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Old 21-11-2017, 16:06   #1
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OK to run motor with hot water tank empty

The Volvo Penta engine is cooled via raw water. It heats fresh water in the hot water tank. If I have the hot water tank drained as part of winterization, can I run engine safely?
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Old 21-11-2017, 16:58   #2
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Re: OK to run motor with hot water tank empty

Yes, but just don't turn on the electric heater!
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Old 21-11-2017, 18:34   #3
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Re: OK to run motor with hot water tank empty

Yes, it's OK.

Engine coolant doesn't get hot enough to pose any kind of a problem in the heat exchanger coil inside the water heater.

As noted by others, don't run the electric element...
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Old 21-11-2017, 19:31   #4
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Re: OK to run motor with hot water tank empty

+2

NO problem.
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Old 22-11-2017, 15:58   #5
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Re: OK to run motor with hot water tank empty

Thanks for the input!
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Old 23-11-2017, 10:38   #6
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Re: OK to run motor with hot water tank empty

If you have the hot water tank drained because of the fear of freezing, I would recommend you install a valve on your engine supply hoses for the tank and reduce the chances of ruining you heat exchanger.
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Old 23-11-2017, 19:30   #7
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OK to run motor with hot water tank empty

Quote:
Originally Posted by Hunky Dory View Post
If you have the hot water tank drained because of the fear of freezing, I would recommend you install a valve on your engine supply hoses for the tank and reduce the chances of ruining you heat exchanger.


It wonít. It is exactly the same as a heater core in an automobile that is of course surrounded by air not water, that and a car runs considerably hotter than any Boat Diesel that I am aware of.
However it may be possible to block proper water flow in the engine if you merely close off the water heater hoses. I doubt it, but canít say for sure. Many car heaters control heat by blocking the water flow so itís likely safe.
Our water heaters are pretty much exactly the same as a heater core in an automobile, just heat water of course as opposed to air.

If I were concerned Iíd pull the two hoses off going to the heat exchanger and out a short loop hose in, then your still getting flow through, just not to the water heater of course.

Although as I assume he isnít going to do any extended runs on the hard, and coupled with the fact that he canít put any real load on the engine, itís not likely to get very hot anyway.
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Old 23-11-2017, 19:39   #8
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Re: OK to run motor with hot water tank empty

Quote:
Originally Posted by Hunky Dory View Post
If you have the hot water tank drained because of the fear of freezing, I would recommend you install a valve on your engine supply hoses for the tank and reduce the chances of ruining you heat exchanger.
Simply shutting off the flow to the tank will, in most installations, block the majority of the engine's coolant loop and overheat your engine.

You'd need to *bypass* the tank.
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Old 24-11-2017, 06:40   #9
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Re: OK to run motor with hot water tank empty

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Originally Posted by chris95040 View Post
Simply shutting off the flow to the tank will, in most installations, block the majority of the engine's coolant loop and overheat your engine.

You'd need to *bypass* the tank.
You are correct that one should not stop the coolant flow to the water heater.

However, one does not need to bypass the tank; just run the engine as normal.

The engine coolant (be it fresh or seawater) will flow through the heating coil in the water heater as normal. The lack of potable water in the tank will have no bearing on anything.

Now, if one turned on the electric heating element in the water heater that is another story. That depends on the heat dissipation of the surrounding water to prevent it from getting way to hot very quickly. The likely outcome is a burned out heating element.

But to the original question, if the water heater is drained of potable water, there is no issue with running the engine and coolant circulation through the water heater heating coil (or loop).

Now, in the case of a seawater (raw water) cooled engine, they generally run at a much cooler operating temp (typically around 120F) to prevent calcium precipitation and build up in the engine. So the water heater loop really does not get very hot at all. This is why it takes a long long time to heat water with a raw water cooled engine vs a fresh water (closed loop) cooled engine that most likely runs at 160F or higher.
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Old 24-11-2017, 08:36   #10
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Re: OK to run motor with hot water tank empty

In reply to comments, Dr D has stated the water that comes from the engine to heat the HW tank is raw water, not glycol closed cooling. This raw water will freeze and can cause damage to HW tank heater coil. If the tank is drained because of freezing you should also drain heater core. Shutting the loop to the HW tank will not overheat engine, HW tanks are an accessories that a lot of boats do not have. My boat did not even have a heating loop connected to the HW tank in my boat by Bayliner the manufacture even though the exchanger was integral to the tank.
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Old 25-11-2017, 06:57   #11
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Re: OK to run motor with hot water tank empty

Thanks, Hunky.

As I originally posted, the engine uses raw water for cooling. For convenience, it circulates fresh water from the water heater.

I am winterizing my fresh water system now, but I want to use the diesel engine for a few more weeks. It is that situation that I was asking about and it appears all is OK.
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Old 25-11-2017, 11:54   #12
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Re: OK to run motor with hot water tank empty

Yes, you can. I ran mine that way for weeks until I plumbed the hot water system.
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Old 18-05-2024, 14:46   #13
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Re: OK to run motor with hot water tank empty

I love this forum and google search integration. This is exactly the question, with answers, that I was looking for at 2AM.


Thanks all!
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Old 18-05-2024, 15:08   #14
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Re: OK to run motor with hot water tank empty

Whether closing off flow to the wh will cause a problem depends on how it's plumbed. On my previous boat the only way to connect the wh to the engine made it a part of the cooling loop. Closing a valve on the wh definitely would overheat the engine. "Bypassing" the heater would require actually bypassing it by taking the hoses off and connecting them together. On most other boats that's not the case. But bypassing it isn't necessary, running the engine with the heater empty won't hurt anything. Energizing the electric element will.
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