Pumping Away Question

mpdb2

I have a hot water heating system with 3 zones: domestic hot water with a dedicated pump to an indirect HWH, and two heating loop circuits off a second pump. If I add a loss loss header with a 3rd pump on a primary loop and the primary loop pumps (on the return to the boiler) towards the expansion tank, but the other pumps pump away (on the supply lines), will the heating zones maintain constant pressure, IE: take advantage of the pumping away concept?

Jamie Hall

Constant pressure? No, but they don't anyway. Depends on where the expansion tank actually is. Sometimes it's easiest to make a sketch of the system. Doesn't have to be to scale. The expansion tank is the point of constant pressure -- point of no pressure change (PONPC for short).. Any pump will add pressure. Any length of pipe will cause the pressure to drop more or less linearly along the length of the pipe. So you get a sort of sawtooth pattern. The only two rules, really, are that where pipes join -- a T -- the pressure in the pipes has to be the same, and a pump adds pressure (a boiler acts like a very long pipe from the pressure standpoint).

hot_rod

What type of boiler? If it is a cast iron and you run all one temperature, no need or advantage to a separator.

It possible a single circulator with zone valves for indirect and loads could work fine.

Circulators induce flow in a piping circuit by creating a pressure differential Delta P, ∆P.

A gauge on either side of the circ would read the same psi, until the circulator is powered on. The gauge on the discharge would then show the differential pressure added to the circuit.

mpdb2

I have a set-up now with pumps on the returns of the heating circuit and indirect HWH. I'm switching to add a low loss header and primary loop to free-up the two existing pumps and moving them to the supply side. Sketch of the new layout is attached. My question refers to the pumping away concept that the pressure will drop the most at the expansion tank, which I do not see as a problem on the primary loop. But will it have a negative affect on the heating circuit?

Jamie Hall

There seems to be some confusion; you say that "the pressure will drop the most at the expansion tank", and that is incorrect -- the pressure tank is the point of no pressure change.

Move the expansion tank to the inlet of the boiler circulating pump, between the LLH and the pump. If there is room -- there should be at least 10 pipe diameters distance on either side of its connection.

hot_rod

Here is some additional info and graphic of what Jamie is suggesting.
https://www.caleffi.com/sites/default/files/file/idronics_15_na_2019.pdf

mpdb2

yes, I guess I was confused. I was referring to pressure drop across the system when placing the pump in front of the expansion tank. I wasn’t sure if the same concern applied to a limited primary loop if the other pumps were pumping away.

hot_rod

Basically that B&G drawing shows the concept of pumping away or towards the tank.

In that example the circulator is creating the ∆P by lowering the suction side pressure as the tank establishes the point of no pressure change,

The amount of pressure change would depend on the type of pump and characteristics of the piping circuit it is connected to.

Yes it would apply to all the circulators installed in a piping assembly.

Jamie Hall

I might add that there is very little head loss through a low loss header (that's why it's called that!) so in your setup, with the expansion tank between the boiler circulating pump and the LLH, all of your pumps are pumping away, as they should be.

gennady

my 2 cents

mpdb2

Thanks everyone. I was contemplating the same piping change that gennady made. Trying to limit the amount of re-pining I’m going to do, that’s why I had the expansion tank in front of the primary pump originally. I’ll see how to make this work with limited time and space.

hot_rod

I like to seethe indirect piped off the boiler, before the separator. You will get the hottest SWT and fastest recovery