An indirect water heater is a storage tank with no burner or element of its own. Hot boiler water flows through a coil inside the tank and heats your household water without mixing with it. It only makes sense in a home that already has a boiler, which is uncommon in Arizona.
How does an indirect water heater work?
An indirect water heater works like a heat exchanger with a tank around it. HTP describes them as tanks that "work in conjunction with a boiler to heat the water that's circulated through the heat exchanger in the storage tank."
The parts, in order:
- 1The boiler heats water for the home's space heating, such as radiant floors or baseboard radiators.
- 2A separate zone sends some of that hot boiler water to the tank. Bradford White's manual says the indirect heater "must be installed as a zone separate from the space heating system," usually with its own circulator pump.
- 3The coil inside the tank carries the boiler water through the stored household water, heating it.
- 4A thermostat on the tank, called an aquastat, calls the boiler for heat when the stored water cools.
Because the tank has no flame, it needs no vent or gas line of its own. HTP also notes that "Indirect water heaters require a boiler (sold separately)."
### What is domestic hot water priority?
Priority is how the system splits the boiler's heat between the house and the tank. Bradford White's manual gives two options. With priority, "Demand for space heating is interrupted or postponed until the domestic hot water demand is satisfied." With non-priority, "Boiler output is divided between space heating and domestic hot water heating," so hot water delivery "can be reduced" when both call at once. The manual recommends priority when the boiler's heating capacity is 100,000 Btu per hour or less, among other cases.
Is an indirect water heater a good fit for an Arizona home?
Usually not, because most Arizona homes don't have a boiler. The U.S. Energy Information Administration's 2020 household survey found that 55% of Arizona homes heat mainly with a furnace and 25% with a central heat pump. The share using a steam or hot water boiler was too small for EIA to publish. EIA withholds a figure when "fewer than 10 households were in the reporting sample" or its statistical error was too high.
So for most Valley homes, an indirect water heater would mean adding a boiler just to heat water. That's rarely the simplest path. A standard tank, a tankless unit, or a heat pump water heater does the same job with one appliance. Our pages on gas vs electric water heaters and what a hybrid water heater is compare those options.
Where it does fit:
- Homes with hydronic radiant floor heating or radiators. When a boiler already heats the house, an indirect tank lets that same boiler heat the domestic water.
- Homes replacing a boiler's built-in coil. Some older boilers heat domestic water through a coil inside the boiler itself. Weil-McLain's manual covers this swap: "If water heater will replace tankless coil in boiler, disconnect piping to coil or remove coil from boiler and replace with a cover plate." It warns that plugging that coil can cause "severe personal injury."
- Buildings with a central boiler. A commercial building that already runs a boiler can feed an indirect tank the same way. Our page on types of commercial water heaters covers the other commercial options.
What does the plumbing code require for an indirect water heater?
The code treats it as a water heater and adds rules for the boiler connection. The main ones in the plumbing code Phoenix uses:
| Rule | What it requires |
|---|---|
| Relief valve (504.4) | A temperature and pressure relief valve on the tank, like any storage water heater |
| Heat exchanger wall (608.17.3) | Double-wall construction if the boiler fluid is toxic; single-wall allowed if it is nontoxic |
| Water above 140°F (501.2) | An ASSE 1017 mixing valve if the combined system needs water hotter than 140°F for space heating |
Section 608.17.3 is the one that matters most for health. It says heat exchangers using "an essentially toxic transfer fluid shall be separated from the potable water by double-wall construction," with an air gap between the walls. Those using "an essentially nontoxic transfer fluid shall be permitted to be of single-wall construction."
Bradford White's single-wall tanks spell out the conditions. The boiler water must be "practically non-toxic," boiler pressure must be "limited to maximum 30 psig by approved relief valve," and "The pressure of the heat transfer medium is maintained less than the normal minimum operating pressure of the potable water system." That last point means that if the coil ever leaks, household water pushes into the boiler loop, not the other way around. So be careful with antifreeze or treatment chemicals in the boiler loop, and confirm they suit the tank's wall type.
Section 501.2 covers systems where one appliance does both jobs. Where space heating needs water "at temperatures greater than 140°F," a mixing valve must hold the household hot water "to a temperature of 140°F (60°C) or less."
### How hot does an indirect water heater get?
It can get hot enough to scald, so the tank's setting matters. Bradford White's manual says its tank thermostat adjusts "from approximately 80°F (27°C) to approximately 160°F(72°C)" and is "factory set at 120°F(49°C)." It adds that "Setting the water heater temperature at 120°F(49°C) will reduce the risk of scalds," and recommends "an ASSE approved mixing valve" if you raise the setting.
The boiler water feeding the coil runs much hotter. The same manual caps it: "The maximum boiler water supply temperature to the indirect heat exchanger must NOT exceed 240°F (115°C)." Its rated performance figures were measured with 180°F boiler water, so real output depends on how hot your boiler runs. See what temperature a water heater should be set to for the scald numbers.
### Where does an indirect water heater go?
It goes near the boiler, indoors, where a leak won't cause damage. Weil-McLain's manual says keeping the distance between boiler and tank short gives "minimum friction loss," and that "This water heater is not intended for outdoor installation." It says to install the tank so that a leak "will not cause damage to area near water heater," or else use "a suitable drain pan." Its Plus models need 12 inches of clearance on top for service.
Our page on water heater drain pan overflow explains how the pan works.
Does hard water affect an indirect water heater?
Yes, and makers say so directly. Bradford White's manual warns that "Hard water, sediment, high or low PH and high levels of chlorides in the domestic water should be avoided." It asks for a pH between 6 and 8, dissolved chlorides under 100 ppm, and a filter "where sediment that is greater than 5 microns in size is present." It adds that "A water softening system is recommended for areas with hard water." It also notes that "Damage to tanks caused by improper water quality is not covered under warranty."
That's worth reading twice in the Phoenix area. Our page on whether hard water ruins a water heater covers what scale does to tanks.
HQ Plumbing & Air installs and replaces water heaters of every common type across the Phoenix area, with free estimates and upfront pricing. See our water heater service.
No boiler, no indirect water heater. With a boiler, it's a quiet, flameless tank that rides along on heat you already make.