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Green Building Curmudgeon

Been Having Those Hot Water Blues

Small house hot water distribution issues continue to plague me

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The white CPVC pipe shown here is a reservoir intended to mix hot and cold water to avoid the cold water slug, but it didn't do its job and will be removed soon.
Image Credit: Carl Seville
The white CPVC pipe shown here is a reservoir intended to mix hot and cold water to avoid the cold water slug, but it didn't do its job and will be removed soon.
Image Credit: Carl Seville
My moderately scientific method to check how much time it takes for hot water to get to shower. It was a bit of a juggle with the faucet, thermometer, and timer. Always seemed like I was one hand short.

As part of my renovation project, I needed to move the water heater out of its location in a below-grade recess in my crawl space that I was filling in to eliminate the need for a sump pump to get rid of water that collected. When considering the best type of new water heater, I considered both heat-pump water heaters and tankless heaters.

One thing I like about heat-pump heaters is their dehumidification capabilities, but after some discussion with others who have installed them, I determined that the amount of hot water I use would not cause the heater to run frequently enough to provide any significant amount of dehumidification. It would also have to be installed in my new storage room, which didn’t exist when I had to change heaters — a fact that would have required me to move out sooner than I planned.

My next option was to install a tankless heater on the exterior of my house, right next to the bathroom. This was a task I could do before demolition started, allowing me to stay in the house through the first half of the renovation work.

Considering going all electric

The heat-pump water heater was part of a strategy to eliminate my natural gas use. I had already installed minisplit heat pumps for space heating and cooling, so the only natural gas used is for water heating and cooking. I could have replaced my range with an induction unit and eliminated my natural gas connection altogether.

Down here in coal-burning country, that seems like a poor decision in the climate change department, particularly since I have no solar exposure to take advantage of for PV panels. I can purchase green power from the local utility, which I will get around to doing sometime soon to assuage some of my guilt.

The main reason for dumping natural gas was purely financial. Although we have low natural gas rates, the monthly base charges are pretty high, so although I use about $1.00 worth of gas a month, I end up with a bill of about $28.00 of recurring non-fuel charges. Pretty annoying.

I could have used an electric tankless heater, but I have not heard many good stories about them, so I went with a gas tankless heater. The Rinnai unit I installed was mounted on the exterior of the bathroom wall, a bit closer than the old tank heater, but when I first used it I discovered that the start-up time allowed one half gallon of cold water through the line before any hot water arrived.

I have learned a lot about hot water distribution from my friend Gary Klein, the guru of all things hot-water-related, so I discussed the situation with him and came up with a plan.

A good plan, at least in theory

I installed a main hot water line that ran past the shower, lavatory, and laundry, and ended at the kitchen sink. That’s where I installed a Metlund demand pump with buttons to activate it in both the bathroom and kitchen.

Between the heater and the shower, I had my plumber install a 3’ length of 2” pipe as a reservoir for hot and cold water to mix. Since the gas tankless heater always has the half gallon cold start, the intent of the reservoir was to buffer that cold water “slug” and avoid waiting for hot water at every use.

All looked good until I took my first shower. I ran the demand pump to prime the hot water line. When the pump stopped, I turned on the shower and the water was hot within about 15 or 20 seconds, wasting about 6 or 7 cups of cold water – more than I wanted, but not terrible.

Then about 30 seconds in, the water ran cold again for another 30 seconds. Although 30 seconds does not seem like a long time, when you are naked in the shower and the water turns cold, it sure seems like forever.

Time for some testing and investigation

I ran the shower with and without the demand pump and checked the water temperature with an IR thermometer and a timer. Interestingly, hot water arrived at the shower in about the same amount of time whether or not I used the pump. The big difference that was without the pump, the water heater never turned off, avoiding the ramp-up time, keeping the water hot throughout the shower.

Further discussions with Gary as well as Larry Acker at Metlund led us to the conclusion that the buffer reservoir I installed was both too close to the shower and not large enough to balance out the water temperature. The demand pump works well when running the dishwasher or when I need hot water at the sink, but it just isn’t doing much for the shower at all. If the distance between the water heater and the shower was longer, and the reservoir larger, it probably would have done a better job managing the temperature.

Had I purchased a tankless heater with an internal buffer tank, I probably would have avoided the cold water slug issue entirely, and in my endless quest to waste money on my house, I may consider swapping out heaters and seeing if the demand pump works better. But for now, I will just waste a bit of cold water when I take my showers and only use my pump for my kitchen. Another lesson learned.


  1. subahn | | #1

    point of use and heat recovery
    Thought about tossing in a couple of cheap point of use electric water heaters and drain water heat recovery pipe?

    1. DWHR, $500-$900:
    2. Electric tankless heater, $200-$400/ unit:

    If this configuration was possible and you like DIY projects, it seems like this would take care of cold plugs, eliminate non-fuel natural gas costs, and be very reliable. The DWHR could also let you get away with a slightly less expensive and smaller water heater for your shower.

    I know this approach isn't very popular on this site (compared to heat pumps and solar hot water), but I think it's worth looking into especially if you are paying for electricity on a time of use rate.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Kristopher Steege-Reimann
    Thanks for your comments. I agree that it's worth considering point-of-use electric tankless water heaters.

    You wrote, "I know this approach isn't very popular on this site (compared to heat pumps and solar hot water)..."

    On the contrary: GBA doesn't promote solar thermal systems very much (although they have their applications). You might want to see these articles:

    Solar Thermal is Dead

    Point-of-Use Electric Tankless Water Heaters

  3. davidmeiland | | #3

    I didn't want to do it right the first time either
    When we bought this house, it had a propane tank heater in the laundry room, wasting precious space. I soon got rid of it and installed a Takagi tankless heater in the attic. As Carl describes, tankless wastes more water than tank because it sends you more cold water to start with. The cold water slug is a nuisance when washing dishes by hand.

    Recently, I did what I should have done to start with, and made a proper place in the attic for a tank heater, and we're happy again. I only recommend tankless to people who have no room for a tank, have a huge bathtub, or rarely use hot water.

    Carl, aren't you in a perfect climate for a HPWH?

  4. Expert Member
    CARL SEVILLE | | #4

    Water heater choice
    David - I could have used a HPWH, but as I said, I had no place to put it at the time I was renovating. I needed to fill in the low part of my crawlspace that was collecting water, leaving no room for a tank heater, and although I now have a storage room that would fit a tank heater, it is an addition that wasn't there when I had to replace the old heater. Since I didn't want to move out of my house for the entire renovation, I elected to install the tankless heater on the exterior. Also, I the storage room is not very large and I didn't want to sacrifice any floor area. Had I realized the cold water plug/slug issue I probably would have selected one with a reservoir like the Navien, but that is (hot) water under the bridge.

    Kristopher - I have considered drain water heat recovery in the past, but did not for this house. I take such short showers that there wouldn't be much benefit. As it goes, I will waste a bit of water when I shower and avoid the cold slug, although if someone else takes a shower shortly afterwards, they will probably get hit with it. At this point, I don't need to invest any more money or time in this issue. If I do decide to do something, it will probably to swap out my water heater for one with a heated storage reservoir to avoid the wasted cold water. Maybe Navien will donate one for me to check out and review.

  5. davidmeiland | | #5

    I didn't read carefully enough--thought it sounded like you were thinking of changing out your tankless.

  6. wjrobinson | | #6

    What is saving and what is spending?
    To me, the way to using less water is to use less. And the way to heat less water is to heat less water.

    I don't understand how it can be justified to go to the effort in labor and materials and monthly costs and long term maintenance costs to install lots of equipment, in order to do less of something.

    Am glad you gave this a shot Carl and I think it helps all of us learn what not to do. For where you live and for one person taking short showers a very small electric tank unit would be my install, and then do less with water, use more cold water and call it a day. I do not wait for water anymore. I wash my hands for example with cold. In the last decade my water use is down by half. Biggest problem any of us has is a slow toilet leak or other minor drip. Had one that showed up on my water bill that equaled thousands of gallons. Now I check my water meter when near it as the small wheel shows any water leak even down to just a drip.

  7. user-984364 | | #7

    Taco SmartPlus recirc pump
    Has anyone tried the new SmartPlus recirc pump from Taco? It's a standard recirc pump, but with a temp sensor to learn when the house uses hot water, and only run around those times.

    I put one in during our renovation, and rigged temp sensors & circuit monitors to see how it learns. Problem is I keep accidentally dropping power to it, which causes it to go into a new week-long learning cycle. ;) So I have very little data so far. Eventually if I can leave it alone for a few weeks I plan to write about how well it matches our usage patterns. Seems like an interesting idea...

  8. user-917856 | | #8

    Switch to a different gas provider
    Carl - Take a look at all of your options for a natural gas provider. In Atlanta, at least in 2007-2008 the gas providers were plentiful (and deregulated, or is it regulated?). I compared the different providers service charges and cost per therm of gas and found what fit my household the best. Walton EMC only charged $6 or $8 per month service charge. They might even give you $ credit to switch. You're paying too much!!

  9. Expert Member
    CARL SEVILLE | | #9

    Gas supplier & using less water
    Kris - I've looked around several times at different gas suppliers - the monthly charges end up being about the same. The $6-8 fee is only part of the deal, There are other pass through charges that are the same for all providers, so it is practically impossible to get gas supplied for less than $25 base charge per month. Deregulation, combined with low prices work great for big users, but minimal users like me end up paying through the nose for the little bit we use.

    AJ - I didn't consider an electric tank heater because they are about the least efficient from an energy source point of view. In retrospect, a well insulated low boy tank could have been installed in the crawlspace right below my shower, solving several problems outlined in my post. And the money I saved would have helped pay for an induction range, getting me part way towards eliminating my gas bill, although I still have a gas dryer which I would also have needed to replace. Like you, I use cold water in my sink, take short showers, and wash my clothes in cold water. Many things I try out in my own homes are as much to see how they work so I know how to advise clients as looking for better performance. If I still liked construction, I would have more opportunities to try new things out. These days, I need to experiment wherever I can.

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