Hot water Reciculating loop vs integrated loop question...which?

[danrudy]

Hi All,

need a quick answer ....We are building a house here in south florida. Plumbing almost all done. We were planning to put a recirculating loop for hot water so we could have instant on hot water. The plumber on the job has suggested we put in a Grundfros Comfort System recric pump. Best, I can tell, it will not have a dedicated hot water loop but a valve will move the hot water into the cold water system to return the water to the pump at the heater. Thus, while we would only wait a few seconds for hot water. We would have the adverse effect of having warm water initially at the cold water taps (like when kids brush teeth) which will take a few seconds to cool. While I know the Grundfros can be set up on a timer I assume a dedicated recir loop can also have a timed pump so the notion that it needs to be on 24/7 is not a disadvantage. Is that a correct assumption?

This is a brand new construction....we can do either way. Any strong opinions of dedicated loop versus the integrated Grunfros type system. Obviously, I would like also do to what is more cost effective but also want to maximize the convenience of hot and cold water "instant on".
Is one system preferred to the other....is one better? Is one going to save me more money over time?

For the record, my GC prefers the dedicated pump and the plumber the Grundfros....it is my call.
Help!
Thanks
Dan

[jadnashua]

Many of the systems can work with either a dedicated return line, or use the cold water return. ON new construction, where you have the choice, I'd add a dedicated return line. NOw, consider the layout of the fixtures, you may need more than one return line, and they would require check valves and a valve to help balance them properly. If everything is strung out in a row, you can put the return line at the end and everything before it will be hot as well, but that won't work well if there are significant branches in the system.

[danrudy]

thank you for help....
anyone else?

[jimbo]

JAD says it all

A dedicated return is always preferred to the aftermarker, crossover types.

[hj]

The only advantage of the Comfort system would be if the plumbing had multiple branches that needed the "instant" hot water. If the piping is not installed to make them a continuous "loop", balancing a dedicated return line for these is a difficult process, but with the Comfort, you just install a "bypass" valve at the end of each branch and use the single pump.

[danrudy]

Thanks all...seems that dedicated lop return is preferable. We are building a two story house and most bathrooms are in a fairly straight line to loop except one bathroom which will be a low use room so I do not consider it a tragedy to have that one bath have a 30 second wait if it came to that.
I live in Forida and am ignorant of such things so forgive me in advance for a seemingly dumb question....Will a loop give if any more significant heat then a regular return and generate more electricity costs or are the differences negligible? In Florida I prefer to keep my house cool so am a little gun shy about "heat generators" that will make me turn up the A/C. I figure that if I have either system (loop or return) on a timer for expected use hours (Early AM and PM) then it should not effect AC or electricity bills in any meaningful way but I did wanted to confirm my assumption/..
thanks

[hj]

Neither the circulating loop, or the Comfort series loop, (note that they are BOTH "loops),will "heat" your house. With the dediicated loop, you need a valve between the pump and the water heater, (NOT ahead of the pump aand is in addition to the check valve which you also need), and shut it down until it just maintains the return line temperature. This will reduce your power bill and will prevent copper tubing failure to to high flow velocities "eating away" at the pipe.

[Dana]

No matter what you do, loop, no-loop, dedicated loop, insulate the hot water distribution lines with a minimum R4 of closed cell pipe insulation (and if looped, at least R2 on the return line) which will keep the hot water warm enough to be useful for 10s of minutes after the initial draw (or pump cycle), and lower the cooling load the lossy lines present to your AC system. This dramatically reduces the number of pump cycles in an idling a recirculation system.







R4 is typically 5/8" or 3/4" foam, and not normally found in box stores (but readily available online, and sometimes found in-stock at Graingers). The stuff found at home centers is usually 3/8" R2 goods, which is fine for the more tepid return lines, but it's worth (= cost-effective on energy savings alone in short years) buying the fatter stuff for the distribution lines, as well as all near-tank plumbing, including the cold feed to the heater and any temperature & pressure outflow plumbing within 10 feet of the water heater.

[danrudy]

Thanks!!!
I have passed on the info to my builder and plumber.
We are going to go with the dedicated recirc loop. I sent my builder your replies so that he is aware of pump placement and need for insulation. Funny thing was that the builder was telling me that he is old fashioned and he always preferred dedicated recircs as his dad was a plumber and this is what he used.
Really appreciate the help folks! it made this decision easier.

[Gary Swart]

I don't think being "old fashioned" has anything to do with the question. A dedicated return line a far superior method.

[hj]

The only reason NOT to use a "dedicated" return line is when it is too late to install it, or the piping was not originally designed to take adavantage of it. In fact, the "retro" systems are fairly new and only became available with the advent of the "crossover" thermostatic valves for use at the sink locations. WHY would a "temperature and pressure outflow from the water heater within 10 feet" NEED any insulation at all? WHO cares if it "cools down" faster? And why is 10 feet the "magic number"?

[Dana]

The only reason NOT to use a "dedicated" return line is when it is too late to install it, or the piping was not originally designed to take adavantage of it. In fact, the "retro" systems are fairly new and only became available with the advent of the "crossover" thermostatic valves for use at the sink locations. WHY would a "temperature and pressure outflow from the water heater within 10 feet" NEED any insulation at all? WHO cares if it "cools down" faster? And why is 10 feet the "magic number"?

If it's copper, the thermal conductivity of the dry outflow plumbing is still conducting measurable amounts of heat away 24/365, as is the thermal conductivity of the water & plumbing on the cold feed.

There's no real magic to the 10' number- just a rough order of magnitude. On tank attached plumbing other than hot water distribution lines that is about the limit of what's cost effective on water heating utility savings at R4. If you lived on Block Island or some other place where fuel and electricity prices are more than 3x the national average there may be a case for more (both R and distance). Split retrofit foam is typically sold in 6' sections, so if you're into the penny-accounting feel free to cut one in half and call it 9' if you're in cheap energy country, 12' if you're above the national average. (Or 12' if electric/oil/propane, 6' if natural gas.) If the outflow plumbing is PVC or PEX you might drop back to 3' there, but not on the cold feed- water is still fairly thermally conductive.

The rate at which the water in the lines cools determines how often a recirc system that's maintaining temp at the tap runs, and to some extent how long it runs, and the magnitude of the total BTUs lost/abandoned in the plumbing, depending on the hysteresis of the controls. The BTU/hr loss rate out of the plumbing applies directly to the peak cooling load of the house, but the total BTU loss is also lower, lowering the average contribution to the cooling load as well. A 50-100' of uninsulated pipe that averages 120F a continuous 24/365 is essentially a low-temp space heating radiator that can be felt in bare feet though 1.5" of subfloor & flooring, and R4 of pipe insulation cuts the total heat flux by way more than half.