Can I Put My Gas Boiler on a UPS, and How Long Will It Heat the House?

Yes, most residential gas boilers run well on a UPS, and they are easier to back up than a forced-air furnace. The ignition control, gas valve, any combustion fan and, above all, the circulators stop when the power does, but those loads are modest. The number that decides how long the house stays warm is how many circulators can run at once, and which kind they are.

What a boiler actually draws

A hydronic system's load is a sum: the boiler's own rating from its manual, plus each pump and valve from its data sheet.

Two different kinds of number sit in that table, and mixing them up is how most boiler UPS plans go wrong. The circulator figures are measured power at a given speed: Grundfos lists 60 W on speed 1 and 87 W on speed 3 for the same pump. The boiler figures are circuit ratings. The WM97+ manual explains that its 15.0 A "includes up to three circulators, not exceeding 2.2 amps per circulator", which is a budget for the wiring, not a draw. The Evergreen's 5.80 A is the same kind of figure: the manual uses it as the first line of a worksheet for sizing the circuit, then adds up to 2.0 A for each of four circulator outputs and 2 A for a side receptacle meant for a condensate pump.

The Taco 007-F5 shows the trap from the other side. Its submittal lists amps and horsepower but no watts. At 115 V, 0.71 A is about 82 VA, and for a PSC motor real watts are lower than VA (see VA vs watts), so 82 W is a safe ceiling for sizing, not a running figure. For your own pumps, a measured number beats both; see how to measure power draw.

Sizing the UPS: add it up

Write down every 120 V load that can be on at the same moment, then add a margin. For a typical cast iron boiler with circulator zoning:

1. Boiler controls. On a natural draft boiler such as the Weil-McLain CGa, the controls run from a 40 VA transformer that feeds the ignition module, gas valve and vent damper, so about 40 W is a fair allowance for that side. Sealed-combustion boilers add a combustion fan; a HeatingHelp contributor who measured a few put sealed-combustion systems at "typically somewhere around 80-100 Watts".
2. Each circulator that can run at once. Use the data sheet watts at the speed setting actually in use, or 44 to 45 W for a Taco 007e or ALPHA2 at maximum.
3. Zone valves, if any. They run on the boiler's 24 V transformer, already inside its budget.
4. Extras. A condensate pump on a condensing boiler, an indirect water heater pump, an outdoor reset or zone control panel.
5. Margin. Choose a UPS whose watt rating is comfortably above the sum. PSC circulators draw more current for a moment as they start, so a UPS running near its limit can trip its overload alarm when a pump kicks on.

The combinations in the table below land between about 85 and 300 W, which a 1000 to 1500 VA line-interactive UPS covers with room to spare. The watts are rarely the problem. Battery energy is.

How long will it heat? Runtime by zone count

The table uses the method from why UPS runtime is not linear: 85% inverter efficiency and a Peukert exponent of 1.15 for AGM batteries, on a common 1500 VA tower with two 12 V 9 Ah batteries (216 Wh nominal).

Here is the arithmetic for the three-zone ECM row. The battery supplies 172 W / 0.85 = 202 W, which is 8.4 A at 24 V:

These are worst cases: every pump running continuously. Real systems cycle. If the zones call half the time, the average load halves and, because the current per battery falls, runtime more than doubles: about an hour and a half for the three-zone ECM house and more than three hours for the single ECM zone. The runtime calculator takes your own measured average. Treat the table as a new-battery figure; the same HeatingHelp contributor's rule for UPS runtime charts was to "take the number they show and divide it in half".

Getting hours instead of minutes

For an overnight outage the choices are a UPS that accepts extended battery packs, a large lithium battery system, or a generator on a transfer switch.

Waveform, grounding and flame sensing

Both Weil-McLain condensing boilers here prove flame electrically. The Evergreen manual says that on its smaller models the spark electrode doubles as the sensor: "After ignition, the electrode measures flame signal." The WM97+ control shows a "FLAME SIGNAL" value it uses to decide whether to shut the boiler down. That measurement depends on a sound ground path, and the manuals say so in their own terms:

• Weil-McLain Evergreen: "Must wire ground to this terminal to provide boiler grounding. Failure to properly ground the boiler can lead to abnormal operation and potential hazards."
• Weil-McLain WM97+: when the flame signal is low, the manual has the technician check the ignitor, then confirm that "ground wiring is in good condition, and ground continuity is satisfactory" before replacing the ignitor.
• Navien NCB quick installation guide: power connection of 120 VAC 60 Hz, "Min. 2 Amp current with proper grounding".

None of these manuals states a waveform requirement or mentions polarity. Our recommendation is the same as for furnaces and tankless heaters: buy a pure sine wave UPS. ECM circulators such as the 007e and ALPHA2 are electronic drives, and flame sensing works on small signals. The gas furnace guide explains how flame rectification works and why a floating neutral on some inverters and generators causes lockouts; the same physics applies to a boiler's flame rod. If a generator is part of the plan, read UPS not working with a generator before the first outage.

Connecting it: plug, inlet or transfer switch

The Evergreen manual assumes a hardwired supply, asking the installer to "provide and install a properly-sized, fused disconnect or service switch as required by applicable codes. (15-amp for most cases.)" A hardwired boiler cannot simply be plugged into a UPS, and the fix is not a homemade cord.

The HeatingHelp thread makes the case against improvising. On swapping a boiler's hardwired feed for a plug, one long-time contributor wrote: "it may not meet code, and a building inspector might be perturbed. More to the point, however, is grounding. By using the plug and play option, one has lifted the ground connection to the boiler," which can leave the boiler and its piping at a different voltage from the rest of the plumbing.

Hardwired vs plug-in UPS covers the options in more detail, and using a UPS with a generator covers the bridge setup. Never backfeed a boiler circuit through a male-to-male cord.

Boiler or furnace: why the boiler is the easier backup

A furnace moves heat with a blower; a boiler moves it with a pump rated at no more than 45 to 87 W at full speed in the data sheets above. The furnace guide's worked example, with an ECM blower, draws 340 W while running. The single-zone ECM boiler in the runtime table draws 84 W, a quarter of that, and the same 1500 VA UPS runs it for about 94 minutes. Everything else overlaps: pure sine output, the ground reference, the wiring rules and testing before winter.

Where this answer stops applying

• Electric boilers. The heating elements draw kilowatts. No UPS in this class runs them.
• Oil boilers. The burner has its own motor and ignition, and none of the figures here describe it. Measure before buying.
• Large or commercial systems. Circulators above 2.0 to 2.2 A must run through a relay or starter under both Weil-McLain manuals, and are outside desktop UPS territory.
• Unattended homes. The Evergreen manual warns that power outages "could result in frozen plumbing and water damage in a matter of hours" and recommends a security system that "operates during power outages, senses low temperature". See how much runtime do you need to plan for the outage length you actually get.