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I have read in a maintenance manual that the ideal temperature for incoming water to a dishwasher is 150F.

In my house, the hot water heater is currently set to 135F and it cools on traveling around the house, so the typical faucet temperature is around 120F.

I have considered up the temperature of the water heater to 150F, but in that case it is recommended to have a thermostatic valve reduce the temperature to 120F, which would be no better than what is happening now as far as the dishwasher is concerned.

I could boost the water heater temperature to 155F which would result in 150F water reaching the dishwasher, however, then all the faucets would also have 150F hot water, which is pretty hot.

The only way I see around this is to create a special hot water line that goes to the dishwasher and clothes washer that comes directly from the water heater and put the water heater at 155F, then put a valve on the water supplies to all the rest of the house. Is this is the "proper" solution, or is there another way to get the right temperature to the appliances?

  • What model dishwasher? – manassehkatz May 9 at 16:07
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    Do you really need 150 degree water in your dishwasher ? What "maintenance manual" did you read this in ? – Alaska Man May 9 at 16:40
  • Also, clothes washers generally don't need quite as hot water as dishwashers. Not the same sanitary issue (for most people, most of the time) as with food. Plus the detergents are actually designed to be quite effective in warm or even cold water. Plus clothes often don't handle really hot water very well (colors bleed, fabrics degrade, etc.) – manassehkatz May 9 at 17:04
  • @Alaskaman "Troubleshooting major appliances" by Kleinert, 3rd edition. – Tyler Durden May 9 at 17:38
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    Reading "general" books is one thing, but to make informed decisions you need to work on the exact detail for the specific dishwasher and the precise cycles it operates and how it senses its parameters. – Solar Mike May 9 at 18:02
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You don't. Let the dishwasher do it for you.

I have read in a maintenance manual that the ideal temperature for incoming water to a dishwasher is 150F.

Was it a maintenance manual for your actual dishwasher? If not, I would ignore random advice (no better than advice from some stranger on the internet...).

Most dishwashers now automatically heat water as needed to the necessary temperature for optimum performance. This avoids exactly the problem you mentioned - water hot enough to clean effectively and to sanitize dishes is also hot enough to quickly burn people. Some dishwashers (I think typically European models) are even designed to run with cold water - heating water from cold to very hot as needed, not just hot to very hot. That being said, I would recommend connecting your dishwasher to the hot water line rather than the cold water line, unless your manufacturer specifically recommends otherwise, because:

  • Electric heat is generally more expensive than natural gas or other forms of heating. If your main water heater is electric then any heating by the dishwasher will cost the same as your main water heater, and if your main water heater is something else then any heating by the dishwasher will cost more. I can't see any normal scenario where using your dishwasher to heat from cold to hot will actually save any money.
  • If your dishwasher runs some cycles without heating the water to very hot (e.g., perhaps a regular rinse cycle as opposed the main wash cycle or a final sanitary rinse cycle) then using hot water instead of cold water will be much more effective.
  • This conflicts with what I have read. According to my book on dishwashers, quote: "Most dishwashers have heaters... but this does not fully compensate for low temperature of the water supply." – Tyler Durden May 9 at 16:28
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    @TylerDurden A dishwasher will use a thermostat to determine how long to heat the water. So for cycles where it is heating the water, the incoming temperature will determine how long it takes (and how much electricity is used) but not change the outcome. But for cycles that don't heat the water, the incoming temperature matters. – manassehkatz May 9 at 16:49
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    Or you can overkill the bleep out of it and mimic commercial dishwashers which have a dedicated super-hot water-heater just for the dishwasher, very close by But modern home dishwashers typically have no need for that, as they have one built in, as already discussed. – Ecnerwal May 9 at 18:03
  • I agree, I have installed commercial dishwashers that required cold water I know the last one actually heated the water to 180F , my wife was worried about our dishwasher when we purchased our current home, it is plumbed to the hot but the water heater was set to 125, I checked the water temp during wash and final rinse and both times the water was @145f so the dish washer was heating the water to that specific temp if it was based on time only the since would be hotter than the rinse since the water in the pipes is already warm on the since cycle.+ – Ed Beal May 10 at 19:08
  • Actually I would plumb it to cold. Let the dishwasher do the heating. If you plumb to hot, the dishwasher will fill with the slug of cold water which has cooled off in the hot water pipe. It will have to heat that anyway. The bad thing is it will abandon another slug of hot water in that pipe, and the energy to heat that will be wasted. – Harper Jul 13 at 18:53
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If your dishwasher is already connected to the hot water line you can get a few degrees hotter water by insulating the water line. The greater the distance from the water heater, the greater impact this will have.

  • Insulation that will raise 135 degree water to 150 degree water ? – Alaska Man May 9 at 16:41
  • Insulation can cut the loss, it can't make the water hotter. So if it starts at 140 (the high end of "normal" because of scalding issues), you might have a situation where currently you lose 10 degrees --> 130, and with better insulation you'd only lose 5 degrees --> 125. But that won't get it up to 150. – manassehkatz May 9 at 16:50
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    @manassehkatz thank you for putting into words the point i was implying. "Insulation can cut the loss, it can't make the water hotter" The other implied point is that the "answer" is not an answer to the question of "How do I get 150F water to the dishwasher?" – Alaska Man May 9 at 17:03
  • I thought that would be obvious. The OP expressed a concern about raising the water temp enough to get 150 degrees at the faucet so I suggested that as a way to get hotter water for the dishwasher without making all the faucets scalding hot. – Eric Simpson May 9 at 18:20
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An on-demand water heater is perfect for this

The key is to regulate the rate-of-flow down quite low, so you can get sufficient temperature rise without having to provision a huge amount of electrical service.

Regardless, this is like trying to air-condition a gazebo. The dishwasher itself - box, pumps and racks - has a lot of thermal mass, to say nothing of the dishes! Preheating the water to 150F will do precious little when it equalizes with all that other thermal mass. (Remember the water volume in a dishwasher is not high; it doesn't fill with water like a washing machine). It really depends on the dishwasher's internal heater, which needs either high current, or time.

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