Thread Number: 72678  /  Tag: Modern Dishwashers
Cold water experiment
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Post# 960372   10/3/2017 at 07:27 (354 days old) by Kindalazy (toronto)        

LONG time reader. 1st time to post. My farm home has a commercial on demand Rinnai water heater. The home also has a Grundfoss pump which circulates the hot water in a LONG loop. In an attempt to save some cash on my natural gas bill, I decided to turn off the Rinnai, using only cold water to use the dishwasher. It's a relatively new KitchenAid. So I used the hi temp wash and sani rinse options with the normal program. If the sani rinse doesn't come up to temp, the sani led flashes at the end of the cycle. It didn't flash the past 2x I tried this out. I put the dishwasher on before bed when the hydro rates are lowest and not caring how long it runs. Am I better off from a financial perspective heating relatively small amounts of water in the dishwasher electrically, or heating a long loop of water with natural gas?

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Post# 960378 , Reply# 1   10/3/2017 at 08:21 (354 days old) by Yogitunes (New Jersey)        

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well this is where you would need a watt meter to determine which is saving you money....

and that depends on areas, sometimes gas is way cheaper than least in New Jersey it is....

on-demand water heaters are usually more efficient than standard water tanks....

dishwashers can heat their own water, but may take longer to run in order to get the temperature raised.....most times its just boosting temps 10 to 20 degrees....and depending on how cold your incoming water is...

plus as a dishwasher cycles, the heating element only comes on for the main wash, and final the mean time, the pre-rinse and regular rinses will be cold, so you would be raising and lowering the internal temp...things like greases and oils could be dissolved and redeposit.....

this cold wash attempt would function in a pinch.....can't say it would be something I would do on a regular basis....

your mileage may vary....

Post# 960391 , Reply# 2   10/3/2017 at 09:51 (354 days old) by combo52 (Beltsville,Md)        

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It is probably cheaper to let the DW do the heating, but it will probably put a little more wear on the DWs wiring and main motor doing so all the time.

Could you just have a timer on the pump for the hot water loop so it will shut off after an hour or so, so it does not have to keep the loop hot after the DW is through with most of its fill periods.

Post# 960394 , Reply# 3   10/3/2017 at 10:05 (354 days old) by foraloysius (Leeuwarden, the Netherlands)        

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I've never seen a loop system with a tankless water heater. Overhere those systems are only used with a well insulated tank water heater and insulated hot water pipes to make it as economical as possible. Tankless water heaters are for hot water on demand, that's what they're best at. Using them for a loop system is a waste of energy.

Post# 960398 , Reply# 4   10/3/2017 at 10:25 (354 days old) by Kindalazy (toronto)        

Thanks Yogitunes for your reply. Combo52 (John, I think) I always read and respect your participation. I used to have the pump on a timer. The problem was it had to run between 40-45 minutes per hour to supply the house with hot water. Even then, cold water would get "sandwiched" between the hot water. I installed a remote on/off switch, which helps. Now if i switch it on, the water is heated within 3-4 minutes. This dishwasher like my other KA's is really wonderful at cleaning, and i don't wish to prematurely wear it out - it's replacement (if it comes to that) will likely be a BOL Miele. I'm just scratching my head as to the most economical way to use energy. I'm balancing 2 homes and a condo, and my bank account is depleting 😀

Thought I'd share some pix of the kitchen and house built in 1881 (the windows are being replaced with metal and wood clad with the original style rounded tops as we speak. Kitchen was a 2 year project completed in 2014.


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Post# 960401 , Reply# 5   10/3/2017 at 10:41 (354 days old) by DADoES (TX,U.S. of A.)        

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There are often discussions about tankless water heaters and circulation loops on a plumbing discussion board that I read on occasion. People want the loops when the water heater is located a long distance (plumbing-wise) from hot water usage points. The advisory always is that the circulation loop is an energy-waster if not insulated extremely well, and of course it negates some of the advantage of tankless if the pump runs continuously. I vaguely recall mention of a tankless brand/model that has a built-in circulation pump.

I have a tankless (no circulation pump). It takes 2+ fills for heated water to reach my dishwasher if I don't purge the line first, which I do on occasion depending on winter weather and/or the contents of the load. However, my dishwasher has assured water heating. The main wash and final rinse are extended as long as necessary to reach the target temperatures.

I also keep the tankless set at 102F as the "normal" temperature and raise it temporarily when needed for particular tasks ... but that's a separate discussion.

Post# 960515 , Reply# 6   10/3/2017 at 23:36 (353 days old) by GusHerb (Chicago/NWI)        

You say you turned off the water heater, but do you mean you turned off the recirc pump? I don't understand why you'd turn off an on demand WH completely. If you don't wanna spend money circulating water then just disconnect the pump. I notice no insulation on the pipes in your pics either, if they aren't anywhere then insulating them would probably help a decent amount in cost savings. 


Honestly I feel you're cutting cost in a place where the savings would be little. Surely you're paying a TON more to heat that house than you would to heat some water and run it through a recirc line. 


Louis: I encountered a tankless WH with a recirc line setup once using a 3 gallon electric water heater to supply the heated water for the loop, and they had it on a timer. They were complaining to me that when the recirc pump and WH were off for the afternoon that it would take forever and ever to get hot water. I looked it over and turned out the plumber had piped the water tank in-line between the WH and rest of the house so in order to get hot water you had to purge all the cold water out of that 3 gallon tank first. Kinda ridiculous because they could've easily put that tank on the return side of the loop to prevent that issue. I offered to correct it but they declined. 



I personally like to stick with tank water heaters for consistent water temperature and not having to wait for the burner of a tankless to light up and the HX to warm up in order to get hot water first which would cause more waste. Not to mention a standard tank WH works in a power outage. I've also never seen our gas usage during non heating months to be anything more than a negligible amount while serving a gas range, dryer, and WH. Especially compared to heating which makes everything else look like nothing at all. 

Post# 960572 , Reply# 7   10/4/2017 at 13:16 (353 days old) by Kindalazy (toronto)        

DADoES, we have uninsulated pipes for the loop, and the furthest point is about a good 75 feet from the tank. It also runs through slab since there's no basement on the addition which is about 30 feet. We keep the temp at 125f.

Gusherb , I turn off both the tank and the pump at night, looking to save resources.

It's beginning to seem like 12 of one, and a dozen of the other. I don't wish to wear out the dishwasher. The tank is under a warranty and would rather replace it.

Cheers and thanks!

Post# 960589 , Reply# 8   10/4/2017 at 14:32 (353 days old) by foraloysius (Leeuwarden, the Netherlands)        

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That is indeed a very awkward set up. Akward too that they didn't let you change it. What a wast of water and energy!

Post# 960617 , Reply# 9   10/4/2017 at 16:36 (352 days old) by jkbff (Happy Rock, ND)        

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I am going to express my thoughts on this, but this seems like trying to get milk from a crowbar.

Why isn't the circulation pump on a pressure switch with a time delay? Meaning when a valve is opened, the pump circulates until the valve shuts and x amount of time (five minutes) passes before it stops circulating?

If you are wanting to shut off the hot water to the house at night and still run your dishwasher, you are going to need a dishwasher that is designed for that type of usage. I would imagine that bosch is capable of doing this, I know the Miele EcoFlex dishwashers are designed for this, but so are their other dishwashers.

I would honestly just go the pressure switch / time to off delay route my self.

I plan on doing a setup that you have when I finally redo my bathrooms.

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