Multiple Water Heaters Anyone??

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by John Aull, Apr 8, 2005.

  1. John Aull

    John Aull New Member

    Messages:
    6
    I am finally remodeling my master bath. I've installed a 100 gal. tub along with a separate shower with multiple heads. The house is 3200 sq. ft. with 3.5 baths. I have currently been using a 65 gal. gas hot water heater. Due to the size of the tub and the fact my house is rented twice a year with 5+ people showering every morning I am opting for adding an additional 40 gal. gas water heater instead of replacement. My idea is that I'll have more usable hot water and should have a faster recovery time.

    My question is this: Should I install them in a series (cold coming into tank A, hot leaving tank A and going into cold side of tank B and then hot leaving tank B to the rest of the house)? OR, install them parallel I draw from both tanks simultaneously? If installed in a series, should tank A be set hotter, colder or the same as tank B to get the most effiecent use? Due to the configuration of the house, my only option is to locate H2O heaters in the basement - I can't place one strategically close to a demand point.

    Thanks for any advice you can offer. Regards, John
    Last edited: Apr 8, 2005
  2. etg

    etg New Member

    Messages:
    9
    Interesting question with multiple answers. Probably not what you wanted to hear.

    Heater in parallel have specific plumbing issues which need to be designed in from the start. The supply lines and discharge lines (up to the hot water main supply line) absolutely need to be hydraulically the same. Otherwise one unit will work harder and more often because it is carrying the load.

    In series, the first heater is just a pre-heater that takes the water up 20 to 40 degrees with the second heater making up the remaining temperature. Many of these units will also have constant recirculation (on larger installs) and you will find that is somewhat cheap to run this way.

    Don't confuse no cost or little cost with major cost...you still need to heat 80 to 120 gallons of water and keep it hot. It just costs less on large applications of central water heater banks (with tempering tanks) in condos or apartment buildings.

    Something else for you to consider is that units are usually placed side by side with common venting also considered. You will need to look at your current vent sizing and location.

    Have fun and ask if more questions.
  3. go big or stay home

    Series is the only way to be sure you will literally get the total amount of hot water you think youi should. You will never get the proper draw off two different size water heaters in parellell....cant be done without making it into a mechanical nightmare.


    I would get a 50 just for fun and use it as the pre heater. Set it on med and use the 65 for the main boy.

    as long as youi got room in your chimmney for all this it should fly just fine.

    thats a lot of hot water by the way.

    you will also want to install a couple of thermal exp tanks too..


    series is the only way to do this. and expect it to work right.
  4. hj

    hj Moderator & Master Plumber Staff Member

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    27,247
    Location:
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    wh

    Connect them in series and set both to the same temperature.
  5. thats debateable too

    thats a debateable subject , setting them both at the same temp.

    Honestly , of course it woud work just fine.,

    it just depends on which one you would rather let do all the work.

    the second one in line would not even come on utill the first one was totally
    spent and then the second one would really only raise the temp slightly to get it up to even with the first.

    If you get a 50 and have a 65 already, I feel I would rather have one as a pre heater and left on med or low, and let 65 gallon the outlet one raise it up to the high temp,

    mostly more for gas saveings and cost of keeping them both left on high.


    but of course if you want a 115 gallons of hot water ,
    you probably got 2 Hummers sitting out in the drive way and money is really not an issue. (just kidding)

    The yearly cost saveings of stagggerig the temps on both water heaters just might fill up one of the hummers tanks with gas.

    just remember the expansion tanks.
  6. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    settings

    The only savings from setting one lower is that it will lose its latent heat a bit slower so it would not have to reheat the tank as often. But the difference would be so small that it would only put gas in a "hybird" car for a few days. You still have to heat every gallon of hot water you use whether it is coming from one tank or two.
  7. jrejre

    jrejre New Member

    Messages:
    61
    Location:
    Minnesota
    Series. I'm not a plumber but a DIY homeowner. I have several neighbors that have this setup and it works well. They normally have the first unit (the one closest to the cold water supply set lower than the "final" unit. This does pre-heat the water for the second unit. During periods of very high demand (e.g., lots of company in the house), they set them both to the same setting.

    The issue of which unit "is working harder" is accurate, but a bit silly in my book because both of them are work no harder (and generally less hard) than a normal single unit setup.

    The other option would be to go for a very high recovery, commercial unit. I have looked at the AO Smith unit - Cyclone XHE. See http://www.hotwater.com/frame.html?topage=/commercial/comhome.htm

    I have not had one installed. It's very efficient and can put out a lot of hot water in a single tank. However, the unit cost was about $3,600. The concept is something else to consider.

    Good luck and let us know what you decide to do and how it works out.
  8. King3244

    King3244 New Member

    Messages:
    83
    Location:
    Kelowna, BC
    John,

    I have just what you are talking about. Two 40 gallon in series, the first one set at about 90 degrees and the second one set at what ever temp. you want for final temperature.

    I have this set up in my own home as well as two others in separate rental units. Had two teenage sons at home and never ran out of hot water and never had any complaints from the tenants either. The one rental place had six occupants and no problems.

    Have you thought about putting a tankless water heater behind your original tank? Seems to me that if you preheated the water to the tankless that you would have lots of hot water.

    I know that a lot of people on this site are anti-tankless but give it some thought. I am in the process of swithching to tankless in my own home.
  9. John Aull

    John Aull New Member

    Messages:
    6
    The Decision

    Hey Folks -

    OK, here's what I ended up doing. I went ahead with the second water heater. Have a 65 gallon, added a 50 in series.

    During heavy demand (a lot of people renting the house), I set both heaters to approx. the same temperature. Once they leave, I cut the first one down by half as a preheater.

    During this last furniture market, had 5 people staying here all showering in the AM - not one complaint about hot water. In fact, I was the last to shower and there was more than enough.

    Thanks to everyone for your advice. Having it to do over, I might have installed a tankless in front of my 65 and would have more than likely had enough. But with several showers running at once, the tankless would have been useless (affordable tankless usually only handle 5-8 gal./min. right?

    Thanks again.

    John
  10. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    tankless

    As they approach their maximum GPM, their "resistance" increases dramatically which means the dynamic pressure drops drastically. When that happens, your pressure balancing valves will reduce the cold water pressure to equalize it and your shower flow could drop off greatly.
  11. Sparky2

    Sparky2 New Member

    Messages:
    2
    Location:
    Clarkston Michigan
    Also thinking of adding multiple heaters

    My wife is really ticked off. "I want this hot water issue resolved by the end of the week!!!!" after another cold shower.

    We have a 50 gallon POWER VENT natural gas hot water heater that is on its last leg. We have a family of 5 (3 boys) and she wants more hot water. I looked at a 75 gallon but it has to be a power vent. The cost is also pretty high for such a "rare" unit.

    I want to install two 50 gallons side by side. I have done the calculations and have enough Natural gas line to run an additional hot water heater.

    Now that I have read your forum I have decided to run them in series. My question is can I power vent them both through the same 3" pvc vent? The blowers don't have a CFM rating on them so I can't calcluate the volume of air being pushed through. The outlet of the blower is only about 1 inch in diameter going into a 3 inch pipe. I would think the volume of air would not be an issue. If I installed a flapper valve (if they make such an item) on both units to prevent back flow of fumes it seems like I could get away with it.

    I'm a DIY with a degree in HVAC so I know about the issues involved but rusty (pardon the punn) on some of the codes especially in a situation like this. I like the idea of have two tanks and getting double the capcity for virtually the same money.

    Also what is the reason for the expansion tanks? Do I really need them for pottable water systems? I thought they were only used for boiler heating system.

    Any advice? :rolleyes:
    Last edited: Mar 25, 2006
  12. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
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    Location:
    Cave Creek, Arizona
    heaters

    The heaters normally would not be able to connect to a common pipe because of back pressure issues, and a "check valve" in the flues could create an unsafe condition. As far as the temperature settings of the two heaters, the second one will ALWAYS come on at the point where the tank's temperature reaches its setting, regardless of the incoming water's temperature. A low setting on the first heater will cool the second one down faster, and a high temperature will keep the second one hot longer. I woudl always set the equally, so that by the time the second one kicks on, you would have received about the combined gallonage of both heaters.
  13. Sparky2

    Sparky2 New Member

    Messages:
    2
    Location:
    Clarkston Michigan
    multiple heater venting

    What if I plumb both vents to the one line and then put the blower in that line and wire it in paralel so no matter which heater is on the blower will always turn on.

    But then again it would probably be easier to knock another hole in the wall and add another vent.
  14. Cass

    Cass Plumber

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    5,980
    Location:
    Ohio
    No,
    Run 2 vents or put a 50 gal. electric in. feeding the gas WH.
  15. Bob NH

    Bob NH In the Trades

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    Location:
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    "Also what is the reason for the expansion tanks? Do I really need them for potable water systems? I thought they were only used for boiler heating system."

    When you put cold water in your water heater, then heat it, the volume increases. It is most significant if you run a lot of hot water at once. The increased volume must have some place to go.

    It will push water back into your cold water system or leak out of the relief valve if it has no place to go. You could find hot or warm water where you expect cold.

    You only need the expansion tank if your system is sealed, such as with a check valve or regulator on the supply. You don't need it if you have a pump system with a hydropneumatic tank. It is useless on a municipal system with the ability to accept reverse flow because it has nearly constant pressure, which will cause the expansion tank to have a constant volume of water.
  16. Cass

    Cass Plumber

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    5,980
    Location:
    Ohio
    ..................
  17. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,247
    Location:
    Cave Creek, Arizona
    cass

    .......................?
  18. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    "No, Run 2 vents or put a 50 gal. electric in. feeding the gas WH."

    If I had to put in an electric heater because of venting issues, I would put the electric AFTER the gas, with the gas heater temperature set HIGHER than the electric. I might set the gas heater at 160 F and the electric at 130. That way the lower cost gas is doing as much heating as possible and the electric is there for storage, while the electric heater maintains temperature.
  19. Terry

    Terry Administrator Staff Member

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    ...............I didn't hear that one either.
  20. HGM

    HGM New Member

    Messages:
    1
    Hello Everyone,

    This is my first post after lurking for about a year on and off. Thanks for the help now and in the future. I'm not a pro, but I do have some practical experience, having run a small 18 unit motel.

    We had an 80 gal fast recovery tank that was sufficient most of the time. A friend had a nearly new 40 gal tank freeby, which was put first in line, since then no problems... If it works for me, I am sure it will work for a house.

    Some things that will make this good project more beneficial. These are relatively easy and inexpensive...

    (1) When plumbing the new tank, put in a bypass with valves so each tank can be taken off line seperatly and serviced without shutting off the hot water. You still have hot water if one goes out, and getting a repair done is not a severe crisis.

    (2) While the work is being done, put in a water filter ( I use 5 micron ) on the cold inlet ( washing machine also ). They only run about $20, your whites will be whiter, and my hunch is your water heater will last longer without so much crap in the bottom of the tank (we all know they don't get flushed as often as they should ).

    (3) Make sure the water is contained and has somewhere to go when a leak happens ( washing machine also ). The water damage and cleanup can cost you as much or more than a water heater and is a real pain that can take a week or more to go away. Something as simple as a water barrier and hose can save hundreds of dollars.

    my 2 cents, hope someone finds it usefull.

    - Dave
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