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Flow switch Control

/Flow switch Control
Flow switch Control 2018-08-13T13:12:31+00:00

The Chilipepper hot water on-demand system pump is a great way to save time water and money.  Most users of the system use a button hard wired to the pump at the sink where it is installed.  Some customers use X10 remote controls to control the Chilipepper pump from other locations as well.


Then there are the tinkerers and home automation enthusiasts who use various methods to control their hot water demand pumps such as motion detectors and door switches.

I was just recently asked by a customer who had purchased a Chilipepper pump how to control it with a flow switch.  He wanted to put a flow switch into the hot water line so that when you turned on the hot water the pump would come on without having to push a button.

You could briefly turn on the hot water tap and immediately shut it off, which would activate the hot water demand system.  When you hear the pump shut off you know you will have instant hot water when you turn on the hot tap.

The Chilipepper requires a momentary contact between its control wires.  Typically this would be done with a button, like a door bell button.  When you push the button the contact is made between the two wires, and when you release the button the contact is broken.  Breaking that contact is what signals the Chilipepper pump to turn on.

You can’t however, use a flow switch very effectively by just connecting the contacts to the control wires of the Chilipepper.  It would work fine if you turned on the hot water faucet and then turned it off.  The flow switch contact would be made and then broken.  But after the Chilipepper has finished its cycle and you begin using the hot water the flow switch will again close the contacts, but it won’t break them until you again turn off the faucet.

In the meantime, if the Chilipepper sees the control wires make contact and they remain in contact for more than 15 seconds, the microcontroller in the Chilipepper thinks there is a short circuit on the control wires.  In response the Chilipepper will turn on for two brief spurts about every 15 seconds until the contact between the control wires is broken.

The reason for this behavior is that if a short does develop, or a remote control module malfunctions, the pump won’t just come on and run forever.  The pump tells you something is wrong.

We can overcome this problem by using a capacitor and a resistor.  By placing a  non-polarized 100uF capacitor in series with the flow switch contacts you can create a momentary short circuit on the control wires.  Place a 10K ohm resister in parallel with the capacitor to discharge the cap after the unit shuts off.

When the switch closes the capacitor allows a rush of current to flow and the resulting voltage drop on a resistor inside the Chilipepper circuit causes the input pin on the microcontroller to go low.

In less than a second the capacitor charges up to the 5 volts on the control wires and the microcontroller thinks the contacts have opened and the Chilipepper is then activated.

After the flow switch opens its contacts when you shut off the faucet, the capacitor needs a way to discharge.  Placing a resistor in parallel with the capacitor provides a discharge path.  The control wires consist of a hot wire and a ground wire.

The hot wire is being pulled up to +5 volts through a 5k resistor and also connects to an input pin on the microcontroller.  The pin must be brought down to about 1 volt when the contacts on the flow switch close and back up to about 2.5 volts to indicate to the controller a broken contact state.

A 1uf capacitor in parallel with a 10 k resistor works great.

If you use a non polar capacitor you don’t need to worry about which leads go where, but if you use a polarized cap you will need to use a voltmeter to determine the polarity of the contact wires and connect them to the correct plus and minus leads on the cap.  The resistor doesn’t have a polarity.

It’s a nice solution to eliminate the need for a button if that is the kind of thing you want to do.  However, flow switches are pretty expensive.

Trying to use flow switches for several sinks would be very expensive and you would need to do a lot of plumbing and wiring as well.

There is however what I call a “whole house” solution.  Use just one flow switch at the water heater.  Then the pump will activate no matter where you are when you want hot water at any particular faucet.