Calculating head in primary loop and recommended replacement circulator

Laars has a pump selection table 8

It shows a 1/25 hp circ for the 160, that would be more like an 007. 13 gpm 6'

I come up with 13.3 gpm & 4.02 ft of head

I suspect there are many over pumped systems out there.

If you trust the numbers from the man and @HydronicMike 13 gpm range, less than 5’, then a 0014 is way off curve for the job

Over pumping leads to high flow velocity, which often presents as noise. Does the boiler short cycle, on and off a lot during a load?

Check the delta across the boiler S&R to get an idea of actual run conditions.

That is a pretty good sized boiler for a residence, what size home is it heating ?

A couple ways to install that bypass crossover. Either an adjustable valve, like a globe valve , or Laars has a thermostatic valve to put in the piping

It keeps the boiler from seeing cold return, mostly used on slab radiant systems. It aslso assures the boiler gets some flow as zones close off

If it has run 24 years without it….

@hot_rod "Does the boiler short cycle, on and off a lot during a load?"

my thought also. because that boiler has no turn down, with an injection system that often uses part of its capacity, i.e. couple bedrooms reported as principle regular load, I would say its overpumped and underutilized. but even way wide of specs it has worked fine for 20 years.

the bypass would not be to stop short cycles but, as @hot_rod notes prevent cold return to boiler although I can't quite tell from pics because i'm not sure i'm seeing your secondary pumps, but a primary loop essentially performs that function already unless secondary pumps against their head are are crazy oversized.

now, the 356 should do some cycle flattening and depending where the sensor is located can be set to seek a minimum return temperature to boiler irrespective of the radiant curve target temp. i'm not precisely sure how it balances those when trying to limit short cycling. But a real world way of looking at this, besides checking the delta T is to hang by the boiler and time your cycles.

while that stuff looks very neat that was done before the era of isolation flanges for circulators and i'm not sure bleeding was really well thought out. inter alia, i don't see an isolation valve on the return (labeled "IN" on the boiler I think) and that comes from what I assume is the primary circulator high up next to the wall on the return so you can't cut the boiler off from bleeding water that is higher than the relief valve. and you can't limit the air to the pump change. and the location while efficient in keeping the mechanicals to small corner looks like a real PIA to get up there. without those constraints, after checking delta T and cycles, I'd just through an 007 or maybe a grundfos or Taco 3 speed and then you can try 3 different curves without changing the pump, if you are into empiricism which has, i admit, generally been my methodology.

This setup is kind of one of the best cases for a modulating boiler or what i've always done is just used a domestic hot water storage tank as a thermal flywheel and water separation for floors (many of mine were put in before anyone thought about oxygen barrier and honestly, I like the flywheel effect anyway and you get a used domestic storage tank cheap because people take them out either in favor of combi boilers (which I hate) or stand alone on deman (which I love). I just needed a very specific unit , triangle tube smart40 or the weil mclain equivalent because of the tapping locations, and it took me a month of waiting on facebook and I got one for $50. that will extend cycles and cut starts. starts are not as much of an issue for an atmospheric. it does take up a little room, but I would really want to give some room to the system so the pumps could be installed where taking them off doesn't drip the contents onto other components, esp. electrics. and where they are just plain easy to access.

But if your system worked for 24 years with it sounds like limited maintenance and NO (editted to add, i always leave out the important word) need to open the system, you could leave it and maybe it will work for another 24, seriously. It is good to think ahead about what to have on the shelf if something breaks, but honestly, if anything broke apump is the least of your worries because they are everywhere accessible. if something does need replacement, or if you are getting the 25 years time to prophylactically replace some stuff itch, I'd buy a propress and change the piping around to improve circulator isolation and physical accessiblity, bleeding (i also jump out a tee after the backflow preventer with a ball valve that bypasses the pressure reducing valve for bleeding purposes because those PRVs are one of the weakest links in the system. These valves generally don't get a lot of exercise and when they are need the most, e.g. when I hit the 'quick fill' mode after they have been sitting, half the time it won't shut off, sometimes it doesn't go on. Caleffi's are great, not blowing smoke @hot_rod, but I still pipe around and I like to have bleeding tees with ball valve to pex to an open drain, e.g. laundry or set tub often available, instead of having to hook up a hose to bleed when I give a system I will have to maintain a going over.)

there's my novel, fear and loathing in the boiler room (well i get to make a head to head comparison with fear and loathing in las vegas at AHRExpo this year.)

PS in the NFN department, this system is not configured to pump away from point of no pressure change. i hadn't stopped to think if that is as important with a short low head primary loop. its a little bit of a thread jack but I do mean it in service of larslars, if he ever has to open the system, is that a relevant consideration with any reconfig or given the low loss it doesn't represent that same potential issue as pumping toward the fill and expansion in a non primary secondard approach with a much higher head loss in the heating loops?