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Thread: Tankless in parallel

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  1. #1
    DIY Junior Member cruiser56's Avatar
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    Default Tankless in parallel

    I have an electric tankless that works great, until now. I remodeled the master bath and put in an amazing shower. The problem is I don't have enough flow from WH to take advantage of it. I talked to the manufacturer and was told if I install another WH in parallel I will add the flow of the new. So if my old is say a 4gpm and I add another like it I will have 8gpm. But I don't have a power plant in my backyard only 200amp service. So can I install a gas tankless say about 8gpm in parallel to my electric will I achieve 12GPM. I really need help my wife is irate!!!!
    Thanks

  2. #2
    DIY Junior Member TheLex's Avatar
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    I'm no plumbing expert but I am facing a similar situation right now. We are remodeling a 4300 sq ft single story rancher. Our temps get up to 110F in the summer and down to 25F in the winter.

    All water heaters (WH's) do the same thing - they take the incoming ground temperature water and make it hot, usually up to 110F or so. You live in Wisconsin. I imagine it must get to below freezing there in the winter. Let's say in the winter the water gets down to 40F. If you want to heat that water up to 110F, that's a 70F rise in temp.

    If you look at gas tankless WH's, they are rated at gallons per minute (GPM) at a certain temp rise. Take the Bosch C1050 ES NG for example. It's rated for 10.1 GPM. Sounds great right? But wait....that 10.1 GPM is for a 35 degree rise. So if you want 110F hot water, this unit will give you 10.1 GPM if the ground temp water is 75F. Is your ground water 75F during the coldest part of winter?

    My point is these tankless units will put out less and less flow as the groundwater temp drops and the unit is asked to heat the water from a lower temp. This Bosch unit is rated to deliver 3.9 GPM at a 90 degree rise. All of a sudden it doesn't look that great does it? And this unit costs almost $2k!

    My master bath shower has 3 shower heads that each flow 2.5 GPM for a total of 7.5 GPM if they are all on. That means this Bosch unit will not deliver enough water on a very cold winter day here. Two units hooked together would barely cover it. And what happens if one of my kids runs their shower or my someone turns on the dishwasher. Uh oh.

    So what I've decided to do is go cheap. I'm getting two Bradford White tank WH's. These bad boys are 75 gal each. So I'm going to have 150 gallons of HOT water right off the bat in reserve. And each unit can produce 82 GPH of hot water at a 90 degree rise. That's 164 GPH for the two of them, AFTER I've exhausted the 150 gallons in the tank. Somehow I doubt I'll ever run out of hot water.

    BTW, the total cost for the two WH's is less than $2k. The tankless system would have cost me almost $5k.

    Hope this helps.

  3. #3
    DIY Member Soapm's Avatar
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    Quote Originally Posted by TheLex View Post
    My point is these tankless units will put out less and less flow as the groundwater temp drops and the unit is asked to heat the water from a lower temp. This Bosch unit is rated to deliver 3.9 GPM at a 90 degree rise. All of a sudden it doesn't look that great does it? And this unit costs almost $2k!.
    90 degree rise to 110??? That means the input water is 20 degrees. Wouldn't it freeze at that temp?

    Ideally, set the WH temp to the temp you like your shower and you'll get max flow rate. Luckily, ours has a remote controller in the restroom so we can set it before we turn it on and I've never been starved for flow living in Denver where we just had a below zero spell. But then our frost line is pretty deep so maybe that helps from having 20 degree source water temp.

  4. #4
    In the trades Dana's Avatar
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    In a gusher-shower you can roughly double the showering performance with a drainwater heat recovery heat exchanger, provided you have sufficient headroom on a vertical drain downstream of the shower for 4" x 48" tall (or larger) unit:



    (Best of all, it doesn't increase your power use, or burn any gas.)

    A gas-fired unit in parallel with an electric unit is difficult to control. The major high-end manufacturers of gas tankless heaters have special controllers to be able to parallel them with good results, but it's damn-near impossible to make heterogenous tankless systems "play nice" together, and would likely require a custom set of controls and a man-year of engineering time to really get it down.

    With cold-water only (so that the water heater isn't intentionally restricting it, bucket-measure the true gpm it's drawing- unless you have unusually high water pressure you're unlikely to hit the gpm-ratings of the shower heads, but it'll usually be more than half the rated flow. Shower head rating are a max-number, rated at 80lbs pressure. Most homes have water pressure half that or less, and typical 2.5gpm shower head it typically running in the 1.6-1.8gpm range, or about 2/3 of the rated flow. If your true flow is in the 8 gpm you can probably get there with a 4" x 48" to 4" x 60" drainwater heat exchanger.

    The manufacturer will sell direct at a retail type pricing- if the US distributors & retailers don't stock the larger sizes. Home Depot carries some, and can be shipped to a store near you, but I don't see any thing bigger than a 4 x 48" or 3" x 72" listed. EFI's warehouse is in WI- you can open an account with them over the phone with a cc# to get their wholesale price on whichever models they are carrying.

    There are others in the biz, but you need something with an efficiency better than 50% as tested per Natural Resources Canada, but that also doesn't unduly restrict flow. Something in the 60% range like a 4 x 60" to 4 x 72" would be better, since they're tested at 2.5gpm, and you'll be running more that 2x that flow. A 60% @ 2.5gpm unit would still be delivering over 50% @ 10 gpm.

    If that doesn't cut it, a monster-sized tank is your best bet.

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    DIY Senior Member lifespeed's Avatar
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    You're better off with two gas heaters in parallel. They have greater capacity and one can properly control the other.
    Lifespeed

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    In the trades Dana's Avatar
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    Quote Originally Posted by lifespeed View Post
    You're better off with two gas heaters in parallel. They have greater capacity and one can properly control the other.

    That's definitely true!

    But it's about a $6-8K proposition for the installed pair plus upgrading the gas plumbing to the units to be able handle the ~400KBTU/hr load, and will also likely require upgrading the gas service to the house at some additional expense (which could be cheap, but not always.)

    Coupled with a drainwater heat recovery heat exchanger that can be reduced to one gas-fired tankless, if the electric tankless doesn't quite cut it even with the DWHR unit installed. The home's gas plumbing would probably still have to be upgraded, but probably not the meter or service drop.

    If this house is heated with a gas-fired hydronic boiler (pumped hot water), you may be able to get there with an indirect tank running off the boiler for less cost and less flow restriction than a gas tankless. With an indirect the issue isn't peak flow, but capacity- how long can you shower before depleting the tank, which is a function of the boiler-output, tank temp, and tank volume. Here too hot water performance is dramatically boosted with drainwater heat exchange.

    Payback on fuel savings for drainwater heat exchangers is nearly-never (OK, maybe a decade) for households with hot-water sipping showers (or tub bathers where the return is exactly nil). payback is pretty quick on 10 gpm gusher showers (and INSTANT if it means it keeps the missus happy! :-) ) Since it reduces the size & cost of the mechanicals necessary to support those high flow showers, the payback can easily up-front, as in the paired gas-fired tankless option.

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    DIY Member MikeQ's Avatar
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    Quote Originally Posted by Dana View Post
    Payback on fuel savings for drainwater heat exchangers is nearly-never (OK, maybe a decade) for households with hot-water sipping showers
    Since when is a 10 year payback considered a long time?

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    In the trades Dana's Avatar
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    Quote Originally Posted by MikeQ View Post
    Since when is a 10 year payback considered a long time?
    Behavioral economists have measured homeowner purchasing habits for buying into energy efficiency, and determined that the subconscious discount rates in the net-present-value of most is WAY into double-digits- it's a tough sell getting them to buy in even with a 3-year payback (or even 3 months for a large fraction.) That's why most single family homes in the US have gobs of low-hanging fruit on the air-sealing & insulation of the building envelopes, and why the market penetration of high-efficiency long-lifecycle lighting like LED have been pretty tepid prior to efficiency mandates (love 'em or hate 'em.) With high efficiency lighting the investment and dollar savings is small- too small for many to even care, but the internal rate of return is HUGE. (I could have retired decades ago if my 401K did as well as a twisty compact fluorescent! :-) )

    With drainwater heat recovery the investment is large enough to notice, but with a 50% discount rate purchasing-habit on all things efficiency related 10 years might as well be "forever". It ain't pretty enough to be considered an aesthetic improvement to the house, and home buyers aren't going to be showering during their home inspection tours- limitations of hot water delivery can be pretty hard to spot for those without understanding it. That's how (as in this case) luxury 10 gpm gusher-showers get installed in houses served by 4 gpm tankless hot water heaters, and how luxury spa tubs get installed in houses served by a 40 gallon tank. The average person doesn't bother (or can't) do the math on the hot water delivery systems. It's easy to see the shower or spa, but the mechanicals needed to support them are out of sight, and too often out of mind.

    In a country where the average duration of residency is 6-7 years, something with a 10-year payout on energy use savings and doesn't change the market valuation doesn't have a huge present-value. Spending the same ~$1K you might spend on a drainwater heat exchanger on spackle & paint just prior to selling has a much higher R.O.I., and a much shorter time frame than a decade, something very easy for the home owner to grasp. But when stuck with a gusher shower that you can't actually use, hot water delivery performance trumps any marginal fuel savings issues. Drainwater heat recovery is pretty cheap performance compared to a second gas-fired tankless.

    Whether it'll be enough to get this electric tankless up to snuff is still TBD- it'll be a squeaker if it does, but if it means replacing the electric with just one gas-fired tankless instead of two, it's the cheaper way out. It's a no-lose situation to try just the DWHR approach first to see if the cost of even the first gas fired tankless can be avoided.

  9. #9
    DIY Senior Member Reach4's Avatar
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    Quote Originally Posted by lifespeed View Post
    You're better off with two gas heaters in parallel. They have greater capacity and one can properly control the other.
    How does that one controlling the other work?

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