ONAN generator installation:

[SFH]

In the process of installing our new ONAN genset but struggling a bit with the exhaust and muffler location.

If I have to keep the min. 12 inch drop between the exhaust and the muffler it is impossible to install water separator just above the muffler

If I install the water separator right above the muffler the drop is less than 12 inches

I could really need a bit of advice here...

[a64pilot]

I don’t think that min 12” for the muffler is flexible. I think it’s a requirement to keep from water logging the engine

[jt11791]

why does the separator have to go right above the muffler?

[a64pilot]

Originally Posted by jt11791

why does the separator have to go right above the muffler?

I don’t think it has to be immediately above, pretty sure being offset some as long as you get that 12” rise would be OK too.
I don’t see what that 12” is for myself, as long as you get a good drop and it would stay a drop no matter how heeled over etc. should be OK?
The first 12” I understand, but not the second so much.
Call Onan, don’t go on what I say and maybe ruin your generator.

[svHyLyte]

The water/exhaust gas separator does not need to be directly above the muffler. It can be off-set so long as the height between the muffler discharge and the top of the separator are maintained at or less than the maximum permitted height. The height has to do with the exhaust gas pressure in the muffler being able to push the column of water up and into the separator which is dependent upon the "head pressure" (or height of the water column) in the discharge tubing. Lateral off-sets have no effect on the head pressure.

FWIW...

[SFH]

Thanks for valuable input.

Almost done with the installation. Only struggling with the 12 inch drop between genset and the muffler.

The ONAN muffler and ONAN genset are both sitting on the same level and the drop is not 12inch between the two

Further I have to pass the exhaust pipe from the genset through a hole (pre-cut) in the bulkhead where it drops well but then has to raise it on the other side of the bulkhead again where the pipe enters the mufflers - some kind of a dropping loop which means no straight 12inch drop as shown in drawing.

I will write a mail to Cummins Aftersale support to check if ok and post their reply here

[a64pilot]

You could delete the mechanical water pump and replace it with an AC electric one and connect it to the generator output. This would mean that the water won’t flow unless the generator is making power. That means that as soon as you turn off the generator the pump stops and the engine exhaust blows all water out of the system, and no water is introduced into the system until after engine start and the generator is making power.
So for the first few seconds or so there is no water flow, which hurts nothing cause in just a few seconds there isn’t enough heat.
It’s keeps the muffler and exhaust system dry when the generator is off, or it does on my little 3.5 KW Nexgen anyway.
You also know the exact second water begins to flow as the exhaust tone changes markedly.
You could also put a switch on the pump so that you could turn the pump off a few seconds before turning off the generator, listen for exhaust change, then turn off generator, that way the exhaust system is sure to be dry before engine shutdown.

Of course this is modifying the new generator, and you have to be smart enough to remember to use the switch, but the exhaust is so loud without water, I can’t imagine you wouldn’t notice it.

[SFH]

Originally Posted by a64pilot

You could delete the mechanical water pump and replace it with an AC electric one and connect it to the generator output. This would mean that the water won’t flow unless the generator is making power. That means that as soon as you turn off the generator the pump stops and the engine exhaust blows all water out of the system, and no water is introduced into the system until after engine start and the generator is making power.
So for the first few seconds or so there is no water flow, which hurts nothing cause in just a few seconds there isn’t enough heat.
It’s keeps the muffler and exhaust system dry when the generator is off, or it does on my little 3.5 KW Nexgen anyway.
You also know the exact second water begins to flow as the exhaust tone changes markedly.
You could also put a switch on the pump so that you could turn the pump off a few seconds before turning off the generator, listen for exhaust change, then turn off generator, that way the exhaust system is sure to be dry before engine shutdown.

Of course this is modifying the new generator, and you have to be smart enough to remember to use the switch, but the exhaust is so loud without water, I can’t imagine you wouldn’t notice it.

Hi A64pilot
Brilliant idea. Complicates setup slightly but great idea.
Will still check with Cummins but this comes in handy if alternative is required

[Dockhead]

Originally Posted by a64pilot

You could delete the mechanical water pump and replace it with an AC electric one and connect it to the generator output. This would mean that the water won’t flow unless the generator is making power. That means that as soon as you turn off the generator the pump stops and the engine exhaust blows all water out of the system, and no water is introduced into the system until after engine start and the generator is making power.
So for the first few seconds or so there is no water flow, which hurts nothing cause in just a few seconds there isn’t enough heat.
It’s keeps the muffler and exhaust system dry when the generator is off, or it does on my little 3.5 KW Nexgen anyway.
You also know the exact second water begins to flow as the exhaust tone changes markedly.
You could also put a switch on the pump so that you could turn the pump off a few seconds before turning off the generator, listen for exhaust change, then turn off generator, that way the exhaust system is sure to be dry before engine shutdown.

Of course this is modifying the new generator, and you have to be smart enough to remember to use the switch, but the exhaust is so loud without water, I can’t imagine you wouldn’t notice it.

I studied this modification on your generator with great admiration, and I have resolved to do the same to my 6.5kW Kohler if the impeller pump ever carps out on me

Why don't you post details of your installation?

[Colin A]

Originally Posted by SFH

Thanks for valuable input.

Almost done with the installation. Only struggling with the 12 inch drop between genset and the muffler.

The ONAN muffler and ONAN genset are both sitting on the same level and the drop is not 12inch between the two

Further I have to pass the exhaust pipe from the genset through a hole (pre-cut) in the bulkhead where it drops well but then has to raise it on the other side of the bulkhead again where the pipe enters the mufflers - some kind of a dropping loop which means no straight 12inch drop as shown in drawing.

I will write a mail to Cummins Aftersale support to check if ok and post their reply here

Not an Exhaust expert but I think your asking for troubles with the low spot in the exhaust hose. I have seen a number of hydrolocked engines from things like that over the years. (water gets trapped in the low spot then washes into the engine when it's not running)
Here is a good guideline on exhaust install from Northern lights.
http://www.tadiesels.com/generators/...stallation.pdf
Also I believe a while back there was a detailed write up on exhaust installation in Professional BoatBuilder I think you may be able to get back issues from Wooden Boat Magazine. Dave Gerr also had a few articles but I don't remember if there in books or magazines.

[a64pilot]

A way to dummy proof what I am saying with the electric pump would be a normally closed momentary push button switch. Push the button and power to the water pump is interrupted, the generator blows the exhaust clear of all water, shut down generator. Nothing to remember to turn on during start, just remember to push button until exhaust note changes, then turn off generator.
Best of course is to install IAW directions, but that’s not always possible.

Dockhead I’ve talked about it a few times, and I’m certain that I didn’t think it up, I’m sure I must have read or heard about it somewhere, but it’s simple, just take out your mechanical pump and plumb in an electric AC air conditioner pump, connect it directly to the output of the generator, that way anytime the generator is making power, the pump is running, and when it’s not making power, the pump isn’t.
The only thing to remember is at least a March pump will not self prime at all, they have to be below water line. But that’s easy, just put the pump down near the thru hull, like your air conditioner pump is.