Help with pump selection for indirect DHW tank with modcon boiler

patrickoneal

Earlier I asked for some general advice about piping in a new Navien NHB-80 boiler and got lots of good information. I've got two zones of fintube, and I've been able to select appropriate pumps for those zones, as well as for the primary loop pump, but I'm having trouble figuring out the sizing for the pump for my Megastor MS-40 indirect tank.

I'm using the factory pre-fabbed manifold from Navien to get my plumbing started and it looks like the pump for the indirect tank is ahead of the primary/secondary loops:



With this setup I'm assuming that I'll have to consider the head loss through the indirect tank coil and the piping connecting to it, as well as the boiler heat exchanger?

The example calculation in the manual for the MS-40 shows 3 feet of head through the tank @ 8gpm, and 5 feet through the plumbing connecting to it, for 8 feet total. My setup is going to be pretty similar, so I think 8 feet is a pretty good number for those parts.

Where I'm confused is when I look at the boiler manual to calculate head loss through the heat exchanger. The chart(posted below) for the NHB-80 maxes out at 8 gpm, and indicates 17 feet of head at that flow rate. Adding the numbers together makes a total head loss of 23 feet @ 8gpm, which points me towards what looks like a fairly large pump for my little boiler and indirect tank.

Should I really try to put 8gpm through the boiler heat exchanger? Being that 8gpm is the largest number on the graph makes me question if that is appropriate or not.

Does anyone have a suggestion for a pump that would be appropriate for this application?

Thanks again for any advice.

EBEBRATT-Ed

@patrickoneal

This is an issue with most mod cons.

They are really designed to work best at a 30 or 40 deg TD. When they go on a 20 degree TD baseboard system you have to put more water through them and as a result the head goes up. That's why your maxed out on the graph.


I agree you have 17' of head @ 8gpm through the boiler

3' of head for the indirect sounds about right (didn't look it up)

5' of head for pipe and fittings sounds like a lot to me

8gpm through 1" copper is 4.52'/100feet of pipe

So even if I allow 20' of 1" copper resistance =4.52/100 x 20=.094' of head for the pipe x1.5 for the fittings =1.36' total. Call it 1.5 feet

So now you have 17' +3'+1.5 feet=21.5 feet total


So you need a pump capable of 8 gpm @21.5 feet if you want full capacity on the indirect

That gives you 80,000 btu at the indirect which you probably don't need. Most 40 gallon gas water heaters are only 40000 input.

If you split the difference and move 6gpm through the indirect (which should be plenty)

Then you need a pump that will move 6gpm @14' of head

It's your choice depending on how much hot water you need

mattmia2

Won't the delta t on the indirect be more, or couldn't it be more? If you don't have the tank yet I would think about using the commercial or solar version with dual heat exchangers and maybe drive them in parallel depending on geometry to get lower head and more transfer surface to drive the delta t up.

mattmia2

If you don't have the boiler yet you could use a fire tube like an HTP EFTU that will have a lot less resistance.

patrickoneal

@EBEBRATT-Ed

Thank you for the information.

I live alone, so I can probably get away with a smaller pump.

There's an older Grundfos Alpha pump on my old boiler I could reuse for this. It doesn't quite reach 6gpm @ 14' of head. Is there any harm in trying this pump on the highest setting? If I'm understanding what you're saying, I should be able to get 40k btus @ 4gpm, which this pump would do?

It doesn't cost me anything to try it, and I can replace it if I find the hot water production inadequate.

EBEBRATT-Ed

@patrickoneal

That sounds like a good plan. Worst case yo buy a bigger pump

And @mattmia2 is right the boiler will just go to a higher water temp to satisfy the indirect

With one person living there you will likely be fine with the smaller pump

Should run 1" to the indirect to keep the resistance low

patrickoneal

What's the reasoning behind the design of this manifold and others from other manufacturers? Why is the DHW pump ahead of the primary/secondary loop? It seems like the pump would accomplish the same thing if it was plumbed on the secondary like the zone pumps. Is this setup more efficient in the warm months when space heating isn't needed?

EBEBRATT-Ed

When you heat DHW and your not heating the house the indirect pump pums water through the indirect and through the boiler the boiler pump doesn't need to run so no heat goes to the heating zones, it's the best way to do it