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A Lot of Hot Water!

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Showering cropped

Let’s get real about how much hot water (DHW) you and your neighbors really use daily.  Some of you are abstemious, washing your clothes in cold water with an Energy Star washing machine, washing dishes efficiently with an Energy Star dishwasher, and taking short cool showers and not using the tub for a bath.  More power to you – you are really helping to save our planet’s resources.   My research and experience shows that you are in the minority in the USA.

There are several important parameters you need to ascertain when you talk to a family about their use of hot water.  You need to know about the DHW use patterns for the washing machine and dishwasher and how many times per week they are used.  After that, you need to ask about the shower duration and frequencies of each occupant, what the flow rates are from each nozzle and the preferred water temperatures.  You need to measure the Jacuzzi type tub and figure out how often it is used.  Then crunch the numbers.

You may take very short showers but few do today.  I should know – I interview clients for new solar hot water systems, have done a lot of DHW research, and written a new textbook, Solar Hot Water Fundamentals.  I have concluded that DHW use is now dominated by shower/tub activities – the new washing machines and dishwashers are so DHW efficient, their use is minimal.   I am one of those DHW hogs – a good shower (hot, long, and lots of flow) is one of my little gifts that I give myself for good behavior and feeling good.   And, when I go for a run at lunch, I take another short one too.  OMG, you groan, he’s so old school!

OK – I am – but I am not alone.  Students in my classes and many clients are just like me and often WORSE.  One rasta haired fellow admitted that he enjoyed 20 minute showers at 2-3 gallons per minute and his mom and sisters camped out in the shower for even longer!  I interviewed a rural Rensselaer County family whose Dad works for a solar company (4 athletic teens and one Jacuzzi tub).  Their daily DHW use came to 200 gallons per day – showers/tub use accounted for about ¾ of the household load.  That comes to about 33 gpd per person (gpcd).


“Terrible,” you say?  Get real – there are many other clients I deal with who have showers with multiple nozzles they use a lot and others whose appliances are DHW hogs.  Rental unit DHW use per person can be worse, especially if the rent does not feature a separate utilities charge.  Here’s the spreadsheet that I use to calculate shower/tub use (the biggest DHW demand) after talking to the clients.


But – before you crunch the numbers, you’d better think a little about the family’s future.  Will those shower hog teens be leaving soon for college?  Will the tweens be coming back home after college and maybe with a surprise infant too?  Are there some little ones at home now who will become shower hogs in a few years?  Will that cute retired couple be leaving soon for an assisted living community, selling the house to a family with teens?  Right size the system you design but suggest that your clients consider a changing DHW demand profile.

There are many research studies out there that purport to predict national and regional DHW use patterns.  Chapter 49 of an ASHRAE guidance document from 2007 (Service Water Heating)  presents a graph that shows the DHW use daily patterns, I discuss, really well.  Figure 14 shows the large difference between High and Low Users and Table 6 provides average family use patterns – about 65 gallons per day for a 3-4 person family with the bulk of the use in the morning and dinner time.  




The Building America Study is often referenced in this arena.  http://www.nrel.gov/docs/fy10osti/47246.pdf  They concluded that DHW use is dependent on a basic demand number plus an adder based upon the Number of bedrooms (Nbr) and Number of occupants (Np).  See Table 10 below.  Table 12 shows that an average shower lasts 7.8 minutes (mine are usually 9 minutes when I shave in the shower).   Average flow rate is 2.25 gallons per minute for a total of 28 gallons per day for a 3 bedroom house.



The Syracuse University Center of Excellence has begun a very impressive DHW production comparison study in a modern office building you can see on the south side of I-690.  Nine different DHW heating systems are operating and being closely monitored for energy consumption by CDH Energy Corp. in Cazenovia, NY.  Right now, these 9 systems see a 10 gallon DHW draw six times a day.  New DHW Draw patterns are being developed now based upon a regional DHW use pattern research project.  The results from this groundbreaking DHW system study and the DHW draw pattern research will provide New Yorkers with pretty sound conclusions about the most cost and fuel efficient DHW systems and how much DHW midstate New Yorker homeowners use daily.


Finally, no DHW discussion for your readers would be complete without some discussion of a zero energy house consumption pattern analysis.  Thankfully, I found one in “Building Energy” fall 2012, (Vol. 30, #2) published by NESEA.  Engineer Marc Rosenbaum bought a 10 year old 1589 ft2 house on Martha’s Vineyard and retrofitted it to be much more energy efficient.  He created a serious monitoring system and presented data for a year of post-retrofit energy consumption.  Two people dwell in the house and only use approximately 14 gallons of DHW per day.  

Marc installed an 85 gallon Marathon hot water heater (for storage only) and hooked up the DHW plumbing to a Nyle Geyser heat pump as a side arm heat source.  But since the air temps were low in the winter, he turned off the heat pump and just used the electric elements in the Marathon to heat his hot water.  He found that the elements use about 25% more electricity to heat his DHW than the heat pump did.   He argues that if his household used more hot water each day, energy savings from the heat pump would be larger. 

He presented a monthly energy use profile for his house which is presented below.  (Dan – I need to get a color rendition of the graph – B&W isn’t good enuf)   You can see that even at the paltry rate of 14 gallons per day, production of DHW in a low energy house is a big part of the home’s energy budget (if he used a more normal amount of say 30-40 gallons per day, the energy required to heat DHW would dwarf all the other demands except heating in December-February).   You will also notice that energy use to heat water doubled in the winter – maybe because they took more showers and cooked more and/or because the heat pump was less efficient and more Marathon tank standby losses occurred. 


Conclusions – DHW use is very different for each household and will change over time as families grow and shrink and appliances change.  Showering has become the biggest consumer of DHW in today’s energy efficient homes.

Peter N. Skinner  P.E.  Principal Scientist -  E2G Solar LLC; Custom SHW designs and installation; 2 Snyder Road, West Sand Lake, NY 12196;  e2g@verizon.net ~ 518-369-3208 (cell)

Comments on "A Lot of Hot Water!"

  1. Dan Gibson's avatar Dan Gibson February 15, 2013 at 9:08 pm


    Thank you for enlightening us that the abstemious are few! To be fair, I’m not sure which group we fall into yet, as we have never been able to measure our hot water before, or our cold for that matter, since we get our water from a well. But now I can…

    Amazingly, since we moved in on 12/20, we have used 4139 gallons of cold water and 2084 gallons of hot, exactly half is hot! The amounts don’t mean much yet because we didn’t have a clothes washer until last week and still don’t have a dish washer.

    So I won’t extrapolate or calculate, except to say that we have used 391 kWh to heat the water. If there weren’t any heat losses, that was enough energy to heat 2,353 gallons of water ((391 kWh * 3413 BTU/kWh)/(8.34 lb/gal * 68 Temp Rise)), which mean that 89% (2084/2353) of the energy used went into the hot water that left the 30 Gallon Electric heater. That is a lot higher percent than I expected.

    I am really looking forward to hearing more about the study you mention being performed in Syracuse. That should be very helpful to me and others trying to make decisions on how to heat water.

  2. OEIC default avatar moagman February 27, 2013 at 5:59 pm

    Great information-(Peter thanks!)

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