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Old 02-12-2010, 12:26   #1
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Time and RPM to Heat Water ?

Has anyone found the optimal time and RPM that will get their internal, engine heated hot water system back to hot from dead cold ?
Engine - Yanmar 4JH
Water heater - 6 gal unit ?
Standard Beneteau fit in 411.
I've been running 15 mins at 1500 rpm and this doesn't do it! Luke warm still.
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Old 02-12-2010, 12:32   #2
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Just like your car, how long does it take to feel heat out the vents while at idle from a cold start? The engine will need to come to operating temperature, then long enough to heat water. This will happen faster depending on engine load.
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Old 02-12-2010, 12:48   #3
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I would say it would take a lot more than 15 minutes - more like 30 or longer. I have often been surprised how long it takes my Yanmar (30GMH - 3-cylinder) to even become warm to the touch on the outside of the engine block - starting from dead cold. When you think about it, the block and all the coolant around it first has to heat up enough to even open the thermostat at all, only then will warm water start to flow thru the water heater - and it will be fairly 'cool' coolant to begin with ... When the thermostat does open, it first has to mix in all the coolant that is in the pipes and inside the heat-exchanger/water heater, and all of this has to also warm up to HOT before it finally starts to have any effect on your six gallons of cold water. Try taking six gallons in a monster cooking pot and put it on the stove .. it takes a mighty long time for that much water to get really hot. The reason is the 'Heat Capacity' of water. Water has a very large heat capacity. It can absorb a lot of energy without affecting its temperature very much. This makes water a very good medium as cooling agent.
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Old 02-12-2010, 13:45   #4
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Under optimal conditions, your 4JH should cruise at about 2900 RPM, and almost certainly not as low as 1500.
You shouldn't "idle" your engine to heat water. Generally, you should be operating your engine somewhere near 85% of maximum continuous RPM.
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Old 02-12-2010, 13:53   #5
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Back before I installed the solar and wind generation systems, I could run my 4JH3-TE for an hour to charge the batteries and it wouldn't heat water significantly. Just not enough load on the engine.
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Old 02-12-2010, 14:05   #6
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Quote:
Originally Posted by cmiddelmann View Post
I would say it would take a lot more than 15 minutes - more like 30 or longer. I have often been surprised how long it takes my Yanmar (30GMH - 3-cylinder) to even become warm to the touch on the outside of the engine block - starting from dead cold. When you think about it, the block and all the coolant around it first has to heat up enough to even open the thermostat at all, only then will warm water start to flow thru the water heater - and it will be fairly 'cool' coolant to begin with ... When the thermostat does open, it first has to mix in all the coolant that is in the pipes and inside the heat-exchanger/water heater, and all of this has to also warm up to HOT before it finally starts to have any effect on your six gallons of cold water. Try taking six gallons in a monster cooking pot and put it on the stove .. it takes a mighty long time for that much water to get really hot. The reason is the 'Heat Capacity' of water. Water has a very large heat capacity. It can absorb a lot of energy without affecting its temperature very much. This makes water a very good medium as cooling agent.
Let's all move to the Tropix - and use SOLAR!
Not exactly.

The thermostat blocks the flow of coolant to the raw water heat exchanger. The fresh water circulating pump moves water around the engine block when ever the engine is running. Domestic hot water heaters and cabin heaters are normally plumbed so that coolant flows through them whenever the engine is running. This flow may be all but nonexistent at idle do to frictional line loss. As the engine RPM increases so does pump pressure and flow to these external heaters.

To the OP:
One question that you may need to answer is whether there is enough water pressure / flow to move enough coolant from your engine to your water heater at 1,500 RPM. After 15 minutes is the engine up to normal operating temperature but the hoses from the engine to the water heater both only warm?

Ted
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Old 02-12-2010, 15:19   #7
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Definately need higher RPM to get the coolant moving but another consideration is the ambiant temperature of the seawater you are pumping through your raw water pump. Even with a bypass thermostat setup that most engines run with the seawater temp will impact warm up time. Depending on how your hw heater is plumbed you may not get any circulation in it until the thermostat opens. That will require generally speaking 120-140 degree coolant temps I believe.
Idling your engine at 1500 in cold seawater you will not likely get your thremostat to open for a long time if at all.

FWIW my prior boat in the summer on the chesapeake would make hot water after about 15min of running the engine at cruise RPM of 2200. The tank was well enough insulated that a shower could be warm the following morning after a night on the hook. In the winter however I would have a hard time getting the engine coolant up to much over 120 so the thermostat would barely open and I would rarely get hot water.
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