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Thread: Reduce lag time for tankless

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    DIY Member philp's Avatar
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    Default Reduce lag time for tankless

    We installed a Rinnai tankless hwh two months ago. Now it takes about 30 seconds to get warm water and 40 to get hot. With our old electric hot water tank it used to take under 15 seconds to get warm and 20 to get hot. (I tested this first thing in the morning when the hot water hasn't been used for many hours.)

    I think the main problem is that the tankless hwh is located much further away from all the hot water faucets - but it doesn't matter which faucet I test this on. The "pipes" from the tankless are PEX - about 25' until they tie into the copper - could this make any difference?

    Does anyone have any tips on reducing the lag time from turning on a hot water faucet to actually getting hot water? I guess we could move the tankless closer to the faucets - is there any rule of thumb for lag time to length of pipe run???

  2. #2
    DIY Senior Member zl700's Avatar
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    Your choice includes a external buffer tank and a DHW circ. besides expense, it reduces the warranty.

    The lag time for the pipe runs would be the same with exception to the pressure drop through the heater which slows flow. You don't mention model, pipe sizes or temp setting to assist further.

    The average ignition and warm up adds about 10 seconds with tankless over a tank.

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    DIY Member ChuckS's Avatar
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    25' of 3/4 pipe is a lot of water to clear out before you get to the hot. My installer did the exact same thing. He said it had to be installed near the gas meter which is well away from where we use HW. If I knew then what I know now, I would have insisted he install the heater as close to the point of use as possible.

    We installed a recirculating system that comes on when we turn on the facet. This takes some of the water in our 30' run and sends it back to be heated. I used the basics of this guys loop and adjusted it to work for me and my house.

    http://s480.photobucket.com/albums/rr167/PRYGAARD/

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    DIY Member philp's Avatar
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    Thanks zl700 and ChuckS.

    It looks like I will have to live with the delay. It is not a problem for showers or dish washing - an extra 30 seconds you just get used to and plan appropriately. However, it means I never get to wash my hands in warm water - I'll have to see if I can get used to that.

    By the way the PEX is 1/2". I like the idea of recirculation if it means no wasted water while waiting for it to heat. I'll talk to my installer, thanks.

    We chose tankless because we wanted the extra space (I can build a home theatre in my basement now!) but I wouldn't recommend tankless to anyone who doesn't have a space issue.
    Last edited by philp; 09-01-2009 at 07:19 AM.

  5. #5
    DIY Senior Member zl700's Avatar
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    Being a tankless user for over twenty years now and going through the progressive changes in products, I still would have nothing else.

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    In the trades Dana's Avatar
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    Quote Originally Posted by philp View Post
    Thanks zl700 and ChuckS.

    It looks like I will have to live with the delay. It is not a problem for showers or dish washing - an extra 30 seconds you just get used to and plan appropriately. However, it means I never get to wash my hands in warm water - I'll have to see if I can get used to that.

    By the way the PEX is 1/2". I like the idea of recirculation if it means no wasted water while waiting for it to heat. I'll talk to my installer, thanks.

    We chose tankless because we wanted the extra space (I can build a home theatre in my basement now!) but I wouldn't recommend tankless to anyone who doesn't have a space issue.

    The problem isn't the tankless as much as it's poor location, remote from the points of use. A recirculation system can fix the symptom, but at an energy cost. An under-sink 2-3 gallon electric mini-tank may make better sense for hand washing use. Hand washing short-cycles the burner on a tankless anyway, cutting severely into it's efficiency (they're under 50% efficient on those 2-quart draws, even if they're running 85%+ for showers. It typically takes 3-6 gallon draws to get them up near their steady-state efficiency.)

    If it's the same/similar money up front and you have the space under the sink, go with a well insulated mini-tank rather than a recirculation system. The operational cost will be lower (primarily due to lower distribution losses from all of that plumbing length), and it'll save the tankless from the excessive wear and inefficiency short-cycle burns. With under 2' of pipe between mini-tank and hand-washing faucet your losses are essentially limited to the standby loss, which is a function of surface area & R-value in an electric tank- smaller is better. Heating up 50-100' of pipe for a 2 quart draw is a waste, no matter how efficient the heating source, since most of the heat expended is dissipated between draws. That may be OK (but never great) in a heating-dominated climate if the plumging is all inside of conditioned space. It just adds to the air conditioning load in cooling dominated climates.

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    DIY Member philp's Avatar
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    Quote Originally Posted by Dana View Post
    An under-sink 2-3 gallon electric mini-tank may make better sense for hand washing use.
    Thanks - great idea - I will look into this.

  8. #8
    DIY Member philp's Avatar
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    Quote Originally Posted by Dana View Post
    An under-sink 2-3 gallon electric mini-tank may make better sense for hand washing use.
    There seems to be two configurations for these.
    1. Connected to the cold supply line - so maximum hot water is tank capacity - 2 gallons or so. Is that ok for a bathroom sink?
    2. Connected to the hot line. I guess this would be a buffer for the tankless and by the time the mini-tank ran out the tankless would be providing hot water.

    Does anyone know which configurations works best? The second one would probably mean a cold "sandwich" and would mean that the tankless short cycles every time the faucet is used. Is this a bad idea - anyone have experience with these mini-tanks?

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    Plumbing Designer FloridaOrange's Avatar
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    Quote Originally Posted by philp View Post
    .. but I wouldn't recommend tankless to anyone who doesn't have a space issue.
    My position as well with tankless.
    Matt
    Semi-professional plumbing designer
    Enjoying life in SW Florida

  10. #10

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    Quote Originally Posted by philp View Post

    We chose tankless because we wanted the extra space (I can build a home theatre in my basement now!) but I wouldn't recommend tankless to anyone who doesn't have a space issue.


    I changed to a tankless for the same reason. I needed the floor space for other uses.

    However, I prefer to have the unlimited hot water that the tankless provides.




    Quote Originally Posted by philp View Post
    Thanks zl700 and ChuckS.

    It looks like I will have to live with the delay. It is not a problem for showers or dish washing - an extra 30 seconds you just get used to and plan appropriately. However, it means I never get to wash my hands in warm water - I'll have to see if I can get used to that.

    By the way the PEX is 1/2". I like the idea of recirculation if it means no wasted water while waiting for it to heat. I'll talk to my installer, thanks.


    How many times a day do you wash your hands at home? I guess you are going to have to either wait a few seconds and let the water get warm, or you can simply rough it out washing your hands in that freezing cold water!

    The only time of the year that the "cold" hot water bothers me for hand washing is in the middle of the winter. Most of the year the cold water is at room temperature. Since I am a manly man I rough it out washing my hands with 70 degree water.
    Samuel James Witwicky

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    DIY Member ChuckS's Avatar
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    Quote Originally Posted by Ladiesman271 View Post
    The only time of the year that the "cold" hot water bothers me for hand washing is in the middle of the winter. Most of the year the cold water is at room temperature. Since I am a manly man I rough it out washing my hands with 70 degree water.
    I use cold also, I wish my wife and daughter would but that isn't a fight worth fighting. I actually saw a increase on the gas usage on my bill while my daughter was on summer break. In addition to turning on the hot and waiting for it to get hot to wash her hands she also turns on the shower and does other things while it "warms up". That is one draw back to a tankless, she is now complacent to the shower "warming up" since the hot doesn't eventually run out.

  12. #12
    DIY Member MikeQ's Avatar
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    Default Space issues

    Quote Originally Posted by philp View Post

    We chose tankless because we wanted the extra space (I can build a home theatre in my basement now!) but I wouldn't recommend tankless to anyone who doesn't have a space issue.
    In many cases a tankless heater can be located closer to the main points of use than a tank could. This is particularly true with electric tankless since there is no exhaust to consider. Moving a water heater closer to the most common points of use is the single best thing that can be done for convenience and efficiency. My 28kW electric water heater is not much larger than a VCR and it can certainly crank out large and endless amounts of hot water.

  13. #13
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by ChuckS View Post
    25' of 3/4 pipe is a lot of water to clear out before you get to the hot.
    I get 0.6 gal with 3/4" ID pipe.

    What's the power input to the tankless? If you know the weight of water in the tankless the time to heat can be calculated. 10 BTU per second [~10 kw] will raise 1# of water 10 F.
    Last edited by Thatguy; 09-03-2009 at 01:56 PM.

  14. #14
    DIY Member ChuckS's Avatar
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    Quote Originally Posted by Thatguy View Post
    I get 0.6 gal with 3/4" ID pipe.

    What's the power input to the tankless? If you know the weight of water in the tankless the time to heat can be calculated. 10 BTU per second [~10 kw] will raise 1# of water 10 F.
    I have 35' of 3/4 and mines was taking just over a minute. The recirculation loops cut that down to about 35 secs'. My loop is not on a timer or push button which is why it still takes so long to get HW.

  15. #15
    DIY Junior Member rob27's Avatar
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    Default Hot water up 3 floors

    Quote Originally Posted by philp View Post
    We installed a Rinnai tankless hwh two months ago. Now it takes about 30 seconds to get warm water and 40 to get hot. With our old electric hot water tank it used to take under 15 seconds to get warm and 20 to get hot. (I tested this first thing in the morning when the hot water hasn't been used for many hours.)

    I think the main problem is that the tankless hwh is located much further away from all the hot water faucets - but it doesn't matter which faucet I test this on. The "pipes" from the tankless are PEX - about 25' until they tie into the copper - could this make any difference?

    Does anyone have any tips on reducing the lag time from turning on a hot water faucet to actually getting hot water? I guess we could move the tankless closer to the faucets - is there any rule of thumb for lag time to length of pipe run???
    Link below is for a device I have been using for 4 years. It doesn't speed up hot water but it does reduce water waste by recirculating the cold water until it reaches hot then shuts itself off. By using remotes one just has to click the remote a minute or so before using hot water. It can be installed by user without a lot of technical complications.

    http://tinyurl.com/ycgfh4y

    Rob

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