Circulator Pump Selection for Fan Coil Units - Please Help

What @Alan (California Radiant) Forbes means is what is the head loss for each air handler? The total head the circulator needs to overcome is the air handler with the highest head loss not all of them added together.

If a pump can handle the highest head zone, then it will handle all the other zones that have a lower head.

The part of the pump performance curve that you Add Together is the total of all the gallons per minute of each zone.

If the longest of most restrictive zone or loop has a head requirement of 6 ft, then you need a pump that can overcome 6 ft of head. That's it.

But if each zone required say 4.5 gallons per minute and you have 4 zones, then you need a pump that will move 18 GPM at 6 ft of head.

A variable speed pump is best for your situation since all the zones may not be calling at one time. so you need a pump to handle say 4.5 GPM at say 5 ft of head for a FCU that is right next to the boiler, and you will need say 4.5 GPM with 6 ft head another time, when the FCU that is the farthest away is calling. That variable speed pump can be set at a ∆T or a ∆P based on your circumstances to produce the proper results whenever all 4 zones are not calling. But it must be able to handle the 18 GPM at 6 ft head when all four zones are calling.

Does that make sense?

@E_Guy

What you need is an accurate calculation of the head loss and GPM requirements. Start by determining the AHU with the coil with the highest head loss Calculate the pipe and fittings needed to connect that one loop to the boiler or to the common header.

Then repeat for the other AHUs (chances are the AHU with the higher loop will be the loop with the most resistance) but not always.

Once you have this done calculate the boiler and any common piping with the total flow of all the zones. Take this resistance of the common piping and add the head of the highest AHU loop to that. That is you total head loss

Now take the total head loss calculated and the total GPM required by all the AHUs. This is what your new pump should be selected from.

I would suggest downloading the B & G system sizer.

The Alpha I 15-58 is the ideal choice. Even though the performance curve shows missing your calculated target, I don't think you will notice a difference. Also, the actual performance of pumps always seems to exceed the published data.

If you want super accurate control for flow, I would consider a dynamic ( automatic, or pressure independent PIC) type balance valve. A Caleffi 127. This valve adjust automatically as the zones open and close, unlike a manual balance valve.

Here is a graphic example of both under different zone valve operation conditions.

Build a copper header or manifold, add a zone valve and 127. Or the zone valve and 127 could be at the FCU. That is how some of the factory FCU unit ship to large job. The ZV and flow control is built onto the FCU, sized and labeled to the engineered spec.

A dynamic balance valve is an excellent match with a delta P pump logic. As the valve modulate, zone come on and off, the circ reponds by varying its flow rate. So you never over or under pump, and use the least amount of pump power.

Usually this degree of balance accuracy is seen on engineered designs where maybe dozens, hundreds, or thousands of FCU need exact control. It can make a big difference in performance and comfort, as well as efficiency with many FCUs.

Certainly no harm in adding auto balance valve on your project. Include a Y strainer or DirtMag to keep them clean.

Here's the problem you're going to run into. Zone 3 requires 2.5 GPM, the FCU drops 10.7 feet of head at that flow. The 76 feet of 3/4" PEX is another 3.18 feet, so 13.9 feet total.

If all four zones are on the same circulator, they all have to be on the same manifold so they all have to see that 13.9 feet of head.

The CXI65's on the other three zones drop 4.3 feet of head at 1.3 GPM. Using the flow calculator I linked to earlier, I see that's equivalent to 320 feet of 3/4" PEX. Taking the longest zone with 116 feet of pex, that's a total of 436 feet of PEX. At 13.9 feet of head I get a flow of 2.1 GPM through 436 feet of PEX. So if you apply enough pressure to get 2.5 GPM in zone 3, you're going to get at least 2.1 GPM in the other zones. So you need 8.8 GPM total at 13.9' of head.

Because zone 3 needs so much more head, it increased the flow through the other zones. You could try choking the other zones down with balancing valves, or even just piping them with 1/2" PEX.

If the zone 3 is on its own circulator, it needs 2.5 GPM at 13.9', and the other three zones need 3.9 GPM at 5.86'. That might be more attainable.

I think the Alpha 15-58 is the correct choice. With all zones open if the spec is 8.8 gpm, 13' head?

The autoadapt may regulate the pump accuratly enough without balance valves. If not, three 2.5 gpm PIC type balance valves would give you the exact flow at any point in time.

The pressure independant type of balance valves will have a PS

ID range stamped on them, or assigned to the valve. So in the case of this Caleffi 127, as long as the circ can develop between 2 and 32 PSID the valve will accurately regulate the flow. This will be the case with fixed speed circulators or with delta P mode on ECMs.

The 2-32 PSID should cover a wide range of circs.

A PICV is a pressure independent control valve that can also act as the zone valve. It can be have a typical on/off actuator, or a modulating type actuator driven by a BAS, for example.

I would use a buffer tank as they recommend. So the variable speed circulator they include just handles the heat pump. The heat pump sees the buffer tank as it,s load, not the system, per say.

The secondary circulator gets sized just to the distribution loads, as you have already calculated, and will be much smaller.
Although their schematic doesn’t seem to show a circ for the FCUs?. But one on the radiant?

And I would not have the back up heat piped that way, no need to heat the heat pump with a backup?

I’m not sure what they mean by ‘wild” coil loops. If that is a 4 or 6 pipe buffer, I don’t see any flow getting to the FCUs. The buffer is a hydraulic sep.

I’d use a dirt sep not a y strainer as strainers can plug and reduce flow if not serviced properly.

I would send one of the Siegenthaler piping designs from Idronics to them for approval.

One last thought, you should not use pvc or CPVC with glycol. Glycerin is used in CPVC as a freeze protection fluid, at least in fire protection systems.

other than that🤔

The term wild loop is not unique or new to me. It appeared in the IBR Hydronic Institute trainings back in the 1980's, when I took the course. Theyn showed fin tube wild loops.

It is as you state a loop that has no on/off controls, it flows whenever a circulator is running.

I'm having a hard time with their explanation of the buffer/ hydraulic separator VCT 37 tank. It reads like something the marketing department put together :)

They explain it as a hydraulic separator and that the radiant loops need a dedicated circulator as shown. I agree.

But ignore that concept for the FCUs?

So how does the fan coil circuit get any circulation as the same hydraulic function would apply. How do they size their built in radiant pump? Without any loop design data? It could be a 1 gpm load or a 40 gpm load.

The pressure drop from the upper to lower ports on the left, HP side will be near zero. So flow from HP would just return to the HP.

There must be some head involved with the FC units as the math you worked through above shows 10' head at 2.5 gpm for the zone 3 example.

Here is an example of how the math works out on this 2" Caleffi HydroSep. The diameter of the tank in relationship to the diameter of the in out ports is how separation is accomplished. The "barrel want to be at least 3 times the diameter of the inlet/ outlet piping.

With a 40 gpm flow in and out of the tank, there is no flow or movement to the distribution side.

In fact air rises up through the tank with a 40 gpm flow going through it, due to the diameter 9A area ratio. Dirt drops for the same reason.

It looks like their tank far exceeds that ratio?

I've built a number of clear hydraulic separators to show how this actually works.

I've also tested brands that don't do as they advertize due to the wrong barrel to connection ration. I have videos to show how secondary flow happens when you exceed the 3-1 ratio, by injecting food coloring.

Unless there is a jumper pipe inside, I don't see how any flow happens in a FCU that has a head associated to the flow rate.

Here is their parable of buffer/separation functions.

Forgot about the 19 gallon buffer tank, another $1149.

OK, didn't realize you already had a radiant system. Strike everything I said in my previous three posts, "wild" mode only works if the FCU's are the only emitters and all of that only applies to wild mode.

If you're using radiant tubing you have to have a zone valve and if you have a zone valve you have to have at least one additional circulator and a buffer tank.

One thing to look into is 3-pipe plumbing at the buffer tank. The supply from the heat pump goes into a tee, one leg of the tee goes directly into the circulator and the other into the buffer tank. When the heat pump is running the circulator pulls water directly off of the heat pump, which in heating mode is going to be about 10F warmer than the water in the buffer tank. Since heat transfer is all about temperature delta, and the temperatures you run are relatively modest, this makes a significant difference in your output.

In cooling mode the water will be cooler. This means the FCU's will do a better job of removing humidity, which in my climate is a crucial consideration.

I believe Space-Pak has a diagram for 3-pipe in their manual.