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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???

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    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.

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    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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    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

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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.

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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.
    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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    DIY Senior Member zl700's Avatar
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    Depends on distance and volume required

    It would make no sense to connect it to hot if you only draw 1/2 gallon per use and the hot water from the main heater never makes it to the mini-tank, thus heat is wasted in the piping.

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    In the trades Dana's Avatar
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    Quote Originally Posted by philp View Post
    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?
    You could set it up in a recirculation loop with the pump controlled by the tank's aquastat, making the minitank a buffer for the tankless but that would be more expensive up front in pump + plumbing. But with a 3-4 gallon minimum circulation-draw from the tankless it saves a bit on short-cycling.

    Option # 2 would still short-cycle the tankless, but on such a long run it would abandon most of the short-draw heat in the plumbing between the tankless & mini. For shorter runs it's a way to mitigate cold-water sandwich issues though. The incoming cold water sandwich mixes with the hotter water in the tank, so the temp fluctuation at the tap isn't nearly as severe. (Bosch even markets their pretty-good minis as the solution to the miserable lag on their crummy tankless units. Why sell one heater when you can sell two? :-) See: http://www.boschhotwater.com/Helpful...9/Default.aspx ) As long as the volume in the tank is more than the volume of the plumbing between the mini and the tankless the coldwater sandwich is barely detectable in practical terms.

    Option # 1 is probably the right one. Little wall-hung point-of-use tanks like Bosch Ariston GL series 2.5 and 4 gallon versions fit under most sink cabinets, and deliver 8+ first-hour gallons (a pretty fast recovery for 2.5 gallon one), which is enough for most bathroom sink apps. If you're concerned about capacity, take the 4 gallon version. They make a 6 gallon version, but that's overkill for handwashing- might be worth it if you have 2 bathroom sinks and a kitchen all drawing from it though. It takes up a bit more space than the others though- harder to jam under some sinks.

    Standby losses of a mini are on the order of 30-35 watts, or 250-300kwh/year, so you're paying a few tens of dollars/year for the convenience, but you may be paying as-much or more in lost efficiency & wear on the tankless using the remotely installed unit as-is. Keep the tankless hooked up for the tubs & showers though- any draw over 3 gallons will be pretty efficient.

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    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.

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    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.

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    DIY Member ChuckS's Avatar
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    Quote Originally Posted by Dana View Post
    Option # 1 is probably the right one. Little wall-hung point-of-use tanks like Bosch Ariston GL series 2.5 and 4 gallon versions fit under most sink cabinets, and deliver 8+ first-hour gallons (a pretty fast recovery for 2.5 gallon one), which is enough for most bathroom sink apps. If you're concerned about capacity, take the 4 gallon version. They make a 6 gallon version, but that's overkill for handwashing- might be worth it if you have 2 bathroom sinks and a kitchen all drawing from it though. It takes up a bit more space than the others though- harder to jam under some sinks.

    Standby losses of a mini are on the order of 30-35 watts, or 250-300kwh/year, so you're paying a few tens of dollars/year for the convenience, but you may be paying as-much or more in lost efficiency & wear on the tankless using the remotely installed unit as-is. Keep the tankless hooked up for the tubs & showers though- any draw over 3 gallons will be pretty efficient.
    What do you think of this guys idea of adding a tank to the tankless system?

    http://www.finehomebuilding.com/PDF/Free/021192082.pdf

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    In the trades Dana's Avatar
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    Quote Originally Posted by ChuckS View Post
    What do you think of this guys idea of adding a tank to the tankless system?

    http://www.finehomebuilding.com/PDF/Free/021192082.pdf
    A tiny buffering scheme like that fixes the cold water sandwich and flow issues (some of the tankless heat exchangers have ridiculous head load at 8gpm.) Seems like an expensive solution for those problems. This is the configuration I'd suggested NOT using with the buffer located at the remote sinks due to the high distribution losses of a very long pump loop (twice the plumbing, twice the distribution loss, more or less.) It works though- and done right it eliminates short cycling.

    In order to get the full rated heat out of it you need to really put a monster pump on it or crank up the tankless output temp though. I'm not sure if the hysteresis in the aquastat of a typical electric mini designed to control a 1.5kw element is going to provide suffiently long burns to not short cycle at the full burner rating- there'll have to be some compromising of flat-out capacity to tweak the overall efficiency. It'll modulate some at high flow, but not a lot. Rather than using a power hog like a Taco 09 they used I'd go with something smaller (maybe even a multi-speed to be able to tweak it) and raise the output temp of the tankless to get the required BTU/hour rate into the tank. That would potentially give you a bigger modulation range out of the tankless too, allowing it to better match the instantaneous load. (Some tankless heaters are less flexible than others in their output temp selections, but they've gotten better over time.)

    It's not different from a small indirect tank and a boiler other than in this instance the "boiler" is the tankless, and it's heating recirculating potable water directly (not through a heat exchanger.) At some point, between tankless loop pumps and recirculation loop pumps, buffer tanks & tempering valves etc you have to just get a grip, and ask yourself why you're making it such a g'dawful kludgy Rube Goldberg contraption just for hot water. It's one thing if it were incorporated into the heating system to improve the efficiency of both heat & hot water, but IMHO it's a bit silly (and expensive up front) for just the hot water end of it.

    Some newer "tankless" on-demand heaters now incorporate tiny ~1 gallon header-buffers to eliminate the cold water sandwiches and provide a minimum burn length to increase the net efficiency (no ultra-lossy short cycling), at the expense of a small amount of standby loss. A tiny buffer like that would reduce it's efficiency a small amount in an EF test due to standby losses, but would increase it's efficiency significantly in real-world use patterns. The current EF test draws are all 10.2 gallons at a time- that's exactly how YOU use hot water, right? :-) The very real very significant short-cycle losses of tankless heaters never show on the EF test, a bone of contention with tank heater manufacturers. (The test standards are rightly undergoing some revision...)

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