Tankless water heater and appliance performance

jasperdogJanuary 30, 2013

Anyone with a tankless water heater notice any problems with your dishwasher or frontload washing machine? I've been reading reviews about tankless water heaters and they indicate there might be a problem because the water isn't on long enough in the appliance to fire up the water heater and get hot water to the appliance. It seems to me with the amount of residual water in the water line that problem would occur with any type of water heater.

Sure you could stand at the sink and run the water until it gets hot, but that defeats the purpose of having the delayed wash cycle on a diswasher.

Thank you for reporting this comment. Undo

My tankless is definately sparked on by my front load washer and dishwasher. I do however have a small 2 gallon electric tank to combat any of the shortcomings a tankless has.

    Bookmark   January 30, 2013 at 2:41AM
Thank you for reporting this comment. Undo

I have had a tankless hot water heater for 10 years. The flow rate at which the flame will ignite can vary by the manufacturer. My unit turns on at .5 gallons per minute. As far as hot water getting to your appliance, that will of course depend on the size of the water line, the length of run and the material of the line. There is less heat loss in PEX water lines than copper. One of the problems with some tankless hot water heaters is called the sandwhich effect. This is where the appliance turns off and on like a dishwasher. What happens is since the tankless heater takes a few second to get the water up to full temperature, every time the appliance turns off and on you end up with a section of water that is cold followed by the heated water. Thus the name sandwhich because you end up sandwhiching slugs of cold water in between the hot. I have not noticed this to be of any concern with the appliances as far as performance. The bigger inconvenience is when you are hand washing dishes and you are constantly turning the water on and off to rinse. There are units that have a small tank that is keeps some water heated so that you don't get the slugs of cold water. After all that I would replace my tankless unit with another when my current unit dies. We are a family of four that sometimes takes four consecutive showers in the morning. My unit is not large enough to take more than one shower at a time. It is also not large enough to support a shower and someone running a sink using hot water. My unit is only 125,000 BTU. The next unit I get will be larger. Hope this helps.

    Bookmark   January 30, 2013 at 8:38AM
Thank you for reporting this comment. Undo

Eight years electric tankless here, a large-capacity unit (28KW). I keep it set at 102F to 105F which is perfect for showering.

I may raise the setpoint temporarily when washing clothes depending on the needs of the load. I *always* purge the incoming hot water line of standing cold water before starting a load. An effective way to do that is by running the water IN the machine and draining it via diagnostics/test mode before loading, if one has access to tech info for the machine's diagnostic procedures. If not, then simply running a faucet that's near the washer in the flow-path should be sufficient. Any low-water, HE washer, either frontloader or toploader, will benefit from line purging regardless of what type of water heater is involved, otherwise a relatively large percentage of the fill may be cold water standing in the line.

Many HE washers nowadays fill by cycling the water on/off several times while sensing the load and water level. The cold-sandwich effect that Tom2013 describes can be a factor, but the water flow typically may be off less than a minute at at time (depends on the characteristics of the machine involved) which likely isn't long enough to be a problem once the tankless unit is fully heated-up ... and purging the line is helpful in that respect.

My dishwasher (F&P DishDrawer) has assured onboard heating for the main wash and final rinse so I don't raise the temp for it. I may purge the line at the kitchen sink before starting in some cases of cold (winter) weather or a dish load of considerable greasiness to assist the prewash effectiveness. The machine takes only 0.8 gals per fill, and approximately 2.5 fills would be needed to fully purge the line due to the distance involved.

    Bookmark   January 30, 2013 at 9:30AM
Thank you for reporting this comment. Undo

The OP asked more or less if cycling is the issue with tankless. I don't think so, as described here.

I think the issue is the low flow rates for a dish or cloths washer.

    Bookmark   January 30, 2013 at 2:42PM
Thank you for reporting this comment. Undo

Thanks for the replies. Fortunately the washing machine is in close proximity to the water heater. We may have to consider adding a small water heater under the kitchen sink to address the dishwasher. We will be using a manifold homerun system for plumbing so purging the lines by running a sink faucet may not be an option.

Please correct me if I am mistaken, but several dishwashers with onboard heaters actually allow for the incoming water to come from a cold water line anyway. So it may not be that big of a deal, just less energy efficient.

    Bookmark   January 30, 2013 at 3:55PM
Thank you for reporting this comment. Undo


Doing a home run plumbing system is the only way to go in my opinion. I also did a home run system on everything except the dishwasher. I have this line tied into the hot water run going to my prep sink. My dishwasher is not even beside this sink, it is on its own with its own drain, but I ran the hot line to the dw, and then looped it back to the prep sink so that when I run the hot water it gets to the dishwasher first.

To tell you the truth tho, with a Bosch or a Miele, or any dw with an inline water heater, you don't have to bring hot water to the unit. The inline heater will heat the water all the way up to sanitize if so desired. In fact, running hot water to the dishwasher is largely a North American thing. Most other parts of the world it is standard practice to bring cold to the unit and let the unit decide what temp to heat the water to. Miele somewhat recommends a cold water hook up.

As for the cold water sandwich mentioned above and other small disadvantages with a tankless, they can easily be remedied with a small companion electric tank or a unit with a small buffer tank built in. True you do lose a slight amount of efficiency with that setup compared to a strictly tankless setup, but from my experience it is still less to operate than a large tank. Besides, pretty much the sole reason I went tankless was to get endless hot water.

    Bookmark   January 31, 2013 at 12:39AM
Thank you for reporting this comment. Undo

Thanks tyguy. That's what I was thinking.

    Bookmark   January 31, 2013 at 9:35AM
Thank you for reporting this comment. Undo

Actually, heaters on the units sold in the US are not big enough to heat the water very quickly. The older 220v units could do this but they haven't been available for a while.

    Bookmark   January 31, 2013 at 4:25PM
Thank you for reporting this comment. Undo

Slower heating can be beneficial with today's enzyme-based dishwasher detergents. Detergents nowadays are formulated to work at lower temps than in the past, and slower heating gives the enzymes more time to work on food soils.

    Bookmark   January 31, 2013 at 5:39PM
Thank you for reporting this comment. Undo


This is interested post to me as i am going to switch to tankless in a few months.

Nowadays most Tankless have a gpm rating i.e the rate they can produce hot water. Rinnai being a manufacturer i am looking at which has a 180000 BTU - 7.5 Gallon per minute model and a 199000 BTU - 9.4 Gallon per minute model.

Now if my assumptions are correct with 7.5GPM unit 3 people can easily take a shower as each shower is rated for gpm flow rate and assumption is no one is using only full pressure of hot water.

Now coming to appliances as someone has already mentioned, most appliances have an internal heater so the temperature to the appliance is somewhat a moot point. I keep my high efficiency hot water tank at 120F, which i think is waste of energy as it has to work towards keeping that temperature whereas the appliances are not running 24/7 so it is probably cheaper to let the internal heater do the water temperature increase.

Also as mentioned i always run the hot water before running my Dishwasher and Washer.


    Bookmark   February 5, 2013 at 1:09AM
Thank you for reporting this comment. Undo

You may not be fully understanding the GPM rating. It's a reference to what flow rate the unit can produce at a stated temperature rise.

The tankless has to instantaneously add sufficient heat to the flow of water passing through it to increase the temperature of that flow from its initial source/input temperature to the desired setpoint/output temp.

For a unit of a given heating capacity (180,000 BTU for example), whether it can achieve the setpoint depends on three factors: incoming water temp, flow rate, and what is the desired output temp. Colder incoming flow reduces either A) the output temp that can be achieved, or B) the flow rate to maintain the desired output temp.

A quick search finds a reference on a Rinnai 180,000 BTU unit rated to achieve a temp rise of 35F at 7.5 GPM. If the incoming water is 35F, then the output temp at 7.5 GPM is 70F (35F + 35F = 70F). A 70F shower isn't likely to be comfortable. :-)

It can achieve a 77F temp rise at 3.9 GPM. That'd produce 112F on 35F incoming water.

Hitting a 102F to 105F output temp (which is a nicely-warm shower IMO) at 7.5 GPM (three showers at 2.5 GPM each), the incoming cold water to the tankless would have to be 70F (70F + 35F = 105F).

Does your tap water get colder than 70F, perhaps during your winter season? If so, then you either A) can't run three showers simultaneously at 2.5 GPM each; or B) can run three showers but at a lower flow-rate to maintain a comfortable temperature.

    Bookmark   February 5, 2013 at 7:42AM
Thank you for reporting this comment. Undo

@dadoes: Makes sense, so the GPM value is somewhat useless and variable depending on various conditions.

I will measure my water temperature and post it here as it is winter it should be around 40-50deg i am guessing.

So in reality the value that should be taken under consideration is temp rise at stated GPM and then you can calculate your best and worst case scenarios.

I will report back later on with temperatures from my hot and cold taps.

    Bookmark   February 5, 2013 at 12:11PM
Thank you for reporting this comment. Undo

we also researched this topic extensively before going tankless. yes, depending on the model of your tnakless heater, the distance between heater and the appliance, and appliace manufacturer you might have "sandwich effect" or not optimal temperature. We replaced our washing machine with Bosch - it is a 220/240V washing machine that will heat the water to the temp it needs to do the cycle. You can connect it to the cold water line for all that matter - it will take longer to complete the cycle. Our old Kenmore top load worked fine with tankless but the volume of watrer it took was enormous and it was never an issue with the tankless heater. You may want run the sink tap before starting the cycle to purge the line but in our case it was not necessary. Bosch heats water to 165F very fast on sanitize cycles. In our research - european washing machines (240 V ones) will heat the water.
As for dishwashers - we went with Bosch as well but haven't installed it yet as kitchen remodel is still in progress. It also has an in-line heater and supposedly will heat up the water. Question remains - will it do it fast enough considering it uses so little water in the cycle. I guess purge the line first? But I am really looking forward to testing this set up in my own kitchen, will report back in a couple of weeks

    Bookmark   February 5, 2013 at 2:57PM
Thank you for reporting this comment. Undo

I've had a tankless for 18 years with no issues. We simply don't try to run 3 showers and the dishwasher at once. Yes, I generally do use the kitchen hot water before starting the DW. And, although I can't shower 3 people at once unless they are very friendly with each other, we can do two single shower head showers with the federally mandated 2.5 gallon flow heads. Well, we can in the summer. In the winter, we reduce the flow rate and can only do one shower and a small use faucet simultaneously. But, that's fine in a two person household. In a multi person household, we would have opted for a larger unit. In fact, we may put another smaller unit in when we do the master bath remodel just to be able to have the larger shower with a few extra body sprays. That won't be but an occasional thing though, and it might not be worth it. But since we DIYed the first one, we would only have the materials factor to worry about for cost.

    Bookmark   February 5, 2013 at 5:30PM
Thank you for reporting this comment. Undo

Anyone know what happens if you oversize your tankless unit? Obviously if you undersize it you either get less than optimal temperature or no pressure.

    Bookmark   February 5, 2013 at 11:25PM
Thank you for reporting this comment. Undo

So i guessed right its below freezing here and my cold tap temp is 45F and hotwater is 120F.

so the model i posted above i would get 80 for showering at the advertised gpm!

Interesting caveat! But as live_wire_oak suggested, it is unlikely 3 people will shower at the same time.

I wonder how i can find the final GPM rate based on my incoming cold temp of 45F to produce say 100 or 110 or even 120F.

    Bookmark   February 6, 2013 at 1:00AM
Thank you for reporting this comment. Undo

@jasperdog: the specification you would want to know is the temperature rise at said given gpm....

So 35F rise at 7.5GPM is better than
35F rise at 4.1GPM
30F rise at 7.5GPM.

i think as dadoes illustrated it all boils down to the optimal final temperature requirement which is related to incoming water temperature and then the specification of the unit.

    Bookmark   February 6, 2013 at 1:06AM
Thank you for reporting this comment. Undo

Optimal use of a tankless is that the output is not overheated ... that is, the setpoint is at or not much higher than the desired temp for the heated-water task at hand.

Traditional tank units require effectively overheating the water for sufficient reserve capacity, so heated water doesn't run out during a bathtub fill, a long shower, or several consecutive showers, for example. Tankless units don't require overheating, as they produce heated water continuously, it doesn't "run out."

Many tankless units have optional remote control panels which can be installed in a bathroom or kitchen to facilitate adjusting the setpoint without having to go to where the unit is installed. The efficiency and convenience of "on-the-fly" adjustment is lost if the homeowner/family must go to where the unit is installed (garage, attic, or outside the house, for example).

    Bookmark   February 6, 2013 at 8:06AM
Thank you for reporting this comment. Undo

Dadoes - My question is regarding the size of the unit, ie, is there an issue with getting a 9.5 gpm unit when an 8.4 gpm unit is adequate most of the time. That is different than setpoint, which you can adjust to whatever you want within the specs of the unit.

    Bookmark   February 6, 2013 at 10:44AM
Thank you for reporting this comment. Undo

You're looking at the GPM rating the wrong way. Don't come at from the point-of-view of literal maximum flow rate. Look at it from the aspect of heating capacity. Any unit can be run at 9.5 GPM flow rate if the household plumbing supply and output lines support that volume. It just may not heat much if the capacity is small.

You're (presumably) not going to be running a (single) standard shower or kitchen faucet or washing machine fill at 9.5 GPM. Maybe a multi-outlet body shower could approach that much flow, in which two tankless units would possibly be needed to reach a comfortable temperature.

Getting an "oversized" unit does not present a problem (other than making sure there's enough gas line or electric amperage capacity to handle it). The unit will moderate its heating (electric elements or gas burner) to attain the target output temp (to the capacity that it can) on varying flow rates. There may be a minimum level for a gas burner at which it can't go any lower without the burner shutting down.

My electric unit is rated at 28KW (it has four 7,200 watt elements, that's 28,800 watts of heating). The display/control panel reports a range of operating parameters (setpoint, input water temp, heating chamber 1 temp, heating chamber 2 temp, output temp, flow rate, and percentage heating output of full capacity). A particular one of my showers at full faucet open runs at 1.2 to 1.4 GPM. I don't know why it's low compared to the standard 2.5 GPM but that doesn't matter. It's a hand-held adjustable/massage-type shower and the flow-rate "feels" perfectly fine in use. As an example of usage parameters ... on a 103F setpoint, with input of 67F to 68F, running that shower, the percentage of capacity started at 31% then settled at 22% to 25% after it ran few mins and got stablized ... which is 6,300 to 7,200 watts ... far below the maximum 28,800, so there's capacity to heat to 103F at higher flow rates if needed, or a higher temp at the given flow rate or on much lower incoming water temps. I don't know the formula off-hand for comparing 28KW electric to gas BTU.

    Bookmark   February 6, 2013 at 11:36AM
Thank you for reporting this comment. Undo

"Oversized" is a complete misnomer. As dadoes says, you are approaching the problem from the idea of a tanked heater rather than the actuality of a tankless.

Find out what your current incoming water temperature is right now, in the middle of winter. That's your starting point for the entire conversation. After you find that out, then you know how many degrees of temperature rise that you will need in order to take a comfortable shower. Then you look at GPM for the unit at that amount of rise. That tells you if you can do only that single shower or if you can do other uses at the same time.

I can personally tell you that it's not at all an issue to develop the "serial use" habit rather than the "simultaneous use" habit. At least, in my household of only 2. In a household of 5 kids, you probably want at least 2 large use appliances (shower and washing machine, or 2 showers, etc.) to be available at the same time. That will mean either moving to the larger BTU unit, or if you live in a really cold climate, doubling up the units or switching to a large tanked system.

The biggest issue that I can forsee with having an "oversized" unit is the GPM flow that will trigger it's use. For instance, if you do a trickly of hot water in your powder room, will that be enough to ignite the burner? In the summer, with the warmest water you will see? If either the flow rate or the temperature rise isn't enough to trigger the burners, you may have a problem.

In the summer, we have darn hot water, and I have to up the flow rate in order to be able to have the water not be heated so much. And that's because the water is so warm, that with a slow flow rate, the water gets too hot to mix with very much cold, and if you do too much mixing, you've lowered the demand for the hot below it's point of ignition.

It sounds more complicated than it is, and it's really is NOT a big deal to make that adjustment, once you make the mental adjustment to the tankless. The 20 year newer ones are also probably have all of that programmed into their operation.

But, I like my oldster. It's still going strong with zero issues about scale buildup in the burners like the fear mongers would yell about. And I can have hot water even with the power out because it doesn't need to have an electronic connection in order to work. Now, if you have hard water and it's full of minerals, then you need to invest in a whole home water treatment for the health of any water heater that you might choose, as well as your internal plumbing and fixtures.

    Bookmark   February 6, 2013 at 12:23PM
Thank you for reporting this comment. Undo

Thanks for the replies. I talked to the water company today and they said winter water temperature is around 35 degrees.

    Bookmark   February 6, 2013 at 11:21PM
Thank you for reporting this comment. Undo

@dadoes: when you said "Optimal use of a tankless is that the output is not overheated ... that is, the setpoint is at or not much higher than the desired temp for the heated-water task at hand. "

Are you implying that when you use the shower or use hot water it should be set to the desired temp and no mixing it with cold water?

Because most of the time (i think true for everyone) we always mix hot/cold to get desired temp.

If the above assumptions are false please elaborate.

    Bookmark   February 7, 2013 at 1:39AM
Thank you for reporting this comment. Undo


Yes, my usage pattern for 8 years has been that I run only the hot tap when showering, the rare occasions of filling a bathtub, and even when washing clothes.

My normal setpoint is 103F, which is perfect for me for showering. I don't often hand-wash dishes, but 103F is adequate when I do. I don't need to increase the temp for my dishwasher, which has assured onboard heating. 103F is OK for warm when washing clothes, I may raise (or even lower) the temp for some loads. Fortunately my washer allows selecting Hot temp on all cycles. Washers with electronic controls that don't allow higher than Warm on some cycles and insist on mixing cold + hot may not work so well with my method.

My tankless unit is in a broom closet in my utility room off the kitchen, so is easily accessible for adjustment when needed. Adjusting the temp "on-the-fly" without pressing the Set button is a temporary override that reverts back to the setpoint after 30 mins (or 15, 45, or 60 mins per a programming option).

Tankless usage is self-adjusting to a degree (pun intended!). Reducing the hot flow at the usage point (such as when moderating a shower) in turn throttles the tankless back on how much energy (gas or electric) it pulls to heat that reduced flow to the setpoint. However, this can bring in the issue in some usage conditions on some units of dropping below the minimum flow-rate required to keep the unit activated -- shower too warm, turn back the hot flow, tankless shuts off, shower turns cold, crank the hot back up, tankless turns on, shower gets too warm ... repeat.

My thoughts are that it's silly with a tankless to crank up the setpoint to 125F or 130F or 140F then temper the flow with cold at the usage point. That's required with traditional tank units for reserve capacity since they heat much slower and would quickly run out of heated water. Not so with tankless. In a way (but perhaps not really due to the self-regulating aspect on energy consumption) it's like floorboarding the accelerator on your car and using the brake to maintain the desired speed.

    Bookmark   February 7, 2013 at 11:37AM
Thank you for reporting this comment. Undo

The deal with tankless is that if you turn the temp up to say 125F or 135F and mix with cold in a shower, you may reduce the hot flow below the minimum flow required for the unit to activate. Then you get a cold shower, or a burst of cold from time to time. Not fun.

I can say that having a lower temp (105F) for my dishwasher might be a problem. Eventhough it has an internal heater, it will only heat so much. If it cannot heat the water sufficiently in a set amount of time, it will shut down and flash a code (the 'pull out your wallet' code) for insufficient temperature rise.

I don't know if the same issue happens with newer clothes washers. Since many of them have lowered their temperatures (Hot has dropped from 50C to 40C), it may not be an issue.

    Bookmark   February 7, 2013 at 12:28PM
Thank you for reporting this comment. Undo

Depends on the dishwasher involved. Mine is a Fisher & Paykel DishDrawer, which has assured heating to specific target temps for the main wash and final rinse, no matter how long it takes. DishDrawers can work on a cold connection.

My washer is a Whirlpool Calypso. No onboard heating. It has ATC (controls mix of hot & cold) for warm and cold (also has tap-cold) but hot is full tap-hot (hot solenoid only), and it does allow hot on any cycle, including Delicate, Hand Wash, etc. Regardless of what cycle I'm using, I select Hot temp on the washer, set the tankless to what I want (85F, 103F, 108F, 115F, 125F, 135F, up to the max 140F it can produce) and let 'er rip.

    Bookmark   February 7, 2013 at 12:41PM
Thank you for reporting this comment. Undo

@dadoes & weedmeister: These are some very good and logical points and make perfect sense.

Where i am there is a law/practice that at the hot water output it has to be mixed in with cold water to temper the water to reduce danger of burning one's self. Ridiculous i know (i have mine disabled).

The only concern, as both of you pointed out would be dishwashers and washers dealing with lower temp for running duty cycles respectively.

This adds a whole twist on the tankless usage! Realistically in winter time one does not need to use the cold water tap except for toilets, drinking water etc.

Very interesting indeed i will for sure insist on a model that has a remote temperature override setting.

    Bookmark   February 7, 2013 at 2:08PM
Sign Up to comment
More Discussions
Why are my frozen bananas melting?
Not sure the right forum for this, but I freeze bananas...
48" Wolf, Thermador or Bluestar...or something else?
Looking into purchasing a 48" range. Wolf: Classic....
Best customer service for wall ovens?
I posted this comment to an older thread lamenting...
Mrs. Porchman
24" refrigerator. Liebherr, Fagor, LG, etc?
We live in Boston. We are in the market for 24"....
Need advice on combining induction and gas
This is my first post. Please forgive the length or...
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™