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New build - tankless hot water heater - recirculation pump needed?

Michael Lamb
6 years ago

My builder wants me to use a natural gas powered tankless hot water heater in my new build instead of a tanker, but doesn't think I should get a recirculation pump with it. He thinks it defeats the purpose of the tankless in the first place, and wastes energy. What do you guys think?

Comments (30)

  • Annette Holbrook(z7a)
    6 years ago
    last modified: 6 years ago

    For the first 18 years of this house, we had a standard water heater with a recirculating pump. Did the trick, maybe costly at the outset but hot water was always johnny on the spot. A little over 2 years ago we switched to a tankless and dh and the plumber acted like I was being a total princess by asking if we could get a recirculating pump in this new set up. So I dropped it. So now I curse the entire thing multiple times a day while I stand around waiting for hot water. I generally turn the water to hot and go off and do a few things while I wait. I don’t want to think about how much time, not to mention water, we waste on a daily basis!

  • Pinebaron
    6 years ago

    Tankless with recirculating all the way, don't pay attention to your builder. Our current rentals take ages for hot water to come up from the garages to the third floor where our master is. We are installing two Naviens in our new construction.

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  • vinmarks
    6 years ago

    Can the recirculation pump be added later on?

  • oliviag55
    6 years ago
    in the old days (ten years ago.. ), a tankless water heater and received pump were a no go. today, a few offer timers and programming to allow the pump to only run during shower hours .
    without that feature, a receiver pump is a waste.better to adjust your layout so that no fixture is further than 50 feet from the water heater, anyway.
  • oliviag55
    6 years ago
    I don't love autocorrect. read recirculation for receiver in above post, please. my apologies for not proofing.
  • Oaktown
    6 years ago

    We have tankless with a demand circulation pump (Metlund). Doesn't recirculate until you press the button in the bathroom. Press, wait a couple of minutes (brush teeth, etc.) and then turn on shower. There is a still bit of cold water to clear the pipe but the tankless heater already is engaged. We are more interested in water saving than insta-hot.

    Good luck!

  • David Cary
    6 years ago
    last modified: 6 years ago

    Your builder is absolutely right that a recirc pump does defeat the purpose of tankless. Additionally, a recirc pump will void the warranty on some tankless units - it used to be all the units but that has changed a few years back.

    I had a recirc pump with a tank at the last house. The losses from the loop were around 10 times the tank losses. Insulation requirements on the loop are better now (at least in our area) but you can still expect more losses than a tank would have. Also the tankless unit gets quite a workout as far as longevity/maintenance.

    Now that being said, we have tankless in a small rental. It is terrible. We have to run the water for quite some time and washing clothes and dishes is frustrating because we have to use superheat modes to deal with the cold water loads. The runs are relatively short since it is 1700 sqft perfect rectangle.

    There is no perfect answer. If you can tolerate buttons and waiting then Oaktown's solution is the best. We won't - and we drive electric cars and had solar at our last house. My plan is POU electric at the sinks. These are small tanks under the sink that give you instant hot water without wasting water or waiting. Sinks use so little hot water that paying full electric for it is no big deal. Then we will have a heat pump hot water heater (or solar) for the showers/tubs/washing machine. These will be pretty short runs and direct - ie no branches. Each run is 15 feet or less.

    Still trying to find standby losses for those tanks but they are small enough that it would be easy to encase in additional insulation.

    Bosch is one of the tank manufacturers and they are about $200 a piece. Our master sinks will share one, then kitchen sink, and trying to decide if powder room gets one. Compare that to the initial cost of the pump and plumbing the loop. For us, it was about $1000 many years ago. And that is neglecting the ongoing costs.

    I should mention that climate matters. If you are in a really cold area, the wasted heat isn't really wasted if kept in conditioned space. I live in a heat needed 3 months of the year so the wasted heat was more of an air conditioner burden.

  • Michael Lamb
    Original Author
    6 years ago

    A few facts about my house:

    1. 2600 sqft 2 story with an unfinished basement(will finish in 1 to 2 years)

    2. 3.5 baths

    3. Plan on having two showerheads in the master bath

    4. Family of 4

    A couple of questions(I have no experience with a tankless)...

    A) So how much longer would I have to wait for hot water than if I had a tank heater?

    B) Can I get some sort of timer system so that the recirc pump is not running all the time? Maybe just run it in the morning when people would shower, then in the evening?

    C) If no water is being used, can the recirc pump just run like 60 seconds every five minutes to provide hot water?

  • Michael Lamb
    Original Author
    6 years ago

    What is the cold water sandwich?

  • Pinebaron
    6 years ago

    A cold water sandwich effect refers to the phenomenon where the water from a heater is warm for first few minutes, then there is a burst of cold water, and then the water quickly returns to being warm again. This is a common phenomenon with tankless water heaters.

  • Michael Lamb
    Original Author
    6 years ago

    Does this happen with all tankless heaters? If so why, and how can it be avoided?

  • htwo82
    6 years ago

    We don't have tankless heaters, but we put in a recirc pump and turned it off about 6 months later. Total waste of money, IMO (for the pump and the extra gas). Now we turn the water on and wait 20-30 seconds and it's hot, without having a ridiculous gas bill.

  • David Cary
    6 years ago

    I have a tankless in a rental and do not notice a cold water sandwich because of the unit. It has to be the cheapest one available as this is a tract spec townhouse by Lennar.

    The cold water effect that I do feel is from the unit in the cold attic. So first I get room temp water in the pipes and then I get cold attic pipes and then I get hot water. This would happen with any unit and is a function of placement.

    Modern pumps have timers and thermostats. Mine ran about 5 minutes of every 15. Doesn't minimize the heat loss that much. The problem with a timer for showers etc is that the piping is run in a loop for a recirc pump so you pay a huge time penalty when the pump isn't running. So it maybe 1.5 minutes with a non pump system all the time and 3 minutes if you have a pump system when the pump is not on.

    Our tankless takes about 2 minutes I think and it would be 1 if tanked. Either one is not acceptable in my opinion.

  • Pinebaron
    6 years ago
    last modified: 6 years ago

    Mike: here's an excerpt: There is some water which remains stored in warm condition at the mouth of the faucet or other outlets, which means when someone runs the tap or the shower, he will receive some hot water instantly. However, a tankless heater takes some time to heat up the exchanger and during that time, some cold water may get through the exchanger and reach the user. This is what results in the cold water sandwich effect. This should not be too great an inconvenience if you are turning on the heater for minor use, but you may not like it too much if you are under a shower!
    The phenomenon is more common to electric water heaters than it is with gas powered heaters. The reason for this is that gas reaches the operating temperature almost immediately, whereas electric heaters may take a minute or two to reach the required temperature for heating.

    Water heaters such as the Navien has a small built in storage tank which retains hot water it's tank; when a faucet opens (along with recirculating pump) has an ability to provide a continuous stream of hot water to the faucet hence avoiding a cold water sandwich effect (in theory anyway).

  • dbrad
    6 years ago

    We just purchased a Westinghouse tankless with built-in recirc pump for our new build - for all the reasons already stated in here. The plumber ran a dedicated recirc line (added $700 to the rough-in plumbing bill). I also plan to add pipe foam wrap to all of the hot water lines in the house. By code the plumber has to wrap all lines in unconditioned space so I'm just going behind him and adding it to the rest.

    The tankless unit we bought has some intelligence to it as to when it recirculates - that's supposed to alleviate the energy waste concerns. I'm not entirely sure how it all works but lots of models nowadays have it.

  • PRO
    The Cook's Kitchen
    6 years ago
    last modified: 6 years ago

    I have lived with a tanklesss water heater for 23 years and would never go back to tanked.

    First, you do not “need” a recirculating pump. Using a tankless does not have a longer lag time than tanked does. Where you locate your water heater, tanked or tankless, determines the amount of cold water in your pipes that must be flushed out before the hot water from your source arrives at the fixture.

    Ours just happens to be around 90 feet away from our master. Branch and twig piping system because it was a retrofit. We just start the 2.5 gallon shower head, and then brush teeth, or hair, or other tasks, for the approximately 75 seconds for the 2.8 gallons of cold water in the 3/4 pipe to clear, and then take our showers. With a direct 3/5” PEX homerun, that wait time could be reduced by 10 seconds.

    If you pipe with 1/2” instead of 3/4”, then your numbers will change to little bit longer wait. I don’t consider a wait of under 2 minutes to be an issue. Or the loss of less than 3 gallons down the drain to be a great loss. That wait will exist regardless of production method. A pump is an energy waster. And a pretty big luxury to save you less than 3 minutes. And that is assuming a pretty long piping run

    The cold water sandwich is mostly a myth if the heater is sized appropriately. If it is undersized, it’s a small reality. Large homes may need more than one tankless. Homes in cold climates may have ground water that is too cold to work well with tankless without a solar preheating tank.

  • jn3344
    6 years ago

    I have tankless. 3 years now. I love it. No recirc. However...

    Longest run is to the master. It does take a long time to come out hot. Longer than a tank without recirculation? No. If I had it to do over I would put in point of use for the sink. Laundry? Short run no issues. Kitchen? Little longer. Maybe pou there too. Because it is used so often.

    Good luck!

  • David Cary
    6 years ago

    Tankless absolutely adds some lag. How much probably depends on the unit and of course electric is worse. Obviously water stored hot comes out instantly hot while water that is cold needs some interval of time to heat up. That is physics 101. Now it is usually small relative to the plumbing lag.

    I have to say, my unit/pipes have a terrible lag. Some of the runs are very short but the lag is still significant. But I haven't had a tanked unit here so it could be 90% pipe issue.

    The Cook's kitchen. 1/2" pipe vs 3/4" means less water to flush out and quicker times to hot water. The flow limiter is the shower so you do better with smaller pipes.

    Pumps are a huge energy waster. Biggest mistake with my last house.

    Nothing green about using Natural Gas of course. Most truly green builders build all-electric houses.


  • M
    6 years ago

    Depending on season, our tankless heater resulted in 2-3min lag before the first hot water would show up. That doesn't make a good experience either for taking showers nor for washing dishes in the sink. I am sure this extreme lag had to do with details of how our plumbing was set up.

    When we installed radiant floor heating, we used the opportunity to switch the tankless heater for a boiler with an indirect tank. It now powers both the radiant floors and the domestic hot water.

    The plumber reviewed the existing plumbing and made a couple of changes. Apparently, some of the original pipes had been too big and the extra diameter contributed to the problem. He replaced these pipes, and also installed multiple dedicated return pipes for a recirculation pump. The pump is controlled both by a thermostat and a timer.

    We couldn't be happier. Endless hot water, and it's also (almost) instantaneous. But that's not to say that a tankless heater is a bad idea. Our general contractor bought the old tankless unit from us and uses it in his own house now. As his plumbing runs are much shorter, the tankless unit works fine even without a recirculation pump.

    If you do need a pump though, keep in mind that many tankless heaters aren't designed with pumps in mind. The heat exchanger is more likely to get clogged/damaged without appropriate in-line filters. You might void your warranty. And you almost certainly should get an expansion tank. You might also need a separate backflow-preventer valve; but that depends on the specifics of your plumbing.

    In other words, it really pays to talk to an experienced heating and plumbing contractor. There are lots of different options. They of course all work to some degree. But for optimal performance, you need to take a really close look at how your house is configured and what your priorities are. It could be blatantly obvious what the optimal solution would be, or it could require a good amount of experience to spot the pain points.

    Things obviously get more complicated if you have lots of really long plumbing runs.



  • homepro01
    6 years ago
    last modified: 6 years ago

    I have a Rinnai gas tankless water heater with a built in recirculation pump. The recirculation pump is on a timer and is used that way. The newer models from Rinnai, have a motion sensor to turn on the tankless water heater when you are in the room where the motion sensor is placed. My longest run from the tankless is about 50ft. In the morning, I turn on the shower and in 5 seconds I step into the shower because the recirculation pump is running. My gas bill went down almost 50% when I changed over from an older tank water heater to a tankless unit. I also added a gas fireplace at the same time. Never experienced the cold water sandwich with my unit, I have a RUR-98i. I also have water saving fixtures so the shower heads, faucets, etc are all 2.5gpm or less. Rinnai does not require recirculation plumbing because it has a little cross over that can be installed at the furthest fixtures.

    Rinnai Wifi Recirculation Information

    Good luck!

  • oliviag55
    6 years ago
    we have a rinnai instantaneous water heater, gas. over 4 years old, no recirc . our plumbing runs to 3 showers are limited to under 50 ft. low flow showerheads. if I run the shower for a minute to get hot water, I waste a gallon, at most.
  • PRO
    Jeffrey R. Grenz, General Contractor
    6 years ago

    We just upgraded our specs to tankless with integral recirc pumps. This reduces the need for multiple tankless water heaters in our larger homes. Less equipment, less cost, less water use. Green.

  • David Cary
    6 years ago

    More energy use. Not green.

  • dbrad
    6 years ago

    David, as I understand it the intelligence they are building into these newer recirc pumps should alleviate that concern. They are so new (albeit improving with each generation) that the jury is still out as to whether the numbers will work, but the concept has merit.

  • PRO
    Jeffrey R. Grenz, General Contractor
    6 years ago

    lol... lets go through that without posting my energy calcs.... (btw A plumber friend working for the local green energy group told me in private that tanks were actually more cost effective if you have a herd of teenagers living at the home... as it doesn't allow endless hot showers). My last home had a 75 gas tank with a recirc due to my 4 kids. Built a few with multiple tankless, both with and without recircs, some with added holding tanks to eliminate the cold sandwich ... it sucks to explain that to a client. Eliminating the 2nd tankless may not save gas usage, but eliminates the cost of the 2nd, including upsizing a lot of gas lines. The energy saved by not building that 2nd unit alone will probably cover the pump use for a couple of decades as well as save the cold water poured down the drain. Further reading your previous comments, yes a heat pump is probably the way to go, but that's going to need some selling to our "instant" trained consumers. My current home, a 60 year old "project" actually has a heat pump HVAC, but a gas water tank nearing end of life... so I might integrate a new solution and try it myself. Have a client integrating a pool heat pump with their home solar system shortly, but they wouldn't consider it for the home due to comfort perceptions.

  • PRO
    Springtime Builders
    6 years ago
    last modified: 6 years ago

    For the most affordable hot water in total life cycle costs, check out Stiebel Eltron's 80g heat pump water heater. They are bulky with interior space requirements but come with none of the tankless drawbacks and are well reviewed. A healthier choice dealing with cool, dry air of heat pumps instead of gas combustion exhaust. Perfect for basements, temperate garages and large mechanical, pantry or laundry rooms.

    I think quality heat pump water heaters are among the best investments possible for replacements and especially new homes that account for them in design.

    I LOVE my 10 year old Rheem Marathon, should a heat pump water heater not be possible. Marathons are touted as most durable water heaters available, with best in-class efficiency. Electricity utilities are beginning to use tanks as batteries for excess PV generation, with good sharing incentives. Tanks are the new tankless!

    Reallocate money budgeted for gas infrastructure to efficient, better performing electric appliances. It will cost less, be healthier for inhahbitants and better for society. Electric grid is getting cleaner while gas industry is increasingly recognized as a major polluter; air, surface water, aquifers and wells.

    Too late for most researching recirc systems, but strive for improvement in future construction: smaller, intelligently designed homes (and plumbing footprints) dont need recirculation. If it's too late, explore serving distant problem areas with seperate heaters. Pumps should only be used with timers, buttons and occupancy sensors. Continuously running recirculation pumps is never a good idea.

    Even the best plumbers might benefit with sizing and design help or serious oversight.

  • David Cary
    6 years ago

    dbrad - I will concede it is possible that some intelligence could help. But I will believe it when I see it - not trust what a sales person says.

    My last pump had a stat and a timer. It was worlds ahead of pumps that don't have that - but it was still an issue.

    Quick story. Wealthy guy, 13k sqft, 2 straight electric tanks, recirc pump. $1000 a month electric bill. Became concerned for environment. Put pump on timer. Wife got home at midnight - very pissed wife. Divorce forced house sale (jk on that).

    Once you are set up for a pump, wait times will increase dramatically if the pump isn't running.

    The Nest has some "intelligence", but when you do something out of the usual, no intelligence can fix that. And when that means 3 minutes to hot water, it takes tolerant occupants.

    Physics dictates that hot pipes lose heat. No pump can ram the hot water to the faucet instantly (or close to it) when you turn it on. So a pump will cause excess heat loss. Now - motion detectors help a lot. They don't make it zero for sure. Only a button can do that.

  • conecaster Mcgill
    3 years ago

    I installed a tankless 6 years ago and will never install a tank again. I did it myself and will do it again if I am ever in a new house.


    I have a 120k Noritz unit that performs flawlessly.


    I just don't get it guys. Installing a tankless without recirc is like trying to start your car with a half dead battery. Really!


    "Your builder is right, it defeats the purpose of tankless" nonsense!


    I did not have recirc initially. I learned by experience you do not just replace a tank with a tankless. It is a bad result for hot water supply. Slow and inconsistent.


    Internal recirculation tanks might help but never having used one I have no opinion. But unless you circulate water and insulate your pipes to maintain the heat you are really missing the boat. Recirculating inside the heater means your pipes are cold and it takes longer to get to a tap because unlike a tank there is no pressure in the lines to radiate towards the taps. Why store water inside your heater when you need the heat in the pipes for faster delivery? That is just a limited solution to the issue in my opinion.


    How much does it cost to keep the lines hot. Depends on heat loss and vol. of water in the lines. Insulate!!!


    I would say my pipes take very little in cost to keep hot. Maybe $5-7 a month. I used 3/4 inch pipe for recirculation to store more water for start up and to feed 110 degree water into the heater. I do not have a timer to shut off recirculation at night so I could save a few bucks if I did that.


    Its a simple thing to lay out. You go to the last tap in the system and put a return to the heater from there. You need to have a cold water supply for the recirculation. A one way check valve between the recirc and the street supply prevents hot water from pumping back to the street as hot water pressure rises. A second check valve goes between the last tap and where the street supply feeds the recirc line to prevent cold water pressure from the street from back flowing through the hot water lines.


    Why any builder would not just put a return line back to the heater is beyond me. Its really not a lot of extra work. Especially with all the cheap Pex being used now. A small grundfos 1/25th hp pump is all that is needed in most cases. I use a Honeywell aquastat to trigger the pump. The pump engages maybe once an hour for maybe a minute to raise the water temp from 105 to 110.


    Heat loss? Did you insulate the pipes you say are losing heat from? You can lnstall a timer to turn off recirc in the night so you use no energy at all. You can do the same during the day when you are not at home.


    For some families you could have a daily timer that changes recirc to match your schedule. The hot water works with the recirc off, it just does not work properly as iz well known.


    Tankless is a superior technology. I did not think the endless supply would matter to me, its noticeably better.


    A tank cools as soon as you start using water out of the tank and cold water replaces it. The temp drops and drops the longer it flows. A tankless heats water on demand and it is maintained at the same temp. Its really is an advantage.


    But, as has been stated here by users, un managed hot water supply sucks and who cares if it costs a little bit of money to heat and pressurize your pipes so hot water is instant. Its way less expense than keeping 40 gallons of water hot in a tank.


    I would never heed the advice by someone on this thread that you don't need recirc.


    Have your builder install a true full house recirculation system. Don't pay attention to this "you don't need it stuff."


    If the builder balks or talks down to the idea get someone who knows what they are doing to come install it and over ride their obstinate attitude. Just do not listen to the talk of having a better system is not needed.


    You are going to great expense to have the best hot water supply you can have. Don't let someone with out your best interests in mind wreck your enjoyment of a great tankless system.


    Recirc all the way!!!


    Gee people, why would anyone not do this in new construction when its way easier to do.







  • David Cary
    3 years ago

    The heat loss in a tank is dramatically less than recirc but it does depend on fuel source. The insulation on a tank is far more than on your pipes.

    The best plan is keeping your heat source close to your need for hot water.

    Tanked do allow hot showers in times of no power. Also allow heat pump.

    .