Short version:
I re-plumbed my 2-zone closed loop hydronic heating system this year to a 3-zone system. I get no hot water in the newly added zone. The 3/4" supply line for the third zone gets moderately warm just past the Honeywell ZV installed to control the zone when the ZV is manually opened up and one or both of the other two zones are calling for heat. But the moderately warm heat just after the ZV is not getting past about 5 or so feet, suggesting no flow in this zone. Yet my other two zones are getting hot water.

After troubleshooting and trying a few things to solve the issue, I am now wondering if my grundfos 1/25 hp circ is not big enough to overcome additional head losses introduced with the third zone added. How would I go about to test flow rate to be more confident I require a bigger circ? I am thinking there must be inline flow meters (I think I found a Caleffi and Taco inline flow meter sold on supplyhouse.com's web site) I can install to get a better idea? If so, where would the best place in the system to install them? One for each secondary loop I would imagine, and would it matter whether on the supply or return side, and would it matter how close or far from the primary loop (where the circs are installed)?

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Long version:
I installed a 7 rad-2 zone hydronic heating system over about 7 years ago. I had 5 rads in one loop (main living area), and 2 rads in the second loop (bsmt). I installed an 80k Westinghouse boiler that more than met the ~70k BTUs required for this small house.

Each secondary zone loop was plumbed with 3/4" pex feed-supply lines, with 1/2" taps for each rad along the run.

The primary loop has 2 grundfos 1/25hp circs, one on the supply side feeding into the manifold for the zone loops, each controlled with a Honeywell ZV, and the second circ is located between the return manifold (for the zone loops) and the boiler.

With my old set-up, the system hummed like a well-oiled machine, with the boiler constantly running at low but steady temps.

Then last year I put a wood stove in upstairs in the living space. Afterwards the boiler began cycling on-and-off multiple times throughout the day while the wood stove was heating the main living space, and I guess the 2 rads in the bsmt loop were not enough of a load to expel heat the boiler was putting out, and so the boiler would keep cycling on and off as the return temp was not sufficiently low enough to keep the boiler burning.

So I had the grand idea to move one of the rads from the upstairs loop to the bsmt loop, and add a third loop to separate the 4 remaining rads upstairs into two loops. One loop was for the living pace with the wood stove, which should only call for heat at night when the wood stove was not being used, and the second upstairs loop was for the rads in the 2 bedrooms that would/should call for heat day and night as the bedrooms don't get too much heat from the wood stove. And in theory, the 3 rads in the bsmt should be calling for heat all day and night keeping the boiler running at a steady low supply heat.

Unfortunately, things have not turned out as intended. Although the bsmt and living space loops both receive heat when they call for heat, the third bedroom loop does not.

At first I suspected air locks because when I tried to flush the loop I was not getting anything coming out of the supply side of the primary loop (there are purge valves installed after the supply circ and before the return circ in the primary loop precisely for the purpose of flushing the system if/when required). So I then proceeded to empty the 2 bedroom rads to try and clear the suspected airlocks. But once purged the rads closed again, they would not fill with city water when the loop was opened back up. However, when I opened the supply and return lines on the other 2 loops (I closed them like I have done in the past in order to isolate a loop when working on it like this) the city water did begin flowing into that third zone and the rads filled enough to spew water out of the bleed valves when opened.

A secondary, more minor issue is that I do not get the same degree of heat out of the two zones that are getting heat when they call for it. I have to turn the boiler temp up higher to get the same amount of heat out of the rads I used to get with the original config. Additionally, the boiler is still cycling on and off because the return temps are still too high and causing the boiler to turn off even though the bsmt t-stat is calling for heat.

So I then suspected that perhaps one of the circs (impeller) was perhaps no longer operating at spec. I know they are both working as I can hear the impellers rotating when there is a call for heat, but I thought perhaps one of them (possibly both?) were just starting to lag in performance. So because I keep a spare in the event of a failure, I swapped that spare out, first with the supply circ, and when that made no change, then with the return circ. But that too resulted in no change.

So now I am wondering if the system requires a bigger circ. But the next step up from the grundfos 3 speed 1/25hp circ is their 3 speed 1/6hp. My only hesitation is that I have to change some pipe in the primary loop to accommodate this larger circ, which I have no real issue doing, so long as I am confident a bigger circ will solve the two problems (bedroom zone not getting any heat at all, and the boiler is still cycling on and off with less heat coming off the rads compared to the original config).

I can't find my circ calculations I used for the original setup. But for the new setup, I calculated head losses in each loop, and the max head loss for the system seems to be right around the grundfos 1/25hp circ's max head spec between 2-4 GPM according to its curve spec sheet. The shortest loop in the living space (where the wood stove is located and so least likely to call for heat) has the smallest calculated head loss at ~12.5 H/ft. However, the other 2 loops, which should be calling for heat for most of the day and night, have head loss calcs of ~16 H/ft (bdrm loop) and ~17 H/ft (bsmt loop).

You can run your numbers to determine the required circuit supply temps, flow rates to determine required size of pump and heat source based on your heat loss using these calculators:

https://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Calculator.html

https://www.borstengineeringconstruction.com/Hydronic_Radiant_Floor_Heating_Design_Calculator.html

One normally designs a HR system before constructing it to avoid having a system that doesn't work well or even not at all.

Yes, it sounded like you did some design work but since I don't know any of the system details or to what extent, figured I should recommend that and provide means to do so. Do read calculator instructions before using them.

As you likely know, you first need to determine the required circuit supply/return temps and flow rates to generate required heat gain for each heating zone. Once you have this, you can determine the circuit head losses, total head loss and total flow rate. These totals should be in the center of the performance curve of the pump you select. If you set your circuit balance valves to provide these design circuit flow rates when all circuits are operating, you will be in good shape.

Don't know if you have a pressure differential bypass valve across your main supply/return lines near pump and heat source (good practice to do so to avoid potentially dead heading pump if pump operates when all circuit valves are closed). If so, do make sure it is set properly as it could limit your max pump flow rate to your system by bypassing some of it.