Building a Swimming Pool Timer

an old fashioned timer by a pool

For many years, I’ve had a 24-foot swimming pool equipped with a heat pump to warm it up. A few years ago, I added solar heat to alleviate the burden on the pump (and consequently the power bill!). Nonetheless, when I started up the heat pump every June to speed up heating the water, my power bill jumped almost as high as what I was paying in January. Since the heat pump loses a great deal of efficiency when the temperature gets cooler (especially below 14° C), I was turning it on around 10:00 AM and shutting it off at 7:00 PM.

That wasn't bad on the weekends when I was home, but on workdays, it meant the pump had to be turned on before I leave for work, even when the temperature had dropped into the low teens during the night.

As a result, the heater was burning energy for the first three to four hours of every day while the water was still cooling, due to the solar heater acting as a radiator and dissipating the precious warmth into the air. And if I forgot to turn it on in the morning, I'd get home at night to frigid water. So this was what motivated me to build a timer that would let the pool heating look after itself.

a homemade timer for a pool heating unit

Assemble Your Pool Heating Timer Equipment

I started by considering the three pieces equipment that are required to run the pool and heat it:

The circulating pump running on 120 VAC.

The heat pump running on 230 VAC.

The solar heater running with the circulating pump (although it could also be controlled with a solenoid valve at 24 or 120 volts).

Next up, I listed out the necessary support infrastructure:

A medium to large enclosure box for the connections.

A power relay capable of handling the circulating pump, small enough to fit in the enclosure box, and equipped with four sets of contacts.

Relay Note: Since a heat pump requires 30-50 Amps to run, it’s not realistic to control its 230V feed through an oversized power relay inside the enclosure. Much more practical is to use one of the switches in the power relay, connected in series with the front panel control switch of the unit through a small gauge wire.

Possibly an electric box to accommodate the two switches if they are to be placed a distance from the enclosure box.

Possibly a step-down transformer if the rated voltage of the relay is less than 120VAC.

A single-pole / triple-throw switch or double throw switch with a neutral center position that will provide AUTO ON, OFF, and MAN ON, positions (in that order from top-down).

A single-pole / single-throw switch to turn OFF the heat pump (since it can’t operate when the pump is stopped) when only the circulating pump needs to run (that will prevent having to run out to the heat pump dial to turn it off).

A roll of 14/2 or 16/2 multi-stranded wire long enough to reach from the enclosure box to the heat pump, and also to the solenoid valve (if applicable).

A seven-day or a daily timer to operate the power relay.

Personally, I bought my timer a while back through Amazon for under $10.00 CA. They’re still available for under $15.00 CA as of this writing, in models at 12 or 24 VAC/VDC, and 120 or 230 VAC. If both the timer and the relay run on 120 Volts, a transformer will not be needed.

Plan Your Timer System With a Diagram

The next step is planning where the control panel and switch box will be in relation to each other. It must be in a location large enough to install the enclosure box and the wiring going to it. If the switch box is separate, take into consideration the additional required space for that, too.

It helps to make a drawing or computer image of the system as you want it to be. This will give you a map to follow as you assemble the pieces, and can serve as a guide for any troubleshooting and repairs down the road.

Fig 3

Creating the System

Before attempting any electrical installations, MAKE SURE THE POWER IS TURNED OFF AT THE MAIN PANEL.

This is when you gather the parts and components and prepare the enclosure for the parts that will be housed in it. The timer can be mounted inside the enclosure, just make sure the electrical connections are well insulated and the push buttons are easily accessible.

The next step is to get the wiring through and inside the enclosure. Make sure that the GROUND WIRE GETS CONNECTED TO THE METAL BOX! I took the extra step of adding a manual shut off knife switch close by, to cut off the power to the circuit without having to run to the main panel in the basement every time.

If wiring for 120 Volts on all components, you’ll have the 120 VAC line-in, the line to feed power to the circulating pump, a pair of 14 or 16 gauge wires to control the heat pump, plus if the two control switches are a distance away, a 16/3 wire for SW1 and a 16/2 wire for SW2. Finally, a 16/2 wire can be added if you opt to use a solenoid switch on the solar heater.

If you have components running at 24 Volts and some at 120 Volts, proceed as in the previous step. Next, the step-down transformer is securely fastened inside the enclosure. The common contact on SW1 can now be connected to one side of the transformer’s primary winding, while the other side is connected to the neutral line of the 120 Volts coming in.

One side of the secondary winding is then wired up directly to the relay coil and the 4th (bottom) contactor on the relay, feeding on the other side to one of the wires leading to the solenoid valve (if you choose that option). The other side of the secondary then hooks up to the other side of the relay coil and to the other line feeding to the solenoid valve.

Finally, screw the relay into the bottom of the box, making sure there’s plenty of room for the wiring. It will probably be easier to wire up the power relay before screwing it in the enclosure.

As mentioned earlier, you can now either make holes through the enclosure’s cover and mount SW1 and SW2 right there or wire them up to a separate box in a different location.

With this accomplished, you can now switch on the main breaker and start setting the timer. You’ll appreciate the reliability of the pool temperature, and the savings when your next power bill comes in!