Different Water Heater Recirculation Methods

I have a Noritz gas tankless water heater. I'm wanting to put in a recirculation pump so we have fast hot water throughout my home. It looks like there are 3 methods and I'm not sure what the pros and cons are of each.  The method chosen will be controlled via my HA system (Insteon/ISY)

1st Method: My plumber told me an easy way is to use a pump under the farthest sink and connect the hot and cold water lines there. For this method I would have to use a button, motion detection, etc. I'm not sure I would like to have to activate the pump every time I need it.

2nd Method: This method is called out in the Noritz instruction manual and utilizes a hot water return line. The hot water return line would start at the furthest point on my hot water system and tie back into the Cold water line but not under the sink like Method #1. This system would use a pump, 8-10 gallon storage tank, auquastat etc.

3rd Method: This method is similar to the 2nd method. It would also use a hot water return line starting at the furthest point in my hot water system but it instead ties back to the hot water system. Basically, the hot water line would come from the tankless gas water heater to an electric 2-6 gallon water heater then to the various outlets, then to the pump. After the pump the hot water would tie back into the hot water pipe just before the electric tank. This method would be a hot water loop only and not tie back to the cold water line like the 1st two methods.

I would like to control the system I go with using my HA system (Insteon, ISY, ELK M1G)  ie. turn off when the house alarm is armed away and between 12AM and 5AM, etc.

What is the best method? Which is more economical?  Which would provide faster hot water?  I only have one shot at this so I want to install the most optimum system for my home.

Thanks for your help!

You realize you are losing enegy efficiency with a hot water circulation system.
That may be a consideration if you are using tankless for energy efficiency.


The return loop should feed back into the tank or input side of the heater.   As specified by Noritz.

This circulates the entire system.
Circulating it back into the hot water line after the heater will end up mixing the cooler water in the loop wth the heated water fresh from the heater, eseentially cooling the just heated water (may be only a degree but that is the effect that is occurring).

Lukewarm water in the loop that is never reheated is a breeding ground for Legionaire's disease.

The first method described is usually used for retrofit situations where a hot water return loop can't be installed.
It usually uses a theromstatically controlled valve between the hot and cold line that opens when the water in the hot line has cooled and allows water in the hot line to pass into the cold line.
The pump over pressurizes the cold line and the water flows backward in the cold line and back into the hot water tank to replace the hot water drawn from it.
That method usually uses some kind of timer or manual activation for the pump.
It draws hot water in and cycles it to the cold water line, mixing hot water into the cold water line.
With the pump running, the initial water coming out of the cold water side of the farthest faucets won't be truly cold initially.
But it won't be fully hot either.

With either method 2 or 3 you can rely on convection to circulate the water or a pump.
A pump can be used with a timer to run during high use times.
A pump will not allow convection to circulate the water when it is not running.

You should use a mixing valve on the ouput side of the heater to ensure there is never water above 125° anywhere in the system.

You can control the pump via relay output or something like a UPB appliance module or controlled outlet to incorporate it into your HA system.

Thanks for the replies!  Yes of course I'll give up some efficiency.  I will control the system with my HA system so not timer would be needed.
 
I guess I'm trying to decided between method 2 and 3.  Noritx specified (Method 2) the return line tie into the cold water line not directly back into the tankless water heater.  I would use a pump for method 2 and 3 not convection.
 
I hadn't thought about the hot water being in the cold water line for method 1!
 
Thansk again!

Tying into the cold line the water will eventually find its way back to the heater, or out via a facuet.
The return loop should tie into the cold line close to the feed side of the heater or small tank.
The feed side of the heater is usually close tot eh cold water entry point into the house anyway.
If you don't plumb it that way, you will end up with method 1 with no thermostatic valve controlling the flow.

So why not use method #3?  I got that idea from this Rinnai PDF...
 
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CD0QFjAB&url=http%3A%2F%2Fwww.tractorbynet.com%2Fforums%2Fattachments%2Frelated-topics%2F61100d1158263375-tankless-water-heater-code-question-cold_water_sandwich.pdf&ei=x2T5UZDSIum9yAGYr4HYCg&usg=AFQjCNFQyYT514GoqqmV8sVd7HIXx5H1Hg&sig2=Pkvg4pyRDOC-ZoeTKPWPGQ&bvm=bv.49967636,d.aWc
 
They say it's more economical than method #2.  Doesn't seem like it would be though.  That link has great pictures of Method 2 and 3.
 
Thanks again!

This pic shows the problem with method 1.
 

This shows the general standard new construction install.
Note the dip tube inside the tank connected to the cold input side travels down to the bottom of the tank to a point near where this diagram shows the return to the tank.
If you connected to the cold water input side of a tankless heater, you would essentially be doing the same thing.
However - some tankless heaters sense the temperature of the incoming water and adjust the burner output according to the temperature difference between the input and the desired output.  Others just turn on the burners at a set rate regardless of the input temperature.  The second method may result in water that is way too hot.
That may be the difference between the two manufacturer's recommendations.
 
If you are going to use a small electric heater anyway, then you can plumb the return loop into the electric heater and simply combine the outputs of the two heaters on the hot line - and then install the tempering valve before the final hot water is delivered to the house. 
 

Desert_AIP said:

This pic shows the problem with method 1.
 

Click to expand...

 
Yeah I think Method #1 has been ruled out.

Desert_AIP said:

This shows the general standard new construction install.
Note the dip tube inside the tank connected to the cold input side travels down to teh bottom of the tank to a point near where this diagram shows the return to the tank.
 
http://www.nachi.org/forum/attachments/f22/30875d1250177520-timer-connected-water-heater-capture.jpg

Click to expand...

 
This link describes Method #2 but did you see Method #3 in my linked pdf file above?  They claim Method #3 is more economical as it includes a small electric water heater tank.

Was typing  ^_^
BRB

I installed a return line from the farthest point back to the cold inlet to the water heater.  Then added a check valve (to keep water from bypassing the heater) and small pump.  It is for the master bath.  I have a push button that I use before a shower that runs the pump for about 30 seconds.  It works fine (IMHO) and doesn't waste energy keeping the pipes hot all the time. Even got good WAF.   
 
These types of systems can use (waste) a huge amount of energy.  Think about the surface area of all the piping.  Even if it is insulated it isn't insulated to a very high value and probably has lots of spots where there are gaps.
 
I have a condensing gas water heater that uses a tank so I don't have the "sandwitch effect" or need for a small added tank.

I don't want buttons throughout the house so I would control it via my HA system.  Yes it would waste more energy....it would be a trade off for convenience.
 
Jim, it sounds like you have Method #2, correct?

OK,
 
I misread your description of method 3 as tying the return into the hot water line after the heater output.
That would never reheat the cooled off water.
 
In this case methods 2 and 3 are virtually identical.
 
Both method's diagrammed in the Rinnai document tie into the hot water system and feed back into the heaters to be reheated.
 
One method places the tankless heater first in line and the electric heater second, with the return tying in between them.
This takes the output fromt eh tankless and combines it with the return line as an input to the electric tank heater.
The output from the tank heater is then routed to the house.
 
The second method uses an unheated storage tank as a hot water buffer and ties the return line into the input side of the tankless heater.
So the input to the tankless heater is a combination of true cold water and the warm/cooled hot water return.
The water is reheated then routed to the tank and then to the house.

You'd probably be happiest with the setup diagrammed on page 2 of the Rinnai document, using a small electric heater.
That is plumbed the same as the diagram I posted, with the tankless system spliced into the input side of the tank heater rather than running the cold line directly to the tank heater.
 
See my edited post above, it's critical to know what temperature difference logic and method your heater is using.