Could a 50k btu boiler fire an indirect WH for 3 apts?

Discussion in 'Water Heater Forum, Tanks' started by gellfex, Mar 8, 2011.

  1. gellfex

    gellfex New Member

    Messages:
    56
    Location:
    NJ
    I ended up removing an 8 year old Mini Therm 50 boiler from an apt where it was heating and firing a 36 gal indirect. I'm thinking that in another 3 family house that currently has a regular 75 gal WH, I could replace it when it goes by buying an indirect for not much more than a replacement 75, power it with the boiler, and save gas with a more efficient system. Is this boiler simply too underpowered for the idea? I looked at 75 gal WH BTU's and they're all over the place. The house has 3 2-bedrooms apartments and a washer in the basement. Any thoughts?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Depends on the peak load...a typical WH is normally in the 45-60K BTU, so the boiler is in the ballpark, but more complex. It might be more efficient. A commercial tank that size has a much larger boiler and could provide more first hour hot water, though. I think it may be undersized for that application. Depends somewhwat on the size of the indirect and how hot you keep it. It would likely require a tempering valve, which makes it look like a bigger tank. It may be normal for all occupants to take their showers in the morning near the same time getting ready for work...that can take a substantial system. If recovery isn't a big problem, then a big tank that can be heated with the small boiler would work.
  3. jimbo

    jimbo Plumber

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    BTU's is BTU's. So despite a higher temp setting, plus tempering, I don't see how 50K BTU will provide hot water for 3 two br. apts. plus WM.
  4. gellfex

    gellfex New Member

    Messages:
    56
    Location:
    NJ
    Maybe there's too many variables to have an obvious answer. Of course I was assuming a tempering valve and at least 140 tank temp. There's 75 gal WH with 65-75k btu, it kinda looks like they more or less come 1k per gal. With a big enough tank hot enough, it looks like you could achieve comparable 1st hr output & could shower the house, but does that make sense?
  5. Dana

    Dana In the trades

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    With a big enough indirect, sure, anything is possible. But you could be looking at a 100-120 gallon indirect if that's a 50K/80% efficiency burner. If the heating system is a high temp (150F+ water ) system with a bang-bang boiler (not a mod-con) you may get overall better performance with a "reverse indirect" like a TurboMax or ErgoMax set up as the heating system buffer/hydraulic seperator:

    [​IMG]

    If any or all of the apartments have shower-only (no tub) you could also get by with less tank (and even less fuel) using a drainwater heat recovery heat exchanger. 50K is enough to run a single shower 24/7 with a ~50% efficient drainwater HX at typcial NJ incoming water temps, but it won't add even a gallon to the first-hour rating for tub-filling:

    http://www.efi.org/wholesale/pdfs/power_pipe.pdf

    http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/drain.cfm?attr=4


    [​IMG]
  6. gellfex

    gellfex New Member

    Messages:
    56
    Location:
    NJ
    Dana, some interesting ideas there. I'm struggling to understand the difference between a standard indirect and the Ergomax, is it simply that the 180-200 boiler water is in the tank and the domestic water runs through it in a "on demand" style? In my case there's no heating off the boiler at all, the tenants have their own boilers.

    The heat recovery sounds like a great idea no matter what kind of WH I'm using, the only problems might be the shower drains are on the sanitary lines and those lines are a 4" cast iron stack which most plumbers advise me never to mess with.
  7. Dana

    Dana In the trades

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    Yes, reverse indirects such as the ErgoMax contain a bulk of heating system water, and it's the potable water that's in the coils. That's one method of adding mass to a heating system to minimize short cycling when heating loads are light.

    If you're using a seperate boiler strictly for hot water it's not likely to be much more efficient than high EF forced-draft standalones, and will be dramatically LESS efficient (depending on it's thermal mass and insulation) unless you use heat-purging boiler controls. See: http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf It's on the wimpy side compared to a 75-gallon standalone's burner, but with a 100-120gallon indirect you can probably make the MiniTherm 50 work. IIRC it's a low mass copper-fin water tube boiler(?), in which case it'll outperform the cast iron beasts, but you might still need an Intellicon HW+ type control if it doesn't come with a similar heat-purge controller.

    Drainwater heat recovery heat exchangers are double-walled- the potable-wrap is a separate piece from the copper drain. It usually doesn't present a code-issue with shared blackwater/greywater drains (it passed inspection in my MA home.) They work fine with any HW heater, but only in simultaneous flow mode- if the drain and potable flow isn't running at the same there's no heat being transferred to the incoming water. But longer & fatter==higher heat transfer efficiency. If you have at least 5' of vertical 4" drain to tap into 50%+ return is doable, but if you have more, the performance boost is cost-effective to go longer, since the installation labor is about the same.
  8. gellfex

    gellfex New Member

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    Location:
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    From Laars

    "The MiniTherm residential boilers are Category I (non-positive vent pressure, non-condensing), natural draft appliances. The heat exchanger combines the heat transfer efficiency of integral copper finned tubes with the mass of two cast-iron wet walls surrounding the combustion chamber. The units are available in a range of sizes to suit almost every residential and light commercial heating load while providing operational efficiencies of up to 84.5% AFUE."

    Its starting to sound like I should let go the idea of using the mini therm if it requires extreme measures for minimal returns. The cost of such a large indirect and controls would take a long time to recover if the efficiency gain is low. But a forced draft WH is not possible due to the 3 other natural draft boilers on the same vent.

    Maybe I should just Craigslist it.
  9. hj

    hj Moderator & Master Plumber Staff Member

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    Indirect heaters do not operate with "free heat". If it takes 50k btus to heat the domestic hot water, there will be little extra available for space heating, and/or vice versa. The "heat recovery" on a shower drain is more hype than substance.
  10. gellfex

    gellfex New Member

    Messages:
    56
    Location:
    NJ
    But HJ, there is no space heating requirement for this boiler. The original question was whether there was any reasonable and cost effective configuration where the 50k boiler I already own would be an asset firing an indirect. I had gotten the impression from the thread that the size of the tank required kinda tipped the cost/benefit equation away.

    The shower heat recovery seems like a good idea if it's cheap enough & easy to install. Any rise in incoming WH temps reduces the BTU load right? In my case it would be a nightmare to install, but in new or newer construction, why not?
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
    New England
    Waste water heat recovery works for things like showers, but not for things like taking a tub bath or the use of a washing machine. If you have enough fall and reasonable access. As a retrofit, it pays if you have good access and don't have to tear too much up. For three apartments, 50K boiler is pretty small for hot water use unless you have a large tank, and can afford a moderately long recovery rate, otherwise, you'll have people without hot water and complaints. A typical 40-50 gallon WH often has about the same sized burner and generally would be inadequate. The thing that could tip it is you could use that boiler to fire a much larger tank, but it would take longer, thus the need for the larger tank so you have enough for those peak uses (like in the morning when everyone is taking showers getting ready for work or school at about the same time) - you'd have a long time to reach the desired temp on the tank from the last large use.
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