Energy cost of hot water recirculation
I am considering a hot water recirculation system to get water to a distant fixture (about 30 feet from the WH) more quickly, but am wondering if it makes sense from an energy cost standpoint? I like the simplicity of a thermosyphoning system (like the Nibco device http://www.nibco.com/assets/Justrt.pdf ), but what effect would this have on hot water heating costs- seems to me that continuously circulating hot water is going to increase the heat loss of the entire sytem, thus raising our DHW heating costs…is it worth the water savings to install these devices, or is it being done primarily for convenience (shorter wait times)?
Currently, our house has a trunk and branch layout, so the first 25 feet of hot water is 3/4″ line, then it branches off in 1/2″ runs to fixtures. We have alot of wasted water due to this layout. Rather than a recirculation system, would it be smarter to just replumb this line with a home-run system using 1/2″ (or even 3/8″ pex runs) to each of the fixtures, and skip the recirculation system altogether?
thanks
Replies
Just picked up a pc of literature from Grundfos on their circulation system. Haven't had time to look through it very closely but there seems to be a timer involved. It would be set for prior to normal time of use, saving water and not causing reheating water constantly.
I think.
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Shawn,
We install circ pumps in all our projects. My plumber uses Taco (sp?) pumps. We quiz the homeowner about usage during the day. Often we set the timer on the pump so it circs every 15 mins starting at 6 am to 8am and then again at 5pm to 9pm.
The circ line is heavily insulated to minimize heat lose and it is plumbed to make the drop as short as possible. Most drops to fixtures are 5'.
The system works great and there is little water wasting.
Bruce
I understand that a programmable timer would make a recirculation system more efficient (compared to continuous recirculation), but it would not work for us- our schedules are too varied from day to day.
Regarding the efficiency, I know that insulating the pipes is very important. but no matter how well you do it, it must be increasing the surface area of the hot water and causing more heat loss than the unrecirculated system...just the fact that you set it to recirculate every 15 minutes suggests that you must see alot of heat loss in the plumbing runs, right?
This just doesn't seem like a good idea...Shawn
You can set them up with a switch so when you walk into the bathroom you turn it on, get ready, then turn on the water. A minute or so is all they need.
Think of a recirc system as a small radiant heating system. Obviously you insulate the pipe as well as you can but there is still some heat loss, which the WH makes up for by running more.
If you have the option of running a 3/8" PEX line right to the fixture in question, that will dramatically reduce hot water delivery time. How easy is it to do this, is it in a crawl space or basement?
Our plumbing is all exposed in an unfinished basement, so I have total access right now. My long run is to a kitchen sink, which has a 1gpm flow rate- can I get by with 3/8" pex supply for this fixture? If so, that should cut my wait time down to only 10 seconds or so, which I think would be sufficient. I'd rather go this route than install a recirculation loop if possible.
Similarily, would 3/8" pex be adequate for supplying appliances like dishwashers and clothes washers, or should I use 1/2" runs to these fixtures?
thanksShawn
I would absolutely install the 3/8". Stuff like the dishwasher and clothes washer don't mind the wait, and don't waste the water while waiting. If anything, the larger lines to those machines *save* you some hot water, because more cold and less hot ends up in the machine. Someone smarter than me could tell you the flow rate of the 3/8" PEX, but I know for a fact it will be OK in the kitchen, I have done it. For that matter, check out the bore of your angle stops, or the diameter of the flex lines.... nowhere near 1/2".
If you can take the 10 second delay I think you'll be OK. I've found the calculations I did for a direct-run shower to be accurate -- 8 seconds in my case.The PEX manufacturers have pressure drop tables for the different diameters. For 3/8 at 1 gpm, it's 0.07 psi per foot, or .21 for 30 feet. I wonder what 1/4" would do :).http://www.alliedpex.com/LITERATURES/PEX_Pressure_Drop_Table.pdf
just the fact that you set it to recirculate every 15 minutes suggests that you must see alot of heat loss in the plumbing runs, right?
I can't really answer that as I have not optimized the system (and probably will not). I doubt we loose that much heat as the pipes have 1" insulation and my gas bills during the summer are 1/2 of ones from my other house. I do not know the real losses on the system at all.
Best of luck
Seems like it would be easy to calculate the heat loss in BTU from a recirc loop but you'd have to get someone smarter than me to do it. I've seen some ridiculous recirc setups, long runs of 3/4" or 1" copper running 24/7 uninsulated thru the crawl or attic. Stuff like that is all over this country.
Here in Austin, you are required to have a recirc system if your outlet is more than 40 feet away from the heater in terms of actual pipe length. We install them as a standard procedure and all our clients are very happy.
We also use foam for insulation, 18 and 20 seer HVAC systems and the like. I am not that concerned about heat loss on a well insulated pipe loop in the insulated envelope of a home.
One client had to wait 3:20 minutes for hot water in the master. We installed a recirc and got it to 5 seconds. They were estatic.
There may be some tradeoff in efficiency, but we don't waste 30 gallons of water waiting to take a shower.
I have a friend with a poor well, it produces well under a gallon per minute and he doesn't want to waste a drop. He installed a recirc system and it's run by a motion detector. If you walk into the bathroom the eye sees you and circs the water. Apparently you can walk in, wait about 3 seconds, then turn on the water and it's hot. Next thing will be a control that reads your mind and knows when you're headed for the can.
Do you know who makes the motion detector activated system? Sounds great, probably 'spensive though...Shawn
I would just use a Cooper 6109 and hook the pump up to that.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Just like that one...
I am so pleased that we home ran all the water lines in our home. The wait time is almost as good as a recirc system. Sure recirc is quicker but only by 5-10 seconds at the furthest distance. It also doesn't cost a thing to operate.
The other thing about the home run is that we don't keep the house water system fully charged. It's a weekend place mostly but in the winter I drain the lines (even with the heat left on) but only enable the areas we will be in. This past weekend the guest bath, ice maker, spigots, washer/dryer, master corner sink, are not on. If we have guests it's easy enough to give the manifold a twist of the valves.
I have the grudflos pump set for 6 AM to 8PM - Kit sink is 90 feet from HW heater - it is hot all the time when we need it during the operational window - use to have to wait 2-3 minutes to get the hot stuff -- it has a thermo so it is not running all the time - belive it is the equivalent of a 100 watt bulb on 6-8 --
Our other alternative was to do a second HW heater or a POU unit which I just did not want to do
To answer your question. Recirc systems have 2 purposes: convenience and reduce water wasted waiting for the water to get hot.
Energy? They consume energy: for the pump and for the loss through the piping. Pump energy is fairly small. Piping loss varies.
Timers help. A switch can be convenient. Small lines will often take less than a minute to 'get warm'. Turn it on only when you need it. A crank timer would ensure it gets turned off.
Not running it 24/7 like some systems is considered 'state of the art' ... that is ... the way most would do it these days. Some kind of control that helps you use it only when needed. If your schedule is irratic ... just use a switch.
What is this 'distant fixture' you refer to?
My 'distant fixture' is a 1gpm kitchen sink that is 25 feet or so from the WH. I see the benifets of the recirculation (especially the motion controlled feature!), but don't like the wasted energy and the cost, so am leaning towards just reblumbing with a home-run layout. Switching to 3/8" pex should bring the wait time for hot water to 10 seconds or less, compared to the 30+ second wait currently. That said, I am now wondering if I can get by with even 1/4" pex for a 1 gpm sink? I can't seem to find any data on flow rates vs. tubing sizes...anybody know?
thanks!Shawn