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.
Comparing gas lines, venting, or fittings between the two before you commit? Get the parts to your door in two days — try Amazon Prime free for 30 days.
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.
- 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.