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Condensing combo boiler

jjjj Posts: 9Member
Hi fellow contractors on the wall! I'm converting my cabin from electric basebrd to hot water basebrd and radiant. I haven't used anyone's combination boiler/ water heater wall hung unit. Looking for feedback pros and cons. 1200 sq ft cabin, 2 baths 4 people, nat. gas


  • ColoradoDaveColoradoDave Posts: 54Member
    couple of options

    We had good success with the Triangle Tube 110 Excellence. Not sure how a 110k compares to your heat loss.

    One of our supply houses says good things about HTPs Versa Flame & Versa Hydro series, but ive yet to work witheither HTP product.
  • TomTom Posts: 337Member ✭✭
    Heat Loss

    It really depends on your heat loss, if you only need 35K BTU's then a combo boiler doesn't really make sense (unless you pipe a buffer tank) The problem is, to ensure enough production of DHW the boiler has to be 110K BTU's or so, that means way to much boiler even modulating for your heat demand typically. I have been trying to match the smallest boiler I can and an on demand with it, plenty of hot water perfect boiler size. This is a bit more money but better efficiency in the long run, IMHO.
  • HomeOwner1HomeOwner1 Posts: 134Member
    Size on hot water demand is what I learned

    Just a homeowner but my two cents for what it is worth.

    Combi's get size on max hot water demand needs, not on heating loads.

    Figure out max hot water gpm coming out of faucets and other areas in your worst case realistic example. Then look at gpm ratings for unit at coldest date with the applicable delta t, usually rated at 77 degree delta T from what I have seen. As an example, we have a pretty large home with 3 full bathrooms and the Navien Ch-240 services our hot water just fine, with a 5 gpm at 77 delta T rating.

    We have a Navien and it is great so far but probably too big for you. But they do modulate down to 17 KBTU though.

    Triangle Tube makes nice stuff and probably in that lower range. The modulation may be able to go down low enough to work nicely for you. The have a low-end Challenger and an upper end Prestige at the top end for double the price. We were leaning towards these until it was figured out the flow was not enough to meet our hot water needs for our house.

    Hope this helps.
  • SWEISWEI Posts: 5,201Member ✭✭✭✭
    Size on hot water demand

    was what most of us were taught "back in the day."  We were taught wrong.
  • HomeOwner1HomeOwner1 Posts: 134Member
    Why is that wrong?

    Curious why that is incorrect?

    That logic worked nicely with our installer and setup.

    Wouldn't the modulation option for the home heating side account for the difference between the big blast of heat needed for the more intense hot water heating mode of operation?

    We were told that they had to just be certain the down-side low end modulation band was sufficient to match the heat loss numbers for the home.
  • NYplumberNYplumber Posts: 385Member ✭✭
    Heatloss, location?

    Has a heat loss been done? What part of the country are you in (assuming you are in the USA)?

    For a cabin that small, go with a tankless and a separate small condensing boiler. Lochinvar Cadet or WHN, Viessman 100, Triangle Tube Prestige Solo... That with an on demand water heater will give you the lowest running costs and the least short cycling.
  • Jean-David BeyerJean-David Beyer Posts: 2,472Member ✭✭✭
    Wouldn't the modulation option for the home heating side ...

    "Wouldn't the modulation option for the home heating side account for the

    difference between the big blast of heat needed for the more intense

    hot water heating mode of operation?"

    I am not a heating professional and have experience with only one hot water heating system with an attached indirect fired domestic hot water heater. My boiler is a mod-con with outdoor reset.

    My experience is that even if I overlook the heat demand for the indirect, I cannot get nearly enough modulation from my burner to account for the difference between when both zones are running at the same time to when only the smallest zone is the only heat demand, and it is warm (over 50F) outside. Not nearly enough modulation.

    I am not sure how heating demand "should"? be calculated for an indirect hot water heater. I know my hot water demand is very low, because there is just one of me and I am not a teenage girl. I have one shower with low flow head. I have one wash basin, one front-loading washing machine, an kitchen sink, and a diswasher. I could run all of them at once, though that normally does not happen.

    I sized the indirect by using the W-M calculator form in their indirect customer brochure. It said about a 30 gallon tank would be enough for me, and my contractor said that the cost of an indirect, installed, was mostly his profit and labor, and the difference in cost of the tank itself did not matter all that much. So I got the 40 gallon model (like a Triangle Tube).

    I have not  calculated how long a hot shower on the coldest day of they year would last me, even if I disconnected the aquastat on the heater, I guess I could take a 10 to 20 minute hot shower without emptying the tank. And of course I do not disconnect that, so it tries to keep up with my demand. And the washing machine and the diswasher do not run very long either.

    The Indirect does operate at higher priority than the heating loads, so If I drew water as fast as I could, the boiler would run at  maximum for the indirect until it was satisfied. But as soon as I shut off that shower, the indirect would recover in less than 10 minutes or so. So the heat to the house could be off that long. But in the grand scheme of things. so what if the what if the heat were off that long? Just how much heat loss would it take to get a noticeable temperature drop in 20 minutes?
  • HomeOwner1HomeOwner1 Posts: 134Member
    Sounds like an expensive route to go

    If they went with two units, they could be looking at 3 to 4k in unit costs alone at the very least.

    A Triangle Tube Challenger could be had for a least half the cost, simplify the installation, cost about the same to run for the most part and most likely meet their needs.

    Is it worth the cost not to go with the combi?
  • ZmanZman Posts: 2,384Member ✭✭✭

    Will you be using this full time or is it a vacation cabin?

    If it is for vacation only, you might even consider an electric water heater that you would turn off when you are away. For a full time res, that would not make sense.

    Either way, you will likely have a heat loss in the 20k range. Keep in mind this is on the coldest day. Your typical loss will be considerably lower.

    There is not a combi unit made that will perform well at that output.

     Homeowner 1 has obviously made an excellent choice is the navien, His is ultra efficient at 10 to 1 turndown ,despite what anyone may say. It is also very high in quality and will run flawlessly for a long time. This model unfortunately  is not available to the average consumer.

    For heat, I would look at a small firetube modcon like the triangle tube 60 or lochinvar knight.

    The DHW could be electric, indirect or tankless. I think that choice depends on your usage and budget.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEISWEI Posts: 5,201Member ✭✭✭✭

    That heat loss is going to kill pretty much any mod/con's ability to throttle.  Even the little Cadet CDN040 bottoms out at 9k.

    Heat loss calc comes first, followed by an occupancy analysis and perhaps a degree-day count.  Do you have (or can you arrange) some solar gain?

    More info, please.
  • NYplumberNYplumber Posts: 385Member ✭✭

    Electric tankless wont run all that much. There is no gas line and no chimney and they are as efficient as dhw production gets. Getting a one stop shop combi will mean that the boiler will shortcycle without propper design such as a buffer tank.
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