t pause during hot water calls. The tank stores enough hot water to cover gaps.
  5. Cleanest separation of potable and heating water — The indirect tank isolates potable water in its own vessel. Boiler water never contacts drinking water. Eliminates any scaling concern in the boiler’s primary heat exchanger from potable water.

Cons

  1. Indirect tank takes floor space — A 30–40 gallon tank is roughly 22–24” diameter × 40–48” tall. In a tight mechanical closet, this is a significant footprint that a wall-hung combi avoids entirely.
  2. Highest potential cost — Quality indirect tanks (SuperStor, Triangle Tube Smart) run 900–1,800 more than the Navien combi.
  3. Boiler must be oversized — To heat the radiant floor AND recover the indirect tank simultaneously, the boiler needs capacity above the radiant-only load. May push from an NHB-80W to an NHB-110W, adding $200–400.
  4. Most complex piping — Requires primary/secondary loop piping, a zone valve or priority relay for the indirect tank, a thermostatic mixing valve on the DHW output, and an aquastat on the tank. More fittings and connections than a combi’s integrated design. Highest plumbing skill requirement of all options.
  5. Standby heat loss — The indirect tank loses heat through its insulation 24/7, even when no hot water is being used. In a garage mechanical closet that may not be fully conditioned, standby losses are higher than in a heated basement. A tankless or combi only burns gas when there’s actual demand.

Comparison Summary

FactorNavien NCB-E CombiRinnai i120CN CombiBosch 8000iF CombiDual Navien NHB + NPENavien NHB + Indirect
DIY installed cost$2,700–3,600$3,000–4,300$3,500–4,800$3,600–5,100$3,600–5,400
FootprintSmallestSmallSmallLarge (2 units)Medium (unit + tank)
DHW priority pauseYes (mitigated by slab mass)YesYesNoMinimal
DHW flow rate~3.4 GPM~3.5 GPM~3.5 GPM~8 GPMBurst from tank
DIY friendliness★★★★★★★★☆☆★★☆☆☆★★★★☆★★★☆☆
ReliabilityVery goodExcellentExcellentVery good (×2 units)Excellent (tank lasts 30+ yr)
Gas lines / vents1 / 11 / 11 / 12 / 21 / 1
Maintenance burdenModerateModerateLow–ModerateHigh (2 units)Low (tank is passive)
Single point of failureYesYesYesNoYes (boiler only)
Scaling risk (hard water)HigherHigherLower (aluminum)SplitLowest

Recommendation: Navien NCB-E Combi Boiler

The Navien NCB-E combi is the best fit for this project. Here’s why:

The DHW Priority “Problem” Isn’t a Problem Here

The most common objection to combis is that radiant heating pauses during hot water calls. In most residential applications (lightweight subfloor systems), this matters. In this garage, it doesn’t. A 960 sq ft, 4–6” thick concrete slab is a massive thermal battery. It takes hours for the slab to lose a noticeable degree of surface temperature. A 10–20 minute pause for a shower is thermally invisible. The slab’s thermal mass turns the combi’s biggest weakness into a non-issue.

The DHW Demand Is Modest

A loft bathroom, kitchen sink, and shop sink will rarely if ever draw more than 3 GPM simultaneously. The NCB-E 150’s ~3.4 GPM at Michigan groundwater temperatures covers this. A dedicated tankless or indirect tank would provide 2–3× the capacity actually needed — paying for headroom that adds no value.

Space Is the Deciding Constraint

The mechanical closet under the enclosed stair is compact. It must also house the electrical panel, radiant manifolds, and low-voltage cabinet. A single wall-hung combi (17” × 28” × 13”) leaves room for everything else. Two wall-hung units or a boiler plus indirect tank could force difficult compromises on clearances and serviceability.

DIY Installation Favors Navien

No other boiler has as much publicly available DIY installation content. The NCB-E installation manual is clear, the error code system is well-documented online, and hundreds of YouTube videos walk through every step from mounting to commissioning. For a first-time boiler installer, this knowledge base is the difference between confident installation and guesswork.

Cost Alignment

At 900–1,800 less than any two-unit option and $300–1,200 less than competing combis that offer no meaningful advantage in this application.

Specific Model Recommendation

Navien NCB-E 150 (150,000 BTU input / ~120,000 BTU output)

  • Provides ample capacity for radiant floor heating even on the coldest design days
  • Modulates down to ~18,000 BTU for efficient mild-weather operation
  • 3.4 GPM DHW at 77°F rise (40°F inlet → 117°F output)
  • 95% AFUE, sealed combustion, PVC/CPVC venting
  • Final sizing must be confirmed by Manual J load calculation post-insulation

If Manual J shows the radiant load is under 60,000 BTU (likely with good insulation), the NCB-E 110 (110,000 BTU input / ~88,000 BTU output) could also work at a lower price point. The 150 provides comfortable headroom.

Hard Water Mitigation

If Clare County water is hard (>7 grains/gallon — check with the well or municipal water report), install a water softener on the potable water supply upstream of the combi. Annual descaling of the DHW heat exchanger with white vinegar flush is recommended regardless. This is a 30-minute maintenance task.

Actions

Planning Phase

  • Complete insulation installation — stage:: 3
  • Perform Manual J load calculation (post-insulation) to confirm BTU requirements — stage:: 3
  • Test water hardness at the garage supply (once water service is connected) — stage:: 6
  • Confirm mechanical closet dimensions accommodate NCB-E 150 plus panel, manifolds, and low-voltage cabinet — stage:: 3
  • Finalize boiler vent route and wall termination location (per Navien installation manual clearance requirements) — stage:: 4
  • Confirm gas line sizing from meter to boiler (Navien NCB-E 150 requires ¾” gas minimum) — stage:: 6

Procurement

  • Order Navien NCB-E 150 (or 110 if Manual J supports it) — stage:: 6
  • Order PVC/CPVC vent and intake pipe, termination kit, and fittings — stage:: 6
  • Order gas connector, shut-off valve, sediment trap, and CSST or black iron fittings — stage:: 6
  • Order isolation valves, expansion tank, air separator, and fill valve for radiant loop connection — stage:: 6
  • Order thermostatic mixing valve for DHW output (scald protection per code) — stage:: 6
  • If water hardness >7 grains: order water softener or plan descaling kit — stage:: 6

Installation

  • Mount boiler on mechanical closet wall (verify blocking/backing behind drywall) — stage:: 6
  • Install PVC/CPVC vent and combustion air intake through wall — stage:: 6
  • Connect gas line with sediment trap, drip leg, and shut-off — stage:: 6
  • Connect boiler to radiant manifold supply/return with isolation valves — stage:: 6
  • Connect DHW supply (cold in) and hot water output with mixing valve — stage:: 6
  • Install condensate drain (neutralizer kit if required by local code) — stage:: 6
  • Fill and purge radiant loops — stage:: 6
  • Commission boiler per Navien startup procedure — stage:: 6
  • Test all radiant zones and DHW fixtures — stage:: 6
  • Register warranty with Navien (within 90 days of install) — stage:: 6

Procurement

Estimated Costs

ItemEst. Cost
Navien NCB-E 150 combi boiler$2,200–2,800
PVC vent + intake pipe, termination kit$80–150
Gas connector, shut-off, sediment trap, fittings$60–120
Isolation valves, expansion tank, air separator, fill valve$150–250
Thermostatic mixing valve (DHW)$50–80
Condensate neutralizer kit$30–50
Miscellaneous fittings, hangers, tape, sealant$50–100
Total estimated (DIY)$2,620–3,550

Order Tracking

References

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