Supply or return side of heat exchanger for circ pump

Michael O'Neill

I will be putting in a wood gasification boiler and using a water to air heat exchager in the plenum above my Lennox Pulse propane fired furnace to heat the forced hot air. The boiler man at work stated I would get more heat form the exchanger if I located the dedicated circ pump on that line on the return side of the HX rather than the supply side. Is that true? He said he had seen in his work measurable difference in heat output doing it this way.

Larry C_13

Where is the expansion tank?

To avoid sucking in air at the pump seals, put the suction of the pump close to the expansion tank connection. Search this site for PONP (Point Of No Pressure Change) and the book "Pumping Away" to fully understand the reasons.

Homeowner Larry

Michael O'Neill

Larry, thanks for your reply. These lines will come off the primary loop separately. The primary loop has the expansion tank and primary circ pump located on the supply side of the boiler in that order, as the book you referenced indicates. The separate circuits off the primary loop are to serve only the DHW flat plate HX or the Water to Air HX, and will operate off closely space tees, using their own circ pumps. I need to know if I should locate these circ pumps before or after their respective heat exchangers to get the best heat exchange. What do you or anyone out there think.

hot_rod

pump

away from the primary loop, and PONPC, into the HX (both air and DHW plate) and back into the primary loop.

With a plate HX, for the DHW, be sure you pipe a counterflow. That is opposite flow direction on the A and B side.

I don't see how pumping the return side of the HX into the primary loop changes the amount of BTUs exchanged? The flow rate, and the delta t are the concern. The only difference would be the circ would operate in cooler temperatures on the return side of the HX.

Be sure you have an air removal device on the coil, especially if it is the high point in the system.

hr

Michael O'Neill

Circ pump location

Hot Rod, thank you for your reply. I couldn't see why the location of a pump on the return side of a hx would make for better heat exchange either. On the other hand this was advice I received from a professional, whereas I am strictly an amateur in this field. I wanted to check this advice out with someone who knows what they are talking about, which you certainly do, judging from the things you've contributed on this site and others I've read.Thanks again.