Bypassing Tankless Coil in Boiler

Discussion in 'Boiler Forum' started by pvtjoker39071, Dec 6, 2011.

  1. pvtjoker39071

    pvtjoker39071 New Member

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    PA
    I have a newer model Peerless Boiler, oil fired, steam system, with the tankless hot water coil. Since the system runs all year roudn for heating hot water, the oil bill is killing my wallet.

    My heating oil supplier told me it was risky to bypass it as it could cause a gasket to leak by not running it year round, but something tells me that want my money for oil.

    So I want to bypass the tankless coil and instead use an electric hot water heater for hot water useage, can it be done without issues
  2. Tom Sawyer

    Tom Sawyer In the Trades

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    Your heating supplier is an idiot. Bypass to your hearts content
  3. Dana

    Dana In the trades

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    He may not be an idiot, only greedy(?).

    At current oil prices heating water at sub-40% summertime efficiency in an oil boiler doesn't make sense in any but the most expensive of electricity markets. Depending on elecricity prices it may make sense to plumb the electric tank in series with the output of the tankless coils, since the net efficiency of the boiler probably hit's 70% during the heating season, and pre-heating the feed to the electric tank with the coil may make sense, without heating the boiler merely for hot water. That way the maximum amount of heat is purged from the boiler, decreasing it's standby loss.
  4. pvtjoker39071

    pvtjoker39071 New Member

    Messages:
    2
    Location:
    PA
    I considered the indirect hot water heater option, however to me it really does not make sense. If I can shut the boiler down from spring through fall it would substantially decrease my oil consumption.

    Just as a reference I used 3/4 tank (approx 200 gallons) of oil from June through November and that is crazy considering I only turned my heat on a few times.

    As for electrical rates, I consider them to be average and dont think the hot water heater is going to dramatically increase my bill
  5. Tom Sawyer

    Tom Sawyer In the Trades

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    An indirect is a very efficient way to make hot water and usually more efficient than electric but some of that does depend of the efficiency of the boiler. Your newer Peerless is probably running at around 83% which makes it a good candidate for an indirect though. The only drawback is the initial cost which will be twice what and electric tank runs installed but, you will make that up in less than 5 years of operation.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    If a boiler is setup to only run when it needs to heat the indirect, it's efficiency is quite good. This also depends somewhat on the burner design. Similar to an electric WH, an indirect doesn't have a flue and tends to be well insulated so standby losses are low - some of the better ones are rated at less than 1/4-degree per hour loss. This means that the boiler may only need to run once a day after you've used a lot of water. During the non-heating season, my boiler rarely runs more than once a day and as I walk by, the temp is often sitting at ambient.
  7. Tom Sawyer

    Tom Sawyer In the Trades

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    I can shut my boiler off, go on vacation for two weeks, come back and the water is still hot. I have a Super Store SS40
  8. Dana

    Dana In the trades

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    In hot-water heating only mode an indirect is pretty pathetic with high-mass boilers (40-50%, depending on the mass & insulation of the boiler, even when set up to cold-fire) but still way better than an embedded coil. See: http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf (see table 2, right column.) To get much better than 50% requires a well insulated boiler with heat purge control on the boiler and a relatively small burner. Using the embedded tankless coil you could easily be down there in the mid-20s in summer. (At the bottom of table 2 compare 12a & 12b, the same boiler set up as tankless mode vs. indirect. )

    But assuming you can get 50% efficiency with an indirect, at $3/gallon oil thats 138000/2=69000 BTUs of hot water at the output of the indirect for $3, which is 23000 BTU/$. If oil hits $4 that's 17,250 BTU/$

    At 15 cents/kwh electricity and a EF 0.90 electric tank you get 3412 x 0.9=3071 BTUs at the tank output for 15 cents, which is 20,475 BTUs/$. At 12 cents that's 25,592 BTU/$, but at 20 cents 15,355 BTU/$, not such a great deal.

    During the heating season the average AFUE of the boiler will be more like 65-75% efficiency (depending on the oversizing factors- see table 3 of that document), so taking a middle number of 70%, at $3 oil you'd be getting 96,600 BTU/gallon, which is 33,300 BTU/$ - a better deal than electricity for most, and at $4/gallon it would still be 24,150 BTU/$, comparable to 12-cent electricity in a standard electric tank.

    Which is why plumbing an electric tank in series with the tankless coil (but controlling the boiler only in response to space heating) is probably going to be the lowest operating cost for most people. The extra wintertime of the hot water load improves the boiler's as-used AFUE, and you can just turn it off for the summer without changing any other controls and you'll still have hot water.
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