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Mod con vs combi

347 Member Posts: 137
Hello All,
I'm sure thls has been discussed but I did a quick search and did not find what was looking for.
I have a friend replacing his boiler, he was going to an induced draft boiler (he did not want high efficiency "too many things to go wrong"). Now he spoke to a friend of his (an GC) who has a navian combi in his house which he loves, so now my friend wants one.
I suggested a mod con with indirect wh. I feel this would be a better choice, but I can't give a really good reason. He was told a combi is much more superior.
This house will have two full baths, kitchen and laundry with two people living in it (maybe a child in a year or so).
I have done a heat loss yet but I did a simular house with a load od of 48k, this one should be about the same.
I'm I nuts to suggest a boiler over a combi?


  • Zman
    Zman Member Posts: 7,493
    The biggest problem with the combi units is sizing. You end up grossly over sizing to the heat load in order to match the max DHW.

    The combi boiler ends up running in a constant state of short cycling. This reduces efficiency and with all those start/stops, "too many things go wrong".

    I properly sized and installed modcon with an indirect will be more efficient and reliable than the Navian. As soon as you put draft assist on a conventional boiler, you just added one too many things to go wrong....
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 20,725
    of course every job is unique, so selection is based on your actual wants and needs.

    new combis have 10-1 turndown so mismatched loads are better handled.
    If you can live with 2.5- 3 gpm DHW, possibly a 110 or 120,000 would behave well on both heat and DHW loads.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • gschallert
    gschallert Member Posts: 170
    347 said:

    I suggested a mod con with indirect wh. I feel this would be a better choice, but I can't give a really good reason.

    LOL, that's because the only "really good" reason to go with a tank over tankless is DHW demand, which I don't think you've calculated. Size a combi to DHW load not heating, as hot rod said above, newer combis have turndown ratios that allow for shoulder season heating without short cycling. The NCB-150E or 180E seem like they might be a good match for what you describe.

    To assess whether a tankless (combi) would meet the DHW demand you need to know a few things like:

    water inlet temp (municipal or well & geo location)
    # & type of fixtures (identify large volume fixtures like whirlpool or walk in tubs)
    # of fixtures desired to be run concurrently (two people need to shower at same time?)

    Based on this data use math to calculate the GPM needs and you'll know whether they need a tank or not. Here's the base formula for determining GPM needed:

    P= hot water ratio
    Th= Supply hot water temp
    Tc= Cold water inlet temp
    Tm= Desired mix temp at fixture


    The better choice for your friend would be what meets his heating & hot water needs at the best price and service levels, yes? A modcon with indirect isn't automatically the "better" choice. :)
  • Gordy
    Gordy Member Posts: 9,537
    edited July 2017
    Indirect is best choice. You can have unlimited hot water. You are heating with the efficiency of the boiler choice. Even a mid 80's efficiency boiler is better than a stand alone tank water heater.

    The boiler size is sized to the heatloss of the dwelling. Unless you have a HUGE domestic hot water demand.

    If the power goes out. You still have the hot water in the tank. With a tankless, or combi your screwed.
  • Gordy
    Gordy Member Posts: 9,537
    While this humorous video is about tankless water heaters some of the same things apply to a combi.

  • 347
    347 Member Posts: 137
    Thanks for all your replys. I gave him all of the reasons as you guys stated. I probably could drop down a size ( fixtures are not quite decided yet), it looks like just a standard tub/shower faucet and a second shower, no body sprays or large heads. As for use at one time, I do not have an answer for that. I find find most people really don't know what they need or want, they use want it when it's needed. I feel to oversize DHW is not really a bad thing when all facts are not known.
    As for the unit, he is stuck on what he was told by others. My input was kinda rejected and as they say "the customer is alway right".
    I know some of you would say that I'm the professional and should do what's best, but sometimes with friends it's not worth the argument.
    Thanks again
  • bob eck
    bob eck Member Posts: 930
    A condensing gas boiler or condensing gas combi boiler is only good if it actually condenses. Will his heating system be designed to use low water temps so the condensing boiler or combi boiler will condense?
    If he will have a low water temp heating system take a look at the Lochinvar Noble combi boiler. Their 110,000 BTU input boiler delivers 2.6 GPM and their 150,000 BTU input boiler delivers 3.6 GPM both at 77 degree temp rise.
    You can turn the heating side of this boiler BTU input down to help with short cycling when there is a call for heat but when there is a call for domestic hot water you get the boilers full BTU input so you get enough domestic hot water.
    Remember when high efficiency condensing boilers or condensing combi boilers break down and need parts some parts are expensive. To offset parts needed down the road the system needs to condense and save the home owner money on their energy costs.
    Also high efficiency boilers their heat exchangers need good clean system water and some boilers heat exchangers need the system water tested every year and if needed chemicals need to be added to the system.
  • j a_2
    j a_2 Member Posts: 1,801
    Take a good look at HTP products....Pretty sure they have a sizing helper on there web site...Comi units have limits and once your demand exceeds your load demand you won't be comfortable...in shower....