Using Generator Coolant for cabin heat?

Hi,
We have a 1977 Trojan F36 Tri-fly and we use our boat year around. We do take trips on it but most of the time we go out a couple miles and anchor. We live in Eastern Oregon and we spend our time on the Columbia River. My Wife is always cold. I have a 6.5K Onan water cooled generator. I am designing a heating system similiar to what side by sides and utv use. I intercept the coolant before the heat exchanger. Put a inline thermostat in the coolant hose and divert the hot coolant thru some Heater core units with a fan and thermostat. Then after heater cores it goes back to close system coolant pump. When engine gets to temp the inline thermostat opens and sends excess heat to heat exchanger.

So my question is has anybody done this and their experience with it. I can not find any infomation on this subject. Is there any reason this is not a good idea. My Wife is always cold so I thought this could be a great heat source when generator is running.

Thank you for any info or comments!

My biggest concern would be how big a heater you could run before causing the generator to run too cold. It only puts off so much heat.

Is heat while running or anchored a bigger concern? If it's heat while running, you can take heat from the engine(s), as they'll put off enough that you can pretty much run as big a heater as you want with no issues.

Probably an old MCCK unit. If I remember right, there is a thermostat in each cyl head. I see no need for any other t-stat. I think both cyl head flows go to exh manifold then center outlet from there goes to HX. You can tie in the heater in that exh man to HX line without worry of sub-cooling the unit.

I did this with my main engine. Used a large automotive van heater core and a squirrel cage blower. Works like a champ. My diesel gennie there is no simple way to tie in, and I am mostly under way so main engine is best heat source.

Do you really run your generator so much that it can provide heat when you need it? If you are running the genny you have other options of reverse AC heating or an electric heater. If you end up using the genny as a primary heat source you risk running it underloaded.

Thank you,
Heater would be the same size that UTV/Side by sides use so it is designed for small engines. I also thought about generator running to cool so I am installing a acurate temp gauge. It would be used in temp for 15 degrees on up for winter anchoring out. Main engines are not used much in these situations. Right now generator runs about 150 degrees under load before heat exchanger with no thermostat. Thinking about putting in a 180 degree thermostat. Though I do not know what Onan recommends for that year for temp. Thank you for your input.

It would also be running the electric heaters and charging the batteries.

Stock thermostats on a freshwater cooled MCCK are 180*. If you point an IR gun at the base of the t-stat housings on the cylinder heads, you should see somewhere in that ballpark (plus or minus a few degrees) once it's good and warmed up.

Taking coolant between the exhaust manifold and heat exchanger return is safe, just make sure you plumb it so that you don't create an air lock and prevent flow. And you might need a booster pump to help circulation depending on how far you're pushing the coolant (stock pump is only so strong).

I use engine heat to keep my hydronic boiler hot when cruising in cold weather. And I've used engine heat to run a car type heater in boats of the past. But with larger engines. I don't and didn't use an additional thermostat (just like in a car). And in many cars, the heater circuit bypasses the thermostat. For a car heater I had a wall thermostat and relay turn the fan on and off as needed. Plumbing much like you describe, before the exchanger and returning it to the same area, but with valves to bypass the heater in warmer weather, engine startup, or in the event or a hose failure. 3 valves - 1 in each hose and one to restore the original flow. The engine coolant pump was always enough to move the water and still cool the engine. At most I was moving the coolant about 35 feet, mostly horizontal. Unless under a high load, I don't think a 6.5kw engine will produce much extra heat. Especially if it's gas.

You can make it more complicated with relays, heat sensors and water valve solenoids like my hydronic system, but I don't think you need it.


Most wives are cold, in more than one way.

A gas engine should produce more waste heat into the coolant than a diesel of equivalent power output, especially at light load. Drawing heat from the return feed to the heat exchanger will at least avoid over-cooling the engine. Worst case, you just don't get as much heat as desired.

If you are running the generator, why not use electric heaters? The generator needs a substantial load anyway and built in electric heaters come in all shapes and forms. Look at some under cabinet heaters. We use three of those in our RV in winter. The electric heaters will also work when docked.

gsholz said:

If you are running the generator, why not use electric heaters? The generator needs a substantial load anyway and built in electric heaters come in all shapes and forms. Look at some under cabinet heaters. We use three of those in our RV in winter. The electric heaters will also work when docked.

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I could see wanting to use coolant as supplemental heat.

In the case of my boat, I've got 12kw (2x 50A feeds) for shore power, but only 6.5kw available on the generator. Ordinarily, that's plenty. But if I were in a situation where I wanted to run all 3 A/Cs for reverse cycle heat on generator power, that only really leaves enough for the battery charger and such (the A/Cs draw a little more power in heat mode). Even running the water heater would be marginal (bumping right up against full load on the gen).

So being able to turn 1 or 2 of the units off and use some waste coolant heat to warm the boat instead leaves more power available to heat water, run the stove, microwave or whatever else I need to power. And it's much more efficient and cost effective than putting in a larger generator that wouldn't be necessary most of the time.

No reason not to do this , its called co-generation and is a better level of efficiency than just heating the ocean.

We have the 6-71 that is hooked to a 60,000 BTU heater box.

By installing a small DC circ pump we can run the heater on the stored heat in a ton of DD .Great in evenings that get a bit cool, for a while.

First off, great old boat. I spent a lot of time on various Trojans from the 70s. What engines does she have?

Assuming a gas powered genset, running at lighter loads should be ok, kinda like an idling taxi at the airport to keep the driver warm. A red dot heater or similar is not uncommon.

To me, the elephant in the the room for a gas engine is CO. Insuring a perfect and very good exhaust run with daily checks seems prudent. Place several CO monitors in strategic places as well as gasoline vapor detectors in the bilge.

Buy that real feather blanket, they're wonderful for the PNW winters. A dehumidifier helps a bunch too to make things more comfortable.

Thank you for all the replies,

On the Trojan for engines we have Chrysler 400 280 hp x2. 2 cyclinder 6.5kw gas genset. We love our old Trojan F36 Tri fly. The CO and gas vapor dectectors are great suggestion.

Ordered a booster pump for the system also. Thanks.

Update on using generator coolant for cabin heat.

I have installed a hydronic heater in my galley using the coolant off of my generator. In the Christmas parade in Tri Cities Washington we were 78 degrees in the cabin and 34 degrees outside. I am very pleased with it. I still have left over heat still be put into the river so I am going to but second heater in my aft cabin. I will update when I get it completed. Thanks.

In City Island NY some of the head boats for day fishing hook both engines to a hollow hand rail , so folks can have warm hands .

Has anyone set someone like this up with their main motors? I was thinking of installing a 12v (fan) aux heater in my GB 36. I could mount it under the starboard settee by the helm and plumb it into the starboard Lehman cooling system. Basically the same heat our cars have. Thoughts? Thx.

I use the heat from my Detroit mains. I have a boiler, but I just use it as a tank and for the circulation pump. Both engines are plumbed, but I usually only have one supplying heat. My system is probably more complicated than you want. I use marine grade forced air heaters in each cabin and have a zone controls. I use manual valves to cut in the engines. The coolant leaves the engine just before the thermostat and returns to the coolant pump suction side. It's easy on a Detroit because they already have a thermostat bypass line.

Follow up

I installed second heater in aft cabin. I plumbed it in paralel and was not happy with second heater performance. First heater still put out amazing heat and the aft cabin heater did very little. I could not figure why. I fought it and thought about it for a few days. Finally I decided to try plumbing it in series. After I got all the air bubbles out of system it worked amazing. Both heaters put a abundant amount of heat and generator runs at about 170 degrees water temp with a light load. I would consider a third heater in Vberth but that might be over the top. Thank you for everybodies responses. It was a fun project.

I guess the pressure drop across the heater element is greater than the parallel pipe. In case you wanted to operate the heaters independently, you could always install a bypass circuit with a vale to direct flow past the heater (similar to original arrangement but with shut-off valve)