Quetion regarding ciculator pump sizing

glifen

I have a Navien NHB-080 that was just recently replaced under warranty. After further issues with the new one is appears that the original installation done a few years ago was the cause of the initial failure, an incorrectly sized pump for the DHW to an indirect tank. How do I figure out what the correct pump size should be? The installer is of course not claiming any responsibility in this and I need to try and gather as much as I can.

The indirect tank wants 8gpm, currently its getting under 3. The boiler is constantly throwing errors due to this while DHW is being called. Navien techs said its the boiler turning itself off to try and protect itself from overheating.

Navien NHB-080
Buderus S32 Indirect Tank- wants 8gpm, adds 1.9 feet of head
https://www.bosch-climate.us/files/Bosch_Indirect_Tanks_Sizing_Guide_07.2018_US.pdf
Taco 007-15-71FC Pump
40 feet of 3/4ths copper
16 90degree elbows
1 T

If i look at this chart - http://www.taco-hvac.com/uploads/FileLibrary/100-1.8.pdf - and work backwards. 007 pump with 3gpm would be ~9.5 feet of head. Does 9.5 feet of head make sense with the above specs? or is it possible the taco pump i have is not working properly?

Then, assuming I want 8gpm with 9.5 feet of head, that would be roughly a taco 008? does that make sense?

Ironman

How about posting some pics of the boiler and it's near piping?

A taco 007 may be slightly under-sized, but it shouldn't cause the boiler to go off on high limit or cause it to fail. More than likely, there's a piping issue.

glifen

you can see the piping at the boiler in my original post from a few years ago:
https://forum.heatinghelp.com/discussion/158773/critique-navien-nhb-80-installation#latest

i'll get a picture of the indirect in a few

glifen

Here's some images of the indirect and piping

ScottSecor

Tough to get 8gpm through 3/4 inch copper pipe. It can be done, but you may get noise and other issues, much better off with one inch. Need to know what the pressure drop (in feet of head) for the Navien unit and the other items that may be in this loop.

ScottSecor

just looked at the 007 pump curve, it will produce 8gpm at 8 feet of head. The catch is this assumes you head loss is less than 8 feet. Assuming your pumping away and not operating the indirect at the same time as the heating system (aka dhw priority), you may not have enough oomph to pass enough gpm. Yes, the 008 should do it, I would try it fist (again assuming everything is pipes properly) before up-sizing the pipe to 1 inch pipe.

Ironman

I just checked the specs and that Navien heat exchanger has 17' of head @ 8 gpm. You'd need a Grundfos ups26-99 or Taco 013 to do that.
Yes the 007 is too small, but I doubt that it alone is the problem. During space heating, with only one zone calling, you're probably only moving < 4gpm and the boiler will modulate to accommodate that with no issues.
Check the 007 to see if it's pumping towards the boiler. There's a directional arrow in the casting.
And as Scott mentioned, 3/4" is only good for 4 gpm.

glifen

The pump is currently set up to pump towards the boiler.

How do you determine what size piping is required and how does that play into the pump/feet of head calculations?

Ironman

From piping tables. You need a 1" pipe to carry 8 gpm at under 4 spf velocity. Hydronic piping should kept at 4 spf or less.

SuperJ

Can you lower the boiler temp dhw priority setpoint? 8gpm becomes less important and it will be more efficient if you're not running at 180f.

Zman

The above advise is spot on. Some boilers will let you limit the output during a DHW call. If the Navien will, you could solve this with a simple adjustment of the controller. A call to tech support would give you that answer. I you want full output, 1" pipe would be the best option.

Jamie Hall

glifen said:

The pump is currently set up to pump towards the boiler.

How do you determine what size piping is required and how does that play into the pump/feet of head calculations?

Head loss in pipe is, to a first approximation, inversely proportional to the square of the inside diameter, and directly proportional to the length.

For example -- a 3/4 inch pipe will have, to a first approximation, almost twice the head loss for a given flow and length as a 1 inch pipe would have.

glifen

thanks everyone for the great advice. The Navien does allow me to set the DHW temp, although right now i'm only getting about 2.7gpm so not sure if just lowering it would really solve the issue?

How do you calculate what water temp the DHW should be set to to maximize efficiency?

I looked at the Taco 007 specs and chart again, it maxes out at 10' of Head and on the chart with it, 10' of head is right around 2.8gpm. Seems to make perfect sense why this thing is operating the way it is.

ScottSecor

How are you calculating the gpm?

Zman

I am suggesting lowering the boiler firing rate not the temp. That would protect the boiler from the low flow condition and prevent damage to exchanger.

glifen

The GPM is displayed on the Navien unit. A Navien tech walked me through displaying the info.

As far as the boiler firing rate, I'll check the manual.