There is a lot of hype about the abilities of tankless water heater systems. They can work well, but you must understand some simple facts. The first major effort is to determine your environment:
· How much hot water do you want to be able to use at one time? If you have a big tub, it could take a looonnnggg time to fill it with a tankless system, long enough where you may not be able to keep it warm enough.
· How cold is your incoming water…pick the middle of winter to measure. If you have a deep well, it could be cold all year round. In some places, it could be just over freezing in the winter. This will make using a tankless much harder to meet your expectations. If you live where things are more moderate, it can work out well.
· The things take a lot of heat to produce hot water nearly instantly…you may need to upgrade your gas or (unless you’ve got really inexpensive electric rates avoid electric) electrical supply. Is your supply large enough to handle the new appliance?
· How hard is your water? The harder the water, the more frequently you’ll need to de-lime the thing or you’ll end up with decreased capacity.
· Do you often use warm water? (i.e., a low-rate of hot)
A tankless will likely cost significantly more than a tank to install and will require more periodic maintenance. Also, the expertise to diagnose and repair the thing is not as prevalent as for a typical tank, so you may be without hot water for awhile.
Another thing to consider is that to work, nearly all tankless systems have either internally, or externally, a flow restrictor. This is required so that you can reliably get some temperature rise in your water. I like to relate it to the hand through the candle analogy. There’s only so much heat available, and if you move the thing you want to heat past it fast, it doesn’t heat up much. The more heat you have available, the more water you can heat, and the higher the flow rate you can achieve.
There are some simple physical laws that apply here. It takes energy to heat water. So, let’s talk about that for a moment.
The amount of energy it takes to raise one gallon of water per minute, one degree continuously takes the following amount of energy: 500 BTU or 0.147KwHr.
Let’s take the situation where you want to draw 5 gallons per minute, your incoming water temperature is 40-degrees and the water needs to be 120-degrees. The heater needs to raise the temperature 80-degrees. So, take the 500 BTU and multiply it by 80 and then by 5, and you get 200,000 BTU. That’s the amount of heat applied to the water, and no heater is 100% efficient at it. So, let’s say that your heater is 85% efficient, that means only 85% of the heat is actually making it into the water, and 15% is going up the flue or into the room. So, to have 200,000 BTU in the water, you need to put in 200,000/0.85, or 235,294 BTU, or 68.958KwHr (assuming the same efficiency – electric should be more efficient but still isn’t 100% - some goes into the room). At a 220vac input, that’s 313.45 amps. We’re talking about some serious energy use here. Electrical tankless systems are less common and usually only available or applicable in low-flow or small temperature rise situations. Gas is more common because it is easier to achieve the concentrated heat volume.
There is a minimum turn-on volume required before a tankless starts producing heat. A common one is ½-gallon/minute. Take a typical bathroom faucet that has a flow-restriction itself of a little over 1-gallon/minute. To get hot to work at all, you’d need the valve at about ½ volume. Less, and the heater won’t turn on and you’d end up with all cold. So, there’d be no possibility of running a trickle at warm, and modulating it to a low-flow warm would likely be impossible. How often this is needed, is an open question, but you need to be aware that it is a functional reality.
Most electronic or even mechanical systems tend to fail the more they are used. Ever notice a light-bulb almost never dies after you turn it on, only in the action of turning it on? A tankless turns on every time you turn a hot tap on (assuming you achieve enough flow to activate it – otherwise, you’ll get no hot water at all). So, while electronics are pretty reliable, all of those cycles mean the system has a long-term reliability issue just from this. The tankless won’t work when you don’t have power. A tank system could hold hot water for days, if not longer. A tankless system would be stressed to provide a quantity of showerheads going at the same time or filling a large tub in a reasonable amount of time if your winter water temperature is low.
Bottom line, tankless systems have their place, and it is not everywhere. They have trouble with large flows and low incoming water temperatures, and require more maintenance than tanks. They are more expensive to install and while becoming more popular, not everyone stocks parts or is trained to be able to diagnose them. They can require huge peak energy inputs depending on your use requirements. Assuming you go with gas, the supply and exhaust requirements often may mean an upgrade in your service, and for electric almost certainly.
Evaluate the specs and compare with the pure physics of the situation…the temperature rise and volume available isn’t magic, you need a certain amount of energy to make water hot. A tank system has the advantage of time to store the energy in the water. The disadvantage is it can’t replenish it immediately. You need to determine your peak volume needs or you’ll be dissatisfied.