Below are a few articles I've written as primers for people who want to know more about hot water heating. Most of these are inspired by the fact that I got asked about these topics on a daily basis, working in Wholesale Heating.
Homeowner Primer: Instant Water Heaters vs. Hot Water Tanks
Introduction
I repeatedly get asked the question “Should I install an instant water heater instead of a traditional hot water tank?” The answer is not a simple one, but depends on the homeowners’ lifestyle and expectations.
The Different Approaches of Tanks Versus Instants
Hot water tanks and instant hot water makers do the same job using different approaches. Although it’s not something we think about, we have adjusted our thinking and expectations to the hot water tank way of life – you have a fixed amount of hot water, but you can use it as fast as you want until it runs out. Instant hot water tanks are the opposite: you have a limited amount of hot water that you can use at once, but it never runs out.
Instant Hot Water Maker Output
The output of instant hot water makers vary mostly by the incoming cold water temperature. The National Ground Water Association (ngwa.org) gives an average ground water temperature of about 44°F in the Northern U.S. and Canada, and as high as 72°F in the extreme Southern U.S. While the capacity of an instant hot water maker is dependent on its BTU rating (typically 150-199,000 Btu), its actual performance varies greatly by how much it has to raise the water temperature to get to 110°F, which we consider as “hot” water. For example, a heater capable of raising 60°F water to 110°F can produce 9.8 GPM, whereas if the water starts at 40°F, it can only produce 4.7 GPM.
Consumer Needs and Instant Hot Water Makers
To continue our example, a modern showerhead uses about 2.5 GPM on average. This flow rate means that if you live in the north, you can expect to run two showers simultaneously. If you want to do laundry or run the dishwasher at the same time, suddenly everyone gets less hot water. Although it’s not going to run out, the instant hot water heater cannot generate more hot water at once, and there is no reserve to buffer the sudden peak in the hot water demand. Filling a bathtub becomes trickier – a typical spout flows at about 6-8 GPM, which might be higher than the output of your heater. This means that you end up filling your bathtub more slowly – you have to crank the knob way to the left, and the flow slows down. Yet, you still get hot water even if you’ve just been using a lot of hot water. If you just want to have one long shower, you can literally have one all day and never run out of hot water.
Hot Water Storage Tanks
By contrast, hot water tanks, with their large storage, give you a big mass of hot water you can use however you want until it runs out. Typically, homeowners who frequently run out of hot water simply install a bigger tank, or an additional smaller tank in parallel with their existing one. While hot water tanks are better insulated than they used to be, there are some standby losses associated with storing hot water – heat escapes through the walls of the tank which wastes some energy (usually by overheating your mechanical room).
Installation, Maintenance, and Warranties
Okay, so you like the idea of an instant hot water maker. What about installed cost, maintenance and warranties? Well, from my experience in the wholesale industry, material costs are slightly higher for instant water heaters by a few hundred dollars, and install costs are similar. However, tanks are typically warranted for 3-6 years, whereas instants for only 1-3 years. Furthermore, while tanks might benefit from regular service, instant hot water makers require it, especially in hard-water areas. Without annual flushing and maintenance, an instant hot water maker’s heat exchanger will plug up with mineral deposits in 3-4 years (in areas with hardness over 14 grains, especially), and the unit will fail or at the very least need its heat exchanger replaced. This maintenance offsets most of the cost savings gained by not storing hot water, but you use less energy making the hot water for when you need to use it.
Instant Hot Water Makers Combination Use
I really don’t recommend the use of instant water makers for dual purpose (that is, for heating and domestic hot water). These instant heaters, using potable water through small heat exchangers, are not meant for continuous duty cycles that a heating system requires, and tend to under-perform expectations. Please see Instant Water Heaters versus Boilers for more on this topic.