Feel like you might have spent too much on your Water Heater? Or worse, are you taking cold showers because someone decided to run the dishwasher? Let us help you figure out what size should your tankless water heater be?
Sizing Your Tankless Hot Water Heater
A properly sized water heater will make your house more efficient while fulfilling all your hot water needs. This quick guide will help you get the right-sized heater next time you're in the market.
Eccotemp 20H Outdoor 6.0 GPM Liquid Propane Tankless Water Heater Service Kit Bundle
$739.00
The Eccotemp 20H Outdoor Liquid Propane Tankless Water Heater is one of the largest outdoor models of the Eccotemp tankless line. Rated at an impressive 6.0 gallons per minute (GPM) and 150,000 BTU per hour, the Eccotemp 20H-LP can meet… read more
Tank or Tankless
The first step is deciding whether you want to go tank or tankless. While both are viable options, they use different measures when it comes to power and efficiency.
Tanks
Tanks are the traditional option that most people have. If you'd like to replace your outdated heater with an upgraded version of the same, then a good rule of thumb is every:
1 to 2 people = 30-40 gallon tank
2 to 3 people = 40-50 gallon tank
3 to 4 people = 50-60 gallons tank
5+ people = 60-80 gallon tank
This is an excellent tool to estimate what size heater you'll need, but it's just that – a loose estimate. There are many reasons why you may need more or less water than this graph supposes. If your house has extensive piping, or everyone likes to shower at once, you may need a bigger tank – unless you're fond of ice baths.
Here's a useful calculator to help you out.
Tankless
Tankless heaters can be trickier to size than tanks, but have no fear – we explain it here.
Propane models exist but aren't very common so we won't focus on those. Gas and electric heaters dominate the tankless market – with pros and cons we also won't get into right now. We will, however, break down the sizing for whichever you choose.
Electric
The process for sizing an electric tankless is about the same as a gas; make sure you have enough BTU's to cover your temp rise at max flow. The process can be seen below.
Gas
Gas-fired tankless can produce higher volumes than electric ones quicker. However, even the largest, gas-fired models can't stand electric's efficiency in large households.
Rinnai SE+ Series with ThermaCirc360® 9 GPM Indoor Condensing Tankless Water Heater with Recirculation, NG
$2,605.00
The Rinnai SE+ Series with ThermaCirc360® 9 GPM Indoor Condensing Tankless Water Heater is a high-capacity, natural gas-powered tankless water heater designed for indoor installation. It can deliver up to 9 gallons of hot water per minute, making it well-suited… read more
Sizing Tankless
According to the U.S. Department of Energy, "Tankless Hot Water Heaters (on average) provide hot water at a rate of 2–5 gallons per minute." That includes both gas and electric.
A more accurate estimate can be found by listing the number of fixtures you expect to use at any one time (ex., dishwasher, shower, faucet).
Once you've figured that out – this will take a bit of math – add up the flow rates of the fixtures. They don't have to be perfect, but a rough guess. Aim high if you're not sure. Average flow rates for household fixtures can be seen here. This will give you your max flow rate.
Lastly, calculate your average temperature rise. Subtract the incoming water temp from the desired water temp. Use the coldest temperature your climate reaches to ensure you'll have winter heat. A guide to your groundwater temp can be found here.
With these numbers, you can figure out exactly how much heat you need and how fast – but it never hurts to have a plumber do the math for you!
If you're looking for a tankless water heater, feel free to check out our collection of tankless water heaters or shop by brand with Rinnai, Eccotemp, or PrecisionTemp.
