Primary Secondary Pipe Sizing

Stokehold

Should the primary loop piping be larger than the secondary? If I have this correct, the primary is the boiler loop and the secondary is the zone loop. If, for instance, the primary is 1 1/4" should the secondary be 1"? I was thinking it shouldn't make a difference if they were the same size as long as long as the total secondary load does not exceed primary flow. Also, can a zone pipe be the same size as the secondary loop? If, for instance, the secondary loop is 1", can the zone be 1"?

Zman

Both loops should be sized based on the required flow, which is determined by gpm x delta t x 500 = btu/hr. Most designers size pipes based on 4 fps. There are some exceptions.

Gordy

A hydraulic separator is a big time saver in lieu of P/S piping, and performs multiple functions in the same package. Air removal, x tank location, dirt removal.

MichaelK

A quick video explaining the operations of the hydraulic seperstor Gordy is talking about. In case you're bored and just love this stuff!


https://m.youtube.com/watch?v=qTm2Y6h-aH8

hot_rod

I consider the main loop the primary, and any connections to it as secondaries.

In the top bobCAD the boiler is within the primary loop.

In the bottom the boiler is treated as a secondary, usually a boiler with high pressure drop will be a secondary.

Piping and circ pumps are sized to the load of each circuit.

Agreed the hydro sep solves all the P/S concerns adds air, dirt, & magnetic separation. Less cost and piping labor also.


http://www.caleffi.com/sites/default/files/file/idronics_15_na.pdf

Tinman

I like the idea of using the terminology of "boiler loop" and "system loop(s)". I think its easier for people new to the concept to wrap their heads around.

Stokehold

If the secondary loop can flow more than the primary, wouldn't that cause a reverse flow? For example, if the primary is pushing at 10 GPM and all secondary zones are running and pumping 15 GPM, wouldn't that extra 5 GPM reverse flow across the spaced Ts? If this is so, then cooler water would be entering the secondary and much cooler water would be returning to the boiler which could cause other problems. That's why I was thinking that primary loop flow must be greater, or at least equal, to secondary flow. So, in sizing the primary circulator, wouldn't it be better to size it according to total secondary flow?

hot_rod

Right you are, there can be reverse flow direction in the closely spaced tees.

Ideally the boiler should have return temperature protection installed to protect it, especially if you are connected to radiant slabs or high mass distribution.

P/S piping does not in itself protect boilers from cold returns. One of the other drawbacks of P/S is that series temperature drop as additional take-offs are connected, noted by the color change in the primary loop in this schematic.

A hydraulic separator eliminates the temperature drop to the takeoffs as they are not in series.