Replies

1. 
   Daniel Morrison | Jan 27, 2011 04:31pm | #1

   I'll let the experts answer your question directly, Sally, but here's an article on heating water that you may find helpful in the meantime: https://www.greenbuildingadvisor.com/green-basics/water-heating

   Dan

2. 
   Michael Chandler | Jan 28, 2011 08:32am | #2

   I've been using the Rinnai 156,000 BTU model (about $1,200) and the Quietside 120,000 BTU model ($1,000) and like them both. Rinnai has a better service parts and training system for it's installers but Quietside has less expensive venting options (vents with 3" PVC plumbing pipe up to 30' long so you can easily install in a central location in the home for short pipe runs) Tagaki also makes a good model. but our local distributor (Ferguson) carries Rinnai and handles parts and training so that's pretty significant advantage to me.

   We rarely need parts though, the main problem I see with the demand water heaters is if they are forced to run on very choked down low fire for a long time by a recirculating system they get soot build-up on the heat exchanger which reduces efficiency and can lead to air-fault problems. When this happens we just clean the soot out of the heat exchanger. I have never had a lime accumulation problem with a demand water heater nor have I had internal leak or other issues though I did once have a flow switch go out but I think that was related to use of pipe dope instead of Teflon tape up stream from the water heater which is clearly prohibited in the installation manual and reinforced in the training.

   I've been using a tempering tank to avoid the low-fire issue in some of my projects esp when dealing with Solar preheat and radiant floor or ultra low-flow fixtures.
   http://www.chandlerdesignbuild.com/files/fhbDecJan08.pdf
   http://www.chandlerdesignbuild.com/files/waterHeatersLatest.pdf

3. 
   Allan Edwards | Jan 28, 2011 08:42am | #3

   Michael (and others)

   Given the initial cost of the units and long term maintenance, are tankless really any more cost effective than a storage tank model, especially when you have access to natural gas. I've used tankless in 4-5 homes and I'm not sure they are worth the extra money.

   Allan

4. 
   Michael Chandler | Jan 28, 2011 09:15am | #4

   They do make a lot of sense to me. I've seen some crazy stuff in Texas especially where the inspectors are making people install multiple demand water heater arrays in large houses. Much more effective to use a single 199 kBTU model with a tempering tank or to distribute smaller units where they are within 30' developed pipe length or less of the fixtures. You can also get a lot of bang through smart pipe design by eliminating elbows, bull-headed Tees and all 3/4" hot water pipes except for a dedicated 3/4" home run to the jacuzzi. (plus you can get points for smart design in both NGBS and LEED-h)

5. GBA Editor
   Martin Holladay | Jan 28, 2011 09:58am | #5

   AJ,
   You can always use Google, you know:
   “Bull head Tees are defined as tee fittings where the ‘branch pipe’ connection is of a larger diameter than the straight through ‘run’ connections.”
   http://www.pipetubefittings.com/hose-fittings/tee-bullhead.html

6. 
   wjrobinson | Jan 28, 2011 10:00am | #6

   Michael, If you would edit your second post. It makes no sense to me almost from start to finish. You say do this not this, do that not that, use 3/4 except then use 3/4,

   and bull-headed Ts? What the hell are those? Am calling the counter boys next and asking for bull-headed Ts.

   I have personally plumbed every home I have built and love to learn from others, but you stumped this chump with your last post.

   Edit... OK... I call out my Ts like they are listed on my invoices, 1/2x3/4x1/2, never once saw the word bull-head. Learn something everyday. Still, would like that post further detailed and explained. I know how to plumb more ways than I am old but I didn't follow the post. Thick as a brick sometimes, We are buried in snow up here.

7. GBA Editor
   Martin Holladay | Jan 28, 2011 10:01am | #7

   AJ,
   Michael's advice on 3/4-in. tubing runs makes sense to me: eliminate all 3/4-in. hot water tubing, except for one 3/4-in. tubing home run to the Jacuzzi.

8. 
   wjrobinson | Jan 28, 2011 10:02am | #8

   Yaa, Martin, I did Google, us new guys doing just residential mechanicals up here in the Adirondacks aren't big bull-head types it seems. Starting next visit to the plumbing supply house I am going to rip into the boys for not teaching me proper.

9. 
   Allan Edwards | Jan 28, 2011 10:15am | #9

   How efficient and cost effective are on-demand electric heaters in far reaching locations. We use recirculating pumps, clients like the instant hot water but I have no doubt they are a big energy waster.

10. 
    Michael Chandler | Jan 29, 2011 12:57am | #10

    Recirc pumps can be huge energy wasters.

    Allan - You are correct to be skeptical about recirculating pumps. The demand-type Metlund and (I think Chili Pepper) that move hot water only when triggered by a switch or occupancy sensor are the best of the lot (as compared to passive thermo-siphon systems or clock timer pumps or the Grundfos wax cartridge valve “comfort” system) but the Demand generally requires a 3/4" supply. If you have a single location that can be supplied by a 1/2" line such as a remote bath with shower and two vanities or a kitchen with one of two sinks it may be more efficient and cheaper to feed hot water w/ ½” PEX from a central demand or solar water heater into a 2 1/2 gallon 110 volt electric tank type (Ariston) water heater. Generally a point of use electric demand water heater is too intensive an energy hog to make sense in this context if fed with cold water.

    Gallons per foot for copper is greater than for PEX of the same nominal diameter.

    The IECC is showing ½” PEX at 0.0092 gal per foot ( ¾” is .0184, 3/8” is 0.0050 as compared to “type L” copper at 0.0251 for ¾”, 0.0121 for ½” and 0.00405 for 3/8”) so if the goal to have no more than six cups (0.38 gallons) in the run between the water heater and any point of use then we are allowed no more than 41.3’ of ½” PEX (or 31.4’ of ½” copper) from the water heater to the point of use. The same amount of water is contained in 20.65’ of ¾” PEX or 15.14’ of ¾” copper. IF I have a remote bath with an under counter 2 ½ gallon tank type 110 volt electric water heater I can supply it with 82’ of ½” PEX and all the cold water in that line will be only .76 gallons before the water starts coming in hot so the tank will supply "instant hot" to the vanity and shower but the remote water heater will prevent the little tank from running cold when the boomerang kid wants to take a forty five minute shower. Green indulgence!

    Bull Headed Tees and plumbers

    But research done by Gary Klein has shown that those elbows and bull headed Tees (IE ½ x ½ x ¾ or ¾ x ½ x ½ -called out through by side) cause turbulence, or mixing in the line so adding elbows or bull headed tees (other than ¾ x ¾ x ½ or ½ x ½ x ½ ) can cause mixing and delay the delivery of hot water to the fixture. Imagine a tiny little kayak popping out in an eddy behind one of those elbows or Tees. As the cold water swirls in those eddy’s it mixes with the hot water passing by and the person washing their hands feels the water go from cold to luke warm to warm to eventually hot. The difference between turbulent and plug (or laminar) flow. We want the hot water to act like a plug, pushing the cold water in front of it with the least amount of mixing possible so we eliminate elbows and bull headed Tees and minimize ¾” pipe in general.

    Why we don't build'em like they use'ta

    The way I was taught plumbing 32 years ago (we ran Quest back then with copper fittings by cracky) if I had two bathrooms stacked with a kitchen on the back of one I would run ¾ cold and hot to the furthest tub and take ¾ x ¾ x ½ tees off along the way and might have a couple of elbows coming out of the water heater to get things lined up. I’ll still run the cold water this way but now I’ll home run one of the bathrooms with ½ to the shower and pop off my feeds to the vanities and bidet in closely spaced ½ x ½ x ½ tees Then I’ll home run the hot to the bath behind the kitchen with ½ the same way (and possibly pull to the kitchen off that same feed.)

    I prefer to place the water heater centrally as close to the kitchen as possible and feed out from there since the kitchen is the one place used throughout the day. This is why I’m so keen on the little Quietside ODW 120A condensing demand water heaters, I can put one in a closet in the center of the house and run the vent in 3” PVC thirty feet up to the roof above.

    If the master bath has an 80 gallon tub I’ll feed it on a dedicated ¾ home run. They’ll fill the damn thing 4 times a year if they’re lucky. But it will be totally worth it for those four magical occasions and it will have paid for itself in the first three or four uses.

    PEX is a lousy insulator! Who knew?

    New research Gary (Dr. Klein) is doing shows that PEX is pretty transparent to radiant heat so it loses heat that way rather than by the metal absorbing the heat and then re-emitting it to the environment. So in terms of how long a pipe will keep water hot after the flow is stopped PEX is not a winner, but as far as how much heat will be lost to the pipe as hot water is flowing to the fixture the PEX still wins. But shiny new copper holds heat better than old brown oxidized copper because the shiny quality makes it a bad thermal emitter. PEX or copper, both need to be insulated, CPVC? not on my watch thank you very much.

    So how much of an elbow does it take to cause turbulence in a pipe?

    This is my moment of family pride. My great-grandfather John Ripley Freeman in his book “Experiments upon the Flow of Water in Pipes and Pipe Fittings made at Nashua, New Hampshire, June 28 to October 22, 1892” (catchy title) answered that “little would be gained by making the radius of curvature more than about three times the corresponding pipe diameter” so, for a ½” pipe, turbulence becomes a factor when the radius of curvature is less than 1 ½” for ¾” pipe the turbulence is encountered when less than 2 ¼” so those big copper sweep elbows are a good thing coming out of a water heater if you have to get horizontal with your manifold. He also did a bunch of research on closely spaced Tees (AKA manifolds) but I can’t find that page in my copy of the book here. Leather bound, antique, smells oh so good sitting on my desk.

11. 
    wjrobinson | Jan 29, 2011 06:18am | #11

    Interesting post Michael. They now sell pex that is preinsulated. In the past I have had my pex runs shot with sprayfoam along with drain pipe. The drain pipe is shot with foam to reduce noise.

12. 
    draginfly58 | Jan 29, 2011 10:59am | #12

    I never realized how complicated something as simple as plumbing could be.

13. 
    Michael Chandler | Jan 29, 2011 01:50pm | #13

    The plumbing code is like dungeons and dragons in 3-D