Water heater exchanger not working

[ClarkePrescott]

We have a '93 Hunter Passage. And a 6 gal Seaward S600 water heater )also known as a "hot" water heater. 😊😊 Water heater works like a champ when we are plugged in. There is a coolant hose running to water heater. However we get no hot water from engine when we are underway. Someone suggested low coolant. (Nope it's topped off). Another suggested air lock at water heater (nope coolant present when hose taken off). So my learned friends, what else do you suggest? Not keen on using propane stove to heat water when system should work.

[Tetepare]

Originally Posted by ClarkePrescott

However we get no hot water from engine when we are underway.

Semantics perhaps as it seems you already know the hot water comes from the tank/ heater not the engine.

So:
1. Hot water is not circulating through heater
2. The water entering heater isn’t hot
3. Problem is between the heater and faucet

2 should be easy to confirm, feel the hoses on both sides of the heater.

3 also should be easy, is there hot potable water entering the output hose?

Process of elimination

[OldGreyB]

Things to test:

1) Does your marine motor obtain correct operational temperature when motoring? This is important as the actual coolant temperature and time applied has a direct relation to the water heater exchange and potable water temperature. Ensure you have a good engine thermostat and optimum running temperature as per your engine owner's manual.

2) With the motor up to temperature, and running for say 30 minutes, go to the water heater and place your hands on each hose. Are both hoses close to equally hot? If yes then that is good, as potentially the coolant water is flowing through the water heater circuit, and the potable water is up to temperature too.

3) If no to question 2 , then with engine stopped and cold, take off both hoses from the engine block that go to the water heater. Open the pressure cap. Place a bucket under the one that is the return line. Take the one that is from the water pump end and using a garden hose, flush the complete line, so all the coolant in the lines ends up in the bucket. Did you observe full flow through the lines at the same rate as the garden hose? All good if you did. If not then need to find the blockage in the lines to and through the water heater. Also at this stage also check that the nipples on the engine block and water pump, are clear from any coolant or rust buildup. Use the garden hose with a quick burst to the engine block water return nipple and you should hear / see water rise in the pressure cap opening. Once resolved or confirmed OK, refill attach the lines and fill system with your collected coolant, ensuring no air bubbles.

4) If full flow through the water heater coolant circuit from question 3, then with the return line attached to the engine block, disconnect the water heater line from the water pump outlet. Some motors the auxiliary outlet is directly connected to the engine water pump,(Image attached) in others the heater supply is just close to the water pump outlet. Connect a similar size hose say 1m long to the engine block heater supply nipple and point skywards, then into the bucket. Have someone run the motor for 30 seconds to 1 minute. You should see a strong flow of water from the pump through the hose. If not, then the pump auxiliary outlet may be partially or fully blocked internally. This can happen if the engine has been running at sub optimum temperatures due to a faulty thermostat as in Q1. This may require you to remove the water pump and clear the internal blockage. Once resolved or confirmed OK, refill attach the lines and fill system with your collected coolant, ensuring no air bubbles.

5) Test run system, checking twice at least after engine is up to temperature that coolant levels are correct.

[Tin Tin]

I had a Yanmar once that had a little valve on the hot water T fitting on the engine so the hot water flow could be turned off or varied, if yours has one it could be blocked.

[BobHorn]

On a much much older boat, the heat exchanger coil in the water heater was completely blocked with deposits. There was also a valve on the engine for one of the hoses which was also blocked. Think it was caused by the boat sitting unused for an extended period of time. Had to replace the water heater.

[Capt Clark]

“There is a coolant hose running to water heater”

Clarke,

There should be 2 hoses between your water heater & engine. It’s a very simple design and if the engine is getting to operating temperature and the water is circulating, it will heat the water. Sounds like a blockage or the plugs weren’t removed on the heater.

Capt Clark

[Cheechako]

Originally Posted by Tin Tin

I had a Yanmar once that had a little valve on the hot water T fitting on the engine so the hot water flow could be turned off or varied, if yours has one it could be blocked.

Yes, look for a shut off valve.

Yes, there should be two hoses, one returns back to the engine or nearby.

How far is the faucet? Are you running water long enough for hot to show up?

Feel the output hose from the heater, it should be hot near the heater.

Some raw sea water (only) cooled engines dont get hot enough to do much. Your engine has a heat exchanger I imagine?