Dedicated vs cold water recirc line?

Users who are viewing this thread

Ramias

Member
Messages
33
Reaction score
0
Points
6
Location
Alexandria, Virginia
Inching closer and closer to going tankless. Have a quote for a Rennai with recirc from a local company where all they do is tankless with Rennai and Navian. Quote is to use cold water line for the return.

I Have a plumber coming out for separate work. Should I have him run a dedicated recirc line with Pex (rest of the house is copper. It I think pex will be easier to run) if I can pull this off with minimal drywall repair? Or just use the cold water return? I don’t want to contaminate my cold water lines with luke warm legionella or something similar if that can happen. But if that is not a concern, warm water for a few seconds until it goes cold is better than cold water for a few minutes before it gets hot.
 

Jeff H Young

In the Trades
Messages
5,706
Reaction score
1,280
Points
113
Location
92346
Ive never heard that circulating through a cold line was a health issue. circulating that way works ok but not as good as a true circulating system though you may be satisfied
 

breplum

Licensed plumbing contractor
Messages
1,049
Reaction score
391
Points
83
Location
San Francisco Bay Area
Dedicated recirculation PEX (3/4") is the way to go if the loop comes off the far end run.
Won't be ideal if two branches go in opposite directions.
Can't beat Navien NPE A series (built in pump and minitank) and it is the ONLY product we install, but there are other opinions.
 

jadnashua

Retired Defense Industry Engineer xxx
Messages
32,750
Reaction score
1,170
Points
113
Location
New England
Using recirculation with a tankless must be done very carefully, and some do not allow it. See what the installation manual for the ones you may be interested in say about it. On some models, it will void the warranty. Depending on how setup, it may be enough to trigger the tankless on numerous times, which can put undue stress on the things. A buffer tank is sometimes used to minimize that.

If you can retrofit a dedicated return line, you may like that better. That would have been a really difficult thing to do on my place, and I use the cold water line as the return. Most systems do not run until the full hot level is attained, and shut down the loop earlier. This is done for at least a few reasons:
- the higher the delta temperature, the higher the radiation heat loss and thus expense
- if warm at the furthest end is attained, hot is quite close so it doesn't matter.

In mine, using the cold line, if I flush the toilet, it pretty much purges all of the warm water out of the cold line, but it depends on how things are setup.

The performance of a tankless heater can vary considerably based on the temperature of the incoming water, so what might work great in the summer might disappoint in the winter. Where I live, I've measured my incoming cold water supply at 33-degrees F after a cold spell, and a tankless would be hard pressed to deliver much of any hot water, at least at any decent volume. Alexandria probably won't have that issue, but it depends...
 

Jeff H Young

In the Trades
Messages
5,706
Reaction score
1,280
Points
113
Location
92346
ok so three recomendations for a dedicated Hot water return line.
Ramias I sure dont think youll get diseased piping it like you want it just wont work as well
 

jadnashua

Retired Defense Industry Engineer xxx
Messages
32,750
Reaction score
1,170
Points
113
Location
New England
You want to insulate any and all pipes in that hot water loop to minimize the energy hit. You do not want a big HP pump for this, and a larger pipe doesn't really help either. There's almost no pressure loss when you use a reasonably sized pump. One company of a recirculation system uses a very large pump (50x more power than the one I have). Keep in mind the Copper Institute's guidelines on the water velocity on a copper hot line that says to never exceed 5fps. That's easy to achieve with a larger pump. IF it's running as a demand thing, to keep the wait time low, you might want a bigger pump to do it faster, but that has its risks. The pump on mine works just fine, and only draws 8W. A HP = 746W, so 8/746=0.01Hp. Mine's on a timer, so the first time it runs in the morning, it might take close to a couple of minutes or so, but after that, it's always hot as it cycles on/off maybe 4-5x per hour for maybe 45-seconds. Most all of my hot lines are insulated, which helps it hold the temperature.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks