Replies

1. GBA Editor
   Martin Holladay | May 30, 2016 05:25am | #1

   Scott,
   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 | May 30, 2016 07:07pm | #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 | May 30, 2016 09:57pm | #3

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

4. GBA Editor
   Martin Holladay | May 31, 2016 04:41am | #4

   Scott,
   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 | May 31, 2016 09:00am | #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 .

   https://www.rinnai.us/boiler/product/qp85n#tabs-2

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