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condensing boiler

alv54 | Posted in General Questions on

I read previous articles here regarding boiler oversizing and proper setup.
I had a boiler installed in sept 2013 a US Boiler AlpineALP 150 gas condensing 95% eff.
todate 11/9/2017 it has cycled 103800 times and has logged 6400 hours.
I am in Westchester NY my home is stucco no insulation approx 1900sq ft.
5 zones , dhw zone
My question is that the boiler was oversized and therefore cycles constantly, who can help make adjustments to help improve the performance of this system. I am homeownew with hardly any knowledge of how to correct this

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  1. Expert Member
    Dana Dorsett | | #1

    What type, and how much radiation do you have? (Broken down by zone.)

    Is the radiation enough to heat the house at condensing water temperatures, even at the 99% outside design temperature?

    Have you run a fuel use load calculation? What is your design heat load? (I'd expect a stucco clad 1900' house with at least some storm windows & some attic insulation to come in between 50-60,000 BTU/hr @ +10F, and surely under 75,000 BTU/hr, but there are exceptional cases both above and below those numbers.)

    Have you run the napkin-math on the zone radiation and water temperatures required?

    You can always run at a high enough fixed temperature that it never short cycles, sacrificing some (or all) condensing efficiency. You can also add a buffering thermal mass as the hydraulic separator to limit the number of cycles.

  2. Jon_R | | #2

    +1 for adding a buffer tank so that it isn't cycling 16 times per hour. You could also modify zoning such that zones switch between partially open and fully open (vs closed and open).

  3. alv54 | | #3

    thank you for quick response, I hope I can provide the info you noted hergoes,
    a 2 story wood frame house attic blown in insulation.
    all dbl hung windows dbl pane glass.
    5zone breakdown,
    z1 2nd floor bedromm 10x10x8 10ft slant fin baseboard.
    z2 2nd floor bedroom 10x14x8 19ft slantfin baseboard.
    z3 2nd floor bath 6x9x8 castiron 9chambers, bedroom 9x12x8ft castiron 23 chamber.
    z4 1st fl lr/dr 18x20x8 32ft slant fin baseboard.
    z5 1st fl kitchen 12x14x9 (toekick heater) hall laundry and powder 6x20x8 11ft slant fin.
    z6 DHW amtrol 20gallon tank.
    I have not done any calculations, after reading your past post on this issue and other parties concerns and replies I feel it beyond me to further delve into this on my own for lack of knowledge on the matter.
    I hope however you may guide me in the direction that will hopefully improve the setup of my system.

  4. alv54 | | #4

    thanks Keith H for the info. Its a lot to absorb so I will start working on it. hope to make some progress over the next few days at which time I will record status.

  5. KeithH | | #5


    I think you'll need to do more detailed calculations on your own or hire someone to do a Manual J (which should have been done before the equipment swap out).

    Here are two self-help resources that might help you. I've used the first and compared it to a professional Manual J with satisfactory comparison. I have not used the second but it looks like a shorthand J that should give you an idea. As for room to room loads, I don't believe these will help but if you have a good whole house load, that's a start. At the minimum, you'll be better educated about what a load calculation looks like. Check out:

    Alternatively, you could look for a local energy modeler (better IMO than a HVAC guy) who performs Manual J. In my area, this service runs $400+ depending on house size etc. I don't think you should expect anyone to do that for free online as it takes some time to catalog and enter all the components like windows etc.

    I used an exceptionally crude (and quick) calculator on for your house size and came up with 95k btus. Crude calculators typically over estimate so that's more confirmation of Dana's intuition rather than saying you need 95k btus.

    Looking very briefly at your model, it looks like it has a significant turndown capacity to 30 MBH. I would think that if your controls were correct, you could make this boiler size (30 - nominal 150) work.

    I think rather than suspect that boiler oversizing is crushing your efficiency, I would look into improper setup or possibly some defective thermostats. I've seen older mercury thermostats fail in 'call for heat mode'. In that case, your boiler might cycle something like: pre-warm exhaust air or fan startup cycle; heat; exhaust fan spindown. That cycle might take some time even if the heating portion is very short. Were the thermostats replaced or recommissioned with the boiler?

    I'd suggest your best recourse is to have the boiler installer out to examine its control setup.

    ** I'm not a pro disclaimer

  6. Expert Member
    Dana Dorsett | | #6

    Forget the Manual-J, you can figure out the whole house heat load by fuel use numbers.

    The real issue is to figure out why it's short cycling. If you had read the condensing boiler sizing blog piece the answer would e immediately obvious:

    The minimum fire input to the ALP-150 is 30,000 BTU/hr, which means even in non-condensing temperatures at 85% efficiency it's dumping over 25,000 BTU/hr into the water. See:

    Even your LONGEST baseboard zone only has only 32 feet of baseboard, which is not capable of emitting more than ~24,000 BTU/hr at even an entering water temp of 210F, the highest temperature that boiler can deliver! See:

    At condensing temps that zone can emit only about 6500 BTU/hr, 7000 at most. At condensing temps the boiler is delivering ~28,000 BTU/hr or more into the system so you have 20,000 BTU/hr going into less than 20lbs of water, a temperature slew rate on the order of 1000 degrees per hour, 17 degrees per minute. I don't remember if you can program the high to low temperature differential around the boiler's setpoint with that unit, but even if it's programmable it's probably no more than a 15-20F spread. If it isn't it's typically a 5- 10F spread. So at condensing temperatures you're looking at best-case about a 1 minute burn, but more likely 30 seconds or less.

    And that's your BEST CASE zone! (Not including the 20 gallon sidearm water heater, which has 167lbs of water just in the tank.) The other low-mass zones are even shorter, with less emittance and less water mass, and will short-cycle even faster. There isn't an adequate description of the higher mass radiators, but they don't sound anywhere big enough to emit the 28KBTU/hr, but may be sorta-OK if there is sufficient water mass.

    Even the SMALLEST modulating condensing boilers are going to short-cycle on zones with only 10-20' of baseboard at condensing temperatures- it'll be bad, just not as CRAZY bad as what you currently have.

    Are you running it fixed temperature, or using the outdoor reset?

    If you combined the whole shebang into one zone you'd have 72 feet of baseboard, a toe kick heater of unspecified output, and a couple of not huge radiators- call it 100' of "baseboard equivalent" as the right order of magnitude. Even that isn't enough radiation to emit 28,500 BTU/hr at condensing temperatures. At 170F (a typical average water temperature for cast iron boiler based system) it's in enough to emit ~50,000 BTU/hr at 180F AWT(a bit higher than typical, 190F water output) it can emit about 60,000 BTU/hr. At 200F AWT (210F out) it would only be emitting about 70-75,000 BTU/hr. So if the system is keeping up with the heat load that's probably the upper bound on the heat load- you don't have enough radiation to emit much more than that at the ALP-150s highest output temperature, even though at high fire the boiler can delivere twice that many BTUs/hr. It's hard to imagine why the contractor would install a boiler that huge. Was the older boiler a cast-iron thing with an embedded coil for heating hot water?

    So... anyhoo...

    If you're going to keep it micro-zoned, it's going to need a significant sized buffer tank, but if it can all be operated as a single zone maybe not, depending on how much thermal mass there is in the whole system, including the water volume of the radiators. One cheap way to get there is to buy a scratch'n'dent 40 gallon electric hot water tank has over 300lbs of water in it, and if the call for heat from the boiler was slaved to it's aquastat, with the thermostats only operating zone pumps & valves sipping from the tank, it can extend the minimum burn times to a handful of minutes or so. It's usually possible to use the heater element ports as plumbing ports for the heating system, and use the hot & cold water ports plumbed to the boiler, or conversely. This is a major hack, best done by somebody doing at least the napkin math, but done right it can cut the number of burns by over an order of magnitude.

    But to get a more nuanced handle on this you're going to have to some of your own napkin math, starting with...

    The EDR square feet of the radiators:

    Look up some radiators that are still being made take a stab at estimating the total water volume.

    Find the model numbers and/or specs on the toe-kick heater.

    Run the fuel use heat load calculation too:

    With more accurate radiation & heat load parameters we can at least take a first cut stab on how to fix this without leaving you in the cold.

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