Need Help Getting Enough Hot Water to New Bathroom

Discussion in 'Water Heater Forum, Tanks' started by stinkpaw, Apr 19, 2010.

  1. stinkpaw

    stinkpaw New Member

    Messages:
    8
    Location:
    NY
    I’m remodeling my master bathroom and am having trouble deciding on how to provide enough hot water. The bathroom specs:

    - 150 gallon Jacuzzi tub.

    - Custom shower with multiple body sprays and showers. If I turn everything on, shower would output 10 GPMs, although my normal operation normally would use 5-7.5 GPMs.

    - I live alone so don’t have to worry about other people taking showers etc…

    I’ve had many different suggestions on how to supply hot water to this beast: two tankless heaters, large storage tank with boiler, two large hot water heaters.

    I have a 405 gas meter, a ~175 BTU furnace, 75 BTU garage heater, and 75 BTU boiler heating an in-law apartment above the garage.

    I’ve been doing research on the internet and think getting two 75-gal heaters with a good recovery rate (although keeping the BTUs reasonable to as not to run into a gas supply issue) is the way to go. I live up north so assume a 90 degree rise to heat the water.

    This is what I came up with:

    AO Smith GPS-75 Power Vent Heater. 80k BTU. 86 GPH refresh and 90 degree rise. If I did the math correctly I should be able to take a 20 min shower running 10 GPMs.

    http://www.hotwater.com/lit/spec/res_gas/AOSRG45600.pdf

    My questions:

    - Is this a good solution or is the boiler/storage tank a better way to go? If so what would you recommend?

    - Reading this forum people seem to be down on AO Smith. What tank & model do you recommended?

    - Should the tanks be in series or parallel? I read something on here about potential problems if you put them in parallel but didnt understand.

    I also dont understand how series would work because once you empty tank #1 wont you keep pulling cold water from that tank as it tries to refill?

    - Can someone explain (or provide a link) the different venting options and the pros/cons? I don’t know why one would pick one type of vent over the other.

    Thanks so much for your help.
  2. Redwood

    Redwood Master Plumber

    Messages:
    7,450
    Location:
    Connecticut
    Sounds like your taking their shower for them... LOL
    Al Gore will be stopping by to talk about your water usage...
  3. stinkpaw

    stinkpaw New Member

    Messages:
    8
    Location:
    NY
    C'mon, I can't be the only person that has a car wash for a shower... :)
  4. Redwood

    Redwood Master Plumber

    Messages:
    7,450
    Location:
    Connecticut
    I guess you have your options pretty well figured out...
    You either need a couple of huge tanks and a timer to tell you when to get out of your car wash...
    Or, some tankless units that will meet your demand...
  5. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    [​IMG]

    Drainwater heat recovery will extend the capacity for the car-wash shower without bumping the burner size, and your recovery time on the tank(s) will be much shorter (not to mention it'll cut the fuel use dramatically for that gusher.)

    Natural Resources Canada maintains a list of 3rd-party testing performance for different models/vendors, http://oee.nrcan.gc.ca/residential/p...ain.cfm?attr=4. In general, the fatter & longer they are, the higher the performance, and it's cost effective to put in the biggest one that fits.

    EFI is the US distributor for PowerPipe, and are pretty easy to deal with over the phone, will open an account for you over the phone with a credit card, and ship promptly with no markup on shipping. Other vendors sometimes sell direct, others not, but don't expect to find this type of thing at the local plumbing supply house (except in Canada.)

    At a 2.5gpm shower flow, a 50%+ drainwater heat recovery unit is like having an extra ~35KBTU of burner in the system during showers, and as the flow increases so does the return, but this "burner" doesn't use any fuel. At higher flow rates than the tested 2.5gpm the recovery efficiency falls off a bit, but at 10gpm a 50% unit will still be delivering at least 40%, if I remember the curves correctly. This means you'd be looking at something like 80-100KBTU/ return from the heat exchanger when in gusher mode, which is more heat output than the burner on a second 75 gallon tank (!). With this kind of monster shower it'll pay for itself in under 3 years no matter what your natural gas rates are if you're running the shower even 10 minutes/day.

    This sort of system doesn't buy you anything for filling the jacuzzi though- you still need enough storage to fill it.

    Boilers with indirect-fired tanks have much lower standby losses than standalone tanks, since they don't have a center-flue heat exchanger sucking heat out when the burner is active. Since you already have the 75K boiler, using it as the burner and zoning a larger indirect as "priority" is going to be a better bet, and will improve the annual AFUE of the boiler by giving it a big load & thermal mass to work with.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,537
    Location:
    Cave Creek, Arizona
    Unless that "heat exchanger has multiple small passages in it the ONLY water transferring heat will be that which is in laminar flow on the inside of the pipe, which will provide very little boose. IF it does have multiple passages then there had better be some provision for removing it to clean the hair, soap scum, etc, out of it. But those same materials will coat the interior of a full size pipe also diminishing its heat transfer capabilities. IT is a fancy way to waste money. A series heater WILL deplete the first tank, but that has nothing to do with the amount of hot water you can use. It is still receiving heat from the burner and the second heater's burner is ALSO heating the water. In a parallel configuration, when the heaters begin to run out of water, only ONE burner is heating any given amount of water flowing through the system. AND you have to have a LOT of faith, that the water will flow through BOTH heaters equally, or one will "run out" and be distributing cold water even though the other one is still supplying hot water, which will give you something between tepid and warm water.
  7. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    hj: Drainwater heat exchangers rely on the water forming a thin film over the interior of the big-bore copper drain pipe (which it does), not "multiple small passages". Getting them fairly close to vertical while installing is important, but the surface-tension of the water film gives it some tolerance. The thinness of the films makes the inner surface of the film not-so-thermallly isolated from the outer, and there is some rollover turbulence in the downward flow, not a pure laminar flow due to the difference in friction at the copper interface vs. the air/gases in the center of the drain.

    The potable flow piping for the bigger units is coiled wraps of squared-off copper piping (to maximize surface area & flow). The inherent turbulence of water flowing through a helix eliminates laminar flows on the potable side, just as they do in many other helical heat exchangers. The bigger better ones use multiple coils to minimize head loss.

    There is something close to a 30 year history on these since the initial versions of gravity-film heat exchangers were patented. While there is a small degradation of performance over time, they don't succumb to crud nearly as quickly as you seem to think. The estimated lifecyle (defined as degradation to 75% of original performance) is on the order of 50 years, but even if it's 1/4 of that, it's cost-effective.

    Some of the recent Canadian studies behind the subsidy-policy initiative can be found here. They are full counterflow heat exchangers, and demonstrably work. The US DOE has tested them (less extensively than the Canadians), and have concluded they're cost-effective within 2.5-7 years based on utility costs & volume of usage. (I doubt the DOE studies looked at 10gpm 20 minute showers though! :) )

    Note: To max out the efficiency of the heat exchanger, it has to feed both the water heater and the cold side of the shower, not just a series-configuration as in the WaterCycle graphic in my prior post. This provides more flow=higher rate of heat exchange at the heat exchanger, but lower flow at the tank (since you're mixing in substantially less hot water to get the right temp at the showerhead when your incoming cold is north of 70F rather than 40F.)

    [​IMG]

    Whether you use one tank or two, in series or parallel, the payback on DWHR is real- far quicker for the car-wash type showers we're talking here than 1.5gpm 5 minutes/day water-sippers. And it extends showering time considerably, while shortening tank recovery time. (There are Canadian studies that measured those aspects out there online too, if you need me to dig up the numbers on 'em.)

    Situations like 150gallon jacuzzis scream "tankless" at me, but if you're maxed out at the meter it's better to impress one of the other burners into service, which is why impressing the 75K boiler into service with a big indirect is a better solution than multiple standalone tanks. Maintaining it at 180F and insulating the hell out of a ~75gallon indirect (and the plumbing) will buy you something, maybe enough if you had a 150K boiler behind it, but a behemoth like the ErgoMax E109 or TurboMax 109 would have sufficient thermal storage capacity for a complete tub-fill at 140F. If used as a buffer for the heating system the fact that the boiler is likely 5x oversized for the space heating load of the-inlaw apartment goes away, and it wouldn't need to be separately zoned to share both loads simultaneously. (Baseboard outputs drop pretty fast below 140F, but you can be happy with tub-fills all the way down to ~110F.) I suspect that as-used the AFUE of the in-law boiler is ~60%, with every cycle pretty much a short-cycle. With a '109's ~120gallons of boiler water to work with it CAN'T short-cycle, and it's AFUE will be pretty close to it's steady-state thermal efficiency. They're not cheap, but neither is a pair of 75 gallon tanks either.
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