Hot Water Recirculation Flows

[daaguirre]

Hi Members,

I am trying to calculate the hot water recirculation flow for several balancing valves. I am following the procedures outlined in the ASPE Plumbing Engineering Design Handbook Volume 2, Chapter 6. However, once you have more than one recirculation path the process becomes a little bit confusing.

For Example: in the system shown, I can calculate a heat loss from the water heater to node C, a heat loss from node C to node A, and a heat loss from node C to node B. I can then calculate the heat flow corresponding to each of these heat losses, and I can assign the flows to each of the balancing valves. However, since the heat loss from the water heater to node C is shared by both recriculation paths, It seems logical to me to assign only a fraction of this flow to each of the balancing valves, i.e.:

Balancing Valve A: Flow corresponding to heat loss node C-node A + fraction of flow corresponding to heat loss water heater-node C?

Balancing Valve B: Flow corresponding to heat loss node C-node B + fraction of flow corresponding to heat loss water heater-node C?

My question is: How do you calculate the fraction of the flow corresponding to the heat loss from the water heater to node C that will be assigned to each balancing valve? The ASPE Plumbing Enginneer Design Handbook Volume 2, provides Equations 6.9a and 6.9b. However, I am yet to understand how to use them.

The question becomes more interesting once you have several divisions of flow and several hot water return paths and balancing valves.

Thanks!

 

[PEDARRIN2]

I cannot see your detail, but typically what I do is find the path that is the longest (or most friction loss). That path I use as the starting design basis for flow/pressure loss/heat loss. I calculate the flow rate for each of the other branches based upon ASPE heat loss information. At the junction of each of these branches to the supply and return, I subtract (supply) and add (return) the flow in the branch to/from the supply main. I then modify the calculation for heat loss and friction loss for the added/subtracted flow in the supply main.

 

[dbill74]

What Pedarrin said.

Additional food for thought, there is no code or rule-of-thumb or anything dictating that HWR piping MUST run parallel and adjacent to the HW supply pipe. A single loop is a lot easier to calculate than having several branches, plus you can save on material by not having the balancing valves. Depending on the situation, you could also save lots of piping as well. In your scenario illustrated in the sketch, run the supply from Node-C (N) to Node-A then around to Node-B and then from there back is all return.

For example, I did a 2-storey hotel. Everything was in a nice straight line so took the HW up to second floor, ran the length of the building with take-offs for the rooms; at the end, dropped the HW to first floor and back down the other way. The 'Return' length was considerably shorter and saved probably 250-300 feet of 3/4" - 1" pipe that would have done nothing except transport water back to the water heater.

To be fair, there are applications that do not lend themselves to a single loop. Had my hotel been 3 or more stories or had a second wing going off in a different direction, I would have had to find a different strategy. The simpler the system, the easier your job, less chance for making errors, and easier to install and maintain.