Dishwasher/washing machine GPM?

Chuck_7

Does any one know approx. instantanious GPM to typical household dishwashers and/or laundry washers?
I am concerned that with a shower running, turning on a dishwasher and washer could over tax a typical on demand water heater.

Mike T., Swampeast MO

Both clothes washers and dishwashers in the 2-3 GPM range depending on supply pressure. (With clothes washer that's the hot side flow only.)

European clothes washers usually heat their own water with a built-in electric element. Their dishwashers likely do the same. American dishwashers have long had a "high temp final rinse" option, but I'm not sure if many or even any are designed to heat the water with each fill (usually 4-5 fills).

larry

I believe that both dishwashers and washing machines have sensors which detect water level. As such, if an on-demand heater doesn't flow as quickly due to other things happening in the house, it should just take longer to fill those appliances rather than them not working at all assuming the appliance has some level of electronic intelligence.

As an aside, the European appliances do have on-board heaters, but some of them (e.g. Miele) will still use hot water supply depending on what wash setting have been chosen while other always start with cold water only (e.g. Asko).

Firedragon_4

You can get 10 gpm out of

a 1/2" pipe at 55 psi and a washer typically goes through 18-24 gallons of water per cycle.

Dave Yates (PAH)

several factors come into play

Showers are supoposed to be rated at 2.5 GPM, but most will exceed that number if the pressure is higher than 50 PSI. It's best to time that while filling a bucket with the adjusted bathing temp to obtain true GPMs. The last one I measured was running at 78 PSI under full flow and even though both the faucet body and the shower head were stamped as being 2.5 GPM max, the actual flow rate was just a hair under 4.5 GPM. Straight out of the box! When I checked the manufacturer's literature, their GPM rate rises as pressure rises - right up until that 2.5 GPM rating was hit & as if by magic, that turned into a flat line as pressure continued to rise. Even at 90 PSI they were still showing that at 2.5 GPM. And that's why the owners were running out of hot water when another faucet, the ACW or DW were turned on. They were already maxing out their on-demand water heater with just the master bath shower.

Next comes the ratio of hot to cold water, which is based upon the systems coldest (IT) incoming water, (ST) storage temperature & adjusted (BT) bather temp & it looks like this: Ratio = (BT-IT) ÷ (ST-IT)

If your storage temp is 140 F & the coldest water arrives at 38 F, the ratio becomes .628 or 62.8% of the flow is hot water if your adjusted bather temp is 102F.

140 minus 38 = 102 degree rise & 8.33 x 102 = 849.66 Btu's needed per gallon of flow.

Take your measured flow x the ratio to determine GPM. Multiply that by 60 or else divide the NET input of the on-demand water heater by 60 and divide that by the 849.66 to see the real GPM numbers.

If you do the math, you'll find most on-demand GPM ratings are based upon a 49 F rise! Let's see - if I get 38 F incoming water under design conditions and raise that 49 F to a scorching 87 F, will my customers think that's hot water or will I be the one in the hot seat?

Gotta do the math to get at the truth! We need to hold all water heater manufacturers to identical standards when rating GPM. That doesn't exist right now, so it's a marketing game.

Some of the claims are outright silly - like making the statement that you will save 60% over a conventional water heater when the split between their stated efficiency ratings is just 2 percentage points and standby flue loses may add an additional 3% to 6%. Or: unlimited hot water at a temperature you set. No mention of GPM rates there.

But one of my favorites states the following: Conventional water heaters last 2 to 10 years while our on-demand water heaters have an average life span of 20 to 30 years! If you're losing a tank every two years, you have water quality issues and no water heater - especially thin-walled copper tube types - will last 20 years or longer if that's not addressed first!

Who get's the angry consumer calls when these stated claims prove to be based upon weird conditions seen only in a lab setting.

When used properly with solid facts to support the required flow rates, on-demand water heaters perform like a champion.



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