The pads are on a switch nobody checks

Somewhere in your rig there is a switch labeled tank heaters, and a fuse for it that you have never seen. The pads it feeds have their own thermostats, which only work if the pads have power, and none of that is visible from inside. You find out it was off in March, when the gate valve will not open.

Holding tanks are slower to freeze than water lines, because forty gallons of anything takes a while to change temperature. They are also more expensive when they go: a split gray tank or a cracked gate valve is a shop job, not a fitting and an afternoon.

So the useful thing is not knowing the tank froze. It is knowing the pads are not doing their job while the tank is still comfortably above freezing.

Reading the tank rather than the bay

a. A probe on the tank wall

The direct reading, and the first thing to buy if you winter in a rig with pads. Tape a wired probe to the tank wall beside the heater pad, or to the dump valve body, and insulate over the outside of it so it reads the plastic rather than the bay air. Warn below 36°F and go critical below 33. A Govee H5044 carries several probes on one device, one per broadcast, which is how you cover three tanks without three devices; an Inkbird IBS-P01B is the single-probe version.

Two further limits worth knowing. It is one spot on a large surface, so it reads the end it is taped to. And the H5044’s decode was validated against a reference at room temperature with the sub-freezing end unchecked — which is exactly the range this whole article lives in, so read the direction of travel as reliable and the last degree as approximate.

b. The bay air, which most people already have

An ambient sensor in the bay, warning below 38°F and critical below 34, is probably already there from the wet bay watch. On a rig with a heated enclosed underbelly it does a second job: it is the reading that says the furnace duct into the basement is not delivering.

It is air rather than tank, though, and that difference cuts the way people do not expect. A tank with working pads sits warmer than the bay around it, so a cold bay is not proof the tank is at risk — but it is proof the exposed valves and lines are, because those have no pads and no mass. Treat a bay under 34 as a valve problem whatever the tank probe says.

c. The Mopeka you already stuck to the tank

A Mopeka reports its own temperature alongside the level, every 300 seconds, and it is magneted to the underside of the tank — which makes it a free tank-bottom thermometer on any tank that already has one. Warn below 36°F. The raw-level caveat that applies to water tanks does not touch the temperature reading.

Same trap as the probe, for the same reason: mounted right on top of a pad it reads the pad’s cycle rather than the tank.

d. Whether the pads are drawing anything at all

Twelve-volt pads pull roughly 2 to 4 amps each and switch themselves on below about 45°F, so on a three-tank rig a working system puts a 6 to 12 amp step into the current on a cold night. That step is visible on a shunt, and its absence is the cleanest evidence there is that the switch is off or the fuse has gone.

It is a chart read rather than an alert. There is no step or rate rule to set — a threshold fires on a level and the furnace blower is a similar-sized step on the same cold nights, so a plain current rule cannot tell them apart. Two more confounders: a converter on shore power masks small loads, and a pad thermostat legitimately holds off while the tank is above 45. Pads with no power and pads with a dead element look identical, because both draw nothing.

If your pads or a bay heater run on 120 volts instead, the pedestal monitor‘s power reading is the equivalent — with the caveat that it is whole-rig power, and a water heater cycling on electric swamps a hundred-watt pad completely.

The combinations

One reading here is nearly useless and two are decisive, which is the cookbook pattern earning its keep.

• Bay under 38, tank probe six degrees or more above the bay air — the pads are working. The tank is fine and the exposed valves are what need attention.
• Bay under 38, tank probe within two degrees of it — the pads are not heating. Check the switch, then the fuse, then whether the pad’s thermostat is sitting against an empty tank.
• Bay under 40 and no current step since it crossed 45 — the 12 volt pads never switched on at all. Switch off, fuse blown, or no pads installed.
• Bay under 38 and the interior below its warning too — this is not a pad problem. That is the furnace, and the pads are a symptom.
• Tank probe under 36 with that tank reading above 60 per cent — a lot of water about to freeze with no working heat. Dump it if you can. A full frozen tank is the expensive version of this.

What none of this tells you

Anything about the inside of a tank or the inside of a gate valve. A probe on the wall is a proxy at one point on a large surface, and nothing at all tells you a valve has frozen shut until somebody pulls the handle and it does not move.

What to do about it

Most of the protection here is not a sensor. Dump before the cold arrives rather than during it, keep the valves closed, and leave the tanks part full rather than bone empty — a pad against an empty tank is heating plastic and air, and a tank with some liquid in it holds temperature far better than one with none.

Then one probe on the tank wall, which is the reading that turns “the switch is on” into “the tank is warmer than the bay”. That comparison is the entire article: not a number, a difference between two numbers you can already see.

And check the switch in October, with the bay cold enough that the pads should draw. It is the one test that costs nothing and the one nobody does until March.

Where to get one

A multi-probe thermometer covers three tanks for the price of one device, which is the only purchase this page really argues for. The compatibility database lists everything else that would do the ambient half.