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boiler size/selection help

I'm looking for some feedback/advice on boiler size/selection. I've tried to do as much background research as I can before posting (this site/message board is really fantastic, by the way), but would be interested in comments from anyone willing to provide some.

Some background info:

Built early 1900s with an addition put on a few years ago, 2 story, ~2100sq ft
Still many original wood windows on the original part of the house (no storms either!)
Some insulation in the walls, insulation in the ceiling of the second story
Located in the northeast.
Planning to gradually improve the building envelope (can't afford to do it all right away).

Heating system:
gas-fired modcon boiler, 150K input BTU
indirect DHW
boiler feeds two hydro air systems, one in the basement, one in the attic (one zone for floor 1, one zone for floor 2)
hydro air systems are:
basement: 83k BTU for 180deg water (45k for 130deg water)
attic: 63k BTU for 180deg water (34k for 130deg water)

The issue: boiler is leaking condensate, and has lots of rust outside. I already know (from pros) that the boiler can't be repaired and needs replacement. The boiler is a bit young (~8yrs). As far as I can tell (and comments from pros coming out to the house), the install was done well. I've contacted the mfg and not gotten much support (new homeowner, warranty doesn't transfer).

At this point, I'm trying to make sure that I have as good an understanding as possible as to why this boiler failed prematurely, so I can select something for the future that will last.

I used the slant/fin website to get a rough heat load for the house. I tried to err on the side of higher heat load when filling out room info and design temps. I get the following:

70deg interior/0deg exterior: 78k BTUs
70deg interior/10deg exterior: 67k BTUs

I also tried to get an idea of heat load from gas usage. Unfortunately the gas company won't give me much data since I'm not the original homeowner. But they did tell me the previous owners used a max of 290therms in one month (Jan 2016). I think that works out to about 40k BTUs on average for the worst-case month.

If you've read this far, thank you! Here's what I'm wondering:

1) Is it possible that the system was designed with lower water temps, and that's why things might seem oversized at first glance? At 130deg water temps, I'm looking at 79k BTU from the air handlers which is pretty close to my worst-case slant/fin calc. But, would you ever size things that way? I thought the slant/fin calc was supposed to give you a design extreme; does it make sense to design things such that the extreme for the system would have such a low water temp?

2) Because of the issues with this particular boiler, and stories on the web, I'm a bit wary of installing a modcon-type boiler. However, I'm not sure if a CI will work well with an oversized hydro air system. Or, at least, it would seem that boiler sizing becomes more critical in that case. If the boiler is oversized, it can't modulate down to stop short-cycling. Although maybe the CI boiler has sufficient thermal mass to prevent that from happening?


PS. It's been mentioned that a good contractor would do a manual J for the house, etc., but none of the ones I've had over have even mentioned it (maybe b/c I'm not looking to do a whole heating system overhaul). A couple have commented that the boiler might be oversized, but that's as far as it went.


  • Leon82
    Leon82 Member Posts: 684
    Well if you only need 79000 BTUs why not go with an 80 tp 100 k boiler.

    But odds are you probably need half that to heat the house
  • delta T
    delta T Member Posts: 884
    edited November 2016
    Be glad you have an oversized hydro system!!! This is what will make your system efficient as you will see:

    Your boiler is WAAAAAAYYYY oversized, and that is likely a huge part of why it failed. As Leon said realistically you are most likely only going to need something like 40,000 or 50,000 btuh, only a proper heat loss will tell for sure though. MOST people oversize boilers. Even the slant fin app pads their calculation. This is purely to cover their a**. If 100000 will heat it, why not put in 150000, the customer will never complain about not enough heat! :/

    Don't let modcons scare you, you actually have a pretty decent setup for one. The thing that kills modcons is oversizing (which causes short cycling), combined with lack of maintenance. If you put in a modcon (properly sized) you should be able to realize actual 95% efficiency. This is key, if you cannot realize a low enough return water temperature back to the boiler (~120 or less) the boiler will not condense and your efficiency will plummet to 86 or 88 which is only a step above a cast iron at 84 or so.

    If you put in a mod con, be prepared to do yearly maintenance on it. This is not optional. Modcons have to be cleaned out yearly or they WILL fail prematurely.

    I would install something like at Lochinvar KHN085 (85000 btuh), (or if you can get away with it depending on the heat loss, maybe even a KHN055 or WHN055). Either of these boilers (and almost all modcons on the market right now) will be able to modulate down in temperature (and total output) as the demand lessens with warmer outdoor temperatures. Keep in mind that the heat loss number you came up with is for the coldest day of the year (Design temperature) and on most days you will need substantially less heat. The KHN085 can modulate down to supply ~7650 btuh, and the KHN055 can modulate down to supply ~4950 btuh. (these numbers take into account an estimate of the efficiency and represent an estimate of the output of the boiler to the system, not the input)

    Both of these boilers (again as well as most modcons on the market) have an outdoor temperature sensor that allows them to automatically and constantly adjust their output and their supply temperature based on the actual heat loss of the structure. Setting this outdoor temperature reset curve precisely is one of the biggest reasons you need a detailed heat loss done. IF you can match the lost heat with heat added from the boiler perfectly, you will achieve the best efficiency your system is capable of. Believe it or not the best thing would be to have the boiler fire constantly day and night, with juuuuuuuust enough heat to match what is being lost. This is realistically not achievable for the entire heating season in all but a few real world scenarios, but it is the ideal to shoot for. In your case you don't want the fans to run all the time obviously, (unless you upgrade your AHU's) so you will be cycling the boiler. The longer the run time of the boiler cycles the happier the boiler will be and this is achieved by proper sizing, proper flow rate, and proper outdoor reset curve settings.

    The lower the water temperature that you can run a modcon at the happier it will be. The closer its output to the actual heating demand, the happier it will be. This is why you should be happy that your air handlers are oversized for the load. It means you can run even less than 130 degree water, and still meet the demand of the heating system on design day, therefor, your boilers will be in condensing range for the entire heating season, which makes everything happy and efficient!

    As far as contractor selection goes, demand that they perform a proper heat loss (Manual J is one system for doing so, there are many) and expect it to take some time. If they say it isn't necessary or "I've done this for 30 years, I just know" or something like that, just move on. Typically a proper heatloss calculation takes about 3-4 hrs or so. Because of this, it is likely that your contractor may want to wait until they have the bid before they do it, however you can't size the boiler or the circulators properly until its done. Usually what I do, is do a rough approximation based on square footage, emitter sizing, and intuition to come up with a baseline for the bid. Spec equipment based on this with the caveat that if the bid is accepted, a heat loss calculation will be performed and it may change the bid price by as much as (X%) in either direction. Whatever you do, ensure that it is going to be done!!! Try searching the 'Find a Contractor' link on this site to find a real pro near you.

    As for your IDWH, the size of the boiler will affect the time it takes to recover lost hot water. Nothing more. If you set the temperature setpoint of the IDWH up higher, say 140, then mix the water down with a thermostatic mixing valve, you will in effect have a larger tank of stored water, and will likely not run into hot water shortages unless you have a huge whirlpool tub or a shower with 5 heads in it or something like that. How many gallons is your IDWH? Do not size your boiler in accordance with what the IDWH says it needs. You will oversize your boiler by a factor of 3 or 4! If you are worried about hot water demand, then maybe size up one boiler size from you heat loss (in your case I think this will be the 85,000 btuh model) and no more. Most IDWH in residentail settings will want something in the neighborhood of 150000 to 200000 btuh to satisfy their recovery claims.

    If YOU are still reading this I hope your eyes haven't crossed yet! Feel free to ask more questions, we are all on here because we love to talk about heating!
  • njtommy
    njtommy Member Posts: 1,105
    I wouldn't be worried at all installing a new properly sized mod con boiler. Your current boiler being 150,000 btus probably only has a 5 to 1 turn down ratio. Meaning the bottom end or lowest firing rate is 30,000 btus. Hydroair system don't have a lot of water volume or mass to them add that to a over sized boiler it's a short cycling nightmare. Short cycling kills the boilers longevity and efficiency.
  • hydro_newbie
    hydro_newbie Member Posts: 37
    Thanks for the fast responses folks.

    @delta T I see your point about the 'oversized' hydro system being perfect for a mod/con, but it seems like a water temp of 130deg for my worst-case day is low, no? Especially since the number is probably padded. I guess the mod/con won't care as long as it has enough turndown (and the top end isn't too high)?

    @Leon82, @njtommy , @delta T : I think the second thing I was looking to understand was whether it's possible to go with a smaller, conventional boiler with the setup I have.

    Right now, if I went with a 150k conventional boiler, I'd probably end up in a situation where the boiler would short-cycle itself to death.

    But if I go with a smaller boiler, I'm concerned that the hydro systems will pull the heat out of the water so quickly that return water temps will always be too low, and then I'll end up with a condensing cast-iron boiler...! I'm not sure if I'm thinking about this right though. If the above is correct, maybe a buffer tank (or the added thermal mass of a CI boiler) might help with this problem?

    I'm still on the fence on mod/con vs conventional, and there's plenty to read on this site and elsewhere on that subject, but first I'm trying to figure out if a conventional boiler is even feasible given the rest of the system.
  • Leon82
    Leon82 Member Posts: 684
    The smallest ci boilers are 30k or so I believe. Most start at 50 or 60k

    The modcon will go down to 8k or so.

    You may endup with short blasts of hot air where with the modcon you can run colder water and longer blasts of warm air on mild days.
    SWEIdelta T
  • njtommy
    njtommy Member Posts: 1,105
    Yes you could go with a smaller CI boiler. Do you have a chimney in the house for venting standard efficiency equipment? Along with is the chimney in good shape? This could be one of the reasons why they went with a high efficiency boiler and Indirect tank. It's something to look at and consider.
  • Paul48
    Paul48 Member Posts: 4,469
    Don't assume anything..........You need to know the outputs of the air handlers at different supply temperatures. That will ensure enough boiler capacity to meet design conditions. You may also need to run 2 different curves, and the that's well within the capabilities of many mod/cons. That information gives you a starting point to set the curves at.
    That's what the original installer didn't do. He just stuck an over-sized mod/con in and used the CYA approach. He broke rule #1, and everything that followed was screwed.
  • Zman
    Zman Member Posts: 7,607
    I agree with the comments above, particularly the oversizing.
    I am puzzled by a condensing boiler rusting out in 8 years. By design, they are fairly resistant to internal corrosion. What environment is it located in? A damp basement?
    Pictures always help...
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hydro_newbie
    hydro_newbie Member Posts: 37
    Thanks for the additional responses…sorry for the delay. I am now +1 kid. =)

    I’ve attached a few photos of the system, let me know if you’d like more, or if you see something amiss. As mentioned above, I'm definitely looking to make sure there isn’t anything else that’s contributed to premature failure beyond the oversizing.

    We just moved in in September. The basement isn’t bone-dry, but I’m not sure I’d say it’s damp. In September, the humidity was about 60% (maybe 65%, I don’t remember exactly), before I put a dehumidifier down there.

    Good question about the chimney; nothing is vented there now so it would need to be checked out. I was under the impression that at least some of the conventional boilers can still be side vented.

    I think I’m still not clear on whether a smaller CI boiler would be ok with the system I have (apologies if I’m being dense…). My concern is what happens in this situation:

    1. Thermostat calls for heat.
    2. Circulators turn on, the boiler begins its startup sequence, air handler blower starts.
    3. Because the air handler can pull so much heat out of the water so quickly, the water temp drops to 130 or below (boiler hasn’t fired yet).
    4. Boiler fires.
    5. Because the air handler is oversized, even at max output, the boiler can’t bring the supply water temp up enough to prevent condensing.

    Is the above a possibility with a CI boiler that is correctly matched to the heat load of the house, but is stuck with oversized hydro systems? I understand that a mod/con might actually excel in this situation, but I'm not certain of the long-term value of those types of systems yet.

    Thanks again for all the help.
  • hydro_newbie
    hydro_newbie Member Posts: 37
    edited November 2016
    Sorry, looks like the photos didn't take. Here they are.

  • Leon82
    Leon82 Member Posts: 684
    It is possible to have a boiler that can't get up to Max temp with lots of radiation.
  • Leon82
    Leon82 Member Posts: 684
    I believe the Burnham iron boiler that can be side vented will also accept lower return water temperature because of internal Plumbing it has.

    But obviously a modcon will love the low temperature