Tankless Water Heaters 103- What Is A Cold Water Sandwich?

Hello folks.  Welcome to the third installment of my series on tankless water heaters.  In the first two posts we found out that tankless water heaters are really more efficient than most other styles and that they truly can offer “endless hot water” if properly sized.  But we all know nothing is perfect, right?  So what is the other side of the story concerning tankless water heaters?

Well, there are a couple of drawbacks in some cases.  Today we will go into some of those.

One of the main stumbling blocks for many when considering a tankless water heater is price.  A tankless installation in new construction can cost 3-5 times as much as installing a standard tank type water heater, and this extra cost is per unit.  So if you need two units it can be a lot more money to use tankless.  The gas line also has to be larger for a tankless and this could also mean more cost.

In a replacement or retro situation a tankless installation can be well over $3000 when it’s all said and done.  The gas line almost always has to be modified and this can be very costly in a finished home.   There are also venting issues that may come up in this type of installation.  All these add to the costs.

The cost difference between tank and tankless gets even bigger if a circulation or return system is involved.  This typically means adding a storage tank to keep the circulation loop hot.  That means you are paying for both a tankless and a tank type water heater.  There are a couple of other alternatives but they have their own pros and cons.  The point is that, in many cases, a tankless installation is not a money saving choice, it is often more for the convenience of never running out of hot water.  And that is OK too.

There is another issue called the “cold water sandwich”.  While not usually a deal breaker, it is a factor to be aware of.  Basically what happens is that as one person finishes a shower  and turns off the shower faucet, the tankless heater shuts off.  If another person gets in a shower while the water in the line is still hot the heater comes back on when they turn on the hot water and the heater senses flow through the water pipe.  There is a brief delay (a few seconds) before the hot water is created inside the tankless water heater, so a little cold water flows out of the heater before it  gets hot.

This cold water “sandwich” flows through the pipe, surrounded by the hot water that was still in the line from the first use and the new hot water.  When the “sandwich” of cold water gets to the shower the person in the shower can experience a brief burst of cold water right “out of nowhere”.  This can be helped by installing an expansion tank or a tempering tank, but that could be more money.

These are the main issues to be aware of if you are thinking about going tankless with your water heater.  None of them are a strong reason to dismiss tankless, except maybe price in some cases. The real truth is that tankless water heaters really do work very well and have very few negatives.   They are certainly not right for every application but they could be a very good choice for many people.  You will have to decide if a tankless makes sense for you.

I strongly advise enlisting the help of a reputable plumber to make the final decision.  Each installation is as different, just as most houses are different.  An experienced plumber can help you make a better decision concerning your specific situation.   A tankless water heater is a big  investment, I don’t advise “winging it”.


Bryan Stevens

Copyright 2008 Bryan Stevens

Related Posts:

4 comments to Tankless Water Heaters 103- What Is A Cold Water Sandwich?

  • Robert Walski

    im having problems with my tankless heater i can get scolding hot water but when i try to turn on the cold water to make warm water, the tankless heater shuts off giving me only cold water

    • BTS


      It sounds like you are having a flow rate issue but I need more info.

      What make and model tankless do you have?

      Are there any error codes showing on your control panel?

      If you can get me more info I’ll try to help.



  • HI, Bryan,

    I bought an in-line water heater about 10 years ago, a French one called Aquastar, from an Oregon company from an American importer in Vermont or thereabouts. $500, I bought one for $550, with valves so I could connect to solar some day. It had a pilot. (Wastes gas so I turn it off when I’m not using it, but had to pay to replace the on-off valve; apparently it’s not made to be turned on and off so much.)The other option was a Bosch for $1,800. forget that, I’d never use that much hot water.

    Where to put it? Indoors could expose me to carbon monoxide, so I put it in the garage on an outside wall because it said I needed outside vents. Wrong, obviously, the soffit vents supply air to the garage, so there’s plenty of air supply.

    The garage isn’t heated, So, the copper pipes in the heater will freeze up and crack, which they did once. I managed to get silver solder to barely work. Maybe I needed a hotter soldering iron? It seemed still to weep, but I had no more trouble with water leaks.

    I left the pilot on when it would get cold, and tried to put an insulation blanket around the box to hold that little bit of heat in and covered those vents in the back through the garage wall that I didn’t need anyway.

    Once, I forgot to turn the pilot on, and I came home to water spraying all over. Turned it off, have done without hot water because I’ve got other things to take care of.

    Maybe you can tell me how to solder up or fix split copper pipe reliably?

    Basic problem with these is having them in unheated space. There’s got to be a better way.

    A couple years after I got this, the importer switched brands and got electronic ignition, getting rid of pilot losses.

    Still: how to keep these things from freezing? Where to put OR insulate thoroughly and have a tiny thermostatted heater?

Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>