Primary-Secondary Pump Application

[mechanicaldude]

This might be a hard question to discuss without graphics, but I'll try. If I want to add a secondary pumping loop to an existing hot water system (due to remodeling of existing spaces) where this secondary loop will be for a new wing of VAV reheat. Does the return line for the secondary loop connect into the supply Main of the primary loop(which will act as the common pipe) or does it connect to the return main of the primary loop. If it connects back to the supply main of the primary loop (which I believe it does) won't that reduce the supply water temperature going to the rest of the system because I'll be mixing 160 degree water with 180 degree water ??

thanks for your help.

 

[ChrisConley]

You can do either method, but as you mention the supply temperature 'downstream' will be blended to a lower temp. The calculation is pretty easy (standard mixing temp) to determine the new temp, and it might be acceptable.

The advantage is that you're not adding any flow or head to your primary pump, which might not be able to handle the additional flow.

The disadvantage is the lower supply temp. This could be compensated by using a slightly higher supply temp (185F). Depending on the relatively flow rates the drop might only be a couple degrees.

If you do connect and return to the supply line, make sure your connections are as close as possible . Any pressure drop between the connnections will cause a 'ghost flow' that will drive you nuts trying to figure it out. Our design standard is to keep decoupling loops to 12" apart, with no valves or fittings between the connections.