Gravity Hot Water Recirculating Loops

We had a hot water gravity recirculation loop installed in our home about 5 years ago for our master bath. Since then I've noticed green staining around the hot water faucet fixtures but never associated that with the loop. This only occurs in the master bath. I thought I might have a PH problem, but it was checked twice and in the normal range. I consulted an expert in water quality systems. The first question he asked me is "Do you have a hot water recirculation line?" It turns out that the green staining is coming from the inside of our copper pipes. Apparently the constant flow of water is wearing them down. This occurs mainly on the elbow joints. I'm now in the process of figuring out how to safely disconnect the line (I say safely, because I want to avoid a potential bacteria problem caused by standing water in the return line). My pipes are inside walls and under floors so I don't want them to last.

Greg,
I would agree with that expert *if* the loop had a medium or high velocity flow. The gravity loops have a flow that might beat a turtle on a good day. Maybe you are getting erosion from the inside of that one faucet. If the expert is right, why don't we see oxidized copper at all sinks? Rhetorical question.....

Thank you for your reply. Let me begin by saying that I'm not necessarily convinced that I've discovered the problem or the sole source of it. I'm certainly no expert on this and I am keeping an open mind. It's been frustrating though as I feel the clock is ticking with respect to the wear issue of our pipes. What I can say is this:

The staining only occurs in the master bath for which the recirculation line was installed. There are two faucets and a shower. We see the staining around both hot water faucets and in the shower. There are two additional bathrooms (one of them is also upstairs) for which we do not experience the staining problem.

I agree that I wouldn't expect a convection flow line to create enough water velocity for noticable wear although I did notice that the valve going back to the hot water tank was nearly full open. I've since closed it way down after giving the faucets and shower a good cleaning so we'll see if that makes a difference (so far so good, but I think it's too early to tell for sure).

Also, from my research, I have learned that there are other factors, such as water quality, that can affect copper pipe erosion. It's possible that there is more than one factor contributing to the problem.

These factors include:

- Poor water quality (dissolved C02, minerals, etc.)
- Low PH (acidic)
- Quality of plumbing joints (deburring, flux cleaning)
- Electrical grounding issues

I've tested my water twice (once professionally, the other time with a home test kit) and the PH is within normal limits so I've eliminated that as a potential cause/catalyst.

Some of the water quality issues relating to "dissolved" materials would require more detailed analysis to diagnose. It probably makes little difference, but we also have a water softener. The softener was installed after the home was 5 years old so the insides of the pipes should be coated providing some additional protection. This makes it even more disconcerting that we are getting staining even with the coated pipes.

I don't know about a potential electrical grounding issue or how to diagnose that. I have heard that a faulty ground connection can create electrical currents which in turn lead to corrosion. I can say that there are no visible signs of any copper pipes touching dissimilar metals (such as ductwork) and thus causing galvanic corrosion.

I will say that the fact that it only occurs on hot water taps connected to the recirculation line leads me to think that the line may at least be a contributing factor.

-- Greg

my h2o heater is in the basement, i have a supply trunk running to the second floor, dipping to the first floor then back to the second floor, then back to the first, then back the the h20 heater. will a gravity system work with that amount of up and down or do will i need a pump?

Rob,
You need a pump and a winning lottery ticket. If the existing line behind the walls is not insulated, your fuel bills will be in outer space.

Tim:

Thanks more than you could ever imagine for instructions how to install a gravity hot water recirculating system. We live in a rancher with our bathroom located at the far end of our home opposite about 50 feet from the where the gas water heater is located in an unfinished half basement below. It took over 45 seconds for water in the lavatory to warm-up. The wait was unbearable.

Now, following your instructions, after running a ¾ inch return hot water line from the existing ¾ inch hot water feed line located beneath our bathroom in the crawl space, back to the drain at the bottom of our gas water heater, we now have hot water in our bathroom in exactly 3 seconds. It is unbelievable, but wonderful.

With the water heater sitting in the basement 8 feet below the first floor of our home’s living area, there is a natural siphon created in the new return pipe. I increased the efficiency of the siphon effect by making a long loop curling downward to the drain of the water heater. Below are pictures of my installation, which ended up costing about $100.

Below is a picture of the connection in the crawl space under the Bathroom. The “Tee” splice in the center runs from the existing insulated hot water feed line to an uninsulated pipe about six inches long to the left, to an elbow that begins the insulated return line back to the water heater. The other insulated lines are hot-water feeds, and the uninsulated lines are cold-water feeds to the lavatory and shower in the bathroom.



Below is a picture of the return feed to the water heater, consisting of a cut-off valve on the top left, a flapper type backflow valve in the middle below, and a tee valve for the relocated drain at the bottom right.



Below, with no insulation for a short distance as you instructed, is the extra loop coiling downward to the water heater above, installed to aid in the siphoning effect.



Thanks for your help.

Sincerely,


Terry L. Cook

I have a ranch with a full basement and it is a perfect candidate for a gravity recirc loop.

All the existing supply lines are copper. Will the gravity recirc work if the return loop is made of CPVC or PEX and is there a need to insulate these materials?

Thanks,

Howard

hi,
is it possible to give gravity hot water return line, when hot water boiler is placed in roof of a 20 storey building. if it is not possible please give me the solution,please note that i just want to use gravity feed hot water return in my building.

Tim,

I am interested in installing one of these systems. One question though.

my water heater is in the basement. I have a bath on the 2nd floor more or less directly above the heater, the kitchen is also directly above the heater on the 1st floor, and there is another bath on the first floor at the other end of the house.

should the hot water flow go straight up to the 2nd floor bath, down to the kitchen then over to the 1st floor bath, finally returning to the heater? or the reverse of that.

in short, is it better to have the hot water path go up rapidly, then meander down, returning to the heater, or meander up, then rapidly down and return to the heater.

So I have been living with the curse of instant hot water at my taps for about three years now thanks to Tim's advice.

Howard, material should not matter. What does matter is insulation and the correct pipe size. Insulate all the way around, except for the last little bit. Tim the builder says the last 15 feet; mine works with just 4 feet of line exposed.

Ever, you are probably going to need a pump if you want to have a recirc loop.

Tom R, you a probably going to need two different recirc loops. I'm going to guess how your plumbing lines run but I would have to say that you will need a return line from your second story bath and a different return line from your far away first floor bath.

Hi Tim,
Thank you, you have a great column.
My return loop is not returning???
I have a HW system as follows: 3/4" from the HWH, up 4' with 3 branches. 1 a 3/4" to sink within 10'. 2 a 3/4" to baths 25'away with a 1/2" return. 3 a 3/4" to bath 45' & a lav another 25', away with a 1/2" return.
All return piping is pitched back to the HWH, the two returns meet about 30" above the HWH & pipe is increased to 3/4" drops 7' to a check valve & into the HWH. Reading your articles, i've noticed you say install a check 5' away. Could this be my problem & why? I've installed many loops in the past & have never had a problem with the check at the base of the heater.
I'll try drilling a hole in the flapper. There is very little demand for HW in the building could this be part of ther problem? I noticed when they use a lot of HW it seams to circulate, then the hole line cools again.
Any ideas are appreciated Thanks Dave

I would like to create a recirculating loop. My existing hot water pipes are half inch. Do I still need to use a three-quarter return pipe?

I live in a 95 year old house. As part of a kitchen remodel I am about to replace greatly circuitous steel water lines with copper where I can reach it in the basement. In planning, I discovered a gravity recirculation loop to the cold water input to the water heater (in basement). I believe it returns from the 2nd floor shower (hot water pipe at the shower is always hot). The return line to the water heater input is warm unless there is hot water demand and then it turns cold. There is a check valve in the 1/2" steel return pipe that makes connection with the cold line that enters the water heater. The Rheem Fury water heater is ancient (first 4 digits of the serial no. are 0675). I cannot open the drain, but it leaks so I capped it. I know, replace the water heater.
It would be difficult to insulate much of the hot water line because it quickly disappears inside a wall to upper floors.
Should I get a new water heater and tap the return into the drain with a proper check valve? Does this system sound inefficient and should I try to remove the return at the shower or cap it in the basement and forget the recirculation loop?

Thanks,

David

can you send me the diagram for the recirculating loop for water gas heater?

Thanks
a

I installed a gravity recirc loop about 5 years ago, based on Tim's article(s). It works great, but I had trouble with the check valve. The check valve, as I understand it, is supposed to keep hot water from being supplied from the bottom of the tank. That is, when a faucet is turned on, the hot water should come from the top of the tank. The check valve was working in tandom with the hammer arresters in the line to cause the flapper to open and close a few times each time the water faucet was turned off. That was very annoying. I replace the check valve with a ball valve, and I turned it almost all the way off. This works perfectly, allowing only a trickle for the loop to operate, yet restricting flow from the bottom fo the tank. Best of all, no annoying ping, ping, ping, ping, every time the faucet is turned off.
Another friend of mine tried it, but put a metal plate inside a union that he installed in the line right near the water heater boiler drain. In the plate, he drilled a hole, about 1/4" in diameter. This performed the same function as my check valve, and he is very happy with the results.

ok to instll check valve in horizontal pipe instead of vertical?on hot water ruturn loop

I would like to install and on demand tankless water heater, but am having the same problem with water taking forever to reach its destination. My fear is that if I install the a recirc line becuase there is no place to store the water when it returns the tankless heater will always be running. Anyideas to stay "green" on saving natural gas and water? Thanks in advance

I set up a system in my garage of 3/4 copper pipe which loops into a wood fired hot water tank, and rises to the second level to a cast iron radiator. The hot is connected to the top of the radiator, the return is connected to the bottom of the radiator. It won't circulate. Is is because the cold return enters the heated water at the same level that the hot side exits the heated water tank?

Tim:

Does the size of the supply line have any effect on the size of the return line? IE. is 3/4" return acceptable with 1" or 1 1/4" supply? The village code likes over sized supply.

Thanks.

James

GRAVITY HOT WATER CIRCULATION;MY HOT SUPPLY IS 60% 3/4", 40% 1/2"..return is 1/2". works fair. if supply was 1/2" would it work better? This would be easier than making return 3/4". Have check valve and return is not insulated last 12'

I just replaced my old electric water heaetr with a new AOSMITH Conservationist electric water heater. The recirculating hot water loop is not working. The only difference from the old connection is that we could not bring the vertical recirculating pipe all the way down to the drain valve in the water heater because it was blocking access to the two covers to access electric/heating elements. The vertical was brought to the bottom and then a 90 degree turn(L shaped, about 5-6 inches long)) was made at the bottom to connect to the drain valve and avoid blocking the 2 covers. The loop did not work. First, we were getting cold water. Then we install a check valve horizontally in the small 5-6 inch pipe that connects at the bottom to the drain valve. The loop is not working properly. The recirculating pipe is not hot when water is not used. If you use a faucet with hot water then you can fill the recirculating pipe getting hot. I think the check valve is closed due to pressure from the water heater side when there is no activity preventing the loop to work.
What should we do? I sometimes think that the small L shaped turn at the bottom is breaking the gravity--is this right? If not,
should we install a pump?
or should we relocate the check valve as you say in your article to 5 feet from the bottom?
I can understand the check valve working horizontally, but if the check valve is vertically I would think it would keep the flapper open.Or is there a special check valve for vertical placement.
Please, advice. Our house is a 2 story and the recirculating loop was working fine before we changed the water heater.Thanks.ernie rodriguez

I didn't find an answer how to bleed the air out of the return loop of a gravity hot water recirculation system.

I recently had my leaking water heater replaced. After that, the previously perfectly working hot water supply started acting up. In the morning I have to drain hot water out of the bathtub faucet into a bucket, before the hot water will start flowing as expected at another shower head. I suspect air got into the system creating an airlock.

Hopefully you can give me some tips to remedy this annoying situation. In the past similar problems had fixed themselves, but this time it is more severe.

Greetings:

About 2 years ago I installed a hot-water return loop and it has worked fantastic since then. I get hot water within 5 seconds.

Last week I had to replace my hot water heater after it started to leak. I installed the replacement heater of the same type & size. None of the plumbing for the loop was changed.

The loop worked fine for about 32 days then stopped for no reason. I'm back to waiting 3 minutes for hot water again. Since I'm on a well & septic, it's costing me money for electric as well as saturating my drain field.

I'm looking for suggestions on what to change. The new heater has plastic heat traps installed in the intake & discharge side of the heater. They are small blue colored sleeves. The manual says NOT to remove them, but I'm wondering if the heat traps are stopping the circulation of water. I don't recall if the old heater had them or not...or if it did, wheather I removed them.

So, I could use suggestions....as mentioned, it worked fine for years, then worked a couple of day with the new heater, then stopped.

REgards,

Jim

Jim,

Yes, that is precisely your problem. The main article is careful to mention there cannot be any heat traps. The only disappointing point is that the manufacturer may not warranty your heater if you go against their instructions of never removing these heat traps. They're doing what they're designed to do, namely keep water from flowing in unintended directions (at least form the standpoint of "standard," no recirc loop, installations).

For a gravity circulation hot water loop you say: "You MUST use a flapper-type check valve, not a spring-loaded one."

I'm having trouble finding a flapper valve. Any suggested manufacturers or suppliers?

It's called a swing check valve. I believe I have bought them at HD in the past. If they don't have them, then any plumbing supply house will.

I will be installing a recirulation loop in a system with two water heaters connected in series. Should the loop return to the first (up stream) or second (down stream)water heater?

John,

I'm not a plumber, but I would recommend connecting the return to the second one in series. It may work connecting to the first, but I would be worried about having to flow up through the first and back down the dip tube in the second to complete the loop. There may be some resistance to flow down through the second tank considering the heat gradient.

I'm currently having a gravity system installed and I will be connecting to the second in series. This is more important in my case as the first is at a lower temperature which could interfere with the temperature differential and also have the negative effect of pushing colder water into the second causing a call for heat.

I have a problem with hot water.
The water is only HOT in the bathroom sink but warm every where else. Problem just started a month ago. Heater is about 8 months old and near the bathroom. also there is an electric pump of some kind on the pipe that is not pluged in. I dont know what it is. I used to be able to have it on a low setting and it would be fine now at the hottest setting it is still lukewarm. The water seems to fluxuate in a cycle of hot cold warm, if you have any suggestions on what this might be please email me

I have a gravity-fed closed loop hot water heating system.I need to replace my water heater. I'd like to use a tankless water heater. Can a tankless unit be used on such a looped system? If so, how should the connections to the new heater be made?

Thanks to all the comments. I finished my loop. The first half is the original 3/4" copper, the last half is 3/4" CPVC. The last 15 feet are uninsulated. I used galvanized from the bottom of the water heater to a tee/brass boiler drain valve and up to a union. After that I went to the CPVC. It works beautifully. I get hot water in all the bathrooms, the laundry room and the kitchen in about 3 seconds.

I installed three gravity recirculating loops in our U shaped house and ran them to a manifold on the bottom of the hot water heater. I used 3/4" copper. It took a couple of days for them to start functioning but they work great and have been in place for over a year.

I have installed a hot water recirc system as you have described and it works well, with one exception. I seems that cold water is entering the recirc loop and diluting the hot water. Does it mater that I put the flapper valve in a vertical line, (Yes the arrow points towards the water heater) or should I have it in a horizontal line?

Swing check valves need to be installed on a horizontal line. Spring check valves can be installed on a vertical line as well, but probably will not work for gravity systems as they may block the flow completely. With spring check valves, the flow would need to overcome the resistance of the spring.

So you should be installing swing check valves on the horizontal line with at least 10-12 diameter of straight before the check valve to reduce turbulence. IE. 3/4" * 10 or 12 = 7.5" or 9" of straight before the check valve.

I have a circulation loop, but can not seem to get it working. It has a 3/4" feed and 1/2" return line.

I've noticed several questions about using Pex for the loop, but have not seen an answer. Is Pex a suitable candidiate for the loop?

Will a gravity loop work with a tankless coil immersed in a hot water boiler? I have a ranch with the boiler in the basement.I would have about a 40 ft. run of 3/4 copper. You mention drilling a hole in a check valve, wouldn't a swing check valve installed near vertical so that the flapper was hung open, provide a lot less pressure drop? Thanks for any advice

AsktheBuilder
18 Jan 2008, 06:55
Rob,
You need a pump and a winning lottery ticket. If the existing line behind the walls is not insulated, your fuel bills will be in outer space.
============
So, install a shut-off valve at any convenient point in the return line so you can shut it off in the summer. In the winter, you will pay for the heat one way or another; why not enjoy quick hot water while you are at it?

I installed a hot water recirc loop last weekend and it works great(instant hot water). I used 1/2 in CPVC to try and minimize the flow and thus the heat loss.

One problem is the water is not as hot at the end of the loop fixutures now. I think it is pulling relative cool water from the bottom of my heater back thru the loop.

Thus my question on the check valve. You recommend installing in the vertical reric run near the heater. I dont follow the logic of the drilled hole if the check is open in the direction of return to the heater? Also, I have only found brass swing checks and I am unsure if the brass flapper will overcome gravity to close in a vertical run.

I have installed a gravity return loop in house .As the house was plumbed with 1/2 copper ,i used same for the loop.I get instant hot ,then cools after running the shower, have to readjust the control valve. what causing this. Seems work ok otherwise .thanks

Hi, we have had a gravity recirculating water system in our house for 20+ years. We just had the water heater replaced a few weeks back and since then our instant hot water is more like warm... and takes minutes to get hot. I have turned the temp up until it is much hotter than before but it appears the circulation is more of the problem. Could there be air in the line? What can I check? thanks!

i have installed return line to bototm of h2o tank with check at inlet no hot return

Hi, I want to remodel the poorly installed PEX system. Currently most of the run have 30+ feet between the manifolds and the appliance.

My thought is to move the manifolds (12 branch each) to a location closure to the appliances and use a recirc loop to maintain hot water at the hot manifold.

I was going to use 3/4" copper from the hot water tank to the manifold which would be about 25 feet away, and either 1/2 PEX or copper for the return.

Both the tank and manifolds are located in the basement, but I have my choice of placement locations for the manifolds--I am thinking they should be overhead as in near ceiling height in order to gain maximum draw back to the tank (i.e siphon). Is this the best plan?

I have also considered using Nibco's Just Right unit which is readily available for $50...

Do you have any words of wisdom about this project?

Thanx

My house is on a slab.The water heater is about 70 Ft. from the furthest fawcet. The pipes are all in the attic ,below the insulation.Will a gravity feed sys function?

The supply in my two-story home ran in a big loop around the basement ceiling before rising to a second-story guest bath, where there was a 90-second wait for HW. I installed a loop back to the base of the heater from the start of the riser, which was as far as I could go without major reconstruction.

Now the wait is reduced to 30 seconds, other fixtures have immediate HW, and I am happy.

One question remains: why drill the hole in the check valve flapper? I suspect the valve is not very efficient at preventing slow backflow of cold water from the heater, but would such a hole improve this? What gives?


Thanks to anyone who can answer, and save me a lot of wasted cogitating.

Cheers!

Thanks for the heat trap tip! I installed a new tank and the gravity loop stopped working. The plumber did not know the reason. Based on this tip I removed the trap and I am in business again!

Hi Tim.

After reading this column several years ago I installed a recirculating loop. My plumber was convinced that gravity would be enough so he convinced me to install a recirculating pump. I turned off the pump and the system worked great and have never needed to turn on the pump again.

Now that my hot water tank needs replacing, I was wondering if there is a instant hot water system that can be substitued in order to save some energy. If not, I will stick with the tank.

Thanks,

Darren

We live in a ranch and have had a recirc loop working on our hot water heater (a power vented model) since we moved in 6 years ago and we had hot water instantly in all our faucets. We just had to replace our hot water heater with a new power vented one and now the instant hot water does not work and we can only get about 110 degrees at the farthest part of the house with the heater set to 130 degrees. I called and was told by the company that installed it that we should have a pump somewhere which we don't. I am sure we have a gravity recirc. Why is this one not working? What can we do about it. The plumber who installed said he would investigate so I thought I would too. Help please.
Linda

I have a convection current in my recirculation line. We get warm water almost instantly in our baths, but barely enough hot water to finish one shower. The top element in my electric water heater appears to never reaches the set 120 degree temperature because water is always being drawn off by the recirc loop. As a result, the lower heating element never gets turned on, and there is lots of cold water at the bottom of the heater (at least 2 large buckets). When I turn a valve to stop flow in the recirc line, we get plenty of water and find there is warm water at the bottom of the heater. Is there a way to have a recirc. loop and plenty of hot water?

The upper and lower elements were just replaced. The upper was encrusted with mineral deposit, while the lower was not. This supports the conclusion that the lower element rarely gets turned on. The lower element is getting power and passed a continuity test. The dip tube was also replaced, even though nothing could be found wrong with the old one.

Well, I installed a 1/2" PEX line from under the second floor sink back down through a flapper check value (with the arrow pointing towards the hot water heater) and into the drain value of my cellar hot water heater, and it didn't do anything. The check-valve is installed horizontally such that it shouldn't take much to open the little flapper, though I didn't drill a hole in it. I installed an air eliminator also in a T by the fitting under the sink. I don't have the hot water heater that hot (like 108) though, and I couldn't insulate the entire run up to the second floor, though I got most of it. It is an AO Smith Voltex Heat Pump, and it says in the installation instructions to install a "heat trap" in the line (basically an inverted "P" trap), but there is none installed. Any ideas or help to diagnose?
--joe

A few comments based on my experience as an engineer and as a mechanical contractor foreman who has sucessfully designed and installed recirculation systems:

To improve energy efficiency and minimize needless heat loss, use thicker R-4.2 insulation. You will probably not need to leave any insulation off the return line when operating at around 125 to 135 deg F. on loops that are around 40-50' or greater in length with R-4.2 insulation. There is adequate heat loss through R-4.2 insulation to provide good circulation. However at around 120 deg F at the hot water heater, there will be minimal recirculation.

You do not need, nor do you want a check valve orifice if you select the correct check valve and mount it properly. Swing check valves such as the MUELLER INDUSTRIES 101-003 (1/2" brass) work well as they require very little pressure to move. Install in the horizontal position and position slightly off level, if required to minimize the differential pressure required to allow flow. Swing check valves designed for both vertical and horizontal installation require more off axis mounting to work properly.

I recommend using the hot water tank drain tap for the return and leaving the thermal check valve in the cold water inlet. Remove the thermal check valve from the hot water outlet.

1/2" return size is easily adequate for typical domestic recirculation loops of 60' (30' of 3/4" supply and 30' of 1/2" return). Vertical risers required to pipe over ducts and piping are OK as convection forces provide the energy to move the water, not gravity.

One additional benefit of a recirculation system is elimination of hot water tank temperature stratification and stacking in the hot water pipes.

Michael Hart

I was wanting to this for my home, but am unsure what size pipe to use. My last fixture uses 1/2". Should i run 1/2" back to the tank and then go to 3/4" for the tie in, or go to 3/4" after the fixture and run that all way back?

Thanks!

I used 3/4" as the main line and used 1/2" take offs to all the fixtures.

Have had this system in place for over 5 years and love it.

Darren