Tankless Water Heaters 102- Endless Hot Water or Shameless Hype?

Hello again.  I hope you are having a great week.  The weather is great in Georgia, it’s finally cooled off a little and feels like fall is on its way.

Last time we started looking at tankless water heaters.  We found out that they really are more efficient than most other tank type water heaters, even if the manufacturers’ claims might be a little optimistic.  I also teased you a little by mentioning that some people’s gas bills actually went up after installing a tankless, so let’s find out why right now.

The reason that some people don’t see any actual energy savings with a tankless heater is actually due to one of the other stated benefits of tankless, endless hot water.  That’s right, some people actually see their gas bills go up because more hot water is being used because more hot water is available now.  (This phenomena has been reported to be linked with the presence of teenage girls but there is no scientific data to support this yet.)  Showers that used to be cut short by the hot water running out can now be extended virtually indefinitely.  So if you want to go tankless to save money you should be aware that this is a possible issue.

That brings us to the other big sales pitch for tankless water heaters: endless hot water.  Can that really be true?  No more cold showers after the tub  has been filled or the laundry has been going all day?  Well, yes…with a few restrictions.

Since almost all tankless heaters are designed to maintain a desired outlet temperature, they have to restrict the flow through the heat exchanger to do so.  This means that there is a limit to how much hot water they can provide at a given temperature rise or “Delta T”.   This basically means if you try to use more hot water at one time than the heater can provide it will limit the output volume to maintain the desired temperature.  This could leave you in the shower, with shampoo in your hair, thinking you have been suddenly transferred to a cheap hotel.

The good news is that this problem can be solved by proper sizing and, in some cases, by using more than one heater.  The bad news is that tankless water heaters are fairly expensive and two will probably cost twice as much as one.  The main lesson here is that it is important to take a good look at your hot water usage habits when you are trying to decide on tankless.  You may decide that it’s worth being willing to modify your habits a little.

I don’t want to make the issue sound bigger than it is.  Most 2-3 bath houses will be just fine with one properly sized tankless water heater.  If you have more than 3 baths or showers and expect them to all be used at the same time, or if you have a master shower with lots of body sprays and extra shower heads, you will probably need at least two units.

It’s pretty simple to do the math.  Just figure 2.5 gallons per minute for each shower head and 1 gallon per minute per each body spray.  Check the literature for the brand heater you are considering and find the output in gallons per minute (GPM) at the temperature rise you need.  A 60 degree rise will cover most locations but if you aren’t sure, call you Water Utility  and ask them the winter water temperature (or check it with a thermometer if it’s winter).

Once you’ve done the math, make sure you will have enough flow volume to meet your needs from one unit.  If not you will need two.  The flow rates I gave above are pretty close for tubs and showers less than 10 years old, if yours are older it is a good idea to  measure how much water they are putting out.  Just use a bucket and a watch with a second hand.

So…so far we have determined that tankless water heaters are more efficient and can provide “endless hot water” as long as they are properly sized.  And as long as you don’t start taking super long showers a tankless should save you money.  So why isn’t everyone putting them in?   Well, there are a few more things to find out in order to make a really informed decision.  We’ll discover what they are next time.



Copyright 2008 Bryan Stevens

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