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Thread: Two HW heaters or higher temp w/mixer?

  1. #1
    Engineer garyl53's Avatar
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    Default Two HW heaters or higher temp w/mixer?

    Hi,
    We may need to increase our HW capacity (50 gal now). Is it more energy efficient to add a second HW heater or to increase the temperature and add a mixer valve to the current heater?
    Any advice appreciated!
    Gary

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    Plumber patrick88's Avatar
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    We need to know how old is the water heater, make and model, and brabd would be help full. Is it power vented or not, and any other type of info you could give would be good also.

    I'm just starting to work with an old friend of mine to bring solar electric and hot water systems, wind turbines, Flex Fuel Boilers, batteries, hydroponic gardening, books, pellet grills and more. Also the parts for DIY installation.

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    Plumbing Designer FloridaOrange's Avatar
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    Layout of your house and how many fixtures would help as well.
    Matt
    Semi-professional plumbing designer
    Enjoying life in SW Florida

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    Engineer garyl53's Avatar
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    Sorry for the delay in getting the information.

    HWH -
    AO Smith Model #FSGT50
    48 gal. capacity
    64.6 gal/hr rating
    gas fuel/ 60,000 btu/hr input/ high altitude orifice
    It is not power vented as far as I am aware.
    The unit is about 8 years old.

    The house is 2700 sf (not too big) and has 4 bathrooms (containing 2 tubs(both 5.5ft long and deep), 3 showers, 4 sinks). Three of the baths are stacked over each other with the master BR bath offset about 20 feet from the others. There is also a kitchen sink, dishwasher and washing machine.
    There has been no problem, in general, at anytime with hot water for showers, sinks, dishwasher, and washing machine. We have only noticed the problem when trying to fill one of the tubs. Generally you run out of hot water before you can fill the tubs. The settings on the HWH are Hot, A, B, C, and Very Hot. I have it set to A so no one gets burned.

    Thanks for any recommendations you can give me.
    Gary
    Last edited by garyl53; 12-25-2007 at 12:58 PM.

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    DIY Senior Member construct30's Avatar
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    My Opinion: You could turn it up and buy new valves for the tubs, they have built in scald guards. In a small house I would not add a second tank, but 50 gallons should be big enough for a regular single family in a small house with a gas water tank, it is turned down too low check it with a thermometer. That is the reason for the new valves.

    You do have a lot of baths for the tank, if you use the heck out of them. Is the tub in question a big whirlpool or soaker? It might need it's own tank.

    In theory I have read that you can turn up your one tank, add a flow check on the hot side, then install a mixer valve to temper the water that is too hot at the out going line. No need for the second tank to temper the water. I've never done it and would check with the local inspector first.
    Last edited by construct30; 12-25-2007 at 01:15 PM.

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    Engineer garyl53's Avatar
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    Hi,
    I actually just finish installing a new valve in the bathroom that the children use so I could adjust the scald guard. I couldn't do that with the old valve. The hot water from the faucets might still be too hot though. If I can add a mixer valve and turn the heater to a hotter setting will that allow me to keep the faucets from getting too hot but give me more hot water for the tubs?
    Thanks,
    Gary

    PS: What is a good temperature to set the hot water? That is, not too hot to scald but hot enough.
    Last edited by garyl53; 12-25-2007 at 01:22 PM.

  7. #7

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    depending on the condition of the old heater you might be able to get away with adding a mixing valve. Turn the heater upto 140 and use the mixing valve to bring it down to 120. This should add some "capacity" to the heater without the fear of being scalded. Be sure you plumber installs a thermometer and check valves in the appropriate places.

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    In the Trades Bob NH's Avatar
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    It isn't the number of tubs that you have, it is how much water you use.

    Things are worse in the winter, especially if you are supplied with surface water which can be about 33 F.

    Two tanks in series will more than double the available hot water because there is less mixing of the cold at the inlet of the first to the outlet of the last. The first one will do most of the heating.

    Because the problem is only occasional, you could put in an electric water heater as the second heater in series. The logic is that it will be supplied with hot water from the gas heater in most cases so it will do little heating, it will lose less heat than a gas heater because there is no convection up the flue, but it will provide additional water when it is occasionally required.

    The electric will cost less to buy and install than a second gas heater. You will need to run a 30 Amp circuit for it.

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    DIY Senior Member construct30's Avatar
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    Two tanks is a good way to go, however it will cost you. Running and maintaining two tanks is not cheap. The second one will run cheaper, but you still have to keep both working.

    How hot is your water running now? I'm guessing on the lowest setting aroung 100-110 F. Jacking up the one tank will cost you too, so take that into consideration. In most areas the cheapest way to heat water is with gas. The electric is close to 100% efficient, and the gas is only around 80%, but to heat a gallon of water one degree is still cheaper with gas, in most areas.

    The cheaper way to go is the mixer valve, but the two tanks is sure to work. If you go the mixer valve, just be sure the plumber installs the back flow preventers between the tank and the mixer valve and the cold line and the mixer valve. The between the cold line and mixer may not be completely necessary with the right mixer valve, but I would do it just to make sure no hot water get back into the cold side.

    If it were mine I would try kicking up the tank temp and adding the mixer valve, but it may not work, I have never done it. It is a theory and it should work, but not sure. The two tanks are a sure thing, I would go gas though. If you go the experimental route get back and let us know if it works.

  10. #10
    Moderator & Master Plumber hj's Avatar
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    Default water

    At the "A" setting, the water temperature is such that you probably use all hot water with little if any cold water mixed in. As the temperature rises you start adding cold water, which reduces the amount of hot water needed. Anything that increases the temperature will give you more "capacity", while a second tank will give you the capacity at the lower temperature while doubling the recovery rate of a single tank. A second tank may require a larger gas line and a second flue through the roof.

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    DIY Senior Member Fubar411's Avatar
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    I'm new and I'll admit I don't know much about plumbing, but here's my 2c.

    This could be an excellent application for a lower-end electric tankless heater. As they only raise the temperature of the incoming water, and this is mostly a winter only problem, you could give your 50 gal enough cushion to fill a tub. I'd plumb it so you'd be able to completely drain it and turn it off during the summer months. They make 30amp double pole switches for turning off a wh.

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    DIY Senior Member construct30's Avatar
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    Quote Originally Posted by Fubar411 View Post
    I'm new and I'll admit I don't know much about plumbing, but here's my 2c.

    This could be an excellent application for a lower-end electric tankless heater. As they only raise the temperature of the incoming water, and this is mostly a winter only problem, you could give your 50 gal enough cushion to fill a tub. I'd plumb it so you'd be able to completely drain it and turn it off during the summer months. They make 30amp double pole switches for turning off a wh.
    I would not be a big fan of that, even with draining the tank it would go bad just from not being used. Many of the LESS EXPENSIVE point of use water heaters are junk. Appliances seem to last longer when they are being used a little once and a while. Electric is an expensive way to heat water.

    If you cannot add the second gas water heater, due to the venting requirement you could go direct vent, if getting enough gas is a problem it could be a deal breaker.

    I think turning up the single tank and adding a tempering, mixer valve is looking to be the better, less expensive option. If it is a winter problem it should work.

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    WhereI live a temnpering vaLVE IS REQUIRED. i HEAT THE WATERT WITH A BOILDRE TO 140 AND TEMNPERT IT TO 120.sORRY, i"m typing this on a train to Boston on a cellphone...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the Trades Bob NH's Avatar
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    If only an occasional problem, the second stage electric would not be all that expensive. It is a maintained storage system with almost all of the heating taking place in the first gas-fired unit. It doesn't use much power until you need it to fill both tubs. If you set the temperature a little lower than the gas heater setting then it will only come on when required to maintain temperature or the gas unit can't keep up with demand.

    The electric requires no vent, and you can add insulation with an external blanket. It is there for what you need; extra hot water when you are filling tubs.

    An instant electric is virtually useless and would use more electricity. Instant heaters must be supplied with cold water and so would be on the incoming line where it would operate whenever you run water so would use electricity at those times. The low end units (say a 30 amp capacity) would have very low delivery capability and would cause you to use electricity rather than less expensive gas.

  15. #15
    DIY Senior Member construct30's Avatar
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    A tempering valve should only be required if for some reason the water at a tub or shower cannot be set to the required temp. Setting the hot water tank to a lower temp does that or having new faucets does. It is not a requirement for vanity sink faucets yet.

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