How to properly size and select a point-of-use tankless water heater
Sizing charts provided by the manufacturer are usually the best way to select a proper model for your application. These, however, can often be confusing for an average layman. Our short guide below in intended to clarify some of these points to make the selection process easier.
Sizing a point-of use tankless heater:
1. Determine groundwater temperature
. Depending on the source of water, the number may fluctuate during the year. With municupal supply, where water is close to the ground surface, the difference in summer and winter temperatures is clearly evident. For properties with a deep well, the temperature is more consistent throughout the year. The lowest reading can be made in winter, but a water utility company servicing the area should have the data available year-round. In the absence of these, a groundwater map supplied by water heater manufacturer should suffice.
2. Determine the fixture flow rate
. Many fixtures (such as faucets) will have the flow rate indicated on the aerator. Others will have this data posted on the manufacturer’s website. A simple alternative is to get a 1-gallon milk/juice jug and check how many seconds in takes to fill it up (fixture fully open). For example, at 60 seconds/jug, the flow rate in 1.0 GPM (gallons per minute). At 45 seconds, it would be 1.5 GPM, and so on. Note:
some POU models come with a flow restrictor or a faucet aerator designed to limit the flow of water so it can get heated - these must be used in order for the unit to work properly
3. Determine temperature rise (or delta T)
. Consult the mfr temperature/flow chart to determine how many degrees the heater can add to the incoming water at a given flow rate. Some units are only capable of adding 40F under optimal flow conditions, while others - as much as 80F. It is crucial to realize that not all heaters will produce 120F hot water - they simply "add" the temperature rise to the groundwater temperature.
4. With temperature rise and flow rate data on hand, consult the sizing chart provided by the manufacturer to select a matching model
. When in doubt, always select a model with higher wattage to offset possible cold water temperature fluctuations.
Important things to know about POU electric water heaters:
1. They have a minimum activation flow rate
, which varies by model (as low as 0.2 GPM / as high as 0.8 GPM). This means that the unit will only turn on when there's a sufficient flow (through the unit/hot water line) and in some instances will not work when only a trickle of hot water is needed.
2. They are not modulating
, meaning that they output a fixed amount of power (kW) to heat the water and are often restricted internally or externally to slow the flow down and allow the water to heat.
3. All manufacturer sizing charts assume that 100% of water flowing out of the fixture is hot water (not hold/cold mix)
, which is most often not the case (especially for faucets).
Frequently Asked Questions
Q: I read a lot of negative reviews online about point-of-use heaters. Why is that?
A: Most of reviews have one thing in common - the unit does not deliver the hot water temperature promised. The issue is clearly that the unit was not sized properly for the application. Therefore we advise to read sizing guidelines carefully prior to making a purchase. We also stand behind the products we sell and can assist in selecting or troubleshooting the unit, or even replacing it under warranty shall the need arise.
Q: Is mini-tank water heater a better option than a POU heater?
A: It depends on the application. Remember that the mini-tank water heater is just a miniature version of a large storage type heater and hot water will eventually run out, while a point-of-use unit will provide a continuous flow of hot water.
The question of choice usually boils down to whether running an electrical line is justifiable and how much hot water is needed. Since most mini-tank units are 120V plug-in, they’re much easier to install, but will also take up more space and require time to recover the used-up hot water.
Q: Are point-of-use water heaters protected against dry running?
A: Models we sell have dry-fire protection as well as thermal safety cutout.
Q: Are point-of-use heaters prone to calcium build-up?
A: No more than any other heaters, whether tank or tankless, gas or electric. With hard water, mineral build-up inside the heating element is imminent, unless a water softener is installed. However, in the absence of such the unit can be easily removed and flushed with vinegar to clean out the deposits.
Q: What is the difference between a thermostatic and mechanically-activated POU heater?
A: Thermostatic models generally have a lower minimum activation flow rate than the mechanical ones. Thermostatic models also usually cost more.