Tank water heaters typically hold 40, 50, or 55 gallons or more. The size you buy depends on the number of people living in your home and your peak water usage. A family of four, for instance, might take several showers, run the dishwasher, and wash a load or two of laundry in an average day, totaling 100 gallons of hot water or more. But that doesn’t mean that household needs a 100-gallon storage tank.
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For storage tank water heaters, it’s important to consider the first-hour rating, which is the number of gallons a water heater can deliver in an hour starting with a full tank. You’ll find the FHR on the EnergyGuide label. To calculate what FHR will work for your home, use the calculator on the Energy Saver website.
As the result of recently updated efficiency standards, water heaters less than 55 gallons now have a 4 percent boost in efficiency and water heaters 55 gallons or more have efficiency gains of 25 to 50 percent, depending on the technology used—heat pump or condensing. (See water heater types, below.)
And don’t assume that a new water heater will fit where your old one was. Because of increased insulation and other efficiency improvements, some newer models may be wider and/or taller than your old water heater.
Tankless water heaters, of course, don’t hold much water, so the number to look for is the gallons-per-minute rating (GPM). That’s the number that tells you how much hot water the heater can deliver over a set period of time. The higher the GPM, the more hot water the unit can deliver. If you have a big family and multiple bathrooms, you’ll need a tankless water heater with a higher GPM. A typical shower, for example, uses up to 2.5 GPM. Newer washers use from 7 gallons (front-loader) to 19 gallons (agitator top-loader) per wash, but older models can use up to 32 gallons per load, as indicated below. Calculate your needs accordingly.
For more, you can check our water heater ratings.
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Your use of "boiler" is confusing because this can describe either a tank water heater, or a hydronic house heating system which often includes water heat also. But either way...
Turn on the boiler every day for a small amount of time (e.g. 30 minutes? this is what I am currently doing and I always have hot water available)
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And you may notice your phone does the same thing - you get to use it anytime you want, but you only have to charge it occasionally.
"Well of course it does that, Harper, it has a battery!" Well, a tanked water heater (often called "boiler) is a thermal storage load. It has a huge and very well-insulated water tank, and acts like a huge battery - just a hot, wet one.
Now, a great many utilities offer variable-rate tariffs based on time of day, market conditions, etc. So you can set your water heater to do its heating during those times, if it is financially advantageous to do so.
If you are doing home heating with your boiler, you could have a very large and very well-insulated cistern where you keep a large mass of water that you heat when tariffs are favorable. In fact British people have storage radiators, right in rooms, which heat up bricks at night on cheap power, and then radiate that into the room during the day.
And that's because water heater insulation is already pretty good. You're welcome to supplement it with more - there's no fire hazard to adding more insulation to an electric water heater, and they make insulation kits for that purpose.
When we've looked at this question in the past, we just didn't find a better answer than that. The idea of shutting it down while not in use and letting it cool down, then having to reheat it, saves a little... but it doesn't save much. Also it lowers the temperature into the "breeding bacteria" range, which is a health risk.
Now if we're talking "water heater" there are complications, because if it's interchanging heat in the utility room it's filling the surrounding space with cold air, which means ???. (party because of free A/C, or run the furnace more, or both depending on season).
However, in the greater space of "boilers" there certainly are outside-air-sourced heat pump boilers, either for water heat or for home heating (though more easily found for the latter, which is typically a hydronic system providing both home heat and water heat). These move the heat exchange outside, eliminating the cold utility room. And yes, better ones can work down to very cold temperatures.
If you want to learn more, please visit our website Wall Mounted Condensing Gas Boiler.