I’m new here but I have done a search and not finding anything similar enough for my project. I have a tankless water heater on my RV and it works great but uses a lot of water. Each time the hot water is called for, it takes about half a gallon to heat up to the set temp. Even if the water is only shut off for a few seconds, the water heater turns off and has to start the whole process again when hot water valves open again. I need some help with understanding if an arduino can handles these requirements. Here’s my idea:
12v pump the turns on when water flow in the hot water line no longer detects flow (installed after the “T” that heads to the faucets)
12v pump recirculates the water back into the cold line heading into the water heater
Value that closes so no cold water enters the line and bypasses the heater
12v pump stays on for an amount of time set by a dial or controller that can be adjusted on the fly (controller would be mounted next to the water heater control in the bathroom)
12v shuts off automatically when flow sensor detects flow in the hot water line
Controller has a digital display that counts down till auto shutoff
Controller has an override that will allow the pump to turn on, even when the flow sensor isn’t triggered
Controller has an override to stop timer/pump regardless of set time or time remaining
Entire system will only turn on as long as the water heater control is “on”
You forgot something: safety.
I think that a low-level protection should be added, to avoid that water will be heated and heated until it boils.
Perhaps the way it works it because of safety, which you want to bypass.
I'm not sure that it will work in the end. There are too many things that needs to be just right.
Do you have programming experience ?
Can you tell more about the heater ?
Does it turn on with a small flow ? Can it regulate the power ? Is there electronics with a control circuit for the heater ?
What stops the hot water being pumped back toward the cold water source?
Im guessing the pump also needs a valve because it allows back flow when it’s off ?
That is a common solution to providing "instant" hot water in apartment and office buildings when the boilers are many tens or even hundreds of feet from the tap. We are planning to add this to our next house that will be built for us.
You don't need the exact water temperature. Just tape a DS1b20 temperature sensor at the cold water in to the water heater and insulate it well. When you see a rise in the return water temperature, turn the pump off.
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This bothers me as it ALSO forces water back into the primary fresh water holding tank for your RV.
Paul[/quote]
if you have a check valve on the cold water line, there can not be a reverse flow.
physics also prevents water from a closed system from leaving that system. to displace cold water creates a vacuum, not possible in a water line.
what is needed is a simple flow chart for the water.
// still have not figured how to quote someone and it seems the old way don't work here
some comments.
Paul_KD7HB is right that as water is heated, it expands. as the water expands, it will move back into the cold water line.
the back flow, or check valve needs to allow pressure to leak back in the line. a pin hole is more than enough.
not shown here is the temperature sensor between the check valve on the recirculating line and the inlet of the heater.
in reality the heater has a flow sensor. it will not allow energy into the heater unless there is a flow signal.
in order to shut off the re-circulation line, you need some way to know cold water is entering and/or water is flowing to the faucet.
I would think that one could press the button. the pump would run, and in a set time the water would be hot. 15 seconds ought to be enough. then a light could come on for the user and the pump be shut off.
I get that and I don’t want it to head back to the fresh water tank either but as it stands now, the water coming from the city connection doesn’t go back into the fresh water tank. That would indicate that it cannot head to the tank.
The hot water is only getting to 124°f so it’s not boiling. I’m just looking for a way for the water heater to keep running so I avoid the drop in temp which in turn, wastes water and takes up space in my grey tank.
like putting your finger on a straw and lifting it out of your soda, there is no fast flow between systems.
so, although the water will migrate , it will not flow .
also, the amount of water will be what ever is contained in the recirculation loop in the pipes. hot water will rise into whatever system gravity allows, but we are, I assume talking 1/4 liter.
heater water can and will expand. 400 PSI is possible in a home water heater if closed. there is usually some means of pressure release. a toilet , a dripping sink, or just use. water hammer arrestors will absorb pressure, etc.
but, the primary use is to pre-heat, so in 15 seconds, all the flow in the lines will be cold and all the water will be replaced by fresh water.
if you press and walk away, the volume would not be much.
The water heater has its own safety settings and doesn’t go above 124°f.
The flow rate on the heater must be .9 gallons per minute before it kicks in. In some campgrounds the pressure isn’t above 40psi and it takes so before the heater fires up.
0.9 GPM seems really high.
hand washing is typically 0.5 GPM and most commercial hand washing (single sink) Insta-hot water heaters (EEMax, Chronomite, etc) have a 0.3 GMP flow rate before the heater turns on.
A water heater should not have a check valve. The entire water system is a closed loop.
The act of filling a vessel with ground temperature water and then heating it falls into Charles Law.
400 PSI or greater would be expected.
The T&P or relief valve would be in use frequently.
The standard practice is an unrestricted domestic water system.
An instahot water heater only heats when water is flowing. No flow no heat no pressure no water expansion.
This project would create a micro storage water heater so pressure does come into play. A method of addressing that is required but a flexible hose would suffice. We are talking single digit mm of movement.
The way I understand the issue the pump as drawn will not do what you need.
I would think the issue is the line from the Hot water heater out to the faucet.
OR
Does the flowmeter control the water heater? If so you could make a device that bypassed the flow meter for say 30 seconds it would give the water heater time to heat up before the flow.
