I will be crawling under the house this week end to start tying in all the water lines. We are planing to run a hot water recirc. system and the pipe supply size is now in question. Since the line runs in series from one fixture to the next and then back to the tank, my first guess was to run the whole sytem under the house in 1/2″ copper. The reason for this, whether I am correct or not, was that I would have less heat loss through a smaller pipe. Then I started thinking about having the whole house connected on one 1/2″ pipe, which would then cause too much pressure drop when two or more hot water fixtures are used. Should the supply run under the house be 3/4″ or larger and then run the risers from under the floor to each fixture in 1/2″?.
Basically this is a 1800sq’ single story home with 2 baths, laundry and kitchen pretty much spread through out the house. I would roughly figure about 180′ of pipe that starts at the hot water heater and ends there.
Thank you for your replies in advance.
Replies
Migrainem,
IMHO, you only need to run 3/4 to the first 2 fixtures, 1/2 to the last 2, and 1/4 recirc return.
Don't bother recirc'ing the washer, if it is last on the feed.
The kitchen is the fixture that benefits most from a recirc'ing (instant hot water) system.
If the baths are only 20' or so after the kitchen, I would put my recirc loop at the kitchen and allow a little cold water to accumulate in the feed line to the showers.
SamT
>> ... would have less heat loss through a smaller pipe.
Interesting issue. I think the smaller pipe will cool faster because it has more radiating surface area per unit volume. In other words, the larger pipe will radiate more heat, but the temperature won't go down as fast because it starts off with a lot more hot water than there is in the smaller pipe.
Heat loss is a consideration, but temperature loss is why you're doing this.
If the loop is insulated, I don't think the size will make much difference. You are going to insulate the loop, aren't you? At least all the straight runs?
One notion I considered but never ran the numbers on was running the outbound and return pipes in the same insulation jacket.
Heat loss from a water pipe is a complex issue.
Pipe size does effect the rate at which heat is lost. The bigger the pipe, the faster heat is lost due to the increase in area, but the slower the temp drops due to the increase in volume. A bigger pipe will lose heat faster, but the smaller will drop temp faster. This assumes that there is no flow thru the pipes after they have reached a "hot" temperature.
With flow as a consideration, we are concerned with temperature at the fixture. Here, time is of the essence. Once the two pipes have reached temp equilibrium, the faster water flows, and the smaller the area, the less heat is lost from the water, and the hotter it will be at the fixture. Smaller pipes mean hotter water. Not that you would normally feel any difference, but it is measurable.
With an "instant hot water" recyling system, very little flow (the addition of heat) is required to maintain a constant temp in insulated pipes. However, once the return loop is reached, all work done by the recycling water has been done and the main consideration is preventing further loss, so the return pipe should be small to speed the flow and small to lessen heat loss.
Klear as mudd, no?
SamT
For the situation you describe, I'd run the supply as 3/4 and the return as 1/2.
With everything running in series (which is what you want with recirc), you may need the extra supply to avoid pressure drop. You could do the risers in 1/2, like you say.
Heat loss is not going to matter, due to surface to volume ratio. Insulate the loop well.
The 1/2 return is just pretty standard, and works well with design of most recirc circulators. Otherwise, you could certainly get by with a smaller return.
Edited 3/31/2004 1:13 pm ET by csnow
"I'd run the supply as 3/4 and the return as 1/2"
I agree. 3/4" to all but the last fixture than 1/2" to the last and back. plenty of flow at the all the fixtures.
Someone suggested a 1/4" return. I wouldn't - where do you find tight-fitting insulation for a 1/4" pipe? It is all you need for the flow, yes and if you use tubing, there's no joint for the whole run. But 1/4" tubing won't take a hit or a bump, 1/4" fittings cost more than 1/2" and, again, 1/2" insulation is available everywhere for cheap.
Of course, insulate the out and return in a recirc scheme. If you are really concerned about heat loss, you could put insulation for 1-1/4 or 1-1/2 pipe over the 1/2" pipe insulation. As far as I would take that is to put 10" lengths of the bigger stuff over the joints in the 1/2" pipe insulation.David Thomas Overlooking Cook Inlet in Kenai, Alaska