What the Pulse Filter actually does.
We have had reports of the controller activating the pump when no one was drawing hot water. We typically recommend increasing the sensitivity setting to address this. However, this may not always be possible as increasing the sensitivity also means greater flow is required to trigger the device.
As we researched this issue we determined most false positives are a result of pressure fluctuations in the water line either from the utility company or from a valve in the cold water line suddenly opening or closing.
Examples of this are: solenoid valves for sprinkler systems, spray nozzles on hoses and toilet bowl fill valves.
While water is incompressible and therefore pressure fluctuations cannot cause flow, most plumbing systems are fitted with what are known as “water hammer arrestors”. Water hammer is a result of the fact that water has mass and that as it flows through an open valve it has momentum.
If the valve suddenly closes stopping flow abruptly, the momentum causes a huge spike in water pressure and can result in the piping rattling in the floor joists, ceiling rafters or wall studs. It sounds like someone is pounding on the plumbing with a hammer hence the term “water hammer” and can over time result in damage to the piping and solenoid valves.
A “water hammer arrestor” is a device that is installed at the end of dead end plumbing lines in the home where water is supplied to a solenoid device such as a washing machine or a dish washer.
These devices consist of a short section of piping that has air in one end which is separated from the water by a piston that slides back and forth.
When a valve is suddenly closed the momentum of the water will push against the piston and, because air is compressible, compress the air in the end of the arrestor thus absorbing the pressure spike in a controlled manner and prevent the pipes from rattling.
However, after the air compresses it then rebounds pushing the water back generating oscillations in the piping and generating false flow readings in the flow meter. As these devices do not have damping valves in them as would be found in a shock absorber in an automobile suspension, the oscillations continue until the friction of the plumbing dampens them out completely.
From our observations these oscillations start at a very high frequency and slow down as they die out. The cutoff period of these oscillations seems to be around 19ms.
Enabling the “Pulse Filer” will cause pulses that occur at this frequency or faster to not be counted towards the pulses that trigger the controller. This is the default value and can be modified up or down in tenths of a millisecond, but if you increase the value too much then pulses generated from the normal flow of hot water via a faucet may be ignored and can result in difficulty triggering the controller on-demand. If you decrease it too much you may start to experience false triggers again.
Some Calculations for Geeks
By default the “Flow Meter Delay”, which is the period of time the controller uses to count the pulses, is ¾ of a second or 0.75 seconds. If the number of pulses in this period of time exceeds 20, the default sensitivity count, then the controller will turn the pump on.
All pulses, even line oscillations, are counted. This results in a pulse period of 0.75 sec/20 = 0.0375 sec or 37.5 ms.
So if there are 20 consecutive pulses with a pulse period this length or shorter then the pump will turn on. However, the “Pulse Filter” sets a lower limit on the pulse period such that any pulses with a period less than 19 ms will not be counted towards the sensitivity count.
The intention is to filter out the line oscillations. So calculating in the other direction 0.75 sec/0.019 sec = 39.5 gives us an upper limit on the sensitivity count over which the pump will not turn on. So by default the flow rate has to be between 20 and 39 to trigger the controller.
As the “Pulse Filter” period increases the upper value decreases. The maximum value we allow the filter to be set to it 30ms which gives a very narrow range of flow. At 30ms the sensitivity cutoff is 0.75 sev/0.030 sec = 25. So you can see setting this value too high can create a very narrow range of flow that will trigger the controller.
Lets convert the pulse count to gallons per minute (GPM) to give us another metric to understand these values. Using the formula for a ¾” flow meter X gal/min = Y/23.9 + 0.208 where Y is the pulse count.
So a pulse count of 20 equates to 20/23.9 + 0.208 = 1.04 GPM and a pulse count of 39 equates to 39/23.9 + 0.208 = 1.84 GPM. So using the default values in the controller, if the flow is greater than 1 GPM and less than 1.8 GPM the controller will be triggered.
If you open a faucet that flows over 1.8 GPM, like a tub fill faucet, you may flow higher than this count and the controller may not trigger.
A pulse count of 25 (the max value allowable for the Pulse Filter) equates to 25/23.9 + 0.208 = 1.25 GPM. This could work well for a home with 0.5 GPM aerators on the faucets with the sensitivity set down to 8 or 10. These calculations can be repeated for the 1″ flow meter using the equations on our post Flow Rate Calculations in the Live Data App.
