As we’ve mentioned in our extensive sizing guide, with tankless water heaters it doesn’t really matter how many people are there in your house. What matters more is how many of them are going to use hot water at the same time.
Let’s talk about this in detail.
What’s the difference?
First of all, you need to understand the difference in sizing methods between tank water heaters and tankless water heaters.
With old storage water heaters, your showers and hot water usage were limited to the amount of the preheated water in the tank.
Tankless water heaters provide endless hot water on demand. And I literally mean it. Once the hot water reaches the fixture, you and your family don’t have to worry that it’s ever going to run out. It’s heated on the go. So as long as you have electricity on and the groundwater keeps flowing in, your hot water is going to be there.
But as great as it sounds, there are some limits to tankless water heaters. Their performance totally depends on the amount of heat they can provide to heat the water on the go. In other words, the more BTUs, the better.
But two similar tankless water heater models may have the same BTU, although one of them may be more efficient than the other.
That’s why it’s more convenient for a consumer to look at the flow rate at the given temperature rise. It shows how much hot water it can supply per minute.
And the temperature rise is defined by the difference between the output water temp and groundwater temp.
Sounds way too complicated, isn’t it? Let me make it easier for you.
How big of a tankless water heater I need?
We’ve already covered most of the necessary info in our detailed sizing guide. If you have time, we recommend checking it out.
If you don’t, I’ll try to explain everything really short.
So, to determine the right size of a tankless water heater for a family of 5 you’ll need to calculate the peak flow rate in your house. And find a tankless model that would be able to handle the load at the needed temperature rise.
Let me show how to do it on an example:
Say, you live in California or Texas. With a big family of yours, you’ll probably want to be able to use two or three showers simultaneously. Also, you’ll want to leave some additional capacity for unpredictable situations. Like using a washing machine or a kitchen sink.
The average flow rate for showerheads is 2 GPM. The total flow rate is:
(2-3 showerheads * 2 GPM) + 2 GPM for washing machine or a faucet = 6-8 GPM
And the groundwater temp at your region is somewhere between 57°F and 67°F. So the temperature rise will be 110-57/67= 53°F to 43°F.
Now that we’ve calculated the requirements, let’s try and find the best tankless water heater that would meet these requirements.
In my opinion, the best gas option would be this:
- UEF (Efficiency Rating): 0.96
- Product dimensions: 10 x 18.5 x 26 inches
- Activation Flow rate: 0.26 GPM
- Max Flow rate: up to 7.7 GPM at 43°F temp rise
- Energy Star certified (may be available for rebates)
If you’re looking for electric tankless water heaters options, consider this one:
- Max. Flow Rate: up to 6.5 GPM at the 43°F temp rise
- Activation Flow Rate: 0.25 GPM
- Dimensions: 17 x 19 x 3.8 inches
- Service Requirements: 150 AMPS DEDICATED! (4 x 40A breakers)
- 99.8 Energy efficient
But REMEMBER: electric tankless water heaters almost always will be more expensive in the long run. They usually require serious electric service upgrades and cost more than gas tankless water heaters. So, if the gas or liquid propane are available options, I’d go for them.