Plumbing Both Engine and Genset Into Water Heater Heat Exchanger

[waterman46]

Sure you can do this but think it's a lot of trouble for little if any benefit.



Your description did not include the generator output power. This is a most significant factor here. Most inboard will be at least 5KW. The electric element on your water heater is only 750W so when running the genset you'll probably also have more than enough to run the reefer (250W?), charge the batteries (2KW?), run the microwave (500W?), watermaker, etc. all at the same time. Use all that power while you have it.



Your main problem is a long run time to heat your hot water. If your water heater would be done heating in much less time then you wouldn't have any reason to look for alternatives to electric heating.



You say it takes 5 hours to heat your water! I find this very strange. Our electric water heater is hot in about 20 minutes, long long before the batteries are charged. If it takes 5 hr to heat up your water, then a much simpler solution would be to replace that heater with a better one. Also 55l seems quite large unless you sail with a big crew. I think ours is 10gal (about 37l?) and that seems to be more than adequate for the two of us. Hot water is mixed with cold for showers, and it is super hot, so probably less than 1 gallon of hot water is needed per shower. A smaller heater might heat up a lot faster.



Now, don't bother about the engine being heated by the hot water heater, as some have suggested. An engine cannot be heated to any extent by the hot water heater - there is no circulation in the engine water coil of the heater when engine is not running!



If you have a much smaller genset, say 2.5KW that can't do much more than just heat your water - then the answer is different.

[stormalong]

Originally Posted by waterman46

Now, don't bother about the engine being heated by the hot water heater, as some have suggested. An engine cannot be heated to any extent by the hot water heater - there is no circulation in the engine water coil of the heater when engine is not running.

I am always amused when someone tells me that something I have been doing for years won't work.

[KadeyKrogen38]

Let’s take something simple and make it unbelievably complicated! Why? If you are running gen set why not just use electric element to heat water?

[waterman46]

Originally Posted by stormalong

I am always amused when someone tells me that something I have been doing for years won't work.

Maybe in cold weather I might be able to detect some heating of the engine, but in our case it is not noticeable relative to ambient temperature where I cruise.

[stormalong]

Originally Posted by waterman46

Maybe in cold weather I might be able to detect some heating of the engine, but in our case it is not noticeable relative to ambient temperature where I cruise.

Please don't make blanket statements about what will or won't work based upon your boat. Reread my post and it describes exactly my the situation where it does work. Obviously your setup is different. In my boat with the water heater close to the engine the block gets warm to the touch.

[CaptTom]

Originally Posted by waterman46

Your main problem is a long run time to heat your hot water....
You say it takes 5 hours to heat your water! I find this very strange. Our electric water heater is hot in about 20 minutes, long long before the batteries are charged.

This is my experience as well, on a number of different boats. I'm sure someone will come along and do the math, but I generally expect the water heater to cycle off after about 30 minutes, maybe a bit more if it's being heated from a very cold state.

Originally Posted by waterman46

Now, don't bother about the engine being heated by the hot water heater, as some have suggested. An engine cannot be heated to any extent by the hot water heater - there is no circulation in the engine water coil of the heater when engine is not running!

Here's where my experience differs. My main does indeed get quite warm via natural circulation if the water heater is on and the engine cold for any length of time.

[flippercarson]

Just checking back in on the thread...

There are a couple of comments about heating time and genset use.

For anyone interested, in how long it takes to heat water or how much energy it requires, it is a simple calculation. It takes a 750w heater almost 5 hrs to provide enough power to raise 55l of water from 75f to 180f at 100% efficiency. It's just a simple calculation and I provided the link to the same in my second post. There's no magic here.

Our tank is actually 50l, not 55l as I indicated earlier, but for 55l the calculation is what it is.

As for our genset, we have an 8kw Westerbeke. It makes lots of power for our needs, but that's beside the point. Fuel consumption is tied quite directly to power output. Northern Lights, Westerbeke, Onan, etc. they are all about the same. It takes about a half gallon fuel to create 4KW of generator power when the genset is running efficiently (If it's running inefficiently or lightly loaded, it would take somewhat more.)

Assuming efficient operation and that we need to make water every second day, that's 180+ gallons of diesel a year at $5 a gallon (in most of the world). The cost is not huge, but it's noteworthy. More importantly, it's wasteful if it's unnecessary and if you're hanging out in the middle of nowhere for months on end, the consumption can add up and put a dent in your diesel supply.

Other factors to consider are additional run time on the genset, wear and tear, oil changes, and noise.

I guess many others have wanted to do the same thing I do, as they do make the water heaters with the double heat exchanger I'm looking for, just not in the size I have (unfortunately). I suspect for many boaters or part-time cruisers, it's not worth it, but for us, it would be great.

With that said, I didn't really start the thread to explain why we want to do it, or justify our cruising needs to anyone, I was curious if anyone had tried this on their boat, and if I had the plumbing right.

I'd still welcome feedback related to my original goal. Thank you.

[flippercarson]

Originally Posted by foufou

Wouldn't a check valve in the place of the three way valve acomplish the same thing but automatically?

Which three way? Where would you insert the check valve? Thank you.

[CaptTom]

Thanks, your math is correct. I guess I'm used to 1250W elements, higher starting temp and lower ending temp. I'd suggest a higher-wattage element, but that doesn't really solve your problem, just reduces the heating time by close to half. Likewise, if you say there's absolutely no room for a larger water heater, I'll take your word for it, although I've been able to squeeze in 11-gallon units where the manufacture had installed a 6-gallon.

All that said, I'm actually interested in your project. I think it's very workable with relatively little time or money. I like those kinds of projects.

Personally, I'd go with only one "operating" valve on each loop, to open or close the flow from each engine. You just need to stop the circulation, not completely isolate the non-running engine. To me the check valves would be redundant. Instead I'd put a second pair of manual valves, close to each engine, which would only be closed in the event of a failure, to stop any coolant loss.

Replacing the two operating valves with solenoid valves would be fairly trivial, and not very expensive. My 12V solenoid valve is powered from the "ignition" switch which powers the engine gauges and senders when the engine is running. For the genset, you could also go with an AC mains-powered solenoid valve. Or even a zone valve from a home hydronic heating system. Whatever way you choose, it's a relatively easy, inexpensive and reliable solution. Go for it!

[Dockhead]

I use a 1500 watt immersion heater, upgraded from a 750 watt one. There's basically always enough power left over from other loads (6.5kW generator limited to 5.5kW) to run it from generator. Yes, it costs some fuel compared to free waste heat but as I calculated the cost of the installation to capture waste heat would basically never be recovered in fuel, plus the load is healthy for the genset if other loads are light.