tankless H2O heater + H2O efficient dishwasher = problems

bevangel_i_h8_h0uzzJuly 4, 2010

Just figured out that my water efficient Kenmore dishwasher (not top of the line) doesn't draw enough water to keep my Rennai tankless water heater (propane) fired up. Dishwasher has a heating element but manual says to supply water at 120 degrees so apparently the dishwasher's internal heating element can't heat cooler water enough to get the dishes clean.

What is my best AFFORDABLE fix? At this point, anything more than about $250 to fix this means I'm just gonna have to wgo back to washing my dishes by hand and the money spent on the dishwasher was wasted dollars.

I've heard of water recirculating systems but don't know how expensive they are or how difficult to install. Plus, I don't really understand quite how they work. If they basically keep your tankless in "on" mode all the time, seems like that totally defeats any energy savings potential.

On the internet I've also seen some very small electric tankless heaters for under the sink that one can apparently just plug into the wall. But I can't find any info regarding minimum flow rate to keep even those fired up. And, in listening to my dishwasher run, it sounds like it spritzes water in VERY short cycles. So, I'm not sure that even those would work to keep hot water flowing to a dishwasher. Any one know for sure?

Is my best option to get a small water heater tank for under the sink? If so, can anyone suggest a particular model/size? Is there one that will heat a tank of water in, say, an hour or two so I could turn it on while fixing dinner and know that the water would be hot by the time the dishwasher was loaded? Even better would be one that I could put on a timer to cut back off once the dishwasher cycle was done.

Seems like the tankless water heater manufacturers and the water efficient dishwasher (and clothes washer) manufacturers would get together and figure out how to make these "green" products work WITH each other!

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Easiest solutions is to run water at the sink until you get hot water prior to starting the dishwasher. No matter what hot water heater you have, your dishwasher will be mostly filled by the time hot water reaches it if you start with cold water in the pipes.

    Bookmark   July 4, 2010 at 11:31AM
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Install a 2 gallon electric tank heater under the sink

    Bookmark   July 4, 2010 at 4:23PM
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aliceinwonderland, I tried running the hot water at the sink and then starting the dishwasher but 10 minutes later, the water at the sink was once again cold. The dishwasher just isn't drawing enough water flow to KEEP the tankless water heater going. It cuts out and, apparently, stays out, so that the little bit of water that is delivered to the dishwasher arrives there cold. The only way to keep the water heater heating water is to keep a significant flow of water (about a gallon per minute) running at some other tap in the house the ENTIRE time the dishwasher is running. Since the dishwasher cycle take almost 90 minutes, I'd have to waste something like 90 gallons of water to use my water efficient dishwasher!

zl700 - I think you're probably right. A small tank heater is the way to go. Any recommendations re brand/model? I want to make sure I get a good one that will deliver enough hot water to the dishwasher but use as little energy as possible.

    Bookmark   July 5, 2010 at 12:19AM
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This is a very interesting post. I'm trying to figure out what's going on in my newly plumbed remodel, and wow, I am amazed that the tankless systems aren't made to work with the water efficient appliances we're all installing.

In our house we have remodeled with PEX and a tankless system for the kitchen & laundry only. The laundry seems to work fine. The utility sink gets hot water. The island prep sink gets hot water. The main kitchen sink? Not so much.

I haven't checked out how our new Kenmore d/w is managing, but the last two loads have come out ok, so I think we're doing fine.

Our plumber's comment about the kitchen sink is that the 2.2 gpm faucet is just not drawing enough flow to turn on the heater. That just seems crazy! The tankless systems are supposed to turn on with any flow, right?

I'm in Houston, so the water supply is probably pretty warm from the city's main. I'm just flabbergasted by this whole scenario.

    Bookmark   August 23, 2010 at 5:22PM
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karen....your plumber is wrong. Really stupid statement, actually. So stupid, I would not hire that person again. Bad if they're that dumb. Bad if they're deceiving you intentionally.

If you had a conventional tank, you'd have exactly the same problem. If there's a long run of pipe between the heater and the device that uses hot water, all that water in the pipe -- probably cold water -- has to flow in first. The length, diameter, and composition of that pipe will be the governing factors.

The only solutions are recirculating systems (if you have a tank) or having a heater closer to the water-using appliance.

This is not new. It has always been so.

    Bookmark   August 23, 2010 at 7:29PM
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Asolo, you're right. Our plumber and the GC's supervisor are notoriously dumb about this stuff. When I got home last night I found there was, indeed, hot water coming out of the kitchen sink.

I think the main issue for DH (and me, really) is that the flow at this sink seems less than adequate for clean-up. It's supposed to be 2.2 gpm. I'd bet that if I put a gallon container under this faucet and measured the time to get 1 gallon of water, it would be significantly longer than 30 seconds. This is a separate issue from the tankless system heating or not heating the water.

    Bookmark   August 24, 2010 at 10:45AM
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2.2gpm is a good, substantial flow from a kitchen faucet. Very easy to determine, of course, via exactly the method you described. Why don't you do that? Takes just a minute, and then you'll know.

Another experiment you can do.....starting from dead-cold.....measure the amount of cold water that flows out of the faucet before the hot water hits. That will tell you how much water is sitting in that pipe between the heater and the faucet.

Your tankless heater should kick in via a pressure drop instigated by MUCH less flow than 2.2 gpm.-- should not require full-flow at the faucet in order to operate properly.

    Bookmark   August 24, 2010 at 11:10AM
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Maybe even before doing a bucket test, I'd check the cutoff valves to make sure that they're all the way open. And if there's a flex line, check that it isn't kinked.

I'd also drop the heater setpoint. Ours is at 105F for 2 bathrooms and works fine. Just means you run more HW than you would if you're mixing down at e.g. the clothes washer to get a certain temperature.

Our Noritz, which is ~5 years old at this point, has a ~1gpm turn on flow and either 0.5gpm or 0.8gpm to shut off. We have issues with the thermostatic valve in the shower during the summer. After a few minutes, the valve shuts flow down low enough on the hot side to cut the heater off. That's with the heater temp set at 105F. For me, that means some combo of quick shower or endure a little cold water. Not taking the flow restrictor out as we're both in the water conservation biz.

    Bookmark   August 24, 2010 at 12:19PM
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Results of bucket test - 1.5 gpm.

The Grohe Minta faucet is supposed to deliver 2.2 gpm.

The flex hose coming from the supply is .375", but the fitting on the faucet is 0.25", so the plumber made a transition to the .25" hose. The builder's supervisor says the .25" hose is the reason why our flow isn't what it should be, but if the Grohe faucet has a .25" fitting and still says it should deliver 2.2 gpm, what's the deal?

I do need to check the shut-off valve, but it's at the PEX manifold in the garage....will get to it later.

    Bookmark   August 24, 2010 at 10:05PM
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You're on you're way. You've got it, now. Don't take any prisoners.

    Bookmark   August 24, 2010 at 10:24PM
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Asolo, now that I've totally hijacked this thread (sorry OP and any future readers!) I'm here to report that my Grohe Minta faucet is performing EXACTLY as it's supposed to. Apparently we installed the green version, which is specified to provide 1.5 gpm as a water savings over their non-green version.

The question is, can I live with the lower flow? I think so. But I'd rather have the lower flow on my prep sink than my clean-up. Another failure in communication on my remodel!

    Bookmark   August 25, 2010 at 4:38PM
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"...hijacked this thread..."

Well, you did do that. However, the info is still good.

    Bookmark   August 25, 2010 at 8:55PM
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Back to the OP's question(s).

About which under-sink heater to buy: The choice is between "performance" and efficiency. A 2-gallon unit stays hot all the time (potentially wasting energy when no one is around to use it or dishwasher not running), but an on-demand heater might have a lag-time such that her water-saving dishwasher won't get [as much] hot water as ideal.

Maybe reconsider the re-circ option. There are small point-of-use recirculating pumps that might do a good job here, the trick being to signal the re-circ a few seconds before the dishwasher wants water. I've considered installing a motion-sensor to turn on my whole-house re-circ pump (which currently runs 24/7 much to my energy-saving chagrin.) But for one dedicated to a dishwasher, it might work to have one of those twist-timer switches. You would have to remember to turn on the timer, but it would become habit.

Finally, a remark about Thull's turning down his water heater's thermostat to 105! I've never heard that recommended with a straight face before, and in fact turned mine up recently after reading http://mises.org/daily/3987 mainly because it pointed out that yeast thrives at temperatures around 100, but dies at 140.

Here is a link that might be useful: The Turn of the Screw (water heater temp.)

    Bookmark   September 29, 2011 at 4:21PM
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This is an old thread, but there's some misinformation / misunderstanding cited by the OP.

Tankless water heaters, of course, DO NOT stay on UNLESS there's water running. That's their nature. Water is heated while it flows, the unit turns off within moments after the water flow stops.

A dishwasher runs water in for a discrete period of time (usually 1 to 2 minutes) for each fill until the proper water level is reached, then the machine's pump sprays that water over the dishes for the duration of the wash or rinse period. The machine drains, refills. Repeat for however many wash and rinses periods comprises the complete cycle.

The only way to keep the tankless generating hot water for the duration of the dishwasher's operation is by running the kitchen faucet (or some other nearby outlet, or a recirculation pump) at a flow rate just enough to keep the tankless activated. But that goes counter to the reason for having a tankless water heater.

Regards to setting a tankless water heater to 105F ... it's reasonable for tankless use, but won't have an effect on the dishwasher situation. My tankless has been set at between 102F and 105F majority of the time for 6 years to no ill effect. I don't think there's a risk of contamination being as a tankless water heater does not have 30 to 50 gallons of water stored at 105F to become contaminated, as would be the case of a traditional tank unit. The tankless heating chamber is fully flushed every time water is run.

    Bookmark   September 29, 2011 at 8:00PM
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Echo what Dadoes said:

1. When considering tankless, be sure to check the minimum flow on the heater you are considering. There is usually a turn-on and a maintain flow. Lower is better. Once you get much above 0.5 gpm to activate you start running into problems with low-flow fixtures.

2. Insulate your hot water lines.

3. Be aware of potential issues with high-efficiency appliances (washers, dish washers). They need to be able to heat the water.

4. Tankless heaters can be safely set at 105F since they don't store the water. In fact, many of them have a bathroom control to allow you to set the water temp for your bath/shower.

    Bookmark   September 30, 2011 at 9:31AM
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If it were my kitchen, I would install a 2g or 5g 120v electric heater under the sink with a wall switch. I would turn it on prior to needing the DW and off at other times.

BTW: speaking of electric heater efficiency, I have a 50gal heater now using (I think) 4500watt elements. I have it on a timer that allows for power 3 hours in the morning and 3 hours at night. I have a power monitoring device where I have been observing power usage over the past few months. I recorded that the heater activates for only 30 minutes a day. That's $6.75 a month at my $0.10/kwh rate.

    Bookmark   September 30, 2011 at 6:20PM
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Just a follow up note: There is very little standby heat lost from a well insulated an electric tank heater.

I wouldn't bother with a switch (in either case) to save electricity.

    Bookmark   October 3, 2011 at 9:13AM
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