Redoing tankless hot water recirc system

[standyh]

I have a Triangle Tube tankless hot water heater in a recirculation setup. In order to avoid the "cold water sandwich," there is a 6 gallon hot water heater as well. Since the recirculation return comes back into the cold water side of the on-demand heater and may come out of it at a higher than expected temperature, there is a mixing valve that is supposed to output a steady temperature of water.

When hot water is being used in a bathtub, cold water is pulled through the on-demand heater and now I'm getting ~120 degree water going into the mixing valve, which adds in some cold water (no matter how I have it set) and gives me much cooler than expected temperatures at the tap.

See "current.jpg" for how it's set up now. The black triangles are check valves. I've also simplified it a bit by leaving off the expansion tank (and aquastat for the recirc system, etc). This seems less than ideal because it's not really necessary for the on-demand heater to be running the whole time the recirculation system is getting up to temperature - I think the slower heating from the electric tank would still be totally adequate for the volume.

I'd like to change it to "proposed.jpg," which gets rid of the mixing valve, since there will always be just cold water coming into the on-demand system and I can set it to 120 degrees out. Additionally, it would let the small tank do the work of getting the recirculation system up to temperature.

Are there any issues with my proposed setup? I have left the expansion tank off that diagram as well, for simplicity.

Thanks in advance!

 

[John Gayewski]

Triangle tube will have a diagram for recirculation on their tankless heaters. I suggest you find one and use that.

Different tankless heaters have different programming parameters for their logic. When to fire when not to fire and what not. You will have to see how they recommend for their heater. I know how I install Navien, but we generally only install them so we use their diagrams.

I suspect the designs will be very close to the same as all manufacturers copy each other especially when someone is as popular as Navien, but unless someone here installs triangle tube regularly or wants to research for you, you might be better off calling triangle tube and let them steer you to a diagram they will have posted online somewhere. These heaters are evolving pretty fast and change regularly so a call to them is probably best.

 

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[standyh]

I attached a screenshot from the application guide that I found online. Thanks for the tip.

It’s pretty confusing to me.

I suspect it was confusing to the previous (professional) installer as well, which is how I ended up with what I have.

 

[John Gayewski]

They probably thought it was a lot of extra stuff like I do. Do you currently have two circulators for this like the diagram shows?

 

[standyh]

I do not - only where "pump" is labeled in "current.jpg" on my initial post, right where the recirculation return comes out of the wall.

It looks like they're assuming you aren't using the tank for heating - a lot of the complexity in the application guide seems to be from routing water from the recirculation system back through the tankless heater, as well as keeping the tank up to temperature that way (see the domestic circulator labeled "12").

They basically have layed out two overlapping recirc systems - one to keep the pipes in the wall hot, and one to keep the tank hot.

 

[John Gayewski]

I do not - only where "pump" is labeled in "current.jpg" on my initial post, right where the recirculation return comes out of the wall.

It looks like they're assuming you aren't using the tank for heating - a lot of the complexity in the application guide seems to be from routing water from the recirculation system back through the tankless heater, as well as keeping the tank up to temperature that way (see the domestic circulator labeled "12").

They basically have layed out two overlapping recirc systems - one to keep the pipes in the wall hot, and one to keep the tank hot.

Some tankless systems aren't really made for recirculation as the pump will set off the flame becuse the heater thinks there's flow. That's becoming less of a thing as the newer ones have recirc logic.

I would call triangle tube and ask them for the correct diagram.