Indirect heater temp?

Joewiarton

I've got a Bradford White SW2120L indirect hot water heater I'd like to put into my geothermal system that is running at 110F. I would like to use it to preheat water going to my DHW tankless heater. Can I safely run this unit at less than 120F or will bacteria be a problem?

Ironman

That’s the perfect temp for breeding Legionella.

We set indirects at 140* and mix it down to 120* with a tempering valve.

dko

It won't have the effect in increased gpm you are expecting unless you are also going to also increase output temperature. Also the minimum fire may bring your temp over your setting into scalding temps or not fire at all that close to set point letting the water bypass the tankless.

WMno57

https://noritz.com/legionella/

"According to the American Society of Plumbing Engineers (ASPE), the ideal growth range for Legionella is between 90°F and 108°F, but the bacteria can survive in a wide range of temperatures:

68 ℉ and below — bacteria dormant
77 to 113 ℉ — growth range
90 to 108 ℉ — ideal growth range
118 to 122 ℉ — can survive but not multiply

Depending on the strain, Legionella bacteria will begin to die over time when temperatures rise above 122 ℉, with the bacteria dying instantly at 158 ℉. "

Even if your tankless can output 140 F DHW, the problem is the upstream storage. Based on the above, I would say NOT SAFE because your indirect storage temp is in Legionella growth range and probably ideal growth range. Also, you cant assume 100 percent uptime for your geothermal system. What if the geothermal is down for some amount of time? How will that affect temperatures?
Don't do it.

Hot_water_fan

Does the geothermal have a desuperheater? Those usually have high temp outputs, which could then be routed to the preheat tank.

WMno57

And if the geothermal system has downtime for any reason, he now has an indirect tank full of warmish brackish water. Possibly full of Legionella. Bad idea for a number of reasons.

hot_rod

So pre heat to a tankless running 140f. No problem with that, unless the tankless fails It is a common solar preheat application 

Residential dhw systems turn enough water on a daily basis to present serious legionella concerns

Its buildings with dead ends or hotels with empty floors  for extended periods that cause issues

You are more at risk walking by a fountain in the mall, or downtown San Francisco

EdTheHeaterMan

Residential dhw systems turn enough water on a daily basis to present serious legionella concerns

@hot_rod Bob, did you mean to say "Prevent". One letter reverses the meaning of the statement

hot_rod

EdTheHeaterMan said:

Residential dhw systems turn enough water on a daily basis to present serious legionella concerns

@hot_rod Bob, did you mean to say "Prevent". One letter reverses the meaning of the statement

Prevent it is