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Alternating water temperatures aka: “Cold Water Sandwich”

This is the first in a series of blog posts on tankless water heater problems that I have run into over the years, starting with the more common and more “Real” ones.  The Cold Water Sandwich (CWS for short) is a phenomena associated with tankless water heaters that is usually not a large problem once understood by the Homeowner (HO for short)  It is actually a FAIRLY RARE problem but for someone who has it, understanding it is the first part of the solution.  Here is what can happen in some homes and it seems to be more prevalent in homes with insulated hot water piping.  I’m going to pick some numbers out of thin air for this explanation.

One person showers or otherwise uses hot water for a time at a far end of the home. (Long pipe run is usually a factor but not always)  They get out of the shower and go about their day.  About 10 or 15 minutes later, maybe longer, someone else goes to get into the shower. The water is already hot so they get in and begin their shower.  About 2 or 3 minutes later they feel the water temperature drop considerably!  This last for around 15 - 30 seconds or so and then the temperature climbs back up and sometimes it’s even hotter than it was before.  One, COLD WATER SANDWICH!  This is what just happened in the plumbing system:

Let’s say the water heater is set at 125*F. Person #1 showers.  This heats up all the hot water piping and leaves the system full of hot water at 125*F.  When they finish their shower the tankless water heater turns off.  100 pipe feet of ¾” hot water pipe holds roughly 2-1/2 gallons of water that is now hot.  The water sitting still in that pipe begins to cool. Plastic piping has a bit more insulating qualities than copper so let’s say 10 minutes goes by and the water in the pipe cools to about 110*F.  Person #2 starts the shower.  110* is plenty hot and still requires mixing a little cold so the person sets the shower temperature at a comfortable level of around 105*F mixing in some cold. 110* is coming in the hot side of the shower valve and let’s say 60* water is coming into the cold side.  They are mixing only a slight amount of cold water.  A standard shower head rated at 2.5GPM (at 80 PSI in a lab) actually flows around 1.5 GPM in a home full of lots of plastic water pipe. We are mixing in about 10% or so cold water so we are flowing about 1.2GPM or so of hot.  There is 100 pipe feet between that shower head and our tankless water heater.

The tankless water heater takes about 5 seconds, some models a bit longer to fire up.  So for the first 5 seconds of flow COLD UNHEATED WATER flowed into the hot water line, followed by nice hot water at 125*!  Person #2 is in the shower and this slug of cold water is doing some mixing with the warm water in front of it and the hot water behind it. It takes a minute or two, maybe more depending on the actual flow to get there…but it’s coming.  Poor person #2 feels that temperature drop drastically.  Maybe not to 60* but if your shower temp drops even 20* believe me you are going to get out of the way!  Then they feel it warming back up so the slide back under the water, only to feel it get too hot!  They get out of the way again, adjust the shower temperature and continue their shower.  That friend is a Cold Water Sandwich!

What do you do about it?  Well first off let’s talk about things that make it worse.  Higher hot water temps make it WORSE! Turn your heater down.  That will help. You don’t need to heat water to 125, 130 or 140 only to cool it back down to 105 to shower.  Try setting your tankless at around 115* or even 110*.  That can help a lot.   Today’s bigger and bigger homes and lower flow fixtures make it worse too. More footage of smaller diameter water piping feeding fixtures using very little flow of water join together to make this the problem it is.  Sometime even with tank water heaters!  Insulated hot water pipes can make it worse because they don’t cool off for a long period of time.  If you know your home has this problem, try avoiding the situation.   If you know a couple people are going to shower back to back keep the times between them to a minimum, or make the time between them longer.  Hot water lines usually cool down pretty quickly unless insulated.   Uninsulated pipes probably cool down in less than 30 minutes.  Insulated pipes make take an hour or more depending on the temperature around them.  Basement or crawlspace pipes are going to cool off a lot faster than ones in ceilings or walls.  Not much you can do about it now other than understand it. 

Now the first and most obvious solution is to turn on the hot water and let the temperature stabilize before you get into the shower.  That however means running the shower for a few minutes to get all of the tepid water and CWS out of the pipes.  (Wasting water)  It works, but some people will not want to do that even though you really are only flowing 2 or 3 gallons down the drain.  After all it does all add up. Another solution is to install an on demand hot water recirculation system like the Enovative “On Demand” circulator (www.enovativegroup.com) and use either a push button or motion sensor to activate it.  This will push that tepid water and the CWS back to the water heater, filling up the piping will hot water just in time for you to use it.  Another solution may be to install about 6’ of large diameter pipe right after the tankless water heater (2” or 3”).  This will hold some hot water and act as a buffer for the CWS reducing it considerably or possibly even eliminating the problem. 3” pipe 5’ long will hold over a gallon of water.

So that’s what can happen with a CWS.  Again it is a pretty rare and very minor issue all things considered.  After all, there is no tank to bust and flood your home. (much more common than CWS) and no running out of hot water so once the temperature stabilizes, you can stay in that shower as long as you want. Check out my YouTube series from the link above and I hope this helps.  Stay tuned for more coming.

Rinnai has introduced a new adapter to allow the use of PVC on CONDENSING units ONLY!  Many will say “It’s about time” but I still maintain that the Rinnai concentric vent is easier, cheaper and much faster especially on shorter runs!  It is faster to simply push one joint on one pipe than it is to cut-prime-glue-push-hold…on two joints!  Plus if you need a long vent run the first thing to look for is where else you can put the unit.  Sometimes it is unavoidable and on very long runs certainly PVC will come up less costly.

The Maximum vent length will vary based on vent size (3” or 4”) and type of fuel.  Consult your models instruction manual for more!  I’m not listing that here because it’s too easy for readers to confuse it with other models or manufacturers and it may change as models change.  Also, the concentric venting on natural gas fired units can vent longer as well.  Again, refer to your manual or the Rinnai Website for specific information.

Stay tuned for more updates and check out the youtube videos on tankless water heaters on the Scott Gregg Channel.