Rethinking the PONPC

hot_rod

As many know, older "packaged" boilers shipped with the circulators on the return pumping into the boiler. Various reasons for this location, easy of packing and crating, cooler temperature for pump seals among them. Low head 3 piece circe developed low head so no issues with excessive ∆P.

So we then embraced the pumping away concept, locate the complete package air separator, expansion tank and circulator on the supply. Mainly we do the best air removal at the hottest point in the system. Really no advantage to the circulator and expansion tank at that location.

Fast forward to mod con technology with high pressure drop heat exchangers. The high pressure drop, along with pressure lock out switches mandates the circulator be located pumping into the boiler. The ∆P developed by the circulator adds to fill and assures the boiler performs properly. Combis and some boilers come equipped with boilers mounted inside, all are pumping into the boiler.

This seems to confuse many installers, where is the correct location for all the components, can the be separated, etc.
Obviously reading the manual clears up what the manufacturer requires for boiler circulator.

So why not pump all boilers with the circulator and expo tank on the return? Low pressure drop style, cast boilers for example would work fine, many mod cons insist on return piping, no reason fire tube design could not pipe the same.
With a low pressure drop type boiler even less potential for excessive pressure causing the relief to seep or pop.

Most of the boilers I spotted at AHR that were imports look to be former tankless HX design, small passage tube style, so they want "pumping return" and either a P/S, or hydraulic separator.

Advantages include both circulator and expansion tank are in the coolest fluid, generally anywhere from 10- 40° cooler fluid temperature. When multiple circulators are used the hydro sep or low loss header becomes the PONPC for all the various circs.

Standardization would certainly help the industry. Some examples:

The Steam Whisperer

While we do only a little hot water work anymore, here's a few thoughts

From what I've remember, for most high pressure drop boilers, it is prefered to pipe them P/S, especially if using TRV's or zone valves on the system. With the Expansion or compression tank and pressure reducing valve on the System loop, the return mounted boiler pump is pumping away from the PONPC, helping purge the boiler of air.

On many systems, having the pump on the supply with the A/S right behind it will allow you to completely purge the system of air just by turning on the pump and will tend to inherently keep it clear of air.

I've also found no real need for fancy air separators when set up pumping away. I prefer the B&G IAS....they seem to be much more effective than an air scoop, but are still only a chunk of iron requiring no cleaning or servicing

EBEBRATT-Ed

Is the reason I like primary secondary for almost any job. No blowing relief valves. Just make things easier IMHO

MikeL_2

Hot Rod,
I think you mean combis & some boilers come equipped with circs mounted inside?

Leon82

I used the 4th diagram for mine

Tim McElwain

Hot Rod which Idronics book do those illustrations come from?

flat_twin

Our firetube modcon is piped direct with the circulator on the return side and is pumping away from the expansion tank. I was aware the diagram showed the pump and expansion tank on the supply side but the installer wanted it on the return to satisfy a pressure switch. I don't know how valid his point was but there's been no problem with a pressure switch or air in the system. Delta between the supply and return is only 3 to 12 degrees so any benefit from the pump being on the cooler side is minimal.
FWIW, the circulator on the old boiler was pumping toward the expansion tank. Startup in the fall always involved bleeding some air from all the radiators. I still bleed them out of habit but only get a bit of air from the one radiator on the 2nd floor.

hot_rod

The Steam Whisperer said:

While we do only a little hot water work anymore, here's a few thoughts

From what I've remember, for most high pressure drop boilers, it is prefered to pipe them P/S, especially if using TRV's or zone valves on the system. With the Expansion or compression tank and pressure reducing valve on the System loop, the return mounted boiler pump is pumping away from the PONPC, helping purge the boiler of air.

On many systems, having the pump on the supply with the A/S right behind it will allow you to completely purge the system of air just by turning on the pump and will tend to inherently keep it clear of air.

I've also found no real need for fancy air separators when set up pumping away. I prefer the B&G IAS....they seem to be much more effective than an air scoop, but are still only a chunk of iron requiring no cleaning or servicing

If the circ was on the return with the exp tank upstream the system air removal would be exactly the same as the circ and expansion up top on the supply, little to any pressure drop in a sectional or fire tube boiler to change the ∆P added by pumping into the return.

For high performance systems and high efficiency boilers I believe a micro bubble purger is beat. Any wide spot scoop or camel back is not adequate.

I suspect many of the over-heated and failed water tube boilers could be traced to air pockets trapped in some of the coils.

I suppose that is know by Spirotherm, sense the invention of micro bubble resorbers.

hot_rod

Tim McElwain said:

Hot Rod which Idronics book do those illustrations come from?

12, 15 & 19.
Proven Hydronic Systems
Hydronic Fundamentals
Separation in Hydronic Systems

Make sure you have the latest, should be 26 in your library now.

hot_rod

EBEBRATT-Ed said:

Is the reason I like primary secondary for almost any job. No blowing relief valves. Just make things easier IMHO

Always safe to pipe via P/S or with a separator. With a cast iron boiler it is not really needed. Also there is always some temperature blending in either unless both flows, primary & secondary are exactly the same gpm.

hot_rod

This drawing is what I feel is the best expansion tank location, also the air separator is in the very hottest fluid. When the boiler goes to high temperature for an indirect load, for example, that 180F goes thru the sep before it hits the loop or any load circ. and secondary.

Gordy

Does it really need to be rethought, or just better understood?

All your examples work. However it’s which example to use in what application that needs to be remembered.

Obviously the closer to the x-tank with the circulator inlet the better.

The Steam Whisperer

HR: If the circ was on the return with the exp tank upstream the system air removal would be exactly the same as the circ and expansion up top on the supply, little to any pressure drop in a sectional or fire tube boiler to change the ∆P added by pumping into the return.

However, on the supply the water temp would be higher for better air removal

HR: For high performance systems and high efficiency boilers I believe a micro bubble purger is beat. Any wide spot scoop or camel back is not adequate.

The B&G IAS is not either of these. From looking at how it is built, I believe it uses the Bernoulli effect to remove air very effectively. From my experience much more effectively than scoops.

HR:I suspect many of the over-heated and failed water tube boilers could be traced to air pockets trapped in some of the coils.

I suppose that is know by Spirotherm, sense the invention of micro bubble resorbers.

Cosmo_2

As long as the circ is pumping away from the expansion tank I doubt it really makes much difference.

PONPC as I see it is not bound to return or supply side location on a heat exchanger.

Cosmo

Ironman

We use the DirtMag when doing replacements, unless a LLH is necessary. The DirtMag has a 1/2" connection on top which is ideal for connecting the expansion tank to. Then the circulator can be placed on the return or supply with a fire tube boiler.

In a system with CI rad's, an MBR does little good and is not necessary since air goes to the top of the rad's and stays there until it's manually bled. Using the DirtMag on the return not only protects the boiler, but also provides the ideal place for the PONPC that you're describing.

GroundUp

As 98% of my jobs are mod/con and radiant, my $.02 goes to the circ on the supply due to the pressure drop through the piping and equipment. Maybe I'm nuts, but I feel the flow will always be better with the circ pushing into the high loss areas (as we do with boiler circs when using P/S) with less chance for cavitation in the event of a leak/ low pressure or faulty expansion tank.

hot_rod

Correct the air purger always goes on the supply, the hottest point in the system

On a chiller it goes on the return connection

GroundUp the small passage water tube boilers are often the highest pressure drop, that us why manufactures want you to pump into them. The system piping will also see the delta P added by the circ

If you pump away from the boiler, that high pressure drop is on the suction side, gauge one and see the pressure relationship. With a high head circ and high temperatures you could drop below NPSHR, and have more trouble purging the air from the boiler, possible cavitation

ch4man

so its a consideration of delta T's, hi head loss boiler designs such as water tubes vs cast sectionals and fire tubes. and systems designs such as poorly built pex radiant panels vs gravity conversions.
let the pressure differential work for you not against you

GroundUp

Primary secondary piping on these high pressure drop boilers eliminates that issue. The primary circ pumping into the boiler is hydraulically separated from the secondary loop, so the secondary (load) circ should be pumping away from the boiler and into the high pressure drop load piping the way I see it- not trying to draw through it. Maybe I'm crazy

hot_rod

> @GroundUp said:
> Primary secondary piping on these high pressure drop boilers eliminates that issue. The primary circ pumping into the boiler is hydraulically separated from the secondary loop, so the secondary (load) circ should be pumping away from the boiler and into the high pressure drop load piping the way I see it- not trying to draw through it. Maybe I'm crazy

Understood. Sometimes it’s easier to explain with a drawing.