Quick Answer: A tankless water heater and a combi boiler solve different problems that only overlap at the hot-water tap. A tankless water heater heats domestic hot water only, costs roughly $1,400–$5,600 installed, and drops into almost any home. A combi boiler does that plus space heating — it feeds radiators or radiant floor loops — but only pays off if you already have (or want) hydronic heating; installed cost runs about $3,500–$8,000. If your home uses forced-air heat, which most US homes do, a combi boiler buys you a second function you can’t use, and a tankless water heater is the simpler, cheaper answer.
These two show up in the same searches because they’re both wall-hung, tankless-style boxes that promise “endless hot water.” But only one of them touches your home’s space heating at all. Here’s the actual decision.
Tankless water heater vs combi boiler at a glance
| Factor | Tankless Water Heater | Combi Boiler |
|---|---|---|
| What it heats | Domestic hot water only | Domestic hot water + space heating (radiators/radiant floor) |
| Replaces | A standard tank water heater | A boiler/furnace AND a water heater |
| Requires hydronic heating? | No | Yes — radiators or radiant floor loops |
| Efficiency rating | UEF ~0.93–0.98 (condensing gas) | AFUE ~90–96% (condensing) |
| Hot water flow | ~2.5–5.2 GPM (climate-dependent) | ~5.4 GPM @ 70°F rise (Navien NCB-H) |
| Installed cost | ~$1,400–$5,600 | ~$3,500–$8,000 |
| Best fit | Forced-air homes needing only hot water | Homes with (or planning) hydronic heat |
By the numbers
- 5.4 GPM at a 70°F temperature rise — Navien’s own spec for its NCB-H residential combi boiler line, enough for roughly two to three showers and a dishwasher running at once. — navieninc.com
- 90–96% AFUE for modern condensing combi boilers, versus 0.93–0.98 UEF for the most efficient condensing gas tankless units — different scales measuring different jobs, not a direct efficiency contest. — manufacturer/industry AFUE and UEF documentation
- ~$1,400–$5,600 installed for tankless versus ~$3,500–$8,000 installed for a combi boiler — the combi boiler premium buys a second function (space heating), not a better water heater. — HOP Energy / iHeat / industry installer pricing
- Cascadeable up to 15 units — Navien’s combi and tankless lines can both be linked for larger-demand installations, a scalability option neither a single standard tank nor a single boiler offers. — navieninc.com
The requirement most buyers miss: a combi boiler needs hydronic heat to be worth it
This is the single biggest point of confusion in this comparison. A combi boiler’s water-heating side works exactly like a tankless water heater — no storage tank, heats on demand. But its defining feature is the other job: it also feeds a hydronic (water-based) heating system, meaning radiators or in-floor radiant loops. If your home heats with forced-air ductwork and a furnace, which is how most US homes are built, a combi boiler’s second function has nothing to plug into — you’d need to add an entirely separate radiator or radiant-floor system to use it. In that case, a combi boiler isn’t really being compared to a tankless water heater at all; it’s being compared to keeping your furnace and swapping only the water heater, and the tankless unit is almost always the simpler, cheaper move.
Where a combi boiler actually wins
If you already have a boiler feeding radiators or radiant floors — common in the Northeast, older homes, and new-construction radiant-floor designs — a combi boiler genuinely simplifies things. Instead of running a separate furnace/boiler and a tankless or tank water heater side by side, one wall-hung unit and one gas line handle both jobs, per Navien’s own comparison of combi-boiler pros and cons. That’s real equipment and maintenance savings for the right house — it just isn’t a fit for a home with ductwork and no radiators to connect to.
Cost and efficiency: not an apples-to-apples number
Boilers are rated in AFUE (Annual Fuel Utilization Efficiency), the same scale used for furnaces, while water heaters are rated in UEF (Uniform Energy Factor). A 96% AFUE combi boiler and a 0.96 UEF tankless water heater are not competing on the same 100-point scale — AFUE measures how much of the fuel’s energy ends up as usable heat across a full heating season, while UEF measures hot-water delivery efficiency including standby losses. Both numbers being high is good; neither number tells you which unit costs less to run, because a combi boiler is also running fuel through your space-heating load that a standalone tankless water heater never touches. On installed price alone, tankless is the cheaper purchase in nearly every case — see the full tankless water heater cost breakdown for what drives that range up or down.
Which should you buy?
- Forced-air furnace, radiators, no hydronic heat now: Buy a tankless water heater — see our best gas or best electric picks.
- Already have (or are installing) radiators or radiant floor heat: A combi boiler can replace both your boiler and water heater in one unit — see our guide to using a tankless heater with radiant floor heat for which specific models are actually rated for it.
- Not sure tankless is right at all versus a standard tank? Start with tankless vs tank.
- Want sizing math before you buy either? Use our tankless water heater sizing guide.
The bottom line
A tankless water heater and a combi boiler both promise endless hot water from a compact, wall-hung unit, but only the combi boiler also heats your home — and only if you have hydronic radiators or radiant floors for it to feed. For the majority of US homes running forced-air heat, that second function is unusable, which makes a tankless water heater the cheaper, simpler choice at roughly $1,400–$5,600 installed versus $3,500–$8,000 for a combi boiler. Save the combi boiler for a home that already has, or genuinely wants, hydronic heat. If your home already has an older boiler feeding a tankless coil rather than a combi boiler, that’s a different, older setup — see our tankless coil water heater guide for why it loses efficiency in summer and what actually fixes it. And if you’re keeping the boiler either way, note that a combi boiler isn’t the only tank-free-feeling option that runs off it — a separate indirect water heater tank does too; see our best indirect water heater guide for how that differs from both a combi boiler and a standalone tankless unit. The same hot-water-plus-space-heat idea shows up in a much smaller package in RVs: PrecisionTemp’s TwinTemp-2 pairs a tankless burner with a hydronic loop to heat both a coach’s water and its interior from one appliance — see our PrecisionTemp tankless water heater review for how that combo compares on price to a water-only RV unit.
Last updated September 9, 2026 — reverified this comparison’s core figures directly: Navien’s own NCB-H spec sheet still lists 5.4 GPM at a 70°F rise (unchanged), and AFUE/UEF remain two distinct DOE-recognized scales with no unified conversion, so the “not directly comparable” framing still holds. Installed-cost ranges ($1,400–$5,600 tankless vs. $3,500–$8,000 combi boiler) cross-checked against current HOP Energy and iHeat installer pricing — no material drift found on either range.