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Indirect size w/ smallish boiler.

TAGTAG Member Posts: 216
My new build has two bathrooms w/ showers. No bathtubs. Just the two of us -- with occasional guests. The Viessmann B2HA -19 boiler is rated at 67k . Guess I could step up from the 42g to the 50g (52?) Viessmann indirect. They have a new 42g -- but, I think it's just the color that's "new" . Thoughts? This house is in PA with a well.

Our studio in the city has a typical 40g gas Bradford White ... think it's just a 26k burner and I never have an issue. Granted -- just one bathroom. My other houses have larger boilers .. the smallest being an 80% 115BTU cast iron coupled to a Buderus indirect .... I do keep it at 140. Again .. never an issue. City water.

It's hard to figure out the recovery as they all list larger boiler sizes. the larger tank will give a bit more reserve .. but, the recovery will be longer. Viessmann sells a boiler w/ 26g tank with what looks like the same output ... expensive product.


  • flat_twinflat_twin Member Posts: 259
    Two of us with one shower. Never run out of hot water with a 40 gallon indirect set at 140 degrees mixed down to 122. I've seen five showers back to back etc, still good. I've also watched the boiler and indirect while shower was going. Recovers within just a few minutes. Boiler is limited to 70k btu max.
  • ZmanZman Member Posts: 5,865
    Consider that a typical gas water heater produces ~30K/btu and a typical electric ~16.5K/btu.
    I am thinking with no big tubs or high flow shower heads a 42 gal tank will work great.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • unclejohnunclejohn Member Posts: 1,571
    edited November 2019
    Pictures of piping. Is the tstat set high enough? Air in system. I have a 40 gal smart tank and five girls and no problems.
  • TAGTAG Member Posts: 216
    It's the old problem -- you don't know what you don't know. One installer said I needed a 160k boiler with an indirect. Now, I know that's not correct !

    I have never used an indirect with a boiler this small (<60k).

    Back in 94 I did a Buderus CI gas boiler with matching indirect. The indirects back then were thermoglaze w/ very heavy steel tank. The thing looked like a diving bell. It lasted until about 2 years ago (coil must have failed) -- the new Buderus SS tank that replaced it is much taller/ thin and not as heavy or insulated. It also does not recover as quickly --- I can only assume it's a combination of overall mass and coil design. The boiler will go out on high reset before the tank is satisfied. I'm sure the heavy steel of the old tank was able to temper the incoming water. The new tank is really light
  • ZmanZman Member Posts: 5,865
    You are making this more complicated then it needs to be. There are only 3 variables in this. Boiler output, Indirect storage capacity, and indirect HX surface area.
    Almost all indirects have adequate HX surface area to support a 60kbtu boiler so it is probably not a factor in your case.
    I have heard the discussion that the indirect needs to be "paired" with a certain size boiler. This is just not true. The boiler will actually run more efficiently with an oversized indirect HX, as the return water temps will be lower.
    The boiler produces BTU's and they are transferred to the hot water in the tank, that's it. If an installer does not believe this, ask them where they think the BTU's will go?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,081

    You already answered your own question. You said 1 property with a 40 gallon gas water heater was satisfactory.

    If thats the case a 40 gallon indirect with a boiler with at least double the input of the water heater will be no issue
  • TAGTAG Member Posts: 216
    The house with the standard brad white 40g .. is one bathroom. The new build has two .... so it possible with guests we could have 4 showers .. 2 with some time to recover and then 2 more.

    What I find interesting are the discussions regarding going larger .. 42 to say 52. it seems that you get a bit more hot water ... but the recovery is now less. So ..I'm thinking maybe the smaller is better as it will give you the two showers and allow for faster recovery for the next two.

    And yes .... I do overthink ..!!
  • flat_twinflat_twin Member Posts: 259
    Zman questioned the use of high flow shower heads and that's something to consider. IF either of the showers is a "human car wash" then you probably need a larger indirect. Otherwise you'll be fine.
    I forgot to mention supply water temp for our indirect is 160. I could turn it up for quicker recovery but I don't feel it's necessary. Nine minutes is quick enough.
  • ZmanZman Member Posts: 5,865
    I like to think of it this way. The storage is your buffer for when your demand is higher than your supply.
    The boiler dictates both the supply rate and the recovery rate. The supply rate or the amount of continuous hot water that is available is governed by the universal hydronic formula BTU/hr = GPM x delta t x 500. In your case 67,000 / 70 / 500 = 1.9 GPM continuously.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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