Have tankless Rinnai--ideas for improved performance

ralleiaDecember 21, 2006

Yes, I have read the incredibly long-running discussions about the pros and cons of tankless water heaters. I don't want to re-discuss 'em. We HAVE a tankless Rinnai, and I want to figure out how to live better with what we have.

Generally, we're happy with the unit. We've made several modifications to how we use hot water, which are acceptable. There's just one main thing I'd like to solve.

The chief issue is the amount of time it now takes hot water to reach the tap in the kitchen. I mistakenly thought a hot water recirculator would solve the issue, but of course it couldn't start up the water heater. I considered the solution of installing a 6-10 gallon tanked water heater in series, but decided against the whole thing for two reasons: (1) the potential for creating a hospitable breeding ground for Legionella in the warm pipes and (2) the fact that it would defeat any energy savings.

I'm now thinking of installing a very small (1-3 gallon) tank water heater under the kitchen sink. Hot water use at the sink is very limited, but in a busy place like the kitchen, when I want it, I want it NOW!

Has anyone done something like this?

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This company has a couple high-capacity recirculation pumps that supposedly work with tankless systems. They're expensive, but if you are more concerned with having hot water NOW as you say, perhaps it's justifiable. They're thermostat controlled, only run long enough to sense a temperature rise, which helps make them suitable for tankless. Can be put on a cycling timer to keep the lines primed.

http://www.metlund.com or http://www.gothotwater.com

    Bookmark   December 22, 2006 at 1:02AM
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Before one of our tanks sprung a leak, we had two tanks and a hot water recirculating system for instant hot water. We replaced both tanks with a tankless system...Takagi I think. Anyway, the plumbers checked into how to install with the recirculating system and they were able to set it up. So we do have instant hot water with our tankless system. Want the phone number of my plumber?

    Bookmark   December 22, 2006 at 11:42AM
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Over the years I have heard some dumb ideas but using a high velocity circulator to operate a loop with a tankless has got to be the best one.

The singular advantage that tankless has over a tank type is no standby loss when there is no demand, of course with the insulation that they are putting in tank units now standby losses are nearly none existant so the savings are marginal at best, then someone comes up with the idea to use a high velocity pump which then makes the entire hot water distribution system a continuous radiator.

The solution to Releia's problem is simple. The demand at the kitchen is minimal so you install a small point of use tankless in the kitchen then you can discontinue the hot water line from the primary tankless to the kitchen entirely.

    Bookmark   December 23, 2006 at 3:24AM
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My solution is even simpler - I just live with it! Granted I am no VIP, so the "NOW" and the "real time" are just amusing sounds to me. On a pracical side, my runs are short and pipes are insulated ;-)

    Bookmark   December 31, 2006 at 2:22PM
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lazypup, the recirculation pumps I mentioned aren't intended for continuous circulation with tankless, although they can be used that way if desired. They're intended to be triggered just before a hot water task is run to get the supply line purged of standing cold water. In any case, they have a thermostatic cut-out and won't run once heated water reaches the pump, until the sensor cools down again, which is why they have to be run on a timer for "continuous" recirculation since they won't restart by themselves.

handyal, your solution is also mine.

    Bookmark   December 31, 2006 at 4:25PM
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I just had a chance to live with a gas tankless for a week (Paloma) in a summer cabin I am house-sitting. It was interesting since I've only been around electric demand heaters. My experience is probably old news to some of you, but it was kind of new to me. The first strange thing is the lag time with gas. Electric comes on really instantly. Gas sort of does. When I turned the hot on in the kitchen of the small house, I could hear the Paloma in the adjacent bathroom come on after about 3 seconds. There's probably no way to shorten this time lag.

Not counting the ignition delay, you're limited to what you can do. You either need 1) less water in the line between the tank and the faucet, 2) higher water production at the faucet, 3) less heat loss in the pipe, or 4) less pipe. Best would be all four, although you'll have to do a cost/benefit analysis of the expense of the change verses cutting seconds off the hot water delivery time.

The solutions are 1) use 3/8th inch delivery line (having 1/2 as much water in the pipe means 1/2 the delivery time), 2) remove any restrictions on the faucet (an air entraining filter might reduce the flow by 1 gph so removing it will cause splashing but reduce delivery time by several seconds), 3) change from copper to plastic pipes or insulate the copper (assuming you have copper), 4) reduce the amount of pipe between heater and faucet (by snaking PEX pipe instead of using rigid pipe). Most of these "changes" are easily installed in new construction but difficult for a retro-fit that cuts delivery time from 30 seconds to 10 seconds.

The house where I just stayed has copper pipes that run under the house to the kitchen sink. Total distance as the crow flies is less than 15 feet. Because the copper pipe drops down under the floor into semi-conditioned space, and it was in the mid-30's, the delivery time for true hot water was over 35 seconds. You could tell it was getting warm at about 30 seconds, but full hot water was at least 40 seconds. I could hardly believe it.

Because of the lag time during ignition, every time the hot was turned on/off meant there would be a 3 second slug of cold coming down the pipe. The slug would cool off the pipes a little, so full hot was obtained again only after the cold slug was gone for 5 seconds.

A gas heater with copper pipe passing through unconditioned space made for a clunky system, but as was posted above, the best solution might be to learn to live with it. My wife and I managed just fine after figuring out how to set the shower temperature and to not use any other fixture when somebody was in the shower.

Another alternative might be installing one of the really little demand "hot coffee" type of 120V heaters in the kitchen, depending on why you might want a little hot water really quick. If you're filling the sink for washing dishes, it really doesn't matter if the first seconds are cold.

Mark Fleming

    Bookmark   January 1, 2007 at 10:33PM
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The scenario and solutions presented by Mr. Fleming present a number of problems.

Quote-"When I turned the hot on in the kitchen of the small house, I could hear the Paloma in the adjacent bathroom come on after about 3 seconds."

The Plumbing Codes prohibit installing a gas water heater, tank or tankless, in a bathroom, Bedroom or a room which opens into a bathroom or bedroom.
Ref: International Residential Code 2005.2 & Uniform Plumbing Code 509.0

He suggested reducing the line size from 1/2" to 3/8". He is correct that this would reduce the time required for the water to get from the heater to the point of demand however a tub is required to have a minimum flow rate of 4gal/min.
3/8" pipe contains one gallon for each 175' of pipe so a 4gpm flow rate would be equivalent to 4 x 175' per minute or 700ft/min. 700ft/min divided by 60sec/min = 700/60= 11.66feet/second.

Code prohibits water velocity in excess of 10feet/second because at higher velocities pipe wall erosion occurs.

Quote-"remove any restrictions on the faucet (an air entraining filter might reduce the flow by 1 gph so removing it will cause splashing but reduce delivery time by several seconds"
Wrong,,,removing the screens would have absolutely no effect on the volume of flow.

    Bookmark   January 1, 2007 at 11:25PM
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I've suspected that complaints about tankless performance are primarily in regards to gas units, and the majority of tankless units ARE gas because people are afraid of electric.

    Bookmark   January 2, 2007 at 1:12AM
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The ChiliPepper circulating pump does up to 3 GPM, which should be enough to activate the flow-sensor in the tankless heater. It is less than $200 and has a thermostat, so when it senses that the hot water has arrived, it quits pumping.


Here is a link that might be useful: http://www.chilipepperapp.com/

    Bookmark   January 8, 2007 at 12:03PM
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We are looking at replacing our water heater possibly with a tankless. Our house has 2 water heaters, one serves the master bedroom/kitchen, the other the laundry room/other 2 bathrooms. The one that needs replacing is the kitchen/master bath heater.

3 years ago when we remodeled our kitchen. One of the things we had installed was a dedicated return line for a circulating pump to move the cooled off hot water back to the supply line. The pump is activated by a switch in our kitchen. I use this often when working in the kitchen. I was afraid this unit wouldn't be usable with a tankless. But, after checking their site, http://www.gothotwater.com/ it looks like the unit can be used with the tankless.

On another note, a friend of mine was told that the tankless heaters were noisy. I haven't read anything about this on thathomesite.com so I was wondering just how much noise do the units make?


Here is a link that might be useful: Metlund d'mand system

    Bookmark   January 10, 2007 at 1:22PM
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I am contemplating installing a Metlund hot water recirculator in my tankless no dedicated return line hot water system. I am sold on the product but cannot get an answer on one point I'm concerned about. Can hot water be mixed with cold when there is a demand for cold water from irrigation,kitchen,etc? the device has a normally open "thermostatically controlled valve" which shuts off when it senses a temperature rise. Being normally open wouldn't the valve allow flow from the hot water line to the cold water line until it senses a temperature rise? This condition would cause the water heater to cycle when hot water is not wanted.

    Bookmark   September 27, 2007 at 7:26PM
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