When it comes time to replace your water heater, it is essential that you choose a unit that can fully meet your hot water needs. At the same time, it is also important that you don't choose too large of a unit as an oversized unit will cost more and use much more energy. In most cases, people simply choose a new unit that is the same size as their current unit, but this isn't always the best method. Water heaters have advanced quite a bit in recent years. As such, you may not actually need as large of a unit as you currently have as most modern units will heat more quickly and effectively than older units. For this reason, we always recommend that you do a bit of calculating to ensure that you get a unit that meets your needs without overpaying for a larger unit than is necessary, and here is everything you need to know to do just that.
Tank vs. Tankless Water Heaters
The first thing you will need to decide is whether you want a traditional tank water heater or a tankless unit. The type of unit you choose is important as this will dictate what you need to focus on when calculating what size water heater, you need. With tank water heaters, you will want to focus on how many gallons the tank holds as well as a measurement known as first-hour rating. If you decide to opt for a tankless water heater, you will instead need to look at the unit's flow rate or how many gallons of hot water it can supply per minute.
Calculating Your Peak Daily Hot Water Demand
In the past, most plumbers calculated what size of water heater was needed simply by looking at how many people lived in the home. The general recommendation was that you needed a 40-gallon unit for one or two people, 60 gallons for three people and 80 gallons for four or more people. While you can still use this method, it isn't all that accurate and will often end up with you choosing a unit that is larger than what you really need.
This is why we always recommend instead calculating your peak daily hot water demand, and then using this to determine what size of unit you need. Calculating peak demand means determining the hour of the day when you use the most hot water and then estimating how many total gallons you use during this peak demand time.
For instance, if you have a family of four where everyone showers around the same time each day, this will be your peak demand time. The average shower uses around 20 gallons of hot water so if you have four people that shower in an hour, your peak demand will be approximately 80 gallons. If you were also to wash clothes or run the dishwasher during this time, you'll want to add around 15 gallons more to your peak demand.
Understanding First-Hour Ratings
Just because you may use 80 gallons in an hour doesn't necessarily mean that you need an 80-gallon unit, which is why it is also important to look at the unit's first-hour rating. The reason is that as soon as you turn on the hot water, more cold water will flow into the unit's tank, and it will immediately start reheating. This means that most any modern water heater can supply more gallons of hot water in an hour than what its tank holds, which is why first-hour ratings are so important.
First-hour rating is a measurement of how many total gallons of hot water the unit can provide in one hour when starting with a full, hot tank, i.e., how much hot water you can use in an hour before you run out. The first-hour rating for many tank water heaters is often 50% higher than the unit's storage capacity, and in some cases, it may even be double.
For instance, you can easily find a 60-gallon unit with a first-hour rating of 80 to 90 gallons. This is important to think about as it means that the 60-gallon unit could still meet your peak daily demand of 80 gallons. As such, we always recommend focusing more on first-hour rating than tank size since you may be able to save some money on a smaller unit and still have it fully meet your peak hot water needs.
Calculating Hot Water Flow Rates
You will also want to calculate your peak daily hot water demand when sizing a tankless water heater. However, in this case, you don't need to focus on how many total gallons you may need in an hour and instead how many gallons you could need at any one specific time. Tankless water heaters don't store hot water so you never need to worry about running out, but they can still only provide so many gallons of hot water at one specific time. If you try to use more hot water at a time than the unit can provide, the water won't get hot enough at all your faucets since the unit can't keep up.
This is where flow rates come in as the unit's flow rate will determine how many gallons it can provide per minute. Calculating what flow rate, you need is slightly more complicated as you will need to determine how many different fixtures or appliances you may need to use at one time and how many gallons each one uses per minute.
The flow rate for a shower is typically between 1.5 and 2.5 gallons per minute (GPM), while the flow rate for a sink is around 2 to 3 GPM. A dishwasher typically has a flow rate between 2 and 4 GPM, and the flow rate for washing machines is normally between 3 and 5 GPM. The flow rates for sinks and showers are fairly standard, but dishwashers and washing machines can vary quite a bit depending on the age, size and type of unit. For this reason, we would recommend consulting your owner's manual as this should include information on the unit's flow rate.
Once you know the flow rate for all your different fixtures, you can use these average numbers to then determine how many total gallons per minute you may need your tankless water heater to supply. For instance, if you were to shower at the same time as both your dishwasher and washing machine were running, you would need a tankless unit will a flow rate of anywhere between 5.5 to 11.5 GPM depending on your specific fixtures and appliances.
If you're still unsure what size of water heater you need, you can count on Apex Plumbing, Heating, and Air Pros for help. Our professional plumbers have years of experience with water heater installation and can quickly calculate what size of unit is best for your home. We can also assist if you need water heater maintenance or repairs or any other plumbing, heating or cooling service in Columbus or the surrounding areas. For more information on our water heater installation services or to schedule an appointment, give us a call today.