Dishwasher drying element checks out, but draws less than 800 Watts!

[whetstone]

My dishwasher has always had trouble drying. The heater element is good. I replaced it with OEM part several months ago hoping that would help. It didn't. I know a number of factors contribute to poor drying but what I'm focusing on right now is the wattage of the element. It seems way too low!

The element measures 13.4 ohms. I don't now what the original measured (it had failed). Next, I checked power consumption with a RMS power meter at the AC outlet -- only 780W during the dry cycle! That's half a hair dryer! What's up with that?

Also, if you work back from wattage and resistance, the circuit that supplies the element apparently only provides just over 100 volts (Ohms law: 780W @ 13.4 Ohms = 102 VAC, 100% power factor). I don't have a schematic but it doesn't make sense that the voltage would be stepped down for the heater element. The model is Kenmore 587.1515, circa 2005.

Is 800W typical? What about the stepped down voltage supply to the element?

[Chat_in_FL]

Why work backwards? what was the voltage and amp draw?  If memory serves me, 13.4 ohms and 8-9 amps is in spec)...

 

Most inexpensive meters use an averaging technique for determining the RMS reading of an AC voltage or current. This method gives you an accurate reading when you’re measuring an AC voltage with a sinusoidal waveform, but if the waveform of the AC voltage or current you’re trying to measure is not a pure sine wave, the reading will be inaccurate, usually low. Depending on the type of AC waveform, the reading might be up to 40% low.

Edited December 9, 2017 by Chat_in_FL

[Budget Appliance Repair]

Tech sheet doesn't show an OHM rating for the heater but does say the heater is 900W.

http://manuals.frigidaire.com/PDFnotFoundCheck.asp?path=ftp://ftp.electrolux-na.com/ProdInfo_PDF/&folder=Kinston&PDFtype=Wiring&PDFfile=154497501.pdf&model=587.15152401

The above is a link to the tech sheet if you haven't already found the one that should be hidden behind the front door panel, (removed by removing the two screws - on one each side and sliding front panel down to disengage from hooks on door).

[whetstone]

@Chat_in_FL, yeah, I should have measured the voltage but it's tight to reach those connectors in situ and not being familiar with the layout, I didn't want to stick my hands back there while the unit was energized. The d/w is located in a rental house I own in another town and I forgot to take my alligator clip leads. But based on the schematic (thanks, @Budget Appliance Repair!), it's clear there's no voltage step-down (no surprise), so I gotta wonder about the accuracy of my meters.

Still, even 900W seems too low to do much good, but this is my first experience troubleshooting a dishwasher.

Referring to the timer switching table, I noticed the heater element is energized to heat the water during the last two rinse cycles. I didn't realize it had that feature. Since rise water temp has a big impact on drying, I should verify this. It's easy enough to test. If the timer is failing to heat the water, that would explain a lot.

I have to return to the house next week to replace a part on the range. This time I'll bring a different multi-meter and some alligator clip leads.

Thanks!

[Rhubarb Tau]

You're assuming the resistance of the heating element is constant, but it isn't; it increases with element temperature. Nichrome heating elements (Calrods and similar) have a positive temperature coefficient of resistance (or tempco). If you measure the current for the first minute or so that a submerged heating element is energized, the Amp draw will start higher and quickly drop ~25%.

It sounds like your heating element is working just fine. I'd start with the basics. Are you using rinse aid (like Jet-Dry)? Is your incoming water temperature at least 120*F, and are you purging the hot water line before a dishwashing cycle by running hot water at a nearby faucet until the water is hot?

As an interesting tempco curiosity, the silicon carbide used in oven ignitors actually has a negative tempco; resistance decreases as temperature rises. That's why, when you're measuring current draw to a hot surface ignitor, Amp draw climbs for the first ~30 sec. until topping out typically at 3.4A - 3.5A.

[whetstone]

@Rhubarb Tau, thanks for reminding me of tempco. I have a graduate degree in engineering and deal a lot with RTD's so you'd think I would have realized that! 

As I said, I'm already aware of all the various other factors that impact drying and have advised my tenant accordingly. I just didn't think 800W seemed reasonable for a drying element. Now I know.

Although I'd prefer not to spend the money on a new timer, I'm sorta hoping it's malfunctioning (re: not heating rinse water) so I can actually do something to improve drying performance. Part of the problem is they have a large family so the d/w gets really loaded. Also, I suspect incoming water temp may be an issue. The water heater is conventional size electric so I'm sure it has trouble keeping up with HW demand for five occupants. In particular, I suggested they make sure the water heater has time to recover before staring the dishwasher.

[evaappliance]

16 hours ago, whetstone said:

Still, even 900W seems too low to do much good

There are no good dishwashers anymore !  If the water is hot, use rinse aid , and heat dry selected, they will clean almost as good a person could do in their yard with a waterhose !!!!  Lol !  Seriously though, it is most likely doing all it is capable of per government regulations

[Budget Appliance Repair]

Also in dry cycle the heater is not on 100%, it cycles on /off thru the dry cycle.

You could also have a hi-limit safety t-stat failing, it is in the left rear corner of the tub on the bottom of the tub, these Frigidaire units seem to burn this t-stat out quite often, (crappy manufacturing - they don't stake down the terminal rivet good enough so the terminal is loose from the rivet and contact inside the t-stat and it burns up the connector and t-stat).   Before they completely burn the wiring up and stop working completely they can work at the first part of a cycle and then as the heater is cycled and the connection heats up it quits working.