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Thread: Boiler help

  1. #61
    In the Trades Tom Sawyer's Avatar
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    I am dubious about drain water heat recovery for residential purposes unless the DVW is specifically engineered so that other cold water sources like toilets and sinks bypass the exchanger. Also, the distance from the source to the exchanger will greatly reduce its effectiveness. IOW the cost of the equipment, additional piping and installation will be much higher than its payback. We do use the technology for large commercial applications though. Particularly hotels and hospitals where the laundry generates quite a bit of recoverable heat.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  2. #62
    In the trades Dana's Avatar
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    Quote Originally Posted by BadgerBoilerMN View Post
    Dana;

    A low efficiency tankless with recovery coil... I wouldn't believe the numbers even if they came from other than the manufacturer. Were it insulated, maybe.

    Poor guy has all that fancy equipment and can't afford sheet rock?
    They do things a bit differently up in the wilds of Canuckistan, eh? ;-) (Notice the code-inspired moisture trap of the poly vapor barrier on an insulated studwall? That would be a mold-disaster in my neighborhood. At least they're letting the footing dry into the basement by stopping both the insulation & vapor barrier a foot or so above the slab.)

    The numbers on the efficiency of the drainwater heat exchangers are all from 3rd party test houses following a standard protocol developed by Natural Resources Canada using realistic drain temps, and IIRC 40F incomding water temp. NRCan, maintains a periodically updated list of performance by model. Since the output of the HX is roughly room temp (when there is 40F incoming water), there's no point to insulating them. In a MD location the incoming water temps are higher, the summetime output temps somewhat higher than room temp (depends on the efficiency and flow rate) and the net efficiencies slightly lower than specified by NRCan due to the lower delta-T, but they are still substantial even with 60F incoming water temp (the annual average incoming water temp in MD will be about 55F.)

    If they didn't work I'd have heard about it from my spouse by now, given where I'm running the temps of my kludge of a combi system. The (measured) peak output of the modulating burner as-operated is nowhere near enough to support a continuous ~3 gpm (bucket-measured) gusher shower on it's own, but with the 4" x 48" DWHX it definitely been working. Without the time-out on the occupancy sensor for the bathroom lights my kid would still be in there! :-)

    From an economic payback on fuel use it's not necessarily worth it at buck a therm gas, but if you subtract the difference in cost between upsized boiler or indirect tank it does. Payback on never running out of hot water during showers even with a 50KBTU/hr boiler is immediate. Estimated lifespan to where performance has degraded to 80% of it's initial performance is between 30-50 years (based on US D.O.E. work done on first-generation versions back in the mid-1980s.) We'll see- we're coming up on year 5 on mine.

    Tom: It's not about the payback on fuel savings, it's about the performance. Even if the thing was 50' away the performance hit isn't very big, given the modest temperatures. At the ~100F drain water temps even if you fed the entire cold water distribution with the output of the DWHX it's not going to overheat the toilet or cause other issues- the output of a ~50% efficiency unit is very tepid water at best, even with 60F incoming water. If people in FL can stand 75F incoming water temps on the cold feed, so can the rest of us, and since it's only going to happen DURING active showering, the inconvenience is very limited. About half of the residential installations I've seen don't isolate it's output to just the HW heater and shower, and I've NEVER heard anybody complain about the tepid water on the cold side.
    Last edited by Dana; 08-13-2013 at 08:24 AM.

  3. #63
    In the Trades Tom Sawyer's Avatar
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    I'm not worried about warm water to the toilet. What I'm saying is that unless you can isolate the heat recovery drains from the rest of the system, every time someone flushes the toilet or runs cold water it will drastically cool the recoverable drain water when they mix.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  4. #64
    In the trades Dana's Avatar
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    In my direct experience, the immediate effect of even a 5 gallon flush of room temp water from the antique toilet upstairs has no perceptible consequences for person in the shower, and it would take a whole lot o' continuous flush to take the temps down substantially over time- the slug of water just isn't spending that much time in the heat exchanger.

    A code-max 1.6 gallon flush of room temp water mixing with the ~100F drain water is a very momentary and very shallow event, and the effect on temp of the gallon or so of potable side water in the heat exchanger is quite small. You just can't "drastically cool" the 75-80F exiting water by dumping a slug of 68-70F water into the drain mix. You'd probably feel the brief dip in temperature if you dumped 5 gallons of 40F water down the drain though. The larger fraction of the DWHX output is going to the hot water heater which either buffers the temp shift with it's internal mass or ramps the firing rate in a tankless case. Less than 30% of the DWHX output is going directly to the cold side of the shower, where it might have a perceptible effect.

    The gpm rate of a toilet flush is way out there off the right edge of the chart on that return-efficiency curve.

  5. #65
    DIY Junior Member Steve Lanham's Avatar
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    UPDATE>> having a new 125k gas atmospheric boiler and a 40gal. indirect installed this week...religned my chimney and removed the old oil boiler with asbestos...I want to thank all of you for your input...I was surprised how much time you guy's spent trying to help...It must be a labor of love for some of you...Thanks again !!!!

  6. #66
    In the trades Dana's Avatar
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    I don't quite get why you're still installing a boiler more than 2x oversized for the 46K (probably overestimated) heat load, given the whole prior discussion. That's probably a 5 or 6 plate sucker, and even a tiny 3-plate beastie would have covered your loads with margin to spare, and (unlike the oversized 125K burner) would actually hit it's AFUE numbers. Even at 60-70KBTU/hr output the smaller boiler would deliver more hot water, with faster recovery rates than any standalone water heater- upsizing the boiler rather than the tank for big hot water loads is almost never the right thing to do.

    By going substantially bigger than the peak heat load you've signed up for 20-30 years of lower than nameplate efficiency, especially if you go ahead and improve the thermal performance of the building in the intervening period. The only way to buy some (but not all) of that lower as-operated efficiency back is with smart boiler controls, but those would still be a good idea even on a 3-plate boiler with a 46KBTU/hr design-day load.

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