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Gravity Hot Water Recirculating Loop Questions

#1
01-17-07, 06:42 AM
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Join Date: Jun 2006
Posts: 25
Gravity Hot Water Recirculating Loop Questions

So I have to wait too long for hot water. I've been looking at recirculating pumps, various brands, various methods. Read and article about gravity hot water recir. loops. I understand the theory, but am wondering if two loops can work.

Cliffs on info:

You run a return hot water line from furthest fixture. You plumb it into the hot water heater's drain spigot. You remove any backflow preventer on the hot water supply side. Thermal properties of heated water take over and hot water rises out of HW heater and the cooler water in the return lines "sinks" into the bottom of your water heater. This slow circular motion keeps hot water available at furthest point. You must insulate the lines, except for last 15ft of return.

So my water lines look like the below diagram, but I would like Hot water at both ends of the line, and am wondering if two loops would work, and if so how to plumbe them correctly.

http://img.photobucket.com/albums/v228/harleysilo/Basement/plumbingsketch.jpg

#2
01-17-07, 07:37 AM
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Join Date: Jun 2006
Posts: 25
So I found another article...
http://www.contractormag.com/article...fm?columnid=62

Main thing I picked up on is it states I should add the volume of water now contained in the loop to my calculation for the expansion tank size....

So for me one loop of 100' of 3/4 would be approx....5 gallons.....

#3
01-17-07, 08:24 AM
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Join Date: Jan 2007
Location: Central Indiana
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I would think your two-loop idea would work if you kept both return loops separate until the last possible point where they then entered the return of the hot water tank.

#4
01-17-07, 09:28 AM
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Join Date: Jan 2006
Location: San Jose
Posts: 2,175
I think it's 2¼ gallons not 5... but the big question is how much of a drop do you have? Most graviity systems work best when they are square or taller, meaning they should go up at least as much as they go out.

Why no do it with a pump? Less than \$200 for a really good one and with it set to run half the time, that's \$10 a year in elecitricity - also, having everything insulated will probably save more than that \$10/year just during the non-heating months.

With gravity, you almost have to picture and pipe it like a ferris wheel. Any rises as the return is coming back down and boom... you have a thermal trap.

(0.75/2 x 0.75/2 x Pi x 100' x 12")/231 = 2.3 gallons

231 cubic inches per gallon

#5
01-17-07, 09:37 AM
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Join Date: Jun 2006
Posts: 25
It could be plumbed with no rises. But plumbing it as you describe, would not work.

I have to cover about 35' with a vertical height of 9'. I guess I don't completely understand the concept. If it cools in the last section of the return, would it not "pull" syphon the water through the parts that are level. Pic will help, in this diagram would both options work, or just the one on the left?

http://img.photobucket.com/albums/v228/harleysilo/hotwaterheaterdiagram2.jpg

Why no pump? Well I could do it this way for cost of pipe and my labor and a few fittings. Pump is \$200 + and might have to be replaced in future.....that is all.

#6
01-17-07, 10:07 AM
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Join Date: Jan 2006
Location: San Jose
Posts: 2,175
True enough... you get a gravity system going and that's that - I tyotally agree. Still I'd do it with a pump with a switch upstairs by the front door and really insulate everything well and still make thermal traps near the WH.

As for the two diagrams I would think that since they both have the same drops, either one would work just as well. Running it back on an angle would probably help.

One thing to remember is that pipes have friction, smooth pipes have a wee bit and fittings have more. The gravity loop has to overcome that. The farther you get from having it go straight up and down, the more you are going to have to coax that water to cycle. It might prove undoable in your application. If you can work it, then you are going to be returning water to the tank that is much cooler.

Just food for thought... as a compromise, maybe just think about where you'd add a pump and then pipe it for gravity. If you can get it to work to your satisfaction then excellent, otherwise you know where to throw in the pump.

If you start shopping for a pump, I really liked the Grundofs 10-16 model. It's a 25watt stainless steel pump and comes in different versions including one with a variable return temp sensor to shut it off once the recirc line is heated and also a simple 24 hours timer divided up into 20 minute DIP switch controlled periods. It connects by plug and goes on the 1/2" return line. I just bought one for \$200 Cdn with all the featurs. I'm still redoing the domestic so it'll be a few weeks before I can stop having to leave the tap on while I get the water hot enough to shave and finally try it.

#7
01-17-07, 11:22 AM
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Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,493
I'm with Who, pipe it in and if it works by gravity then great. If it doesn't work then add the pump. I would bring the separate loops about half way down the side of the water heater and install ball valves before joining them together. If you then discover you need a pump then install the pump below this point where the two loops are connected.

Be sure to add a valve before entering the tank bottom so that you may totally isolate this system to allow for pump installation or replacement, tank replacement or for tank drainage and flushing.

One thing. A gravity system will be working all the time and you will not be able to insulate the return lines. Your cost of water heating WILL go up. If you install a pump you may then insulate the return lines and add a thermostat and/or timer to control the pump.

#8
01-25-07, 09:35 AM
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Join Date: Jan 2007
Posts: 1
Gravity recirculating hot water

Hello,
I have been thinking about doing this in our house and have a quick question: In the gravity hot water recirculating loop system, does the water continuously circulate through the pipes, or does it only move when you turn the faucet on? Thanks!

#9
01-25-07, 12:44 PM
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Location: San Jose
Posts: 2,175
Actually, it circulates whenever a tap isn't on.

#10
01-30-07, 05:38 PM
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Join Date: Dec 1999
Location: San Diego Ca. USA
Posts: 1,039
Hi, I did this a couple years ago. The house is on a slab. I ran a new plumbing system in the attic. The heater is on the ground floor. I know the plumbing has to be above the heater. The heater is on one side of the house, the last tap is on the other side of the house. I ran a 3/4 home run the length of the attic and tapped off where nessary for other taps and returned with a 1/2 in line installed with a slight pitch down. I installed 3 valves to isolate the system if necessary. I installed a check valve at the water heater and plumbed it to be able to install a pump. All the connections were at the drain on the tank. Works great. A few ideas on different sites were to use 1/4 copper coil said would use less heat loss. May work, I don't know. My system works great but sure was a hard job installing. They make sysems that connect the hot water and cold at the far end with a pump you may want to check out.
Good Luck Woodbutcher

#11
02-12-09, 04:54 PM
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Join Date: Feb 2009
Location: Grand Island, NY
Posts: 1
ShillelaghMike

What hardware do I need (besides pipe, fittings, etc.) to install a gravity recirculating loop? I have a gas fired hot water tank. Shouldn't there be some kind of a "one-way" valve somewhere in the loop to keep water from flowing out of the drain spigot connection? Can someone help? Thanks

ShillelaghMike

#12
02-13-09, 02:06 PM
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Join Date: Jan 2009
Location: Texas
Posts: 897
running 2 gravity loops would not work unless they were the same distance and had the same restrictions (90's and such) or the longer one could override the shorter one

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