It’s a reasonable question — a combi boiler and a standard tankless water heater look almost identical on the wall, both promise “endless hot water,” and radiant floor heat is common enough in new construction and the Northeast that the two searches collide constantly. But the answer is a firm no for the unit you’d buy off a “best tankless water heater” list, and the reason is about certification, not just performance.
By the numbers
- 0 — the number of standard DHW-only tankless brands whose warranty covers a radiant, in-floor, or closed-loop installation; Rheem’s own warranty documentation explicitly excludes it across the Rheem, Ruud, Richmond, Paloma, and EcoSmart tankless lines. — Rheem
- 0.4–0.5 GPM — the minimum flow a tankless unit’s internal sensor needs before the burner or element fires at all; a single small radiant zone often flows under that threshold, which is what triggers short-cycling. — manufacturer flow-activation specifications
- 120,000 BTU — the dedicated space-heating output built into the Noritz NRCB199DV combi boiler, on top of its 199,000 BTU / 11.1 GPM domestic hot water side — the kind of separate, purpose-built capacity a standard tankless heater doesn’t have. — Noritz
- $3,500–$8,000 — typical installed cost for a combi boiler serving both radiant heat and domestic hot water, versus $1,400–$5,600 for a standard tankless-only install, since the combi boiler also needs hydronic piping, a circulator, and controls. — industry installer pricing
Why the warranty says no
Standard tankless water heaters are engineered, tested, and certified for one job: heating potable water on demand. Rheem’s limited warranty documentation for its Rheem, Ruud, Richmond, Paloma, and EcoSmart tankless models specifically excludes coverage for any unit installed in a system supplying radiant heat — in-floor, baseboard, radiators, snow melt, or any closed-loop system, and running glycol or non-potable water through one voids coverage outright, per the manufacturer’s own warranty terms. Other major brands write comparable exclusions. This isn’t a soft recommendation — it’s a hard certification boundary, and it’s why plumbers won’t plumb a standard Rinnai, Rheem, or EcoSmart unit into a radiant zone even if the BTU output would technically cover the load.
What actually works
| Unit type | DHW capacity | Space heating | Equipment cost |
|---|---|---|---|
| Navien NCB-H (combi boiler) | 5.4 GPM @ 70°F rise | Yes — built-in | ~$2,600–$3,200 |
| Noritz NRCB199DV (combi boiler) | Up to 11.1 GPM, 199,000 BTU | Yes — 120,000 BTU dedicated, built-in pump | ~$2,785–$3,055 |
| Seisco Radiant Heat Source (RA series, electric) | 1.5–3.0 GPM (RA-28, 240V) | Yes — purpose-built hydronic + DHW | Typically $800+ |
| Standard DHW-only tankless (Rinnai/Rheem/Navien NPE/EcoSmart) | 2.5–11.2 GPM | No — warranty-excluded | $500–$1,900 |
Noritz NRCB199DV Combination Boiler
- Delivers up to 11.1 GPM of domestic hot water while running a dedicated 120,000 BTU circuit for radiant floor, baseboard, or radiator loops, per Noritz's own product specifications.
- Ships with a built-in circulator pump for the hydronic side, so a single unit and gas line replace both a water heater and a separate boiler.
- Rated for a 0.4 GPM minimum activation flow — still worth pairing with a properly sized manifold or buffer tank on very small single-zone radiant installs.
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Navien NCB-H Combi Boiler
- Rated 5.4 GPM at a 70°F temperature rise for domestic hot water, per Navien's own product literature — enough for two to three showers and a dishwasher running at once.
- Handles radiator or radiant floor loops as its second function, the same category we cover in our tankless vs. combi boiler comparison for buyers deciding whether they need the second function at all.
- Cascadeable up to 15 units for larger-demand installations, a scalability option neither a single standard tank nor a single boiler offers.
For all-electric homes without a gas line, Seisco’s Radiant Heat Source (RA series) is built the same way — a single 240V electric unit engineered for both hydronic space heating and potable water heating, rather than a standard electric tankless pressed into a job it wasn’t certified for.
The short-cycling problem most buyers miss
Even with a combi boiler or hydronic-rated unit, radiant floor zones create a flow problem that whole-home domestic hot water doesn’t. A tankless-style burner or element needs roughly 0.4 to 0.5 GPM of flow across its internal sensor to fire at all — the same activation threshold that makes standard recirculation pumps incompatible with tankless heaters. A single small radiant zone, especially one running through a mixing valve to drop the supply temperature for in-floor tubing, can flow well under that number. Left unaddressed, the burner fires and cuts out repeatedly instead of holding a steady flame — wear the unit’s warranty doesn’t anticipate. This is exactly why combi boilers like the NRCB199DV ship with dedicated space-heating circuits and a built-in pump rather than relying on the DHW side to also serve the floor loop directly; larger multi-zone radiant systems typically add a buffer tank on top of that to smooth out the flow further.
The DIY heat-exchanger workaround — and why it’s not the safer bet
Search long enough in radiant-heating forums and you’ll find installers running a standard tankless water heater’s output through an external plate heat exchanger, keeping glycol or non-potable loop water on one side and the tankless unit’s potable water on the other. It’s a real technique, and it’s how some off-grid or single-zone supplemental setups work around the lack of a combi boiler in the budget. But it isn’t a configuration any standard tankless manufacturer tests, endorses, or warranties — adding a heat exchanger, circulator, and controls to a unit rated for potable-only use doesn’t restore warranty coverage, it just adds cost and complexity to a workaround. For a single small supplemental zone where a warranty claim is an acceptable risk, it’s a documented option. For a whole-home radiant system, a combi boiler or hydronic-rated unit built for the job costs more up front but is the actual manufacturer-supported path.
The bottom line
A standard tankless water heater — the kind on every “best tankless” roundup, including our own — is certified for domestic hot water only, and Rheem’s own warranty language spells out that radiant floor, baseboard, radiator, and closed-loop installations are excluded from coverage. If you need one unit to handle both hot water and radiant floor heat, buy a combi boiler built for it: the Navien NCB-H or Noritz NRCB199DV on gas, or Seisco’s Radiant Heat Source line if the home is all-electric. Whichever you pick, size the space-heating side to your zones and add a buffer tank if any single zone runs under the unit’s minimum activation flow — that’s the detail that turns a correctly-chosen combi boiler into a short-cycling one. For the full standard-tankless-vs-combi-boiler decision, see our tankless water heater vs. boiler guide, and for sizing a DHW-only unit once radiant heat is off the table, start with our tankless water heater sizing guide.