Tankless recirculation loop design questions

[Brad Littlejohn]

My tankless (outdoor/Natural gas) has a manifold layout out for cold and hot. Every fixture is single-homed from a manifold in the center of the crawl space. I am planning to leave the cold water manifold alone if possible.

Should I:

- Put all the hot water connections on one big loop throughout the first and second floor with 1/2" branches off the loop a few feet from each fixture? I know that 3/4" in the min for the trunk. But, would 1" be better? I want to prevent noticeable pressure drop when several fixtures are being used at the same time.

- Create a parallel manifold system for the hot water that would connect to the existing hot water line at each fixture. I would think that physics would come into play here and hot water recirculation would take the least path of resistance. The first fixture in the loop.


-Install a hot water bypass valve at each fixture to the cold water line . Once the closest bypass valve gets up to 120 degrees it will close. the next least path of resistance would be the second fixture. Then after a couple of mins, all bypass valves would be at 120 degrees blocking recirculation. Then bypass valves would randomly open and close as they cooled off.

- One bypass valve from the furthest fixture.


I have a 2250 sq ft house with two floors. 6 hot water connections on the second floor and 4 hot water connections on the first floor.

I have been trying to find an article on the best option. But, no luck.

 

[Fitter30]

Guess your running the pump 100% of the time? If you are it's hard on the pump not moving water. They will overheat motor and impeller. Other problem cold water will get diluted. Turn on a faucet might take a minute to get cold.

 

[Sponsor]

 

[JustWantHotWater]

My first post - be nice

If you put a bypass valve at every fixture, would that not make all of your cold lines hot too? That does not sound very pleasant to me at least. It is one thing to waste cold water for the hot water to come, but to waste hot water (that you have spent resources to heat) to get cold water seems extremely counter productive and even more wasteful.

It is my understanding that tankless manufacturers do not recommend multiple bypass valves as that will confuse the tankless internal pump logic. However, I have also heard that a work around is to schedule the recirculation instead and hence turn off the "smart" logic that tankless manufacturers are putting into their units. Additional controllers for tankless units may cost more money (Rinnai wifi module for example).

If you go more than one bypass, the pump would obviously have to be on long enough to move through all the number of lines a bypass is installed; otherwise, if it is not on long enough, the "hardest" lines will never get heated. I am still back to where I started here though in that your cold lines will turn to hot.

Depending on the run length I think there are limitations, if the unit has an internal pump, too on how long the recirculation loop can be depending on the run diameter (1/2 inch versus 3/4 for example). You may run into limitations the more bypass valves you add.

 

[John Gayewski]

My tankless (outdoor/Natural gas) has a manifold layout out for cold and hot. Every fixture is single-homed from a manifold in the center of the crawl space. I am planning to leave the cold water manifold alone if possible.

Should I:

- Put all the hot water connections on one big loop throughout the first and second floor with 1/2" branches off the loop a few feet from each fixture? I know that 3/4" in the min for the trunk. But, would 1" be better? I want to prevent noticeable pressure drop when several fixtures are being used at the same time.

- Create a parallel manifold system for the hot water that would connect to the existing hot water line at each fixture. I would think that physics would come into play here and hot water recirculation would take the least path of resistance. The first fixture in the loop.


-Install a hot water bypass valve at each fixture to the cold water line . Once the closest bypass valve gets up to 120 degrees it will close. the next least path of resistance would be the second fixture. Then after a couple of mins, all bypass valves would be at 120 degrees blocking recirculation. Then bypass valves would randomly open and close as they cooled off.

- One bypass valve from the furthest fixture.


I have a 2250 sq ft house with two floors. 6 hot water connections on the second floor and 4 hot water connections on the first floor.

I have been trying to find an article on the best option. But, no luck.

Your design is bad if you want recirc.

Are you building this project or is already done and your trying to make it better? If your just trying to improve your hot water comfort I would just pick one fixture that you want to be hot instantly and pull a recirc line to it. For instance if your wasting a lot of water for one of your showers or you want insta hot at the kitchen sink. Why would you want instant hot water at your washing machine?

Other than picking a fixture or maybe two, you'd be better off redoing your plumbing for this purpose.

 

[Brad Littlejohn]

Your design is bad if you want recirc.

Are you building this project or is already done and your trying to make it better? If your just trying to improve your hot water comfort I would just pick one fixture that you want to be hot instantly and pull a recirc line to it. For instance if your wasting a lot of water for one of your showers or you want insta hot at the kitchen sink. Why would you want instant hot water at your washing machine?

Other than picking a fixture or maybe two, you'd be better off redoing your plumbing for this purpose.

 

[Brad Littlejohn]

I am looking to for the best design for recirc. I am up for ripping out all the hot water lines or something in between. I came up with three ideas. But, I was looking for someone that has done it and what worked for them.
"Why would you want instant hot water at your washing machine?" Wife likes to wash clothes in hot water. Right now when the washer machine calls for hot water. It only gets cold water since the tankless has to run a while to heat up with water. I think it is a waste to call for hot water only for the washer to fill before the hot water get there. I end up with hot water in the plumbing line but, none it the washer. Same with the dishwasher. I just plumbed the dishwasher to the cold line since it has an electric element to heat the water.

 

[John Gayewski]

If you want instant hot water get rid of the manifolds and run a trunk and branch system

 

[Brad Littlejohn]

If you want instant hot water get rid of the manifolds and run a trunk and branch system

Looks like that is the only solution. I was thinking that someone might have tried one of the other solutions but, I guess not.