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Old 05-29-2020, 01:04 PM   #1
Bus Nut
 
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Recirculating pump for hot water

Typically, when you turn on the hot water, you have to wait several seconds for hot water to come out the line while cold water goes down the drain. To solve this, lots of folks advocate recirculating pumps to make sure hot water is in all the pipes before you need it.

However, I don't quite understand how this is set up.

I have an Eccotemp L10 and two Shurflo water pumps. I also have a bunch of 12 volt switches, most of which are rocker switches, but one is a momentary-on switch. The simplest design is that I just loop the hot line back into the hot water line through a pump back to the hot water heater inlet.

1. To prevent hot water from entering the cold line, I can use a check valve. Is it a problem have multiple check valves in my system? Would this cause water hammer?

2. The water pumps only pump water when there is a pressure drop in the lines. Will it work on a close loop system like this?

3. Can water flow through the water pump if it is turned off? My expectation is that when I turn off the recirculating pump, nothing flows through the recirculating line.

4. If you recirculate water too long, the eccotemp will just keep heating the same water until it is scalding hot and damages your equipment. What hardware would I need to prevent this? A thermostatic temperature valve? A temperature cutoff switch? I'm confused...

Attached is a crappy hand drawn diagram.
IMG_20200529_130225405.jpg
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Old 05-29-2020, 01:58 PM   #2
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My plan is to have a momentary contact switch (like a doorbell) to open a normally closed solenoid valve that will dump hot water back into the water tank.

What I like about this plan is that there's minimal plumbing, no additional pump, and the water doesn't get reheated in a small loop (with the potential for steam generation). When you need hot water at the faucet you push the button for a few seconds to give the heater a chance to turn on and start heating, then release the button and turn on your faucet.
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Old 05-29-2020, 02:40 PM   #3
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A few thoughts: Item 2; your closed loop system will shut off once water pressure limit is reached (faucets closed) which will turn the heater off until there is flow. Item 3; water will not flow through either my Surflo or Flowmaster pump when they are off. Item 4 is true but can be cured by placing a thermostatic switch in/on the heater output line that turns on or shuts off the heater pilot lighting system at a preselected temp. I installed one on my system and it cycles the water temp + or - 2 degrees C from my preferred temp setting on the outlet thermostat.

I saved item 1 for last. Use one pump only for cold water and another only for hot. In your sketch replace the "second" pump with a solenoid valve that dumps back into the water tank rather than directly into the heater. Yes, it will add a little heat to the water tank although I haven't noticed the temp increase except when the holding tank is nearly empty. Because of the thermocouple at the heater outlet, the ever increasing temp cycle doesn't occur. I added a 3 way "hallway" switch so I could turn the solenoid valve off either in the kitchen or the bath when I want full flow at the tap. A 4 way switch could be used to allow a third on/off site. In practice it turns out that I have about as much water coming from the tap as I want to use even with the recirculation solenoid on.

Jack

Hallway switch with guide light
.

Water heater master on/off (top), Thermostatic switch (bottom)
.
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Old 05-29-2020, 02:58 PM   #4
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AZ, your plan will work but only if your return pipe comes back to the tank from the faucet furthest from the heater--so same amt of plumbing as I used.

The use of a second pump assures plenty of water flow to the heater under all conditions when the water heater is on without having to monitor cold water use to preclude low flow to the heater which will shut it off.

An off/on switch serves the same end as your momentary switch but frees you up to do other things while the water heats which seems to take forever especially with very cold inlet water.
Jack
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Old 05-29-2020, 03:15 PM   #5
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Sure, that makes sense, ol Trunt! I was going to suggest one solenoid valve for each outlet but I didn't want to complicate things. I'm converting a very small bus with hot water at the sink only so the one valve is all I need. One things for sure: having hot water while camping is going to be SO NICE!!!
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Old 05-29-2020, 05:24 PM   #6
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Interesting thread here for sure.


The 2 pumps seem to be required. The pump on the hot side concerns me as to longevity as compared to cold water pumps normally used. They are ok for cold water pumping but don't last under high temps.


Any suggestions for the best pump for hot water application? I think the booster pumps used for the cabin heat might be the better way to go?

jack? Anyone have thoughts on this potential weak point of a recirculating system?


John
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Old 05-29-2020, 06:11 PM   #7
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Originally Posted by BlackJohn View Post
Interesting thread here for sure.


The 2 pumps seem to be required. The pump on the hot side concerns me as to longevity as compared to cold water pumps normally used. They are ok for cold water pumping but don't last under high temps.


Any suggestions for the best pump for hot water application? I think the booster pumps used for the cabin heat might be the better way to go?

jack? Anyone have thoughts on this potential weak point of a recirculating system?


John
I think if I am understanding this right with two pumps the hot water one would be before the heater, and the hot water would just go back to the tank for a few seconds, so the water pump would never be subject to hot water. If instead of going back to the tank and it went in a closed loop then it would need to be a pump that can handle heat. The coolant pumps would handle heat but will they produce the needed pressure of about 35psi? Would be interesting to run a pressure test on one.
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Old 05-29-2020, 06:18 PM   #8
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BlackJohn, actually the "hot" pump only pumps water from the water tank to the heater not away from it. These little 12v pumps have a long life expectancy and are cheap and easy to replace--probably a good idea to carry a spare just in case though.
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Old 05-29-2020, 07:00 PM   #9
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Thanks Ronnie and Jack, it's becoming clearer now.


"The use of a second pump assures plenty of water flow to the heater under all conditions when the water heater is on without having to monitor cold water use to preclude low flow to the heater which will shut it off. "


This explanation didn't register till now.



If I have an electric tank for storage only, no power on, that would also help temp stay warm rather than going back to the regular water holding tank, just a closed loop.


Taking this info to the drawing board, ty!


I have no idea how much pressure a recirculating cabin heater pump has Ronnie. Imagine the same as engine water pump? Would that be sufficient for the tankless to operate in such a project?


John
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Old 05-29-2020, 07:58 PM   #10
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BlackJohn, I went hunting for info on high temp use and Flojet and Shurflo water pumps. The first will stand temps to 130 degrees F and the second to 135 degrees F so both are probably OK for our use.

I also looked at the circulation pumps you mentioned and it seems that their flow rater and or max pressure don't meet the requirements of Ecotemp. There are a couple of $500 120v circulation pumps that could possibly be used but they are large in size.
Jack
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Old 05-29-2020, 09:58 PM   #11
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I've already plumbed a small elbow fitting into the top of one of my freshwater tanks for a getting-hot water return. I plan on using a small brass button valve such as the Interstate Pneumatics VBT44 that will be immediately after a Tee fitting on the bathroom basin's hot water line: the branch off the Tee will supply the faucet, and the straight-through run will supply the button valve and its return line to the tank. How will this work? Easy - turn on hot water normally at the faucet, hold down button valve (so water mostly flows through it and not to the faucet) until I can feel it getting warm, then release the button valve and the water will flow to the faucet. This is fundamentally simple and should work fairly well, and it doesn't need any fancy electronicals or extra pumps or whatnots; it just relies on the brass button valve itself getting warm from the water passing through it, and one's finger then feeling that warmth. I'll give this a try, and if it works well I'll report back in a year or so! And if it doesn't, then I've not spent more than a few bucks for a button valve and a Tee.

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Old 05-29-2020, 10:27 PM   #12
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Lots of ways to skin a cat. Are you going to do this on only the bathroom or all the taps?
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Old 05-30-2020, 09:14 PM   #13
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Lots of ways to skin a cat. Are you going to do this on only the bathroom or all the taps?
Jack
I'll need it only for the bathroom - the shower and kitchen sink are directly above the water heater so there will be very little water in their short length of pipes, and the washing machine won't be used enough to justify a water saver gizmo.

John
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Old 06-01-2020, 07:20 AM   #14
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Thanks for the advice everyone.
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Old 06-01-2020, 01:55 PM   #15
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Quote:
Originally Posted by AzironaZack View Post
My plan is to have a momentary contact switch (like a doorbell) to open a normally closed solenoid valve that will dump hot water back into the water tank.
Quote:
Originally Posted by ol trunt View Post
AZ, your plan will work but only if your return pipe comes back to the tank from the faucet furthest from the heater--so same amt of plumbing as I used.

The use of a second pump assures plenty of water flow to the heater under all conditions when the water heater is on without having to monitor cold water use to preclude low flow to the heater which will shut it off.

An off/on switch serves the same end as your momentary switch but frees you up to do other things while the water heats which seems to take forever especially with very cold inlet water.
Jack
It must be Monday because it took me 45 minutes to understand all of this. I think I'm understanding the two pump concept as well. My question is what to do when hooked to shore water. You can only dump so much water back into the tank before it is full. If I understand correctly, these water heaters are triggered by pressure and not temperature. Or is it a combination of both? If it's pressure drop, having a recirculation pump won't activate the heater, you need the solenoid valve to dump water into the tank.
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Old 06-01-2020, 04:48 PM   #16
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Actually it is the combination of pressure, flow and temp that allow the heater to turn on. The 3 gpm pump recommended by Ecotemp seems to be able to handle the pressure and flow rate requirements even with the solenoid recirculating valve open as well as all the water I choose to run at the tap. I suppose if I wanted both the shower and the kitchen sink hot water on at the same time I'd have to shut off the solenoid (wouldn't need it anyway 'cause the heater would be going full blast anyway). If for some reason too much water was being dumped back into the tank to allow the heater to run you vould simply use an adjustable in line valve to slow the flow.

As to being hooked to shore water, whether or not you have a water heater, you need to have the shore water enter AFTER the water pumps or you will over flow the water tank. A couple of manual ball valves will take care of that. No need to use the recirculating system when you are on shore water --so what if a couple of gallons of water go down the drain. You are correct though. If you did use the recirculation circuit while on SW you would have a flood.
Jack
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Old 06-01-2020, 05:21 PM   #17
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As to being hooked to shore water, whether or not you have a water heater, you need to have the shore water enter AFTER the water pumps or you will over flow the water tank. A couple of manual ball valves will take care of that. No need to use the recirculating system when you are on shore water --so what if a couple of gallons of water go down the drain. You are correct though. If you did use the recirculation circuit while on SW you would have a flood.
Jack
Ah, you are correct, being hooked to shore water would negate the need to utilize every drop of water possible. The more I think about it though, that would be an easy, or rather lazy, way to fill the holding tank without having to use a gravity fill connection. In fact, I think I may just add another solenoid and a few more check valves to the system so I can fill the tank the lazy way all the time. I can probably also tie the solenoid into the fill gauge so it won't over fill the tank.
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Old 06-01-2020, 06:00 PM   #18
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I like the way you are thinking.
Jack
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Old 06-06-2020, 04:44 PM   #19
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So on mine, I want the hw recirculation system to work on shore water or on local. I want the ability to push a button and have the hot water lines filled with hot water and then stop.

I have a HW tank with two extra heat exchangers. one will be tied into the bus coolant lines (thermally controlled) and the other one will be tied into my solar water heater (thermally controlled). In addition, there is a 900W electric element in the HW tank.

So, I will have a recirculating pump tied into a controller that once started (by pushing a button), it circulates water until the temperature at the end of the return line is "near" the temperature of the outgoing hot water from the tank. Then it will turn off the pump. I can use this at any time to pre-heat my hot water (probably mostly for showers). This will never "waste" any water and will work regardless of the water source...
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Old 06-06-2020, 06:17 PM   #20
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I'll need it only for the bathroom - the shower and kitchen sink are directly above the water heater so there will be very little water in their short length of pipes, and the washing machine won't be used enough to justify a water saver gizmo.

John

Why not just install a point of service water heater? Think about the relative power requirements, maintenance, etc.
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