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Combi's

Ron W.Ron W. Posts: 13Member
I have heard that the combi is not the best idea for baseboard heating systems as running the temps in the boiler over condensing temperature compromises the savings the customer expected and paid for. Please enlighten me.

Comments

  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,951Member
    Domestic hot water loads are usually large but for a shorter duration than heating loads which are more moderate and steady.

    With a standard boiler or a mod con boiler with an indirect hot water tank domestic water is stored so that when there is a call for domestic water some of the stored water is used and the boiler then reheats the tank. A small boiler sized for the heating load can do this

    With a combi there is no domestic how water stored so the boiler must be larger than needed to heat this instant load.
  • HVACNUTHVACNUT Posts: 2,396Member
    Because a combi needs to be grossly oversized to meet domestic hot water needs, I'm not a fan, unless there's serious space constrictions for the boiler location. A mod con with a high turn down ratio like 10:1, outdoor reset and an indirect water heater, having fin tube baseboard shouldn't be an issue. Worst case it will need a buffer tank. And dont forget about the heat loss calculation.
  • hot_rodhot_rod Posts: 11,750Member
    The Combis have the same turndown as the boiler only versions. 80K is about the smallest mod con I have seen with 10-1 turn down. 80K will turn down to 8,000. There are smaller mod cons but not with 10-1. a 110K is about as small as you might go with a combiner's for adequate one or two faucet draws.

    Ramp delay and anti cycling settings will also help limit short cycles on either type.

    The combi will handle small loads exactly the same as a mod con, boiler only.

    A buffer can be added to either if you have frequent micro loads.

    The key to to do an accurate heat load, especially on replacement boilers.

    There certainly are tradeoff with any combination product. To me the bigger question for the HO is if they can live with the DHW production they actually offer, not the cycling so much.

    If they need or want a 199K for large amounts of DHW and have a 20K, multi zoned heat load, that may be too much of a mismatch without a buffer. A separate boiler and tank would be a better choice.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • LuckyDogLuckyDog Posts: 22Member
    edited July 17
    Hope I am not hijacking this thread....

    My home is a 50k load. All low temp radiant heat. Radiant Floor and a couple of panels. Three zones

    My boiler installer REALLY wants me to put in a combi. A Bosch Greenstar combi 100 p or Lochinvar NKC110L.

    What happens, say, after the wife draws her tub but there is a call for heat? Does that high temperature water then go into the heating system?

    Does the boiler stay at a high temperature like the oil burner did when I was growing up? (180 day or night 24/7/365)

    A plumber friend told me he hates "tankless hot water" because of the "cold sandwich" effect. He told me the water heater would purge, and when he got into the shower after his wife; there would be hot water still in the pipe, a shot of cold water due to the heater being cold and just starting, then all the hot water he needed.

    If this might happen to my wife, then I don't want a Combi. :#

    ETA: corrected model of the Bosch.
    HomeOwner



    Building a house in NH
  • JohnNYJohnNY Posts: 2,318Member
    LuckyDog said:



    A plumber friend told me he hates "tankless hot water" because of the "cold sandwich" effect. He told me the water heater would purge, and when he got into the shower after his wife; there would be hot water still in the pipe, a shot of cold water due to the heater being cold and just starting, then all the hot water he needed.

    If this might happen to my wife, then I don't want a Combi. :#

    We install small 10-gallon electric water heater buffer-type tanks on the outlets of tankless water heaters to resolve this. It works every time but adds a bit to the costs of installation and operation, obviously.
    For installations, troubleshooting, and private consulting services, find John "JohnNY" Cataneo here at :
    "72°F Mechanical, LLC"
    Or email John at [email protected]
    John is a professional Master Plumber, licensed by The Department of Buildings of The City of New York, and works extensively in NYC while consulting for clients in and out of state.
  • DZoroDZoro Posts: 786Member
    Lochinvar Noble has a setting to prevent the cold sandwich, and it works great. Can also set the max btu's for heating as low as 10,000 or 15,000, or 19,000 depending on which model you have.
    D
  • Le JohnLe John Posts: 138Member
    I have a Lochinvar Noble NKC 199. It provides endless hot water and can say that it maintains the set hot water temperature very very well. I have never noticed a fluctuation in hot water temperature. Great boiler.
  • HenryHenry Posts: 914Member
    We stopped installing combi units in 2005. Our water at the end of January is 33F through the end of February. The combi units cannot keep up with the demand no matter what the size.
  • hot_rodhot_rod Posts: 11,750Member
    The basic BTU formula will indicate what it takes

    500 X 3gpm X [33-123] = 135,000 btu

    A 155k would cover that

    It always comes down to expectations and crunching the numbers
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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