why does my Hobart AM15 dishwasher keeps steaming when cycle is complete?

[guest]

Hobart replaced the tank and acidic water or maybe a bit of both.  it was around 6 Megaohms.  maybe I could switch sensors.  The sensor by the vacuum breaker is exactly the same one.   

[fixbear]

Where exactly are they developing.  If near the mount or a weld area,  it could also be influenced by pressure changes. Making the tank sidewalls expand and contract.  You may want to verify the water supply for both pressure and line size.  All the way back to the source for the 3/4 diameter. 

 

Some years ago I had a customer that had run a 1/2 copper line to his dishwasher 170 ft. He didn’t think it would be a problem since line pressure was 120 psi.  After replacing a few regulators and solenoid valves he finally re-plumbed with a PRV and 3/4 plex.  Took a lot of drawings and all to get it threw to him.  Yes we can get the volume thru the small pipe,  but the speed of that water column is like a big ass hammer when you stop it.  Ask anyone who has slammed shut a fire-hose on a long line.  .

[genemattice]

I get what you mean about the hammer.  I’ve had problems with the hot water tanks in the basement and have put in an expansion tank to help.  The kitchen is quite a ways from that  and the sprayers which are as bad as solenoids slamming closed.  The first pin hole was right in the front of the tank above the thermistor where the tank starts to bend.  No weld there but I’m sure it is stretched when stamped out.  The second hole seems to be in or near the weld on the side.

[fixbear]

I have never needed to replace a tank.  This is troubling to me.  Was the removed tank examined for the cause?

 

Ohm driffting, pinholes?  thinking electrolysis.  With stainless, the first one may have a a undetected inclusion.  It’s common in lower grades. But not a second one too.

[genemattice]

I just switched rinse thermistors top to bottom and booster tank is no longer over heating .  the final rinse temp is reading 108 which tells me that my one week old, ohm drifting  ohm  has gone bad.  As far as the old tank, I kept it and am cleaning it up now.  It looks more like a crack .  will let you know. I will plug it and charge it with air.

 

My understanding of electrolysis is that it is caused by 2 dissimilar metals coming in contact causing corrosion. Or is this an electrical thing where a small current flow from stray voltage could cause it?

[ectofix]

AH!  I wanted to say “maybe a bad probe”.  But, you said you’d replaced it.  I “assumed” that it was good, then.  You know what they say about when you “assume” something.

 

I sure wished I had a table to use for testing Hobart thermistors out of a service manual.  I think they’re 100KΩ thermistors.  Negative temperature coefficient ones.

Off the shelf, the best numbers I came up with through my own tests were:

20kΩ @150°F

13kΩ @165°F

That’s all I got.  Did that a few years ago.  I need to fire up a fryer in the shop with water in it and do more tests for 180° or so.

[ectofix]

The only built-in boosters I’ve replaced were ones in a Jackson door-type machine (never identified the cause) and…in a Hobart undercounter LXi.

 

On the Hobart, I discovered that it wasn’t installed with a PRV.  The LXi doesn’t have one built in and apparently was overlooked during installation.  It just so happened that I ran a call on it some time after the tank was replaced.  While running it, I saw that the rinse pressure was WAY too high, so looked at the plumbing…and found NO PRV to regulate incoming water pressure.

I don’t know how that would’ve been overlooked by the Hobart-trained technician who installed it.

 

That’s just testament to the fact that ANY technician who’s bearing that title and strives to live up to it, should KNOW the equipment and attentively observe EVERYTHING – from when it’s turned on, through ALL aspects of its operation…and when it’s turned off.

Every click, tick, hum, delay, rush of water, gush of steam, light, fan, control panel display, guage, drainage, VOM reading…or whatever.  Every breath that machine takes.

 

Be like Scotty with it:  “Captain!  I’m giving it all she’s got!” 

[fixbear]

It can be either/or or both.  The latest frigate built for Australia is full of holes after 6 mouths.  Someone hooked up the anode system with reverse polarity.  Any time there is a current flow threw a metal object that has different dielectric ratings,  there will be a transfer of metal. If you have a acidic or saline solution and a flow of current, electrolysis is happening. Water heaters have a magnesium rod or chain down the middle just for that reason. Sea water heat ex changers use zinc rods.  I don’t believe that the dielectric strength of the Cal rods and the stainless are that far apart.

 

But back to your problem.  Stainless is very prone to cracks. after all, that’s what sank the Thresher.  It does not like pressure and heat change together.  Technically the booster is under 20 to 25 lbs.  But if the prv is set high or the rinse piping/wands are restricted it may see more. As I recall they are 2 spun cylinders that are welded together with a bracket. Stress would be at the bracket welds. Or the threaded taps.  I would really examine the water supply to the machine and the prv setting.

[genemattice]

Okay,  I just want to run this by you.  It seems to me that there is never any actual pressure on the booster tank to speak of.  There is no condenser on it. It has a hood so that solenoid in the rinse line is not there.  Is this correct or am I missing something?  It seems to only push water through the tank when it goes into rinse and the solenoid opens.  I believe that is why when to booster wouldn’t shut off the water was pushing out the wash  arm.  Also, there couldn’t be any water hammer on the tank in any way so I’m thinking that it has to be electrolysis.   Make sense ?

[genemattice]

Oh,  something else I noticed when I was looking at the dishwasher was that the pressure reducer was set very high so I turned it down to 20 pounds right in the green area on the gage.  The next morning the dishwasher wouldn’t fill and showed the EE code so I turned it back up and it worked just fine.  Hobart technicians have been the only ones that have ever worked on this machine until now.