A tankless combination boiler water heater provides space heating and domestic hot water from one appliance. Of these options, the Rinnai I-Series Plus and Noritz NRCB180DV-LP are the clearest combination-boiler choices. The Rinnai i150SN is a space-heating boiler, while the Rinnai RX130iN and A. O. Smith ATI-540P-N are tankless water heaters, not combination boilers. Choose based on whether you need both functions or only one.
| Product | Best For | Important Distinction |
|---|---|---|
| Rinnai I-Series Plus 150k BTU | Heating and hot water in one appliance | Combi boiler; 96% AFUE listed |
| Noritz NRCB180DV-LP | Propane combi-boiler buyers | Simultaneous heating and domestic hot water |
| Rinnai i150SN | Whole-home space heating | Boiler listing does not describe domestic hot water production |
| Rinnai RX130iN | Tankless domestic hot water | Not a combination boiler |
| A. O. Smith ATI-540P-N | High-flow domestic hot water | Not a combination boiler |
Rinnai I-Series Plus 150k BTU Combi Boiler
This Rinnai is the most direct match for shoppers seeking a tankless combination boiler water heater. Its listed purpose is to provide home heating and domestic hot water in one appliance. The product information lists a 96% AFUE rating and Smart Sense Gas Valve technology for energy management.
Best for: Homeowners who want one appliance for space heating and hot water and are comparing condensing combi boilers. It was selected because the listing explicitly describes both functions, rather than just a boiler or a standalone water heater.
Consider: The listing describes natural gas or propane operation, but buyers should confirm the exact model configuration, fuel availability, venting requirements, and installation needs with a qualified professional before purchase. The supplied information does not specify domestic hot-water flow rate.
Noritz NRCB180DV-LP Propane Combi Boiler
The Noritz NRCB180DV-LP is an indoor propane combination boiler intended to provide domestic hot water and space heating. Its listing gives a maximum domestic hot-water rating of 180,000 BTU, up to 9.8 GPM, and 100,000 BTU for space heating. It also describes simultaneous hot-water and heating operation and domestic-water temperature stability.
Best for: Buyers specifically looking for a propane combi unit with stated domestic-hot-water and space-heating capacities. Those figures make it easier to compare the two duties separately when planning a system.
Consider: The listed heating capacity is lower than its maximum domestic-hot-water rating, so confirm that the space-heating output suits the home’s heat loss. The product name mentions standard-vent convertibility, but the listing details provided do not explain the installation requirements or conversion process.
Rinnai i150SN Condensing Gas Boiler
The Rinnai i150SN is described as a whole-home natural-gas boiler with a stainless-steel heat exchanger and a modulating wire-mesh burner. The listed features include multizone control capability, direct connections for thermostats and integrated control pumps, a digital display, and an outdoor reset sensor. Central-heating temperature adjustment is listed from 104°F to 180°F.
Best for: Buyers who need a space-heating boiler and value multizone controls and adjustable heating settings. It was included as a closely related boiler option for comparison, not as a confirmed combi water heater.
Consider: The supplied product information does not state that this model produces domestic hot water. Do not assume it can replace a separate water heater; verify the system configuration and intended use before choosing it for a hot-water application.
Rinnai RX130iN Condensing Tankless Water Heater
The RX130iN is a standalone tankless gas water heater, not a combination boiler. The listing states that it can use natural gas or propane and can be installed indoors or outdoors with the required configuration. It lists a maximum flow of 7.1 GPM, a UEF of .97, an easy-to-use interface, and a Quick Flush feature. A Controlr module for Wi-Fi control is not included.
Best for: Buyers who want tankless domestic hot water but do not need this appliance to provide space heating. Its stated flow rate and fuel flexibility are useful comparison points for a water-heater-only purchase.
Consider: Outdoor installation requires an RX-OVC vent cap, according to the listing. Maximum flow does not guarantee a particular number of simultaneous fixtures in every home; incoming-water temperature and household demand affect the usable flow.
A. O. Smith ATI-540P-N 10 GPM Water Heater
This A. O. Smith model is an indoor natural-gas tankless water heater, not a combi boiler. Its listed maximum flow is 10 GPM, and the product information says it may suit homes with three to four bathrooms depending on climate. It uses marine-grade 316L stainless steel and includes an integrated hot-water recirculation pump. The listing says recirculation may require a bypass valve, which is not included.
Best for: Buyers prioritizing high domestic-hot-water flow and recirculation features in a standalone water heater. It was selected as a useful alternative for households that need hot water but do not need the appliance to heat the home.
Consider: The listed flow is a maximum, not a promise of that output under every home’s conditions. The bypass valve may be an additional system requirement, and this model’s listing does not describe space-heating functionality.
Buying Guide: How To Choose A Tankless Combination Boiler
Do you need space heating, domestic hot water, or both?
Start by identifying the appliance’s jobs. A true combination boiler supplies both space heating and domestic hot water. A boiler may provide space heating only, while a tankless water heater may provide domestic hot water only. The Rinnai I-Series Plus and Noritz NRCB180DV-LP are the listed options that explicitly describe both functions. The other three products should not be treated as combi units based on the supplied descriptions.
If you are replacing an existing system, check whether it uses radiators, radiant floor heating, or another hydronic arrangement, and determine how hot water is currently produced. A combi appliance may consolidate equipment, but the product listing alone cannot establish whether it is suitable for a particular home’s piping, controls, or demand.
How should you compare heating and hot-water capacity?
Combination appliances serve two different loads. Space-heating output relates to the home’s heat requirement, while domestic-hot-water capacity relates to how much hot water the unit can deliver at a given time. Compare each rating separately. The Noritz listing gives 100,000 BTU for space heating and up to 9.8 GPM for domestic hot water; do not treat one number as a substitute for the other.
For a proper sizing decision, have a qualified installer assess the home’s heat loss and expected hot-water use. The number of bathrooms alone is not enough to determine capacity because climate, incoming water temperature, fixture demand, and simultaneous use can change what a home needs.
Which fuel and installation setup does your home support?
Confirm the appliance’s fuel type and the fuel available at the property. The Rinnai I-Series Plus description mentions natural gas or propane, while the Noritz model listed here is propane. Other models have their own specified fuel configurations. Do not assume that a model can be converted or configured for another fuel unless the manufacturer’s information confirms it.
Also verify whether the product is intended for indoor or outdoor installation, which venting arrangement applies, and what accessories are required. The RX130iN listing, for example, says an RX-OVC vent cap is required for outdoor installation. Installation details should be checked against the exact model and local requirements before purchase.
What do efficiency figures tell you?
The Rinnai I-Series Plus listing gives a 96% AFUE rating, while the Rinnai RX130iN listing gives a UEF of .97. These figures use different rating measures and describe products with different jobs, so they should not be compared as if they were identical measurements. The RX130iN is also a water heater rather than a combination boiler.
Look for the efficiency measure that applies to the exact appliance type, and consider whether the unit’s capacity and intended use match the home. Efficiency by itself does not show whether a model can meet peak heating or hot-water demand.
Which controls and convenience features matter?
Control features can affect how a system is managed. The Rinnai i150SN lists multizone control capability, a digital display, temperature and pressure information, and an outdoor reset sensor. The I-Series Plus lists Smart Sense Gas Valve technology. These features are not interchangeable, so compare the controls against the planned heating zones and the installer’s system design.
For water-heater-only models, consider features such as recirculation or flushing access. The A. O. Smith model includes an integrated recirculation pump, while its listing notes that a bypass valve may be needed and is not included. The RX130iN lists Quick Flush and notes that a Wi-Fi control module is not included.
What should you confirm before ordering?
- Confirm the full model number, fuel configuration, and whether the product is a true combi boiler.
- Ask an installer to verify heating load, hot-water demand, venting, connections, and control requirements.
- Check whether any required vent caps, bypass valves, or other accessories are included or must be sourced separately.
- Review the manufacturer’s installation information for the exact model and local code requirements.
- For a replacement, confirm how the new appliance will connect to the existing heating and hot-water system.
Frequently Asked Questions
What is a tankless combination boiler water heater?
It is an appliance designed to provide space heating and domestic hot water without storing the home’s hot water in a conventional tank. Product descriptions should explicitly confirm both functions before you consider a model a combi unit.
Which listed products are combination boilers?
The Rinnai I-Series Plus and Noritz NRCB180DV-LP descriptions explicitly state that they provide heating and domestic hot water. The other listings describe either a boiler or a standalone tankless water heater, not both functions together.
Can a regular tankless water heater heat my home?
Do not assume it can. A standard tankless water heater is designed for domestic hot water unless its product documentation specifically identifies space-heating capability and an appropriate system configuration.
Is a higher GPM rating always better?
No. A higher maximum GPM can indicate greater potential hot-water flow, but actual output depends on operating conditions and household demand. For a combi boiler, also check the separate space-heating capacity.
How do I know what size combi boiler my home needs?
Have a qualified professional evaluate the home’s heating load and expected simultaneous hot-water use. Bathroom count or a single product capacity figure is not enough to size a system reliably.
Does the A. O. Smith model provide home heating?
The supplied listing describes it as a tankless water heater for domestic hot water and does not state that it provides space heating. It should not be selected as a combi boiler based on the information provided.