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No hot water - mercruiser 5.0mpi

hutch1nson`

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17 Dec 2020
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10
Hi all, I need some help.
I get hot water off electric but not from engine power. It is a 2006 Mercruiser 5.0mpi, feed to the calorifier comes from the inlet manifold (top of engine) and returns to the side of the water pump on front of engine. When I take the return hose off the water pump (and plug the water pump) hot water flows through the calorifier and all hoses get hot, when I reconnect the return to the water pump water stops flowing and the pipes aren’t hot. It seems like both engine take off points are positive pressure and cancelling out each other so no flow. Does anyone have any advice or an alternative negative pressure connection point.
 

dpb

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14 Feb 2007
Messages
1,009
Location
Poole, Dorset
The connection points sound right though the the engine to calorifier hose can be connected to the thermostat housing.
Need to make sure no air lock in the return hose.
Did it use to work before you did something to it?
 

hutch1nson`

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17 Dec 2020
Messages
10
The connection points sound right though the the engine to calorifier hose can be connected to the thermostat housing.
Need to make sure no air lock in the return hose.
Did it use to work before you did something to it?
Thanks, it hasn’t worked since I bought the boat, I have fitted an air bleed in the feed and return hoses. I haven’t done anything to it? It’s baffling. Next step is to remove the calorifier I guess?
 

hutch1nson`

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17 Dec 2020
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Thanks, I have just removed the heater so definitely no blocked line, I don’t think air because I have fitted bleed valves to the feed and return. Also, the lines in the attached image (to exhaust manifold) are not getting warm. Engine is running fine and sits at 175 degrees F all day, could this mean closed thermostat?
 

Attachments

spannerman

Well-known member
Joined
30 Nov 2002
Messages
2,943
Those two hoses should never get warm, they are constant flow seawater to keep the risers cool. If the thermostat stayed closed your engine would run too hot.
 

hutch1nson`

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17 Dec 2020
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That’s very helpful thankyou. Any thoughts on why there is no flow through calorifier?
 

LBRodders

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3 Oct 2018
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1,681
@hutch1nson` Has it ever worked? Or is this something inherited?

Its quite rare for a raw water cooled engine to heat a calorifier. If this calorifier is old and the engine a newer installation it may be a mismatch from a fresh water cooled engine.

We removed ours and installed a small 12v calorifier electrically powered from the engine or shore supply. It heats to 85'c then mixes. Will do a couple of short showers. (y)

edit - updated link! don't buy the eBay one
 

hutch1nson`

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17 Dec 2020
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Thanks for this, the calorifier is 15 years old but no signs of any issues, the boat is also 15 years old. I have never known it to work but have only had the boat 6 months. When I open the bleed on the return pipe, the water that flows (downstream of calorifier) is hot, almost to hot to touch so it seems like it could work, but as soon as the bleed is closed water stops flowing.
 

LBRodders

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Thanks for this, the calorifier is 15 years old but no signs of any issues, the boat is also 15 years old. I have never known it to work but have only had the boat 6 months. When I open the bleed on the return pipe, the water that flows (downstream of calorifier) is hot, almost to hot to touch so it seems like it could work, but as soon as the bleed is closed water stops flowing.
Disconnect the calorifier, stick a garden hose on the inlet and see if water comes out the outlet.

15 years of warm sea water may have corroded it. Another reason why they aren't generally raw water 'heated'
 
Last edited:

hutch1nson`

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17 Dec 2020
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Thanks for the reply.
Yes done that, flows freely in either direction. Hot water from the engine also flows through it when I disconnect the outlet at the engine or open the bleed on the return pipe, as soon I connect to the engine water stops flowing.
 

LBRodders

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1,681
Thanks for the reply.
Yes done that, flows freely in either direction. Hot water from the engine also flows through it when I disconnect the outlet at the engine or open the bleed on the return pipe, as soon I connect to the engine water stops flowing.
Is the water pump you mention the one on the engine or an additional 'lift' pump to circulate through the calorifier?

Mercruiser have never made a specific add on for raw water cooled engines so every fit out is a 'rabbit'.

If the engine runs on load at temperature, then one would assume the thermostat is fine, although maybe worth inspecting.
It's difficult to advise without seeing the particular set up, because it would be unique.
 

hutch1nson`

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17 Dec 2020
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It is the water pump on the front of the engine. I have ordered a new thermostat as I have no record of it being replaced so will change it, I will also try rerouting the return pipe to the thermostat housing - I just need somewhere that is negative pressure for the return pipe, if that fails then I will be changing to 12v! Thanks again.
 

LBRodders

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It is the water pump on the front of the engine. I have ordered a new thermostat as I have no record of it being replaced so will change it, I will also try rerouting the return pipe to the thermostat housing - I just need somewhere that is negative pressure for the return pipe, if that fails then I will be changing to 12v! Thanks again.
When I looked into it, I am sure that I would have needed a lift pump specific to the calorifier. This would be wired into the ignition so it ran when the engine did. I just cant find the detail now other than the previous link.

Just be careful that you don't interrupt the cooling flow too much. Like I said, these engines aren't specifically designed to re route the [raw water] cooling. Good luck!
 

seastoke

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16 Mar 2013
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Location
boat conwy
When I looked into it, I am sure that I would have needed a lift pump specific to the calorifier. This would be wired into the ignition so it ran when the engine did. I just cant find the detail now other than the previous link.

Just be careful that you don't interrupt the cooling flow too much. Like I said, these engines aren't specifically designed to re route the [raw water] cooling. Good luck!
If the stat closed would the electric pump run dry.
 

LBRodders

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If the stat closed would the electric pump run dry.
No it'd be on recirc with the rest of the engine.

Like I said, all a bit of a rabbit. There isn't much detail of how to do it by the book. We didn't go there because the engine was under warranty.
 

QBhoy

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11 Mar 2016
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1,508
Hi. Just a couple of things. Unless I’ve read you wrongly, the water take off for the domestic system shouldn’t come off the intake manifold at all. It comes from the main circ pump housing (there is a blank fitting there) and returns to the starboard side top area of engine.
Also, if your raw water cooled 5.0 mpi is actually running at 175o. It’s on the verge of overheating or even is. These should run around 160ish. Mine runs around 152-156 normally.
hope that helps and I’ve picked you up accurately
 
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