Part IV: Solar Water HeatingChapter 7. Active Domestic Hot-Water Systems

Controllers, Sensors, and Overheat Protection

Differential thermostats, sensor placement, and what the system should do on a hot empty afternoon.

Difficulty: Intermediate
Cost: $40–150

published

The controller's job is small. Turn the pump on when the collector is usefully hotter than the tank. Turn it off when that lead disappears, and turn it off when the tank is as hot as it is allowed to be. Everything else is a way to get those two comparisons wrong.

A differential of the high teens of degrees F is a typical turn-on setting. A few degrees, smaller than the turn-on number, is a typical turn-off. If the two settings are the same, the pump chatters on and off. The exact numbers live in the controller manual. Get the order right: on-difference greater than off-difference, and a high limit that stops collection when the store is hot.

A hobby board running a line-voltage pump is not a controller. Use a listed differential controller. If you are not qualified to land the line-voltage side, an electrician lands it. The low-voltage sensors are the part a careful owner can place.

Difficulty

Level: Intermediate
Time: Half a day to mount sensors and an electrician for the pump circuit
Skills: Placing a sensor where it measures water, not sunlight

Cost estimate

$40–150

A differential controller with two sensors often sits in this range. The pump, the line-voltage circuit, and any heat-dump hardware are extra.

Piece	Typical cost
Differential controller and two matched sensors	$40–150
Outdoor-rated sensor cable	A small addition
Electrician for the pump circuit	Separate, and worth it

Materials

Item	Qty	Notes
Listed differential controller	1	With a high-limit setting. Not a microcontroller project on a potable system.
Collector sensor	1	On the absorber outlet pipe, under the insulation, not taped to the outside of the glazing in the sun.
Tank sensor	1	Low on the tank, in a well or strapped tight and then insulated. The bottom third, not the hot top.
Outdoor-rated cable and a drip loop	1 run	So water cannot follow the cable into the controller. Keep sensor wire away from line-voltage runs.

Where the sensors go

The collector sensor should see the fluid temperature at the outlet. If you clamp it to a pipe, sand the pipe, use the clip the controller maker expects, and bury the sensor under the same insulation as the pipe. A sensor sitting in the air inside the collector reads too high and short-cycles the pump. A sensor on top of the glass reads the weather.

The tank sensor goes low. The bottom is the water you are trying to heat. A sensor at the top will shut the pump off while the bottom is still cold, because the top stratified and looked "done." Mark which wire is which. Swapping them makes the pump run at night and cool the tank.

When you commission, compare each sensor to a thermometer on the same pipe. If they disagree by more than a few degrees, fix the contact before you trust the turn-on setting. The measurement page is that comparison, written as a checklist.

Collector sensor at the outlet, tank sensor low, controller in between, pump as the only thing the controller is allowed to start.

The hot empty afternoon

Set a high limit near the tank's safe storage temperature, often somewhere in the 160 to 180 degree F range for a glass-lined water tank. Read the tank maker's limit and use the lower of the two. The high limit protects the tank and the fluid. It does not protect the person in the shower. The mixing valve does that, near 120 degrees F at the fixture.

On a drainback system, hitting the high limit means the pump stops and the collector empties. That is the clean version of overheat protection. On a closed glycol loop, stopping the pump leaves a full collector in the sun. The fluid will stagnate, and glycol that is cooked over and over becomes acidic and eats the loop. A closed loop needs a second plan: a cover, a larger store, or a heat dump that is itself freeze-safe and powered in a way that does not depend on you being home. Do not keep the pump running to boil the indoor tank. Do not circulate overheated glycol through indoor plastic pipe.

Vacation is the design case, not a surprise. If you leave for a week in July, a drainback can sit empty. A closed loop cannot sit and hope. The freeze page is the matching problem in January: a protection that needs power will fail in a blackout. Control logic alone cannot freeze-protect a closed loop.

Fail toward off. Unplug the collector sensor, using whatever lockout the wiring requires so you are not in a live box, and confirm the pump stops. If an open sensor turns the pump on, the controller is in the wrong mode. Fix that before the first hot day.

Buy a listed differential controller
On-difference larger than off-difference. High limit available. Manual in the folder with the plumbing drawings.
Put the collector sensor on the outlet pipe
Under the insulation. Not on the glazing. Drip loop on the cable. Outdoor-rated wire on the roof.
Put the tank sensor low
Bottom third, tight contact, then insulation. Label both cables at the controller.
Set the high limit and decide the vacant week
Drainback may simply stop and empty. A closed loop needs a cover or a dump that works without you. Mixing valve still required.
Prove a failed sensor turns the pump off
Then compare both sensors to thermometers on the first sunny day and write the numbers down.

Difficulty

Cost estimate

Materials

Where the sensors go

The hot empty afternoon

Buy a listed differential controller

Put the collector sensor on the outlet pipe

Put the tank sensor low

Set the high limit and decide the vacant week

Prove a failed sensor turns the pump off

Further reading