Who’s up on their plumbing physics?…
New construction- want to install convective recirculating hot water loop to 2nd floor bath from basement water heater (NO PUMP). The water heater and fixtures will be placed almost perfectly vertically aligned with each other. Want it to be as energy-conserving as possible, although the system does rely on energy-inefficiency (loss) to work.
Questions: Would putting insulation around the outside of the pipes change the efficacy of the system. It would reduce the temp difference, and thus the convective forces, but because of the insulation the water in the pipes will remain hotter, and thus the need for convective forces would be reduced, while still allowing hot water to be supplied to the fixtures. This is of significance because I would like to reduce heat loading during the cooling season (don’t want the system heating the house up more!)
: Should the up (hotter) pipe be put into a different stud cavity than the down (cooler) pipe to reduce the loss through heat exchange between the two?
:Would using a larger diameter copper pipe rather than standard 1/2″ pipe improve the efficacy of the system? Maybe using a larger diameter pipe would be more “forgiving” of having insulation on it?
:These questions seem to be beyond the area of expertise of my local plumber. Any experts out there with some assistance?
Thanks in advance – Brian.
Replies
Just an idea but you could look into the pumps used by some solar water installations. It is a small, 12v, solar powered unit. A small, about 18 by 24 inch solar panel and battery , for nighttime and rainy weather, sits on the roof and the pump is on the supply line. First time I saw the setup I wondered what I was looking at. The homeowner was a professor and quite proud of his setup. He used for a solar water heater but I would think it might work for your use.
Dear Mark,
You're looking for "instant" hot water at the tap? I don't think this is going to work for you.
With insulation, the temp difference is tiny. A liter of water on the "hot" side may weight just a few grams less than on the "cool" side. Gravity can't exert enough force on that tiny mass to accelerate it past the frictional resistance in even 1" tube. The whole loop will go cold and it will take longer to get hot water to the tap.
You could do some things to develop more temp difference. The fun part is, that when you run water out the tap with this system, you get a cooler mix from both sides of the loop. So the concept is self defeating.
Sorry, Fred
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Mark: Fred missed on this one (which is unusual). You (Mark) are right about insulation - it both reduces the driving force (hot, less dense water versus cold, more dense water) and insulation reduces the need for as much driving force. Since thermosyhponing happens in the laminar regime of flow, the effects are fairly linear and I would expect pretty similar temps at the point of use either way. Obviously, insulating only the outgoing line helps reduce heat loss (compared to totally uninsulated) and helps boost the point-of-use temperature. But that does leave the return line dumping a bit of heat during the cooling season (not an issue for me and my thermoshyphoning instant hot).
Here's what I'd do: Use insulated 3/4" all the way till you branch off 1/2" for the each fixture (normal plumbing practice so far). Minimize the pressure drop by minimizing fittings, using two 45's to offset instead of two 90's, etc. On the return side, place a 3/4" brass swing check valve pointing down. It will hang up (open) during non-use times but close when you draw water. This eliminates the mixing that Fred describes. And the check valve keeps the return leg from filling up with hot water.
Regarding insulation of the return leg: ideally you could have an access door where you could strip the insulation off if the thermoshyphon isn't as hot as you want as quickly as you want.
Sounds like you have an ideal set-up for thermoshyphoning. If the horizontal offset was sigificant, I'd consider 1" pipe despite its greater cost, both and poorer availability. If it is important to you that this works well while insulated, then go 1" on the loop, but the copper pipe fittings and insulation will cost 3-4 times as much.
Someone mentioned a solar-powered circulator pump. I've installed a bunch of those (for solar systems). I'd suggest a circulator marketed for instant hot applications instead because the instructions will be more germaine and they come with timers to reduce electrical use and heat loss. Or get the smallest (like 1/40 hp) $70 circulator pump from Graingers or HD or anywhere and plug it into a $6 lamp timer (that fool burgulars by cycling your lamps on and off). But, IMO these pumps lack the elegance of a thermoshypon system for upstairs bathrooms.
Much like power car windows, TV remotes, sex, and under-the-sink instant hot water dispensers; you don't strictly need instant hot in the bathroom. But once you have it, you'll wonder why you didn't do it sooner.
Edited 4/5/2002 4:16:38 PM ET by David Thomas
Mark
I agree with David, Fred did miss on this one. I have installed many recirc. lines lots have not had the best of conditions and I have had to install a pump but the configuration you have described if I am reading it right should convect. on its own. If not I have found the grundfoss recirc. pumps to work great they come with or without a timer and they are silent too.
http://www.grundfos.com/web/grfosweb.nsf/Webopslag/UGRD-4FVQNU
Thanks for some good info. I don't want to use a pump at all; energy consumption, complication & maintenance reasons. Just not my philosophy. Hope to construct this, maybe tweak it a bit, then forget it. I plan on leaving an opening in the basement so I have access to the return pipe to slide insulation up or down on it.
I have designed the house so that the domestic water heater and baths are vertically aligned with a thermosiphon in mind. (I must say that I am surprised at the number of house designers who do not consider positioning the water heater close to the points of hot water use, let alone close enough to use thermosiphoning. My architect's attitude was to put the bathrooms wherever and let the plumber decide to run pipes zig zagging all over creation).
Also, A/C is being roughed in, although I am incorporating a system by which incoming city water will pass through the radiant floor tubing before entering the water heater, thus slightly cooling the house in the summer (as well as "prewarming" the cold water before it enters the water heater). The house is being built on the north shore of Lake Erie, so we are hoping to not need the A/C. Hence my concern for not contributing even a small amount to the heat loading of the house by this system.
So I take your helpful suggestions, gentlemen, with gratitude.
Best regards, Brian.
I remember seeing a system that used a small pump at the bathroom, with a temperature sensor. It was triggered by a motion detector, so when someone entered, the pump would run water through the loop until it sensed hot water had arrived. The idea was that you waste no water by running it down the drain until it gets hot, and you waste no heat because the loop only runs briefly until the hot water gets there, and then only when there's a person who might want to use it. The thing was about $400+, too rich for my blood.
-- J.S.
This is all too technical.
There are only two things Plumbers need to know.
1. Pay day is Friday
2. #### doesn't flow uphill.
Use 3/4" supply line to 2nd floor... return leg can be 1/2". Insulate return till except for 4'.Lakeside...On the mountain, near the stream,aj Builder of Fine Homes & Tennis CourtsAnd featuring; Great Camps of the Adirondacks
2nd idea. Use individual supply lines to all fixtures... Use 3/8 lines. then you will have Quick warm water. Use the PEX even if illegal... Do after inspection. Do not sue me.Lakeside...On the mountain, near the stream,aj Builder of Fine Homes & Tennis CourtsAnd featuring; Great Camps of the Adirondacks
and most importantly #3 - DON'T CHEW YOUR FINGERNAILS.
Hi Mark,
You will find the product for exactly what you want to do at web site: http://www.nibco.com On the home page click on new products. On the new products page scroll down and click on the Justright system. All your questions are answered such as size pipe, insulation with 3 step installation, no electricity needed. A family of four will save from 7,000 to 14,000 gallons a year. Also this web site should get you there also: http://www.nibco.com/newproducts/justright/ Hope you will post again if you install it. Larry
> A family of four will save from 7,000 to 14,000 gallons a year.
At Los Angeles DWP prices, this is about $20 - 40 per year.
-- J.S.
It is interesting what others are saying. Similar to what you are describing, I built a 2 story house for ourselves (plus basement) and put a 1/2-inch hot water return line from the top floor bathroom back to the water heater in the basement. I also insulated this return line to avoid losing heat. I built in a small pump (about 40 watts of power). However, the convection circulation alone does the trick. I get hot water quickly and cannot tell the difference when the pump is running or not. Now, it's always unplugged. Works great!
Scott
Hi Scott - What would you estimate the approx. horizontal distance between your second floor fixtures and your water heater? I wanted to do just the second fllor master bath, but I do have a second second floor bathroom which I wasn't overly concerned about doing this to, because I'm unsure of how well it will work over the extra horizontal distance. I don't want to get too greedy with this, but I'll give it a try if I think the odds of success are good.
Larry - That Nibco device is an interesting idea - using venturi effect to boost the flow when cold water is used in the house. Was Joe at Nibco refferring to not insulating the justright, or thermosiphons in general? (Scott's system works and he insulated.)
Regards - Brian Mark.
Hi Brian,
Joe at Nibco said to not insulate from the return loop far faucet to the Justright system. It's okay to insulate your existing hot water runs to all the faucets. You could insulate and remove it if it didn't work out. He also said the Justright has a one way check valve that prevents cold water from entering the hot water loop whenever the cold water enters the water heater. He said his system needs little difference in temperature to work properly. He also said no roller coaster runs from the vertical pipes to the far faucet. Nibco has toll free number 1-800-234-0227 Larry
Edited 4/10/2002 9:28:19 AM ET by Larry
Edited 4/10/2002 9:30:54 AM ET by Larry
Thanks Larry I'm curious nowas to the price. I'll have to check that out.
- Brian Mark
Mark,
Sorry I'm so late getting back here. My second floor bathroom is about 10 feet horizontal distance from the water heater. It also works great for a first floor bathroom about 16 feet horiz distance away.
Scott
Thanks Scott - now I have some idea what does work.
Hi Mark,
I have the same application as you and called Nibco, talked to Joe in the technical section. He instructed to not insulate the return loop. The water heater and the far end loop must not be over 60 feet away. You have some posts that say insulate. Larry