Water Heaters – Tanks Or On-Demand

There are numerous complaints about tankless water heater under-performance in the online blogs. It is true that quality may vary from one manufacturer to the next, but performance of the water heater should not be a common issue. Said differently, if expectations are properly matched with the capabilities of the tankless water heater unit selected, then performance ought to be quite satisfactory.

Sizing Tankless Water Heaters

Sizing tankless water heaters is a necessary first step in selecting the unit for your particular application. Some manufacturers, Rinnai for example, have greatly simplified how they articulate the capacity specifications of their units for the consumer homeowner.

From an engineering perspective, tankless water heaters are specified by the maximum temperature rise possible, at a specific given flow rate. In order then, to technically specify the capacity of one of these units, one must know the flow rate you’ll need (in your house, for example), and the temperature rise needed. First- Inlet Water Temperature

The temperature rise starts at the normal unheated water temperature. If you live in a warm climate, that temperature will be higher than if you live in a cold climate. It’s also generally lower in the colder months than in the warm Summer months. Clearly we’re dealing with averages here, and if you’re not in the habit of routinely measuring your local water temperature, just assume the incoming water temperature to be 50 degrees F. Second-

How Hot is Hot?

So how hot is the water produced by the tankless water heater? Most people shower with water at around 104 degrees F. Most people think it would be hotter than that, but this is really the average. This 104 degree water is of course, mixed with a little bit of hot water, so the hot water from your water heater is actually warmer than that. A good rule of thumb is to assume 120 degrees F as the heated water temperature. Next- Flow Rate

In spite of the math involved in the temperature estimation above, the flow rate of the tankless water heater is the metric that really draws a lot of fire (no pun intended).

We need to estimate how much Geen warm water we’re going to be pumping out of this thing. To do this, we will want to add up the flow rates from all the things in the house we expect to be using hot water at the same time. After all, if all these things are on, flowing hot water, then in sum total, that’s what we want our water heater to produce (constantly, in theory).

Here is where expectations become unrealistic. If you have a five- bathroom house, a dishwasher, and a washing machine,… is it really likely that all of these are going to be running at the same time? Probably not, and here is where you need to be pragmatic. Two showers at exactly the same time?… that’s not at all unlikely. Will the washing machine be doing a load of clothes when the two showers are running? ‘could be. So let’s make a realistic expectation that we want hot water for two showers and a washer at the same time. That’s a pretty high demand on our tankless water heater, but not too unrealistic as an expectation.

A single shower head typically has a flow rate of 1.5 gallons per minute. Some may be higher, some may be lower (especially if you’re using the low-flow, flow rate restrictor supplied with the shower head). The hot water supply to the washer may be 0.75 gallons per minute. So adding up two of the 1.5 gallon per minute showers and the 0.75 gallon per minute washer gives us a total demand of 3.75 gallons per minute. Putting it all together


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