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Instananeous hot water with a tank

goodheat | Posted in Mechanicals on

I’m looking for the manufacturer of an instantaneous hot water heater with a thirty gallon tank mounted above it. The manufacturer advertises it as a complete unit. Any info will be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like an unlikely combination. Plenty of manufacturers sell 30-gallon electric-resistance water heaters, of course, and if you don't have a big family with lots of simultaneous showers, such a water heater might work just fine.

    If the water heater has a 30-gallon tank, why would you need an instantaneous heater?

    To put it another way: if you have an instantaneous heater, why would you need 30 gallons of storage?

  2. goodheat | | #2

    I'm not sure the exact gallons of the tank. There is a manufacturer making the water heater I just can't remember who it was. I could be a boiler also that heats domestic and radiant.

  3. goodheat | | #3

    I found it, Integrated Boiler and Combination Water Heating with Rinnai Q Premier

  4. GBA Editor
    Martin Holladay | | #4

    I jumped to the idea that you were talking about an electric water heater, not a gas water heater. My mistake.

    Now I understand what you are talking about. This is a combination appliance used for space heating as well as domestic hot water. (Your original question didn't mention space heating.)

    The 22-gallon tank in the Rinnai Q Premier makes space heating possible. (To learn about all the difficulties of trying to design a hydronic heating system without a storage tank, see Using a Tankless Water Heater for Space Heat.) If you are planning to install a hydronic heating system, the tank is a good idea.

    For more information on modulating condensing boilers, see Sizing a Modulating Condensing Boiler.

  5. user-2890856 | | #5

    Scott ,

    If this is going into a well built home proceed with caution on the design of the system for space heating . Low end on this combi is 17000 BTUh , dependant on your low end and high end design loads you will probably need a buffer tank also for the space heating component . Although they say it has everything you need , I assure you it does not . The boiler will short cycle as will all these types of unit . The tank that is included is for DHW storage only , not for space heating . Keep in mind also that although the tank capacity is 24 gallons the storage capacity is not 24 gallons but in fact about .75% or 18 gallons , tanks stratify , no way around it .

    To figure out what size buffer tank you may need you can use the following equation :

    17,000 BTUh - smallest zone = BTUh surplus
    BTUh surplus x desired run time ( usually 10 minutes or greater ) = cycle factor
    cycle factor / Delta T x 500 = tank capacity

    Most folks use the design load smallest zone , I usually use loads at 40* ODT to get the boiler firing for good long cycles for a greater % of the heating season . Room by room heat loss calc and knowing how the system will be zoned is necessary .

    You may find that a stand alone boiler sized properly and a different strategy are better suited for your wants and needs .

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