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A hydronic boiler can provide domestic hot water (DHW) only if its manufacturer supports the specific configuration. The usual options are a boiler designed to make hot water as it is drawn (a combi boiler), or a compatible boiler connected to an indirect water-heating tank. Before choosing either, identify the boiler model, check its installation manual, and have the home’s peak hot-water demand and local requirements assessed. An ordinary space-heating boiler is not automatically suitable for heating potable water.
Can I use my hydronic boiler to heat domestic hot water?
Possibly—but the answer depends on the exact boiler and the way the system is configured. Hydronic space heating circulates heated water through a closed heating circuit. Domestic hot water is water used at sinks, showers and other fixtures, so the design must provide a safe potable-water supply without treating the heating circuit as if it were potable.
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Some boilers are designed to serve both purposes. A combi boiler has equipment and controls intended to heat DHW during a draw. A heat-only boiler may be approved for use with an indirect tank, where boiler-side water heats stored potable water through a heat exchanger. These are different system arrangements, and approval for one model or arrangement does not authorize a similar-looking conversion on another boiler.
For example, IBC’s combi-boiler manual describes connecting an indirect tank to the boiler’s primary loop. Rinnai’s I-Series Plus Solo manual describes DHW production through an indirect tank for that appliance. Those manufacturer instructions are model-specific examples, not blanket permission to convert other boilers.
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What does “on-demand” mean in this conversion?
“On-demand” can mean hot water is heated when a fixture calls for it, rather than being held in a conventional domestic tank. But it is also used loosely to describe systems that respond to demand while still using storage. Identify the arrangement you want before discussing equipment or expected hot-water availability.
- Combi boiler with instantaneous DHW: A boiler designed for this function heats water through a DHW heat exchanger during a draw. The relevant questions include whether the boiler supports the configuration, the DHW flow and temperature rise it can provide, the household’s simultaneous demand, inlet-water conditions and maintenance needs.
- Boiler with an indirect tank: Boiler-side water transfers heat to potable water stored in a tank. This is not instantaneous at the fixture: stored hot water is available for draws, and the tank must recover after use. Capacity, recovery, controls, pump compatibility, standby losses and space all matter.
- Combi boiler with added storage: Some compatible appliances can heat a storage tank through their DHW circuit. The manufacturer must authorize that arrangement and specify the required pump, aquastat or sensor, setpoints and controls. Do not assume that a combi boiler can be paired with storage in any configuration.
None of these arrangements can be selected responsibly from the phrase “hydronic boiler” alone. The model, the existing piping and controls, local requirements, and the household’s hot-water use determine what is feasible.
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What should I check before choosing a setup?
Start with the boiler’s exact make and model and its current official installation manual. Then establish the household’s demand and inspect the existing system; only after those checks can a designer or installer determine whether an approved configuration fits.
- Identify the appliance. Record the manufacturer, full model designation and relevant configuration. A manual for a related product or a different boiler series is not proof that your unit supports DHW.
- Find the manufacturer-approved DHW options. Look for the model’s installation instructions covering combi operation, indirect tanks or tank storage, as applicable. Confirm the required accessories and controls, rather than relying on a generic diagram or another model’s manual.
- Describe actual hot-water use. List fixtures and appliances that may draw hot water at the same time, and note the periods of highest demand. The cited manuals do not establish the right boiler output, tank size or recovery rate for a particular household. Those values require a demand assessment and the relevant equipment documentation.
- Review the existing system. A qualified hydronic or plumbing professional should examine the boiler-side piping, available connections, circulator and controls, as well as the potable-water piping and space for any tank. Whether existing components can remain depends on the approved design.
- Check water conditions and maintenance needs. Ask whether the local water conditions affect scaling or maintenance for the proposed appliance. The available data does not establish a particular treatment method or service interval; follow the appliance documentation and qualified local advice.
- Confirm jurisdictional requirements. Plumbing, backflow protection, fuel, venting, electrical work and scald prevention can be subject to local requirements. The manufacturer’s instructions are essential but do not replace the rules that apply where the home is located.
Do not use a guessed tank size or a boiler’s space-heating capacity as a shortcut for deciding its DHW capability. The capacity question is about the particular approved DHW arrangement and the home’s peak and concurrent use.
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How does an indirect tank connect to a hydronic boiler?
An indirect tank stores potable water and transfers heat to it from the boiler circuit through a heat exchanger. The potable water and boiler-side water remain separate; the connection, pump and controls must follow the instructions for the specific boiler and tank.
In the IBC combi manual’s documented setup, the tank connects to the primary loop using a hydraulic separator or closely spaced tees, with appropriately sized piping and a properly sized pump. The manual also describes DHW priority, in which the controls direct boiler capacity to the tank when there is a DHW call. The exact arrangement and control sequence should be taken from the applicable model’s manual, not copied from this example.
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Some combi appliances can instead heat storage through their DHW connections. IBC’s manual describes a configuration using a bronze or stainless-steel pump and an aquastat. That does not mean those components or that control scheme suit another appliance. Confirm the exact method, materials, setpoints and operating priority in the boiler’s own documentation.
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Can I connect the boiler’s heating water directly to taps?
Do not assume that water in a hydronic heating circuit is suitable for domestic use. Follow the manufacturer’s approved design for separating the potable supply from the heating circuit and for every material or component that touches potable water.
Rinnai states in its I-Series Plus Solo manual: “DHW must not be connected to a system that was previously used with a non-potable water heating appliance.” The manual also requires connected piping and components to be approved for potable-water systems and cautions against introducing boiler-treatment chemicals into potable water. These are instructions for the named product, but the underlying point is essential for any proposed conversion: establish what the existing water circuit has contained and use only an arrangement and components approved for the intended potable-water service.
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How do I control temperature and reduce scald risk?
Scald protection must be considered in the system design, not left to the assumption that boiler controls alone will keep every draw at a safe temperature. The specific risk and required protective equipment depend on the appliance and configuration.
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Daikin’s 2025 manual for its Altherma H Hybrid Gas Boiler Module gives a model-specific warning: under specified high space-heating-temperature conditions, the heat exchanger can exceed 70°C, and a small initial draw under 0.3 L may be hotter than 70°C. The manual recommends a thermostatic valve. Those figures describe that product and those conditions; they are not universal design values for hydronic boilers or a prediction for another system.
Ask the designer or installer how the chosen system limits delivery temperature and addresses any model-specific warnings. Follow the boiler and tank manuals, and confirm whether local scald-prevention requirements apply. Do not transfer a temperature, flow or pressure figure from one appliance to another without documentation that it applies.
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Treat this as a design-and-installation project, not a simple piping change. A practical sequence keeps compatibility and safety decisions ahead of equipment selection.
- Gather the records. Find the boiler’s nameplate information, installation manual, existing control information and any records describing the heating circuit or prior water treatment.
- Ask whether the exact model supports the intended arrangement. Have the installer verify the current manufacturer documentation for the model and its configuration. If the manual does not authorize the proposed DHW setup, do not proceed on the basis that a similar product supports it.
- Define the demand. Describe likely peak and simultaneous draws. Have a qualified designer or installer assess the DHW load against the boiler and tank documentation. Do not select a capacity from a generic rule of thumb not supported by the equipment documentation.
- Choose between approved configurations. Compare instantaneous DHW, an indirect tank, or manufacturer-authorized storage based on the home’s demand, available space, existing piping and controls, and maintenance considerations. “On-demand” alone is not enough to determine the right configuration.
- Specify the complete system. Confirm the required tank, circulator, piping, sensor or aquastat, valves and control settings. Ensure potable-side materials are approved and the heating and potable circuits remain separated as the design requires.
- Resolve safety and code questions. Confirm temperature control, backflow and other applicable plumbing provisions, as well as fuel, venting and electrical requirements with the responsible local professionals or authorities.
- Install and commission to the manuals. The qualified installer should follow the exact appliance and tank instructions, verify the controls and operating sequence, and explain maintenance and any limits on simultaneous use to the homeowner.
The manufacturer manuals cited here provide examples of approved, model-specific arrangements; they do not establish whether an unspecified boiler can be converted. If the boiler model, system history or household demand cannot be verified, the suitability and capacity questions remain unresolved.
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What are the main trade-offs?
| Configuration | How hot water is provided | Questions to resolve |
|---|---|---|
| Combi boiler / instantaneous DHW | A boiler designed for the function heats water during a draw. | Does the exact model support it? What DHW flow and temperature rise are documented? Can it meet simultaneous demand under the home’s inlet-water conditions? What maintenance and local requirements apply? |
| Heat-only boiler plus indirect tank | Boiler-side water heats potable water stored in an indirect tank. | Does the boiler support this? What tank capacity and recovery fit actual demand? Are the pump, sensor and controls compatible? Is there space, and what standby losses are relevant? |
| Compatible combi appliance with storage | A supported appliance heats a storage tank through its DHW circuit. | Does the manufacturer authorize the arrangement? What pump, aquastat, storage capacity, setpoints and priority controls are specified? |
The comparison does not identify a universally best option. Instantaneous production avoids storing a full tank of hot domestic water, while an indirect tank provides stored hot water and depends on recovery after use. The right choice depends on the compatible equipment and household load; the cited sources do not provide a design answer for any particular home.
Frequently Asked Questions
Will adding an indirect tank make my boiler a combi boiler?
No. An indirect tank and a combi boiler are different configurations: one heats stored potable water through a heat exchanger, while a combi model is designed to produce DHW during a draw. Whether a specific appliance can use either arrangement must be confirmed in its own manual.
Can I keep my existing domestic water heater as backup?
The supplied manufacturer documentation does not establish whether a particular boiler can be connected to an existing water heater or how two appliances should be controlled together. Have the designer assess the proposed arrangement, potable-water boundaries and applicable instructions before retaining or reconnecting equipment.
Does a boiler’s heating output tell me how many showers can run at once?
Not by itself. The answer depends on the approved DHW configuration, its documented operating limits, inlet-water conditions and the household’s concurrent demand; a heating-output label alone does not resolve those factors.
What information should I give an installer before requesting a conversion estimate?
Provide the boiler’s full model information and manual if available, describe the existing heating and domestic-water equipment, and outline when and how many fixtures may draw hot water together. Also disclose any uncertainty about prior water treatment or the history of the heating circuit.
