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Old 05-27-2019, 04:33 PM   #1
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Atwood MPD 93756 not turning off

I have a Atwood MPD 93756 6 gallon gas/electric water heater. Last Fall the thermal fuse blew and I replace it with this:
https://www.amazon.com/gp/product/B0007XXTVC/

I opened it this Spring and heater turned on fine to heat water with gas. But I noticed it did not shut off after 45 minutes. I monitored it to see if it would shut off, but then the relief valve started to dribble water.

I ran inside and turned the heater off.

Thoughts on why it did not turn off?
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Old 05-28-2019, 04:43 AM   #2
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Are you sure it was 45 minutes? Water coming out of the pressure relief valve is not uncommon if your tank has lost its air pocket. Maybe try this procedure..
IMG_3382.jpg

Then fire it up again and test the water temperature at a faucet with a meat thermometer after 30 minutes or so.
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Old 05-28-2019, 06:53 AM   #3
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OK I can try that thanks John. I ordered new thermostats last night. For $10 it was worth a try. https://www.amazon.com/gp/product/B0016ZBP1Q
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Old 05-28-2019, 06:13 PM   #4
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Quote:
Originally Posted by markbrit View Post
I have a Atwood MPD 93756 6 gallon gas/electric water heater. Last Fall the thermal fuse blew and I replace it with this:
https://www.amazon.com/gp/product/B0007XXTVC/

I opened it this Spring and heater turned on fine to heat water with gas. But I noticed it did not shut off after 45 minutes. I monitored it to see if it would shut off, but then the relief valve started to dribble water.

I ran inside and turned the heater off.

Thoughts on why it did not turn off?
Hi, See if this helps.

A few things. You listed the MPD 93756 number as your water heater model number. That Atwood number is actually an instructions publication number of the manual for a water heater. Like this one.
Sunline Coach Owner's Club - Sunline Owner's Files - Atwood LP Gas Water Heater Installation, Maintenan

The model number would be on a sticker on the right side of the heater when you fold down the outside cover door. And since you said it was gas/electric and 6 gallon, the model number would start like this: GC6- and then have some other letters and numbers. Just letting you know in case you are trying to find parts for it. You would need the actual model number.

To your water heater issues, if your T stat is working and or replaced and it does not reduce the reheat time for a cold water heat up, you could have a large mineral scale build up.

I'll explain more in the next reply. That is the short and sweet answer.

Hope this helps

John
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Old 05-28-2019, 06:29 PM   #5
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I'll explain a little more what may be going on in your water heater.

Sorry this is so long, but there are common things that many folks do to their water heater not realizing what can happen with a "camper" water heater. I expanded on this for those following along.

There are a few things that can cause what you are describing, long heating time and weeping relief valve.

1. The air pocket being lost that Johnnybgood pointed out, can and will make the relief valve weep. This can come from towing with a full water heater joselting the system dissolving the air pocket, not draining the water heater all season long between campouts or just being in one place a long time, like on a summer seasonal site.

With the air pocket lost, there is no room left in the system for thermal expansion of the water to expand into from heating the water. And when heating the water, you do build a lot of pressure from the heating. This pressure can grow from 50psi cold to 100psi when things are working normal. If there is no air pocket, the pressure builds higher and the relief valve can weep. This especially happens if the water heater is left on all the time, 24hrs a day.

However you also noted the 45 minute time part of this. The problem may be deeper than just the weeping relief valve from a lost air pocket or a bad T stat. The loss of the air pocket does not really affect the time to heat the water even though it can be part of the relief valve weeping.

2. A defective thermostat. You ordered one and that is a cause if the actual T stat is bad. They are preset to 140F in most cases. If we had the total model number I could confirm that. And if the T stat is not touching the tank tight, then it takes longer to get the switch to heat and longer to shut off. You can test the water temp coming out of a faucet with a home meat thermometer in a cup of hot water. It the temp test shows approx. 140F +/- a degree or 2, then that can least rule out that the T stat is at least opening up at the correct setpoint. It the T stat is loose to the tank, that is an issue that needs to be addressed. It has to touch the tank.

If you know the T stat is working creating 140F water, then the issue is a little deeper. This is a process of elimination.

3. Since this was your first spring fill up of the heater and firing it on gas it took 45 minutes, then the relief valve started to weep, you shut it off, there could be a few things going on at the same time.

Was the water you started to heat in the 50 to maybe 60F range? This is raw cold water from a well or some city water supplies. It just takes longer to heat from 50F-60F all the way to 140F then it does from say, 80 to 90F which is what a cooled down water heater can be from overnight and not using it.

I have timed and seen 40 minute reheats from very cold water all the way to 140F on gas only. Where it can only be about 20 to 25 minutes for a reheat top off from an overnight cooled down heater.

Point, very cold water to start with will take longer. 45 minutes is on the long side, but not extra long for a cold heat up and can also suggest that the heater is not sensing the actual water temperature to shut it off. And in your case, you shut the heater down so it could of run longer.

See if any if this fits your case.

This spring I helped a friend service his water heater. It took 55 minutes from the start using gas only, until the gas burner shut down on a well water first heating. The water started at 60F and the heater shut down at 142F and that took 55 minutes. His new higher pressure water pump created 65psi to start with and when the heating was done, the tank pressure was at 122 psi from the thermal expansion with a known air pocket in the top of the tank. It would of been higher with no air pocket.

To me, that was a long time to heat and he had some issues. This was the 2nd time we heated water during this event. The first time, his relief valve started blowing way beyond a weep. It was doing a full out water purging and we shut the heater down. After seeing that, I did the test on time and temperature to try and troubleshoot his issues. Drained the tank when it had cooled, inspected the inside.

I asked him, do you routinely flush out the water heater every spring? He said, no never. He didn't know too.

I asked, where you camp, do you have hard water? He said, we have very hard water, it is direct from a well at our camp and we don't know when we travel, but it could be hard.

I could see a lot of scale in the water weep trail of the relief valve which has been weeping in the past. I asked him about the relief valve if he ever annually tested the trip lever? He said, never, didn't know I was supposed to. Sometimes when it weeps, I tap the stem and it stops. He did say he has had the relief valve replaced a few years ago as it was acting up. So he had fast scale buildup on a replaced relief valve.

I asked, do you turn off the water heater when you are done using it? He said no, never. We turn it on at the start of camp and leave it on all the time. He stated his home water heater is on all the time, so we do this with the camper too and always have.

I asked, approximately how many hrs a day total accumulated do you use hot water in the camper? After he thought about it, well maybe 2 hours, maybe 2.5 sometimes.

Testing the system:

Using an inspection camera inside the heater tank, I saw it was solid white and had a lot of granules, BB size, on the bottom of the tank. The relief valve stem had a lot of mineral build up that could be seen from the discharge port and the trip lever worked very hard.

This water heater is from a 2007 Sunline and is used several months a year on a private well at his camp.

If any of this above sounds like what you have, you could have mineral scale buildup inside the water heater and possibly on your relief valve.

These factors all added up to:

- I verified the T stat was creating 142F water, it was working.

- The T stat was tight to the tank, so this was good.

- A hard moving relief valve lever and a lot of mineral scale buildup in the discharge port.

- The entire inside of the water heater was solid white.

- He never flushed the tank and there was a lot of sand/granules on the bottom of the tank.

- He leaves the heater on all the time.

The issue was, the heavy mineral scale over the last approx 11 years created a situation where the scale was insulating the T stat sensing and the gas heat exchanger tube. (also the electric element) The relief valve had issues as the lifting lever worked very hard even though it did weep.

The fix.

* Replaced the safety relief valve.
* Did an overnight (10 to 12 hours) heated vinegar boil out. (more on that is needed)

That combo changed the same 60F water reheat to go from 55 minutes down to 40 minutes to 140F on gas only and the pressure rise was 65psi to ony 111 psi. His well at his camp also was set at 65. He now saw, that should be reduced down to 50 psi.

Explained the need for annual flushing of the tank to remove the sediment in the bottom, the air pocket formation and loss, the annual need to check and work/test the relief lever on the safety valve and make sure it does not start getting hard to lift.

Explained to shut the water heater "off" when you are done using it. Just turn it on, approx 30 minutes before you need it. In your hard water case, you are creating scale build up all day long baked onto the inside of the tank when you may only use the heater for 1 or 2 hrs a day.

If you want to keep the heater working efficient on your hard water, do the vinegar boil out more frequently than every 11 years.

Hope this helps

John
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Old 06-03-2019, 04:49 AM   #6
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Hi gang,

Replaced the thermostat and ecostat. I also tried to make sure air pocket issue was fixed. I remove bottom drain and drain system between camp outs so do not expect much sediment buildup.

Also correct on model number it is an Atwood GC6AA-8E.

The $10 investment in a new thermostat/ecostat was a quick replace and install. Fired up system and in 1/2 hour the system was hot and shut off on it's own. Thanks all for feedback!
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Old 06-03-2019, 09:33 PM   #7
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Great! Thanks for reporting back.
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Old 06-04-2019, 08:47 AM   #8
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I'm having a real problem with this air pocket thing. The over temp valve can not be above the water level or it would not work it does not sense air temp it senses water temp. Water does not compress so the only pressure would be from air compressing. The hot water out is also not above the water level or all one would get is air. If there is no air somewhere in the system the pump would short cycle. If the outlet was at the top then there no place for air to be. I guess what I am saying the air pocket is fixed there is nothing you can do to change it. As water heats it releases O2 until it eventually becomes steam. When I recommission mine in the spring I turn on the hose open the temp/pressure valve until it spits water then close it, it has never leaked. Think about a house system the valve is in the very top of the tank the air is in a sealed cylinder somewhere in the plumbing system. Am I missing something here?
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Old 06-04-2019, 10:04 AM   #9
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Water does compress, though not easily at all. However heated water definitely expands. So if the cold water level is just below the temp sensor, when heated it will expand enough to make contact.

Some residential heaters do have the T&P valve mounted on top, but as far as I know those are all bladder-type systems.
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Old 06-04-2019, 02:49 PM   #10
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Doh. Strike that about bladders. Got my wires crossed with pressure tanks.
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Old 06-04-2019, 03:14 PM   #11
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Quote:
Originally Posted by Tinstaafl View Post
Water does compress, though not easily at all. However heated water definitely expands. So if the cold water level is just below the temp sensor, when heated it will expand enough to make contact.

Some residential heaters do have the T&P valve mounted on top, but as far as I know those are all bladder-type systems.
Ok for all intents and purposes water does not compress. Can it be compressed yes, miniscule amounts requiring immense mechanical pressure. Yes heated water expands thus increasing the pressure of the air in the chamber. There is not much air in the tank otherwise the pump would run a lot less than it does. Home system have a bladder type pressure tank in line with the pump this keeps the pump from running non stop and/or fluctuating water pressure. The temp/pressure RV valve is very close to the same level as the hot water outlet and because of its position on a round tank it's probe is pointing down at an angle and is submerged in the water. Hot air will not activate the temp/pressure valve. I guess what I am on about no matter what you do or how you do it it's going to have the necessary air cushion if it didn't the pump would cycle constantly same effect as dead heading a domestic water pump. When I fill my system I open the pressure valve for no other reason other than filling the tank some time before dark and keeping air out of the rest of the plumbing so I don't get a face full of water when I turn on the tap. I have a in line home type air tank in the camper the ideal was to reduce battery load it has reduced the cycle time of the pump by a factor of at least 10 to 1.
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Old 06-04-2019, 05:45 PM   #12
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The temp/pressure RV valve is very close to the same level as the hot water outlet and because of its position on a round tank it's probe is pointing down at an angle and is submerged in the water.
Then it appears you've answered your own question (if it was one). I was too lazy to go take a look at mine before my earlier reply, but just did so. Yes, it angles downward.

So with the temperature probe being extended farther into the tank than the "mouth" of the water outlet/valve, it can still be immersed while the valve is seeing nothing but air.

I'm curious whether you did any energy usage measurements before and after installing that bladder tank. While there's no doubt the pump will run less often, it would also run longer when it does.

I've read about retrofitting them and liked the idea, but after our maiden 3 day voyage over the holiday, I'm not so sure I need one. After decades of tenting, we're pretty miserly with our water usage, and I imagine it'll take a while for that habit to die.
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Old 06-04-2019, 11:02 PM   #13
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Quote:
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I'm having a real problem with this air pocket thing.

The over temp valve can not be above the water level or it would not work it does not sense air temp it senses water temp.

Water does not compress so the only pressure would be from air compressing.

The hot water out is also not above the water level or all one would get is air.

If there is no air somewhere in the system the pump would short cycle.

If the outlet was at the top then there no place for air to be. I guess what I am saying the air pocket is fixed there is nothing you can do to change it.
As water heats it releases O2 until it eventually becomes steam.

When I recommission mine in the spring I turn on the hose open the temp/pressure valve until it spits water then close it, it has never leaked.

Think about a house system the valve is in the very top of the tank the air is in a sealed cylinder somewhere in the plumbing system. Am I missing something here?
Hoping this will help. This is good discussion. Makes us all stop and think about this.

I'll add some with pics to help show what I feel is going on inside the water heater, what Atwood says about their heater heating water and the infamous air pocket in the top of the water heater. I'll also add some of my findings along the way that helped me confirm my thoughts of what is going on.

First, how the water heater is made inside. I do not know if every Atwood heater is made like this, but the year 2003 to 2007 model Sunlines are. I suspect many of then are similar.

Here is an outside view of the back of the 2003 redesigned water heater. The electric element is in the "bottom" of the heater in the back and there is a power relay in the junction box to control it. The pre year 2003 if they had the element option would have the element in the same general area, just the controls are different.

I was fixing a leaking bottom plastic tank fitting on my friends camper and upgrading the bypass kit to eliminate the top problematic check valve. The heater was out of the camper which was undergoing rot repair.




I'm showing those to show the relationship of the top hot water discharge fitting in relation to the bottom mounted electric element. We need to see this for the next internal pictures. (the internal pic dates are wrong, the date is not set on my borescope)

This is pic of the top discharge pipe inside the water heater after a vinegar boil out. Yes, that pipe is the discharge fitting on the top outside of the tank


Here is the bottom of that pipe. It is near the bottom of the tank.


Here is a view of that discharge pipe next to the electric element. On the right of the pic, is the large gas fired heat exchanger tube.


Another view


Point being, the top water discharge port actually draws water pushed by system pressure from the lower part of the tank through that internal pipe. No air from the air pocket trapped in the top of the tank will easily fly out the top hole since the outlet pipe draws from water on the bottom area of the tank.

Next is the TP safety relief valve. There is a 4" long temperature sensing probe on the end of it. This is a new valve next to an old one.


Next is a pic from me doing a vinegar boil out. My first one, (you learn more the first time...) I had the above old relief valve out as it was sticking and not opening right. So I thought, I will just add the 4 gallons of vinegar through the relief hole, put the new valve in and top the last 2 gallons with water from the fresh tank.


Well, that was a good thought, but wrong... Nope it's not going to happen. I poured 2 3/4 gallons of vinegar in that hole and it started to flow out the relief valve hole. I had compressed air blew out the heater so there was most likely only 1 qt of water left before the vinegar add. I had to install the new TP valve and then use the water pump winterizing kit to pump the last 1 1/4 gallons of vinegar into the heater and then top it off the last 2 gallons with water from the fresh tank.

Point: The relief valve hole is at the approx 3 gallon full mark on a 6 gallon heater. The TP temperature stem and pressure relief valve is under water all the time once the heater is full.

Water is not like oil that is used for hydraulics which has extremely low compressibility, water does have a level of compressibility in the pressures we are are talking about, under 150psi, granted it is not a lot, but it is there. That said there "normally" is an air pocket in the top of the water heater, it is by Atwood design to be there. But that air pocket will "not" stay there forever or even for after a few campouts if you tow a lot between camps with a full water system. It does get lost. Atwood even declares in their user manual, the air pocket will dissolve into the water over time.

Here is another learning I had along the way.

I upgraded my standard water pump to a FloTec variable speed pump a long time ago. It is another option to adding an air/water bladder accumulator tank. That variable speed pump has an analog pressure sensor in it. When the air pocket is totally lost, that pump wants to about bounce itself off the mounting pad. Since there is no air or very little left, it dead heads and will about kill itself if left on. All I need to do to correct the issue is, regain the air pocket in the top of the heater.

I left the system cool down if the water is full hot. I have a hose and drain valve on my heater. With the pump off and the water cooled some, I bleed off the residual compressed pressure with a sink faucet and close the sink faucet, open the heater drain valve, once it is glub glub flowing due to a vacuum lock, I open the PT valve and let air rush in. After about 2 quarts drained out on the ground, I put the relief valve back to normal, shut off the drain valve, start the pump up and everything works perfect again.

Also learned, some of the newer Shurlfo camper water pumps have an internal pressure bypass built in to help stop this deadheading bounce. It just blows by back to the inlet port in the pump when it reaches system pressure.

This air pocket being lost also showed up as a weeping relief valve other times without the pump jumping around action. After seeing water on the ground from the heater, I screwed a 0-100 gage I had at the time at camp onto the drain valve hose and it pegged the gage. I now have a 0-200 psi gage. It happens, the air pocket dissolves itself.


After finding this out, If I tow several days in a row between camps on a long trip with a water filled system, when I get to camp before we ever turn on the water heater, I just bleed out about 2 quarts of water from the heater by my drain hose with the relief valve open. It re-establishes the air pocket and all is well for at least 8 plus days. We move to the next camp by then so I do not know how many days it takes standing still to lose the pocket. Atwood says to drain it down to the relief valve height, but I find the 2 qts out will solve the issue for my shorter trips.

To this thought, H'mm think about this, I am not totally agreeing with this.
Quote:
If the outlet was at the top then there no place for air to be. I guess what I am saying the air pocket is fixed there is nothing you can do to change it. As water heats it releases O2 until it eventually becomes steam.
Two things, the air pocket will dissolve itself given the right circumstances. I have seen it do it several times until I sorted out what was going on. It does happen.

While heating water may release O2, it will not change to steam until you are at or over 212F and at atmospheric pressure in the tank here on the eastern coast US. Did I miss something with that? Where are you thinking the steam inside the system is coming from?

If the pressure is above 0 psi, then it will take even more temperature to boil into steam. The relief valve is supposed to let go at 210F and if we are at 50psi system pressure, steam is not going flash inside the heater. It takes around 298F before 50psi water will flash to steam. The higher the water pressure, the more temp is needed to allow a steam flash.

See the top right of page 5 under PRESSURE-TEMPERATURE RELIEF VALVE in the Atwood users manual where it talks about the air pocket being absorbed into the water. And how to recreate it. Sunline Coach Owner's Club - Sunline Owner's Files - Atwood LP Gas Water Heater Installation, Maintenan

In your case, by your adding an air bladder accumulator, you actually solved this infamous air pocket issue. You will not find the loss of the water heater air pocket as when your air pocket would ever dissolve, your bladder tank will compensate for the issue. Good Job!! If they would put a simple small accumulator on all campers from the get go, this would be a non issue.

To this
Quote:
Think about a house system the valve is in the very top of the tank the air is in a sealed cylinder somewhere in the plumbing system. Am I missing something here?
In a house system, depending on your source of water, there should be a method for thermal expansion of a water heater to not build excess pressure above the relief valve. And it is not an air pocket in the top of the water heater.

On a well system, there is a modern day compressed air bladder tank to save the water pump from dead heading like you stated in a water logged system. On an old, really old house, there was a pump tank with an air control system that really was problematic as the air control got older. When the air control did not work, the pump with cycle on and off and about burn out the motor. I remember those good old days...

Here in Ohio when we moved into our 1985 built home, we were on county water. Pressurized water from the county came into the house and pressurized the house water system. There was no bladder tank in the system, then anyway. As the water heater heated, the excess pressure would back up all the way into the incoming water supply and not build excess pressure in the house
water system.

Then came new backflow regulations. The county had to install double check valves at every home and the mailed you a letter that on this date, you have to have installed an air bladder tank near your water heater as the pressure will no longer back up into the county water system. So I added an air bladder tank at the water heater.

The house water heater is similar, but yet different then a small closed up system in an RV. The home now have plumbing codes to deal with water heater water expansion. The RV industry just never got the memo I guess.

Another thing I learned along the way is how much pressure builds in your camper from heating water even when air pocket is established in the water heater. A normal 50psi pump setup will rise to 115 psi system pressure with just one heating cycle of the water heater. And if you let it cool down, the pressure drops back down accordingly. With the heater left on, it stays at that pressure until you open a faucet or flush the toilet.

You for sure want to have all good pex swivel gaskets or leaks are going to happen. They built the camper with 150psi piping and fixtures for this reason. And when you lose the air pocket, well there is at least one reason the PT relief valve weeps, the thermal expansion of heating water with no air pocket left is lifting the relief valve stem which with is real close to 150psi.

OK, does that help fill in some of the missing pieces? What am I missing?

Good discussion, let's talk about it.

Thanks

John
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Old 06-05-2019, 07:31 AM   #14
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Having no problems ever with air pockets I guess I have been lucky. When I fill my system I open the tank valve till it spits a little water. So I'm guessing that's all it takes. My plan with the big accumulator tank was strictly about using less battery power. A starting motor typically uses 200% more current than run current so logic would say use the pump less achieves two things longer pump life and less battery draw. My water system is pretty tight with the accumulator I still have water pressure weeks later often the pump will not start for a minute or so after I turn the tap on. Yes the pump runs longer but not dramatically longer then shuts off and stays off thru most of a normal shower. It's also a lot quieter in the camper ( I hate the sound of a thundering pump so I went to great pains to address that too). As you all well know there is not a lot of extra room in a camper so a 2 gallon tank was not a real easy thing to hide the supply side to the tank is right at the pump discharge it's "Ted" into the tank to the camper supply side with braided flex line from the pump.
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