Zone Circs Pumping Away From Boiler vs. Towards Boiler

[velvetfoot]

Is seems that pumping away towards the emitters is preferred, but on my oil system the zone circs pump towards the boiler return. I'm committed to changing things if I have to for best operation/efficiency, but what are the pluses and minuses of each? A fellow at work said pumping on the return used to be common. I can see maybe the pump sees somewhat lower temps on the return.

 

[maple1]

Mine is on the return, pretty well at the very bottom of my system, pulling through my zones - just before a T that goes to the boiler inlet one way & bottom of storage the other. My old boiler had it low on the return line too, so I just duplicated that. Might not be the most efficient way to do it, I don't know - but about all I can say after almost two full heating seasons is that it seems to work.

 

[BoilerMan]

Either way will work. But by pumping away from the expansion tank (which is the point of no pressure change) and also the best location of air elimination as it's the lowest pressure point and the hottest allowing the air bubbles to be the biggest and most buoyant for best elimation. If the circulator is located shortly after the expansion/air elimation location it raises the system pressure toward the emitters. There is no such thing as pulling water, it is the system pressure that has to move water toward the inlet of the circulator.

Suffice it to say the most efficient (watts to water) location and for best air elimation and circulation put the circulator on the supply after the expansion tank.

Either location will work, just one works better. Temperature is not an issue with modern wet-rotor circulators, in the old days with three piece circulators the mechanical seal would last longer in theory with cooler water, but the return is typically only 5-10 degrees cooler........... so not much if any difference.

TS

 

[ewdudley]

I think pumping away, maintaining positive pressure at top of system, gas elimination, pump temperature, and pump cavitation should all be taken into account, but I don't see how watts to water would enter into it as long as there is no cavitation. If it's a circulation loop wouldn't the power needed to move the water be the same no matter where the pump is relative to the expansion device?

Also someone pointed out on an another thread once that although supply side isn't typically all that hot, during certain not un-heard-of episodes it could spike to 230 degF or more.

 

[maple1]

Yes on pumping away from the expansion - my expansion is T'd in just upstream of my load circ.

 

[BoilerMan]

I think pumping away, maintaining positive pressure at top of system, gas elimination, pump temperature, and pump cavitation should all be taken into account, but I don't see how watts to water would enter into it as long as there is no cavitation. If it's a circulation loop wouldn't the power needed to move the water be the same no matter where the pump is relative to the expansion device?

Yes you are correct, but with wood-burning high temps and cavitation would be even more of a concern.

I would argue that the pump's watts adding pressure in the direction of the flow restriction would move more water for the same given wattage than lowering the pressure and relying on the system pressure to bring the fluid back to the pump. This would be very small in a typical residential hydronic system, but I think it still would exist.

TS

 

[ewdudley]

I would argue that the pump's watts adding pressure in the direction of the flow restriction would move more water for the same given wattage than lowering the pressure and relying on the system pressure to bring the fluid back to the pump.

I think all the pump can see is the pressure differential needed to move the water through the resistance of the circuit. If it takes 2 psi pressure differential to move the water at 10 gpm, then the watts to water is determined by how much energy the pump draws to 'lift' that much water at that rate. It doesn't matter if it is lifting from 20 psi to 22 psi, or from 192 psi to 194 psi, the energy required is the same.

 

[velvetfoot]

Well, I guess it's looking like I'll have to make some changes, but it's so purdy.

(The pellet boiler is going where the oil tank is.)