why P/S piping in single zone mod/con system?

joca

i was shown a mod/con combi supplying a single zone heating system which was piped P/S. The boiler's hx is part of the primary loop with a external circulator, a set of closely spaced tees fed the heating zone with it's own circulator.
I'm not understanding the need for 2 circulators serving one zone? seems inefficient.

SWEI

This design is usually required when using a mod/con boiler that has a high head loss heat exchanger. With many of the newer fire-tube HX designs, it can often be avoided.

What kind of boiler was it?

joca

It's an intellihot. It's a tankless waterheater with a stainless steel brazed plate hx for space heating. Shows 15gpm max for hx in specs

SWEI

Might well need a separate boiler pump for that. There should be a published head loss curve or Cv for the flat plate.

joca

Thanks Found the chart . Pressure drop for hx is 1.75 ft @4gpm , doesn't seem high? Trying to see if I should use this install as a model, or can I make it "better". Will have panel rads with TRV's with direct return piping. Without the primary loop, won't the pump deadhead ?

Rich_49

Not if there is a bypass at one of the panel rads . You should have one just so that does not happen

joca

Do you mean an opened bypass at the rad H valve or a diff pressure bypass valve? Was thinking a dpbv near the boiler- but not sure what is "best practice "

Ironman

If you do pipe it p/s, then you could use a Grundfos Alpha pump on the secondary. There's no need for a bypass since the Alpha is a Delta P circ.

Is 4 gpm enough to supply the needed buts? If not, the head loss of the HX will increase exponentially with the gpm.

Rich_49

H bypass @ rad , not DPBV.

Gordy

What model intellihot?

joca

ironman,So the delta P pump would be a solution with p/s or if piped direct thru boiler. it seems that with TRV's , there will always be the potential for deadheading. i'm designed for about 30k btu's at 140 F with 20F drop,so i should be ok?(head at 6GPM is 3ft)
rich, if i use a bypass H valve at rad, how does it get set? won't that rad have a hard time getting up to btu's
Gordy, i200x

Ironman

I use the Alpha with panel rads and TRVs a lot. The pump knows when one or more begin to open and starts and ramps accordinly. It's a smart pump. IDK that I would use it to pump directly through the boiler: you may have insufficient flow and short cycling at times. That's why I said pipe it p/s and use the Alpha on the house loop.

SWEI

This is why we go to the trouble of using DDC on small systems. We zone using properly-sized Belimo CCV's driven by a PI loop modulating to the room temp setpoint -- in effect an electronic TRV. Then, by looking at the valve positions (adding up the percentage open values for all zones) we can manage the call for heat (TT) to prevent short cycling. Still hoping we can turn this into an off-the-shelf solution some day.

Eastman

@SWEI
Does your system manage the boiler modulation directly?

SWEI

It can, but mostly before we got mod/cons with onboard cascade capability. On larger systems, we've been experimenting with running multiple boilers over ModBus. The controller hardware cost doubles or triples just for that, and wrangling integration details with different ModBus implementations turned into a fair bit of work.

On smaller systems, I'm trying to work out ways of speaking on the available analog I/O ports -- managing two thermostat inputs plus a DHW call can sometimes allow us to improve on (or even re-purpose) portions of the boiler's firmware.