Shshs

My guess this system needs a good purge ….

Maybe just move it.

There is absolutely nothing wrong with the size of the pump. There is nothing to gain by changing the pump. I know you said that you're confident there is no air in the system, but you're too confident because there is most certainly air in the system. Unfortunately you did not have this professionally installed because no professional would have mounted the lower circ like this nor would they have foregone a purge station. When he comes back to fix the lower circ, have him install a purge station and properly purge the upper zone- it is undeniably airbound. Most likely on the return pipe above the upper manifold. Cutting in a manual vent there wouldn't be a terrible idea either, seeing as the "technicians" you've had out so far can't figure out how to properly purge the system of air.

You have plenty of purge points. Pump directly through both manifolds with the purgre valves on the manifolds, that is why they are there.

Or pump into the Webstone valve at the small zone, through the piping and loops purge at the blue manifold.

For the upper zone pump into the lower valve left of the expansion tank

For the boiler piping pump into the valve at the bottom, left of the expansion tank, purge at the supply manifold.

The boiler may have a high point air vent inside?

The air purger will take care of the small bubbles that take days to come out of glycol solutions.

An Axiom fill tank would assure you keep pressure in the system as air purges for a few days. Heating the system also helps drive that air out of glycol.

With a wider pic I can maybe explain yet another purge method using the iso /purge valve under the boiler on the factory P/S header. I suspect there are two one on each drop?

One purge valve at the return near the boiler would have simplified it, but what you have will work.

Don't change anything until you inspect the pump and flanges, then another purge.

Hopefully you have a pump able to move 4 gpm or so? A small plastic sump pump from Home Depot works well drop it into the glycol bucket.

The piping is nicely done, maybe the plumber doesn't have a lot of hydronic piping design experience.?

It may not be cavitation but if the system is way over pumped for that particular pipe size it could have to much restriction causing issues.

Closing the valve on the discharge reduces the flow so the pump can get suction. just run it with the valve partially closed.

You need 3 pieces of info to size a hydronic circulator

How many GPM is needed, the total of all your loop required gpm.

Head, the mechanical energy needed to move the fluid

Type of fluid, water, glycol blend

I don't recall your required gpm?? With 1/2' pex .50- .75 is best practice design

Head is determined by the flow resistance in the tubing and piping path, the actual loop length needs to be known

So with the assumption you need .65 gpm X 6 loops= 3.9 gpm

.65 gpm through a 1/2" loop, 300' of pex with 40% PG glycol = 8.1' (4.9 for water)

Use online calculators for the head if you do not have a desigh that gives you this info?

So you two numbers are 4 gpm at 8'

From the PPI, Plastic Pipe Institute, Flow Calculator

Next you find a pump curve for the model you are considering. i think you have a 3 speed Grundfos 15-58

So enter at the bottom at 4 gpm, move up vertically to see where the curve crosses 8'. So you are just above speed 1

An additional step is to develop a system curve, which you plot over the pump curve. This tells you exactly where the pump will run on your specific system. Read through Idronics 16 to learn more about circulator technology and sizing, etc.

Using an ECM as a zone pump will reduce power consumption, it has a more efficient motor. You would adjust it the same way for the gpm and head your system needs.

Without more accurate info on your system, longest loop length, gpm required, and % of fluid all we can do is guess and make assumptions.

Multiple responses from hydronic pros tell you your pump(s) are right for the assumed application.

The installation and confirmation of your job needs to be done onsite, not much more we can do online to help until you supply more info.

The cover pic on Idronics 16 shows pump curves, red lines with the system curve overlaid, white lines. This is an ECM with multiple zone valves. It shows how the pump changes speed as zone valves open and close.

https://www.caleffi.com/en-us/magazine/16-circulation-hydronic-systems

Place a water pressure gauge on the hose bib just before the pump in question to test the cavitation theory. Turn on the pump. If the pressure drops below 5 PSI, then your expansion tank is not properly charged and you could have cavitation.

Of course you will need to open that valve to read the pressure.

If you have any movement at all in the system, you mentioned .7-.9 gpm? The air purger within hours will scrub out this dissolved or entrained air. Glycol takes more time. You might pull the top off that air purger and check for a stuck float or plugged vent.

The piping and components you have should scrub 97% of the air within a day or so.

Closing down a valve on the discharge of a pump will reduce flow, no question about that. But it also will give you a bit of a velocity increase.

A demo of that is a flowing garden hose with and without a nozzle attached.

Without the nozzle you get max gpm. Install a nozzle flow (gpm) decreases but it sprays across the yard. You substituted gpm for fps feet per second velocity.

Basically the principle of an orifice. An orifice in a balance valve increases the speed across the orifice. Read the pressure on both sides of the orifice to convert to gpm.

Mainly a start up purge mechanism, the Caleffi manifolds include float style auto vents to grab air right at the loops.

Loop length info helps with some numbers, thanks.

The radiant delta is more about comfort. The tighter 10° give you a more consistent floor surface temperature compared to 15. However it will cost more pumping power.

For the garage with those long 390 loops a 20- 25° is fine as you are probably not out there barefoot? Two loops in the garage? What is the square footage?

The garage loops at .65 gpm flow will be around 10' head, still within range of your 15-58.

Still not finding any issues with design piping or current pump selection.

I'm guessing your system holds about 30 gallons, 18 gallons in the tubing, misc piping and boiler capacity.

If the high point is 8' above the boiler you need to pre-charge the tank to 8.6 psi, system glycol fill to 8.6 psi.

This assures you have 5 psi at the highest point in the piping.

Water/ glycol will expand as heated, contract as it cools. So pressure will increase.

A properly sized tank would allow pressure to increase to 5 psi below the relief setting, to 25 psi.

Using the online Wessel tank sizer, you can plug in an initial and final pressure :

So from 9 psi fill to 25 psi at 60- 120° would need a 1.2 gallon expansion tank, a #15

If for some reason you wanted only a 1 psi increase from 9- 10 psi across that temperature range you need a much larger 12 gallon tank.

A #30 tank would be my choice for a bit extra capacity, lower final pressure, a wider temperature spread.

The Wessel calculator does not accept a starting ad final pressure of the same valve.

As you heat the fluid, pressure will increase, even without pumps running.

Start the system cold, what is the pressure?

Run the boiler until it reaches setpoint. Turn off the pumps. I'm betting the pressure has gone up.

How much depends on the expansion tank size and accurate matching pore-charge pressure.

The procedure I outlined a few posts up shows a reading across a pump when it is running. That shows you have flow and you can calculate how much flow with that pressure differential number.

If your system reads 8 psi or more cold, no pumps running, and never drops below that the system is pressurized.

But it can be pressurized and still air bound or partially air bound, so little to no circulation.

Reducing the flow, what ever amount you have at the pump discharge will not, cannot increase flow in the loops.

You mentioned .7- .9 gpm on the manifold flowmeters?

you can buy or build a hose connection gauge like this, swap it to a 30 psi gauge

Or build one up eight a hose adapter 1/2 x 1/4 bushing and gauge

Confirm that the boiler gauge is accurate with it, and move it around on all the hose connections you have to observe pressures in different locations with pumps running

You need to start collecting accurate data to answer your questions