sizing of hot water circulation pumps 1

[mechanicaldup]

Hello

What guidelins should I follow when I size/ design a circulation pump for a central hotwater system for a building (hot water is used for plumbing uses showers etc...)?

 

[TBP]

The biggest mistake I've seen is installing way too much pump for applications like this. The system is normally pressurized by the municipal system, so the circ only needs to move a relatively small volume of water around in a circle. (You only need match the heatloss from the system.) If copper pipe is being used, make sure it gets reamed after cutting. The little lip left on the inside of the pipe by the cutting wheel sets up a nice swirl, and in combination with a constant water flow can erode pipe suprisingly quickly. If the circ is grossly oversized, the combination with un-reamed pipe can be particularly nasty. I've seen pipe fail in as little as 6 months because of this combination.

Remember as well, that you're essentially turning the domestic hot water system into a hot water heating system if the lines are uninsulated. I've seen installations where the uninsulated recirculating domestic hot water system was fighting the air conditioning all summer.

 

[mechanicaldup]

thks TBP

you therfore size the flowrate of the pump just to match the hetloss of the system?

 

[TBP]

Yep. BTU/hr = GPM X 500 X delta-t (F). Think of the piping system as a radiator. Once you know the surface area, and delta-t, the heat loss can be calculated. The pump head will be figured off the length of the piping run. I'd bet that the circ pump manufacturers have a calculation, chart/tables or software that will make it easy to pick the right pump.

 

[trashcanman]

Why bother? Specify the use of self-regulating heat tracing tape for the HW lines. You can get it to maintain 110 Deg.F. Just attach it along the piping, and insulate over the tape/cable & pipe. No pump, piping, valves to bother with.

It is made by several cos., such as Raychem & Thermon
A 350' length to handle a HW line will draw less than 20 amp (according to the Raychem program).

 

[TBP]

Not really much bother if you do it right. In some installations, you can take advantage of convection. You don't even need a pump, for those.