it has to finish it's shutdown and restart which can take a couple of minutes to do and then make hot water and push it along 100m of hose which has lost all it's heat,

Hot water in the line from pump to tap will always be lost when loop opens.
I don't understand why heater always runs, won't that just keep heating water when loop is open, recycling hot water back to the heater making water hotter and hotter?

Is the "tank" and actual insulated hot water tank?

 

A process drawing with part descriptions would go a long way...

 

recycling hot water back to the heater making water hotter and hotter?

I'd assume the heater heats to a set temperature and no hotter. The TS has repeatedly said the heater keeps heating, which would make us think the heater would keep heating water to the boiling point. IF there's a tank and it's under pressure the temperature would go even higher. But as you begin using that hot water you'll end up getting steam. In such a circumstance the tank would likely have an over-temp over-pressure release valve to prevent explosion. So I don't think the heater keeps heating willy-nilly.

Like others, I'm wondering if there's a tank. If there is then after a period of non-use when the hose is opened the water IN THE HOSE will be cold. It won't be immediately hot. However, if there is no tank but rather a heater that has to develop heat then flow water through the heater, THAT could take minutes. I believe this is the TS's issue.

I've had a water heater that maintained temperature. Off that water tank I tapped in for a hose. So when I connected a hose and turned the tap on hot water would flow. IF there was water in the hose already, turning the tap on would mean cold water out of the end of the hose until the hot water replaced the cold in the hose.

To me - it's still not clear exactly what the need is. Or what the desired results may be. I've tried to understand it but have only met with frustration.

 

It says it will work at 1Lpm.
The purpose of the relay module which has a delayed turn ON is to help prevent switch "bounce".

Thank you, but I have another question, the delay relay operates at 12v, the batteries fluctuate 11v-15v as they are in a van and being charged, I didn't spot anything, but would this relay work within that range? I wouldn't want to cook it!

OK, it sounds very much like you had the wrong pressure relief valve. The correct valve will open with a small increase, but not rapidly. And it will also close slow enough to avoid that oscillation that was described. It may also be required to add an accumulator so that when the flow is stopped, hot watter can flow into the accumulator for several seconds as the valve opens. That should avoid oscillation quite well.

Would you be able to advise what PRV I would need? I am using 1/2" heater hose so something around the size or smaller (I could put reducers if needed)

Highlighting your words:
"this causes the pump to go in to DE (dead end) and it sits there waiting for the pressure to drop and flow be restored."
I believe the term is Dead Head. You can only Dead Head a positive displacement pump. And as far as that goes - you can't dead head a PD pump. Another pump, such as a centrifugal type which can flow a lot of water but not produce as much pressure as a positive displacement pump. The centrifugal pump won't have back flow preventer valves. When the centrifugal pump stops water will back-flow and drop pressure. Lower pressure starts the pump again. Pressure builds and shuts the pump off. When the centrifugal pump stops water will back-flow and drop pressure. Lower pressure starts the pump again.
Over and over. So your pump must be a positive displacement type.

So I have another question: What kind of pump are you running?

You said the pump runs on 12 volts. AC or DC? Battery or power supply?

Sorry, I guess I'm just trying to get a sense of what you're working with.

This is the pump - https://www.grippatank.co.uk/aquatec-wfp-delivery-pump-100psi - it is definitely dead end as the controllers for this say DE (dead end) and all the documents for these pumps refer to it as dead end

The pump is 12v, the batteries are 2x 105ah = 210ah total, they run between 11-15v

Hot water in the line from pump to tap will always be lost when loop opens.
I don't understand why heater always runs, won't that just keep heating water when loop is open, recycling hot water back to the heater making water hotter and hotter?

Is the "tank" and actual insulated hot water tank?

The heater runs as the supply of water is cold-ish and the supply is only filled once a day.

The tank isn't insulated but in the van it holds the heat very well as it's very thick and not directly on surfaces, but rather on feet.

I'd assume the heater heats to a set temperature and no hotter. The TS has repeatedly said the heater keeps heating, which would make us think the heater would keep heating water to the boiling point. IF there's a tank and it's under pressure the temperature would go even higher. But as you begin using that hot water you'll end up getting steam. In such a circumstance the tank would likely have an over-temp over-pressure release valve to prevent explosion. So I don't think the heater keeps heating willy-nilly.

Like others, I'm wondering if there's a tank. If there is then after a period of non-use when the hose is opened the water IN THE HOSE will be cold. It won't be immediately hot. However, if there is no tank but rather a heater that has to develop heat then flow water through the heater, THAT could take minutes. I believe this is the TS's issue.

I've had a water heater that maintained temperature. Off that water tank I tapped in for a hose. So when I connected a hose and turned the tap on hot water would flow. IF there was water in the hose already, turning the tap on would mean cold water out of the end of the hose until the hot water replaced the cold in the hose.

To me - it's still not clear exactly what the need is. Or what the desired results may be. I've tried to understand it but have only met with frustration.

The heater won't heat to boiling point as it has safety cutoffs.
There is a tank, it is not under pressure. The tank is there as a supply, it is filled once a day and used until empty so the water won't get super heated.

A period of non use will cause the water in the hose to go cold, there is nothing that can be done to prevent this and I am fine with this, but a period of non use causes the heater to shut down, waiting the 10+ minutes for the water to get hot enough to notice out the end of the hose isn't something I am fine with.

What I need is, as per the 1st post, a sensor to detect flow to the 100m hose reel, when flow is detected a solenoid shuts, closing the recirculation loop. when flow has stopped the recirculation loop opens.

 

Thank you, but I have another question, the delay relay operates at 12v, the batteries fluctuate 11v-15v as they are in a van and being charged, I didn't spot anything, but would this relay work within that range? I wouldn't want to cook it!

Should be fine.

 

OK, instead of a rather adjustable pressure relief valve, and instead of a flow meter, a pressure switch that can be set to trigger at a close point. OR BETTER YET, a pressure transducer to make it simple to set the trip off pressure and the switch back pressure. THEN a solenoid valve. And probably an adjustable time delay for the switcher. THAT will allow adjusting to the optimum switch points and the minimum delays required to avoid any oscillation. AND there will be no splicing in a flow meter, which those flow meters may not like hot water, by the way.

 

AND there will be no splicing in a flow meter, which those flow meters may not like hot water, by the way.

The Flow switch listed will work up to 240 degrees F.

 

OK, a flow switch is different. But still it has a pressure drop It could work with a module that included at least one timer. BUT solenoid valves are not cheap. Nor the flow switch, for that matter. At least not here in the USA.

 

The open loop keeps the heater running, but when it gets too hot it shuts down?

I propose a completely different way.

2 pumps.

Make a heater-tank loop ckt with pump #1 (circ), this keeps the tank "hot". When the tank reaches near the heater "off" state the pump turns off (heater goes off), when tank temp cools down enough the circ pump turns back on (heater goes back on).

Pump #2 pumps from the tank to tap, just use a toggle control switch at the reel to turn that pump on, use a pump that has it's own relief that simply loops the Out to the In of the pump.

This way the hot water is a seperate process from the tap flow, and, hot water will always be available.

 

Make a heater-tank loop ckt with pump #1 (circ), this keeps the tank "hot". When the tank reaches near the heater "off" state the pump turns off (heater goes off), when tank temp cools down enough the circ pump turns back on (heater goes back on).

My water heater, and every tank water heater in America simply maintains the water temperature at all times. When the tap is opened hot water is drawn off of the top of the tank and cold water is piped down to the bottom of the tank. There's no pump involved.

I suspect the TS is hoping to not have to run his 100 foot hose until the hot water arrives, and may be mistakenly thinking that the system he's hoping for will provide instant hot water even after sitting for an hour or more.

 

It would help if we had a picture of the heater and its tank. I'm betting it's a high pressure washer with hot water. When not in use the heater shuts down. There is WAY TOO MUCH speculation on what we're trying to work with. Cold hard facts and pictures would be very useful. What the current system is, how it normally operates (though vaguely stated in the opening post) and why the need for the change.

 

My water heater, and every tank water heater in America simply maintains the water temperature at all times. When the tap is opened hot water is drawn off of the top of the tank and cold water is piped down to the bottom of the tank. There's no pump involved.

I hear you, but OP said the heater shuts down when no flow is detected, so to me that sounds like it needs to detect flow to turn back on. I guess it's like a pilot-less tankless heater.

 

Here's proof we're being spied upon. While perusing YT short video's I came across this one.
https://www.youtube.com/shorts/uGYJRxxlUnE
It sounds WAY TOO MUCH like a solution to your problem.

Now, regarding this system, it keeps moving hot water through the household. That means its heat energy is being shed throughout the house constantly, meaning your water heater has to work more to maintain the temperature.

Also, in the video it's shown being installed on the dip tube which is meant to bring cold water to the bottom of the tank.

 

I see no reason at all for a view of the heater or that tank or any other part of the system. It does not appear to me that the TS has a "tank" type of water heater, but rather a "TANKLESS DEMAND" type water heater that is fed from an external tank, located in a van. It is not in any aspect like a home water heater system.
A "TANKLESS DEMAND" type of water heater is totally different in every aspect. The demand type of water heater switches on when water starts to flow, otherwise it sits in a standby mode, NOT HEATING AT ALL!! THAT is the reason for the issue!!! Since it is a gas fired device, the shut down happens fairly rapidly, while the startup runs thru an air purge cycle followed by a pilot light sequence, prior to starting to heat, which takes a lot of time, during which the TS only flows cold water. The "van", with the water tank and hose reel, is some sort of business operation. THAT does not matter at all, except to be the reason that a whole bunch of the suggestions simply do not relate. The heater in our travel trailer works the same way, mostly because there is no reason to burn expensive propane when there is no use of hot water. The difference in our application is thet the water line from heater to the tap is about five feet of skinny water tube.

 

It does not appear to me • • •

This is the problem. We don't have a clear view of what we're being asked to work with. We can develop an image in each of our minds as to what the TS has or is working with or wants for sure. You and I disagree. That's fine. No problem there. The "Problem" is that none of us know for certain what we're working with.

 

We will need to wait for the TS to explain more completely what they are working with. But based on the description we already have, I trust my guess quite a bit: That it is a water tank, about 50 gallons, in a service van, with a propane fired demand type water heater, and with a pump/hose reel closely connected, running on the vehicle battery, and used for some sort of cleaning service. I may be wrong about the tank size and the battery, though.
Now we can see what the TS tells us it actually is. AND, I am guessing that the van is mostly colored white.

 

I am guessing that the van is mostly colored white.

Mauve. I'd have guessed Mauve.

 

Now, regarding this system, it keeps moving hot water through the household. That means its heat energy is being shed throughout the house constantly, meaning your water heater has to work more to maintain the temperature.

This is the problem. We don't have a clear view of what we're being asked to work with.

Interesting idea.
If you use such to heat the space like radiant floor (when needed) to a desired temp, using a PID controller to drive a DC pump with PWM the "loop" is alway "on", you can then add taps and you'll always have "hot" water at each tap. Certainly not a desired state when you need AC.

We can speculate, ideas will come out, wheter or not they help the OP.

 

OK, I seem to have picked up some clues. And at one point a tank in a van was mentioned, which certainly tells me that it is not heating a household. The 100 meter hose reel backs up that guess, as do the comments about a spray nozzle. Also, in a residential hot-water heating system, it is the heater part that gets switched off and on.
I learned quite a bit about heating systems as part of my consulting business that I formed after retiring. so I am not guessing except where I say that I am guessing.