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Magic box

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RayH
RayH Member Posts: 98
edited April 2023 in Domestic Hot Water
I planning to install a Bosch Combi ZWB 28-3 for baseboard heat and domestic hot water in a 1500 sq ft house.  I'm not familiar with the Oxyvent Magic and looking for your thoughts? Apparently the builder bought this unit and it appears to be a little undersized. Anything else I can do to compensate for it being undersized a bit???

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  • yellowdog
    yellowdog Member Posts: 157
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    seems like an overcomplicated gimmick with false claims on fuel usage. once the air is removed, it cannot just reappear like they say without there being some other issue. a spirovent is all thats needed to remove air.
    RayHHVACNUT
  • dko
    dko Member Posts: 611
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    This is all it is

    RayH
  • Derheatmeister
    Derheatmeister Member Posts: 1,550
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    Are you in the UK?
    I am not a big fan of condensing aluminium boilers.

    As for the Oxyvent Magic Box..I think it is a hydronic separator that allows proper flow thru the boiler.

    https://allenhart.co.uk/whats-inside-the-magic-heating-box-sherburn-plumber

    Probably better of with a Low loss header from Caleffi or if you have a two temp system a baunach mixing valve may be of benefit https://www.baunach.net/

    If you are in the USA let me know and i can get you the US Baunach contact information.

  • hot_rod
    hot_rod Member Posts: 22,271
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    Are you in the UK?
    I am not a big fan of condensing aluminium boilers.

    As for the Oxyvent Magic Box..I think it is a hydronic separator that allows proper flow thru the boiler.

    https://allenhart.co.uk/whats-inside-the-magic-heating-box-sherburn-plumber

    Probably better of with a Low loss header from Caleffi or if you have a two temp system a baunach mixing valve may be of benefit https://www.baunach.net/

    If you are in the USA let me know and i can get you the US Baunach contact information.

    That chap is good at grinding foam apart, not as good as explaining hydraulic separation.
    I guess the magic is getting someone to buy that box😁
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    DerheatmeisterEdTheHeaterMan
  • RayH
    RayH Member Posts: 98
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    The job is in Maine. I'm a little concerned about the boiler being a little bit undersized also
  • GGross
    GGross Member Posts: 1,072
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    What makes you think the boiler is undersized? and do you mean undersized for space heating, or undersized for domestic?
  • RayH
    RayH Member Posts: 98
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    I have a 1500 sq ft home in Maine with two bathrooms. Seems to me like a bigger unit would work better???
  • GGross
    GGross Member Posts: 1,072
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    1500 total square feet of heated space, including any basement etc? this boiler will output 84,000 BTU/h (maybe different values are showing up online so it might be a bit more than this) which is 56 btu/sqft that is an incredibly high heating load. In similar climate zone I have seen one other building with similar load load and it was completely uninsulated. My guess is that if your house was built in the last 30 years your load is less than half that. A proper manual J, or radiator assessment, historic fuel usage etc will get a more clear picture of what your heating demand really is.

    As for DHW. Combi boilers are a different type of beast than tank style water heaters. You will be limited by total flow instead of total storage. Personally I like to use a unit that will give me 5+ GPM of hot water based on my regions cold water temp. That unit will not do that. So for me, I would get a space heating condensing boiler, and a nice indirect water heater.

    The issue you run into simply upsizing the boiler for the sake of an on demand DHW load, is that the boiler may be completely oversized for space heating. If you do upsize to a larger combi, make sure it is one with at least a 10:1 turndown ratio, so that the low fire is as low as possible. That being said, you can figure out what your needed DHW flow rate is at any given time and base your combi on that, it is possible that you only need to run a single fixture at a time and could benefit from a smaller combi. My personal preference is that if I am in the shower, and someone turns the hot water on in the kitchen, that my shower doesn't go cold. It is possible the contractors have done this legwork of sizing, I would advise asking them how they came up with their numbers.
    Derheatmeister
  • RayH
    RayH Member Posts: 98
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    This is a two story new construction with hot water baseboard. It has a well insulated crawl space. Can I turn up the aquastat temperature to compensate? Or anything else?
  • HVACNUT
    HVACNUT Member Posts: 5,865
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    Is this a DIY job or is a heating professional installing the system?
    It seems your not familiar with the Bosch boiler. There's no "aquastat" per say but limit temperatures and output parameters. 
  • GGross
    GGross Member Posts: 1,072
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    A higher supply temperature for space heating will allow your baseboards to be able to transfer more heat to the space. It will not increase the capacity of the boiler, but for example if you were sending far too cold SWT to the baseboards, and only transferring 30k BTU to the space, you could increase the SWT and transfer more BTU to the space. But if you were already transferring the maximum capacity of the boiler to the space, and the air temp is unable to satisfy the thermostat, increasing the SWT will not do anything.

    If this is new construction, it should be a prime target to have a manual J performed. You likely have prints available, and insulation values are known. Again based on your square footage, and that it is new, I would have a hard time considering the boiler too small for space heating.

    Your DHW load will be limited more than anything. That boiler can deliver 2.65 GPM at a 72 f temp rise. Now in Maine it looks like your well water averages between 41-44f , but you can confirm your exact temperature. To get you to 120f DHW, which is standard temp, you would need 76-79 f temp rise, your DHW flowrate will be limited to less than 2.65 GPM, and that is with a fresh heat exchanger, which will perform slightly worse over time between cleanings as scale forms etc
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,128
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    RayH said:

    This is a two story new construction with hot water baseboard. It has a well insulated crawl space. Can I turn up the aquastat temperature to compensate? Or anything else?

    So in reference to the vague description of the 1500 Sq Ft house you will need to have a heat load calculation for your home. OR click on spoiler link

    you can answer these questions
    For example:
    1. How many square feet of actual wall surface is enclosing the 1500 sq ft house? (not the interior walls, just the walls exposed to the outdoor temperature)
    2. How much insulation is inside those walls?
    3. How many windows are in those walls?
    4. What are those windows made of? double insulated glass, or single pane in a wood frame or plastic frame... etc.
    5. How many doors are in those walls?
    6. What are the doors made of?
    7. Regarding the two floors, are the first and second floor area the same? 750 sq ft each, or is one floor larger than the other?
    8. Is the first floor over a basement?
    9. Is the basement included in the 1500 Sq ft?
    10. Is the first floor over a ventilated crawl space?
    11. Is the first floor constructed on a concrete slab at grade level?
    12. Does the slab have any insulation on the perimeter?
    13. How many sq ft of roof is exposed to a cold ventilated attic?
    14. How much insulation is in that attic?
    15. Is it a flat roof with no attic?
    16. Is it a pitched roof with vaulted ceilings an no attic?
    17. Are there any Skylights in the roof?
    18. what are the skylights made of? Single glass, Double insulated glass, wood frame or other frame material?
    19. Would you consider the construction to be lose, average or tight in regards to outside air infiltration?

    After you answer all these questions and place the answers on a Form J1 as described in the ACCA Manual J load calculation or use the form 1540 whole house heat loss published by the Hydronics Institute division of Gas Appliance Manufacturers Association (GAMA) which has merged with the Air-Conditioning and Refrigeration Institute (ARI) into the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) several year ago.


    You may find the actual Heating requirements for that structure you believe is too big for that Bosch Combi ZWB 28-3, is actually too small for that appliance. But not really too small because that appliance has a variable input, so it actually adjusts itself to the size of the structure as the outdoor temperature changes. As others have indicated, the boiler may be too small for the domestic hot water (DHW) needs of the family using the building. That can be addressed with a few different adjustments. The simplest one is to install a buffer tank or holding tank before the water heater. This 30 or 40 gallon tank with no insulation will allow the very cold water from the ground, to warm up in the boiler room. This tank will sweat condensation as the water becomes room temperature so a pan with a drain outlet to a floor drain, sump pump or condensate pump might be necessary. Another idea is to use an indirect DHW tank that is operated via the space heating side of the Bosch.

    Do the heat loss first to see if the boiler is really too small

    Mr. Ed

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    RayH