I want to have an analog design that is rugged and will last for 20 or 30 years, and is resistant to voltage spikes, brownouts, over the years, so I plan to use military grade parts.

My 2 bathroom lights are on the same circuit, I have extended wires from both room switches to my utility room to act as a trigger for my circuit to turn on a 110V 20A relay that will switch power ON to my hot water recirculating pump.
I want the power to remain ON for about 2 to 5 minutes (potentiometer adjusted).
After the initial warm-up designed above I want to keep repeating a cycle that turns the pump back on for 2 to 5 minutes (a separate potentiometer) then rests for about 15 to 30 minutes (potentiometer). The cycle repeats while the light switch trigger remains ON.
Entire circuit resets once the light switch trigger is turned OFF.

I'm trying to keep this simple and avoid using a temperature sensor, so I plan to just adjust the timing to get it set correctly. I've done RC circuits but I don't know how to switch from my initial timer to the 2nd endless loop circuit, and I don't know what are the best ways to protect against various voltage problems that can occur over the years.

If you are able to help provide a schematic I appreciate the help, thank you.

 

That is an interesting project, and this thread is going to exceed 100 posts before it is done. First, the basics.

In your environment, "military grade" components have z.e.r.o advantage. Their main characteristics are radiation hardness and documented performance over a wider operating temperature range, which do not apply here. A little transient protection and an oversized filter capacitor should be all that is needed.

Multi-minute R-C timers are a problem because they almost always require an electrolytic capacitor, and those have poor initial value tolerance, poor temperature stability, and notorious value changes with age. Since you don't want to use a uC (a * * * * decision), get familiar with the CD4060. This is a CMOS logic device with an on-board oscillator and 14-stage divider. For a 30-minute timer, this can reduce the capacitor size by over 8000:1, getting it down in ceramic and film cap range for much better stability and aging.

Expect some heat for the non-microcontroller / non-Arduino approach. Ignore them; they are wieners.

Let's get started:

Power
Switch interfaces
Flow chart / state diagram / truth table of the various timers and their interactions
Timers
Output driver

First - What is your skill set for building circuits with electronic components? And, where are you located?

Step 1 - Power. Do you already have a power source for the circuit, or is that design part of the project?

ak

 

Jumping ahead . . .

Why are there three pots? Do the 2-5 minute initial pump period and the 2-5 minute repeating pump period have to be different? This is not a big deal, just that 3 timers are more stuff than 2 timers.

The base design has at least two timers in a loop. Let's call them ON and OFF. Each one triggers the other one at the end of its period. To get the ON timer to have two values, one initial and one repeating, you can either set a flipflop that drives an analog switch to change between two pots, or just have a 3rd timer, INIT.

ak

 

I have done Arduino and Raspberry Pi projects and created code, and if an Arduino will last 20 or 30 years, then I'd consider doing it. But I am skeptical of the endurance over the years will it hold up. I'm not trying to start a feud, it's just my guess that a simple need like this does not need a microprocessor and will outlive the Arduino. In 30 years I imagine the bathroom switches will turn on over 100K times, that seems like a lot of On/Off's for an Arduino to boot up.
Regarding Military parts, I've often seen they are built to last, any automotive or industrial grade is in that same school of thought.
I have experience doing small electronics projects, but this is more than what I've done for analog because it involves 2 stages.
I've built some power supplies for example.
I'm in Fort Lauderdale and appreciate any constructive help.

 

I don't think there is anything inherent in the Arduino that reduces it s long-term reliability. However, it is not a repairable module; once it dies, the only option is to replace it with whatever is available in 20 years. There is an excellent chance that today's code will not run perfectly in that version of the device, and cranking up editing 20 year old code is NOT fun. For this reason, I'd stick with logic chips.

ak

 

Jumping ahead . . .

Why are there three pots? Do the 2-5 minute initial pump period and the 2-5 minute repeating pump period have to be different? This is not a big deal, just that 3 timers are more stuff than 2 timers.

The base design has at least two timers in a loop. Let's call them ON and OFF. Each one triggers the other one at the end of its period. To get the ON timer to have two values, one initial and one repeating, you can either set a flipflop that drives an analog switch to change between two pots, or just have a 3rd timer, INIT.

ak

I chose to not use a temp sensor so the price for this is I have to assume the pipe is cold each time a light switch is turned on. So stage 1 is to warm the pipe up longer assuming cold water in the entire line; I have a long pipe to one bathroom and so I will need it to run for a bit.

Stage 2 is when the light is on for a long time, perhaps several kids in and out with light on. And so it wakes up occasionally to do a quick warmup. I'll likely only need a minute for the warmup, but not sure, and I was trying to design this for others who may have different timing needs.

While it's possible someone leaves the light on for hours, this module will make very light duty for the pump, and so having it run for a few minutes each hour for several hours on an occasional day like that is not a problem for a heavy duty hot water pump; they often run 24x7 all winter long for floor heating.

I did not mention that my design will require adding a check-valve on each of my 2 return water-lines; this replaces the need for a Grundfos 595926 Valve under the sink; I'll locate both check valves in the utility room near the connection to the cold water line, and I'll throw a check valve onto the cold water line as well; and that makes this a closed-loop system which requires a thermal expansion tank attached to the cold water line right where it attaches to the water heater. I wanted to throw this into the notes here just in case someone is not familiar with plumbing.

 

OK, three timers: Cold, Warm, and Off.

When the lights go out, even for 2 seconds, everything resets back to condition 1 - cold pipes?

ak

 

OK, three timers: Cold, Warm, and Off.

When the lights go out, even for 2 seconds, everything resets back to condition 1 - cold pipes?

ak

Thanks for asking, you are right, a better design is to not "reset" with a light switch off until it's exceeded the 15 to 30 minutes setting (or yet another pot set at 15-30 minutes hehe).

Edit: as I ponder my reply, it's not clear that what I mean to say is that if we were 10 minutes into a 15 minute warm cycle and the light switch is off then back on within 5 more minutes then doing a warm cycle is fine; but otherwise it should assume the water is now cold. If this is possible that is great, but otherwise assuming water is cold after a brief time of say 1 or 2 minutes would be fine as well.

 

OK it sounds like what you want is a repeat cycle timer. If you need to set the ON and OFF time independently on a cycle timer then you need to select a product with either Repeat Cycle ON or repeat cycle OFF function. No concern as to temperature keeps things simpler. Unless you plan to start from scratch making your own board I would just buy once and cry once. Macromatic comes to mind. I would visit their website or actually give them a call. You are looking at $100 or less likely. I used their timers on pump systems and when I retired those timers were doing fine 20 years after I designed the pump system.

I also strongly agree with Analog in that going Mil-Spec is not going to make for better. We also ran a system similar to what you describe, a circulating system for hot water. So while the decision to build or buy rest with you there are timing systems which are turn key off the shelf that should suit your needs. Calling Macromatic and speaking to an applications engineer can't hurt. I also see no need to a uC be it Arduino or other.


Ron

 

OK it sounds like what you want is a repeat cycle timer. If you need to set the ON and OFF time independently on a cycle timer then you need to select a product with either Repeat Cycle ON or repeat cycle OFF function. No concern as to temperature keeps things simpler. Unless you plan to start from scratch making your own board I would just buy once and cry once. Macromatic comes to mind. I would visit their website or actually give them a call. You are looking at $100 or less likely. I used their timers on pump systems and when I retired those timers were doing fine 20 years after I designed the pump system.

I also strongly agree with Analog in that going Mil-Spec is not going to make for better. We also ran a system similar to what you describe, a circulating system for hot water. So while the decision to build or buy rest with you there are timing systems which are turn key off the shelf that should suit your needs. Calling Macromatic and speaking to an applications engineer can't hurt. I also see no need to a uC be it Arduino or other.


Ron

I want to help others reading here. This seems very flexible: https://www.macromatic.com/products/time-delay-relay/td-8-series?v=1545
It has multiple functions via dip switch settings. To accomplish what I mentioned it seems this would work with 3 modules:
Interval On module would power my Initial Warm Up stage.
ON Delay/OFF Delay would be set at the same Interval Time for the delay to turn ON, once ON it would power the next module
Repeat Cycle (Off First), this would constantly cycle OFF then ON while the previous delay still has power and also holds the cycle for a delay after the light switch is turned OFF (to prevent reset during frequent light switching).

The benefit here is UL rating is nice since this is for house utility use, the downside is price and also not as fun to build haha

 

OK, three timers: Cold, Warm, and Off.

When the lights go out, even for 2 seconds, everything resets back to condition 1 - cold pipes?

ak

Reloadron made me think of my original specs and I am thinking this is basically an initial ON with a delayed OFF, followed by a repeating pattern of OFF followed by ON.
This keeps the logic as simple as possible. The only fancy addition is to not reset during a fast light switch On-OFF-On again, but I don't have to have that, it's nice to have, but not required.

 

If You want this to be as simple and bullet-proof as possible,
forget the Timers, and especially forget about using a Mechanical-Relay.

Just run the Circulating-Pump continuously directly from the Light-Switch.
The Light-Switch can easily handle ~10-times the existing Lighting-Load,
and your Circulating-Pump probably draws maybe ~2-Amps, if not less,
and is specifically designed to run continuously until the end of time.
( unless You buy a cheap Chinese-copy )

There is no advantage to cycling the Pump on and off repeatedly under any set of circumstances.

If You have 2 or 3 different locations that need to activate the Pump,
use a SSR (Solid-State-Relay) for each of the locations, and wire all of their Outputs in parallel
so that any, or all of them, will power the Pump.

Before this setup fails, Electrical-Power will be free,
so Power-Consumption will not be a factor in the long-run.
.
.
.

 

If You want this to be as simple and bullet-proof as possible,
forget the Timers, and especially forget about using a Mechanical-Relay.

Just run the Circulating-Pump continuously directly from the Light-Switch.
.
.
.

You and Ron won me over. I do want to cycle some, but I'm just going to buy the Repeat Cycle (On First) from Macromatic and go with that. I'll likely cycle 5 minutes on and around 10 off.
One thing I've never heard anyone talk about (so I'm hoping/assuming not an issue) is water flow GPM when the power is off vs on. Does having a pump installed in the line increase flow when on? Decrease normal flow when off?

 

You and Ron won me over. I do want to cycle some, but I'm just going to buy the Repeat Cycle (On First) from Macromatic and go with that. I'll likely cycle 5 minutes on and around 10 off.

Just remember you want a timer where you can set individual On and Off times which just to make sure I suggest calling Macromatic. I actually have one of these which you would be welcome to but in repeat cycle it only has equal time. Heck I would give it to you, postage on me but it won't do what you need.

Again, before buying I would give Macromatic a call and explain in detail what you want.

Ron

 

More Components equals more potential points of failure,
regardless of any possible superior quality of the Components in question.

What will actually happen in the real-World when Cycling the Pump,
and the actual reduction in efficacy in achieving your desired goals,
is yet to be sussed-out,
and in my opinion, is not likely to be in your favor.

The Light-Switch is the perfect "Demand-Indicator" and Power-Supply-control-source.
.
.
.

 

OK, I may have missed a statement, but this sounds like the plan to have warm water at the faucet immediately by recirculating the cold water out of the hot water pipes, back to the heater. That is most often done with a small mains powered pump that runs until warm or hot water is detected below the faucet And the pump is also located under the sink. And there is usually a time switch to only run it certain times of the day. If you have a separate return line then there could be only one pump, back at the water heater, and only one temperature sensor. That makes it simpler except for the added return line. And the check valves can be much smaller and thus cheaper. And maybe that is what the TS is working on. For the timer or timers, industrial quality timers plug into an octal socket and will last a few years at one cycle per minute 16 hours a day. In a home that will be at least 20 years, if there are ten cycles a day. The timers reset to zero with every power interruption, and so it will be important to get users to switch the light off when they leave. Once they understand the reason it will happen.

 

WATER DESIGN: I have a large whirlpool soaking tub in the master bathroom project I'm working on. My goal is to fill it with hot water quickly. I have a 16 gpm/10 bar hot water pump attached to my hot water heater, sending water through a one inch copper pipe for 23 feet to dual 5.5 gpm faucets (11 gpm total), 3/4 inch leading back to the water heater, but a 0.5-3.0 bar Adjustable Differential Pressure Bypass Valve (DPV) is attached to the line just before entering back into the hot water cold side of the tank.
I'll attach the same hot water line to my 2nd bathroom using a Tee at the hot water tank, and another Tee just before the Differential DPV. I have a strong DPV because when the dual tub faucets are open I want 100% of the boosted water pressure to rush out into the tub; my best guess is 1 to 1.5 bars will be enough, but I have up to 3.

Grundfos (and others) sell a thermal valve that attaches under a sink in a bathroom or kitchen and that will work fine for normal applications, but once I added a high-pressure pump for my needs, that required free flowing water when no faucet is open, so I went with my design above.

ELECTRICAL DESIGN: I bought a Triggered OFF Delay relay (with the special jumpers option added to enable to light switch to be both the power and the trigger) from macromatics.com (TD-81622-44T15) who have been very helpful and responsive. I can set delays from 10 to 1023 minutes, I'll likely set the delayed OFF at 30 minutes.
I'm using a motion sensor installed next to the light switch to act as the switch, this enables the system to work at night with the lights off, and it also eliminates the issue where the light switch is left on all day--this allows me to set the delay down to 30 minutes rather than guessing how long a busy bathroom could be in use each day, the motion sensor will just "know" and all is solved.
I bought a 2nd Triggered OFF Delay for the exhaust fan (likely set at 10 to 15 minutes) and plan to have the motion sensor feed power to both time delay relays.
I'm going to locate both of the relays in the utility room so it's easy to swap in a new part in the future if needed.

I think this will work, I'm always open to suggestions. Thanks to the group here for good advice and changing my mind to keep this design more simple. Hopefully, I've explained the goals and design well.

 

Certainly a temperature sensor would allow a simpler control scheme, and while purchased sensors can indeed be expensive, a simple thermistor and an IC amplifier/comparator can provide a means of telling hot from cold well enough. It would only need to sense when the water in the pipe was hot so that the circulation could switch off. THAT simple scheme should cost less to build than an adequate timer..

 

Certainly a temperature sensor would allow a simpler control scheme, and while purchased sensors can indeed be expensive, a simple thermistor and an IC amplifier/comparator can provide a means of telling hot from cold well enough. It would only need to sense when the water in the pipe was hot so that the circulation could switch off. THAT simple scheme should cost less to build than an adequate timer..

I appreciate the alternate ideas. My goal is high pressure water for a large tub, and I don't want that kind of pump power pressing constantly against a mostly closed valve for the return, so on/off sensor solves for that, and the DPV allows for automatic closure of the return when a faucet is open. I'm seeking to fill the tub up on 5 minutes rather than 15.