Tankless with storage problems

BMurrins

Hi all, we just started to manage a 12 unit apartment building and they have a Rinnai CU199in tankless unit recovering an 80 gallon storage tank and I've noticed that the flow rate is only 3 gallons per minute through the heater (Cu199in is rated for up to 11gpm).

I've done tons of troubleshooting but I cant seem to get it to shoot up. (T rise is only ~30 degrees). Theyve got a 2 inch header with a reducing tee to 3/4 for both hot and cold in and out and I think thats got something to do with it. I've drawn something for you guys to get an idea of what I'm dealing with here.

Please let me know what possible solutions you can think of to get this machine to stop underperforming.

Jamie Hall

I suspect your problem isn't so much the piping as the head loss through the tankless water heater. I can't seem to find a head loss curve for it, but it may -- probably is -- more than that Taco can handle.

BMurrins

I think youre right Jamie. The curve for the 0014 is 3-4 gpm at 20-21 feet of head so I guess im around there. If I increase the pipe diameter to 1 inch AFTER the pump, as well as relocating the pump to directly under the machine, I should get 9-11 gpm with no problem right?

delta T

As Jaime said, the problem is your pump sizing, not the piping. That heater, in that application is meant for 5 gpm at 30 ft of head (see attached pages from the manual) and a taco 0014 will not go above 22 ft of head period. A 0013 would be a more appropriate pump for that application (see attached pump curves).

edit: wrong pressure drop curve, please refer to 'Correct pressure drop curve'

Jamie Hall

BMurrins said:

I think youre right Jamie. The curve for the 0014 is 3-4 gpm at 20-21 feet of head so I guess im around there. If I increase the pipe diameter to 1 inch AFTER the pump, as well as relocating the pump to directly under the machine, I should get 9-11 gpm with no problem right?

Nope. The pressure drop is in the tankless, not it the piping. Take a look at the pressure drop curve which @delta T found for you (thank you, sir, I couldn't find it!). You'll never get more than 9.8 gpm out of that tankless, with any pump at all. To get even 9 gpm out of it you will lose 55 feet through the tankless -- and perhaps 5 through the rest of the piping. The Taco circulators have a maximum head of 33 feet. Does Not Compute. The best you are going to manage with a Taco is around 6 gpm -- and that is using one of the big ones.

BMurrins

Ahh okay I got it. Now it makes sense. I'll be picking up a 0013 tomorrow morning

delta T

And just to be clear, be sure to get a stainless or bronze body pump. Model 0013-BF3 is the bronze body, not entirely sure if they make a stainless body for the 0013

BMurrins

I've seen the result of cast iron pumps for hot water. Too funny

delta T

@BMurrins Its always impressive to see a brand new iron body pump completely corroded and absolutely destroyed after a year...

BMurrins

one quick question. how am i even getting 3-4 gpm with the 0014 i have now?

edit: silly question nvm. running on little sleep lol

Jamie Hall

Actually,, if you compare the pump's curve and the tankless head loss curve, that's about where they should intersect.

SteveSan

@delta T Taco doesn't manufacture the 0013-BF3 anymore, we changed over to the SS version 0013-SF3