On our most recent tiny house build, the Open Concept Modern, we installed a full plumbing system for the homeowner, including a shower with hot water.
In an effort to conserve space and energy for the homeowner, we choose a tankless water heater (sometimes called an "on-demand water heater, an "inline water heater" or an "instant water heater").
What is a Tankless Water Heater?
A tankless water heater operates by heating water as it passes through, on it's way to the shower, faucet, laundry or other point of use. Since only the water being used is heated, no standby energy is lost as with a standard tank water heater, often resulting in lower energy costs.
It's a pretty simple concept - when you call for water, it signals the tankless water heater to turn on, and it begins heating water as it passes through. When the water use is turned off, the tankless water heater turns off.
Sizing a Tankless Water Heater
But unlike a tradditional tank water heater, where you simply choose a tank size that is adequate for your household's needs, sizing a tankless water heater is a little trickier. There are three things to consider -
1. Climate. Heating water in Alaska is much different than heating water in Hawaii - in Alaska, our ground water temperature is much colder, so more enegy is required to bring the water to comfortable showering temperature. More energy generally means you'll need a bigger water heater. You can determine your temperature rise by subtracting your inlet water temperature (how cold the water is coming into the water heater) from your desired outlet water temperature (between 110-120 normally).
2. Flow Rate. Not only do you have to consider how much heat is required, but also how fast the water needs to be heated. The flow rate is determined by adding up all possible points of use that could be used at the same time. Most showers require 2.5 gallons per minute, and hot water faucets .75 gallons per minute. Choosing lower flow rate fixtures can result in big energy savings over time.
3. Energy Source. Fuel type, availability and cost can vary greatly depending on where you live, and the type of fuel used will affect the heater's size and energy efficiency. The two most common are fuel sources for on demand water heaters are electricity and natural gas/propane.
- Electric - Available to most everyone on grid in the United States. Often easier to install because no venting is required. May not be able to keep up with large temperature rises or high flow rates for whole house uses in colder climates.
- Natural Gas/Propane - Widely available. Requires venting, fuel tank, and fuel delivery or transportation. Can be sized for whole house use even in cold climates.
Calculating BTUs Required for On Demand Water Heater
We knew electric was out of the question, since electricity up here is expensive, and it might limit the homeowner from going off grid. Since most propane on demand water heaters are rated by BTUs, I just calculated how many BTUs would be needed. It is very simple to calculate BTUs -
1. Find Temperature Rise. Outlet Water Temperature - Inlet Water Temperature = Temperature Rise
2. Find Flow Rate. Add up all possible simulataneous points of use, results should be in GPM (gallons per minute).
3. Multiply Temerature Rise X Flow Rate X 500 = BTUs Required
In my case, we had a pretty big temperature rise because our ground water is so cold in Alaska. But since our client only expected to use one water outlet at a time, we simply choose the biggest (the shower at 2.2 gallons per minute). The BTU requirement is 82,000 BTUs.
We ultimately decided on the EZ Tankless Deluxe.
- It is a compact size for a tiny house
- It was reasonably priced
- It had a vent kit as part of the package (this is actually a big deal)
- It is easy to install
- It had favorable reviews
- The service was great
After installation, we were impressed with how easy it is to install, and how well thought out the vent kit is. We haven't used it yet, hopefully will be able to circle back around with more information when we hear from the homeowner.
Moving on to our big house build, we are considering an on demand water heater to help boost the kitchen faucet, as it is a good 30 feet from a hot water tank. We will let you know how that goes as well.
Your turn - have you had experience with a tankless water heater? Which one did you choose and why? Love it or loose it? Let us know in the comments!