How long is too long for hot water?

Discussion in 'Tankless Water Heater Forum' started by rjordanjr, Jul 30, 2014.

  1. rjordanjr

    rjordanjr New Member

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    2
    I have a tankless water heater that is set at 120F. The unit says it is putting out 118F water. At a faucet about 10-15ft away from the heater it takes 3min 30sec to go from 70F to 114F. Is this too long? Is this acceptable? Its a 2 year old house.

    Are there any standards? Anything to point to that I could give my builder?
  2. Dana

    Dana In the trades

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    A typical/normal ignition delay is 5-10 seconds, and at 2 gpm (a moderate to fast flow for a sink faucet) it takes maybe another 15 seconds to go through 15' of 3/4" plumbing (faster for half-inch). If everything is plumbed & operated correctly you should see 100F+ water in under 30 seconds, not 200+ seconds.

    Make & model of the tankless might be useful information, and if the faucet is a 1-handled mixer type, make & model of the faucet.
  3. rjordanjr

    rjordanjr New Member

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    2
    The hot water heater is a Navien NR-240 and it is set at 120F The faucet is a dual handle. Not sure of brand, Glacier bay?

    It takes 10-15sec for ignition at the hot water heater, but how long does it take for water to get to temp? The tankless hot water heater does put out cold water for a bit, its not literally "instant".
  4. Dana

    Dana In the trades

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    Once the ignition is happening the water coming out of the tankless should be at full temp within a second or two- there just isn't much thermal mass to the very low volume of water in the heat exchanger. The flow rate and the water volume of the plumbing between the tankless & tap determines the delay.

    If yours is a Navien 240A, it has a tiny buffer tank inside, and it needs to be programmed for which hours of the day it keeps the buffer hot. If you're cold-starting it with a buffer tank cold, there will be a substantial delay, since unlike the heat exchanger, the tiny tank has some volume & thermal mass to it.

    With a dual-handle faucet and the cold tap off, there should be only the ignition and flow delay.
  5. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    I've been seeing this when looking at homes recently as well as one we are in. The heat up time you are getting is similar to what I've seen in a much older home (galvanized pipes) retrofitted with tankless. In it we have to waste a lot of hot water for "hot" clothes loads, for the dishwasher, for hand washing of dishes and for showers. To keep the delivered water hot for the appliance fills we have to run hot tap from nearby sinks. It is far less efficient overall than a tank in a colder climate...primarily because most of the lukewarm/hot water goes down the drain rather than being used. The long runs to the showers run fairly low temps, likely about 105 F max in winter and 110 F in summer.

    I've seen about two min. heat up times in two and four year old homes with about 60 ft runs to Rinnai tankless in summer. I didn't measure temp precisely, it was a hand check timed with a watch, and never hot enough that I couldn't hold my hand in the stream at the kitchen sink. I was disappointed with how long it took. Part of the problem is how low the delivered temp is from the tankless water heater. Capping out at 120 on the supply means that delivered temps of 115 are a challenge. I see this lack of useful temp control as a major shortcoming of tankless compared to tanks.

    Compare this with a mid 1990's home using a 50 gal tank and copper lines. I had my tank set between 130 F which allowed for greater than 120 F delivered to the dishwasher (longest run in the home) and clothes washer. It was sufficient to fill a whirlpool bathroom tub without having to let the system recover--125 F setting was insufficient for that. Longest heat up time was at the kitchen sink in mid-winter, something like 45 secs as memory serves. I miss that compared to the three tankless systems I've tested so far.
  6. jasonthompson79

    jasonthompson79 New Member

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    Location:
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    I'm not sure if you have fixed the problem or not but I have done some research on that particular unit and there have been numerous issues. 99% of issues with tankless water heaters come down to it not being installed properly, but that unit may actual have defects from what I've read.

    But to answer your question, it shouldn't take longer than 45 seconds for 100+ degree hot water.
  7. DonL

    DonL Jack of all trades Master of one

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    Location:
    Houston, TX
    In the day a #2 tub took about a hour to heat up, On the wood or Coal stove.

    And all of the kids took a bath in the same water.


    We are ALL spoiled, Now a days, I think...


    Please do correct me if I am wrong. I know that I may be wrong for posting this, but am I right ?
    Last edited: Aug 22, 2014
    jasonthompson79 likes this.
  8. solalo

    solalo New Member

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    It is often said all tankless have 10 second ignition and my experience tells me they fire almost instantly in most cases and the extra delay caused by tankless should no more that 15-20 seconds and that's an extreme scenario. 15 feet is quite a small distance and I would expect to get hot water in less than a minute and more than 3 minutes basically indicates something's wrong with the system. Two problems I could think of:
    1-) The piping route is too mazy for some reason and the water has to travel a very long distance to get to the faucet. I am not sure if there are any standards about piping route but if there is, you have a case.
    2-)Navien 240A models have an internal buffer tank that is supposed to function to reduce hot water delays. Sometimes, whoever is installing the 240A can fail to understand this feature and how it has to be connected to the outgoing hot water pipes. In that case, you will not get the benefit of the buffer tank and as Dana pointed it could even backfire and make the hot water delay longer. In your case, the first thing I check would be the connections of water heater nad its installation guides.
  9. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    I've checked a lot of homes built in recent years now and the tankless units suck at delivering hot water to a kitchen sink in under two minutes...they really, really suck. Tanks do it better (my former home was sub-1 minute at the longest run in a large house with a tank and roughly half of the copper run insulated.) Why? I'm not sure. The PEX lines of modern tankless installs don't seem to be insulated worth a crap...poor assumptions made by builders? The homes in this locale with tanks are older, some have higher temp settings. Every tankless I've checked has been set to 120 F max. And that is one major part of the problem, because it is too low to give hot water at the taps, after running through PEX routing, Maniblocs, etc. While rough calcs suggest the loss/lag time should not be that bad, reality is different. I don't know how much initial firing of the unit slows things in theory (and I'm sure it is 2x or 3x that), but overall system lag is awful and nobody seems to be giving this an honest assessment. The losses are too high.

    No tankless I've tested is delivering anything less than 30 seconds lag anywhere near the unit from what I've seen. I haven't done a process controls instrumentation check of them (although I'm tempted to...) but they seem to be really slow.

    I've been dealing mostly with Rinnai's in this house and the other ones I've been looking at (although a smattering of others had the same problems.) Lucky for me, I finally discovered that by fiddling with a jumper in the cabinet I can get higher temp water out of Rinnai's. I was ready to reject tankless as a failure because of the lukewarm water delivered at the shower, kitchen, etc. However, resetting the jumper allows the set point to be increased from 120 up to a max of 140 F. I'm running 130 now and it is more than compensating for line losses in this older home. (all along I've been able to feel tiles warm below my feet in the kitchen so I know the hot water line isn't insulated below the slab.) So now I can have actual hot showers (longest run) and wash dishes with real hot water. Sometimes the water is actually too hot now without adding a little cold in the mix. I have seen the nat gas usage increase as a result, but I'll take that hit in order to have actual hot water for dishes, hand washing, showers, and clothes washing. The amount of water wasted in these homes for warm up is roughly equal to or exceeding that actually used productively by my estimates.

    Until I get in a newer home with "best practice" hot water lines, and use my high efficiency shower heads I can't really address the relative efficiency compared to my previous home with a tank. And this climate is much more favorable so I should be using far less...rather than more as I've been seeing. I've been running higher flow heads here... and still getting barely serviceable water after 5 mins at 120 F setpoint. At 130 F the warm up is much shorter and I have to trim the shower valve accordingly.
  10. DonL

    DonL Jack of all trades Master of one

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    Location:
    Houston, TX
    Heaters like that are good for Hotel rooms and Cabins.

    But I would never install just one to use in a large house.

    I see no savings, other than they make you not want to take a shower, or wash clothes.

    Both may stink.
  11. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    Having lived with one for a time, I've learned how to make it work. Having to dump water down the drain because of the start up lag is an unfortunate pain though. My rough estimate is that the net loss from waste is as high or higher than the lower thermal efficiency of a blanketed tank and insulated lines. I can't do direct apples-to-apples with my old system but rough checks indicate this is likely.

    Raising the set point by changing the jumpers in the box to allow settings up to 140 does wonders. Even for a tank, 120 is too low. 125-130 works well in my experience with a tank and now with a tankless. No stink. (Dishwashers also work better with higher inlet temp.)

    A primary problem, and the reason I want nothing to do with tempering valves is that the delivered temp at the appliance/fixture is less than that at the water heater discharge. So at best there will be X degrees of offset once equilibrium is reached. And it can take quite awhile to even get that high because of the start up time of the unit as well as losses/thermal mass in the distribution system.

    If you lose say 5 F at equilibrium, and you have an extra 30 secs of lag in the unit, plus the time to heat up the tubing/fixtures, etc. the control equations dictate that you will have a much longer wait until you reach a desired temp not far from equilibrium. On the other hand, if you add 5 F or 10 F to the initial temp, then the desired target temp will be reached much sooner. This is even quicker when the tankless lag is removed.

    Tempering is usually spec'ed at 120 F, while the min temp in a tank to avoid legionella is 120 as well. A tankless shouldn't have the legionella problem, but they are factory set to 120 max for residential. I swear, nobody thinks this s**t through.
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