Cabin Heater Finalized

[Roger Long]

Thanks to all here who helped with comments. I think I finally have a plan for my cabin heat. You can see my web site page here if you aren't sick of hearing about it already.

http://www.rogerlongboats.com/10Winter.htm

When I think of all the 6 and 7 figure decisions I've made in my career on pretty much a "Hey, let's do it this way." basis, I feel kind of silly at how much agonizing goes into making decisions about my own boat. If I'd charged for all the time I spent on this, it would probably have paid for the system three times over.

It's been a complicated thing to figure out though. Having my existing hot water heater at the other end of the boat and too low for natural convection to make the cabin heater water loop work without the current draw and sound of a constantly running pump took some head scratching. I think I finally have a good solution that will also give me a bit of heat in the head. You can see it at the link.

 

[Ross]

Roger, I don't see any flaws in the design. Have you figured the maximum water temperature that you can get if the Dickenson were to run constantly?

 

[BobM]

Roger, have you thought about taking S2's approach for the table? My table sits on two removable metal legs which fit in fittings mounted in the sole. Up when you want and down when you don't want. The table, which has leaves and folds up like a cockpit table, stows upright against the cockpit in the quarter berth.

 

[Roger Long]

Have you figured the maximum water temperature that you can get if the Dickenson were to run constantly?

Piping, and heat transfer calculations are just about my poorest point of sail when it comes to things marine. There were always other people around to do those things. I've pestered Dickenson about several things but not gotten answers as clear as I would like. I can't blame them. They just build the heaters. People like me are supposed to be able to figure these things out.

 

[Roger Long]

Roger, have you thought about taking S2's approach for the table?

I've thought about it but I use a cabin table a lot since I don't have a real nav area or other place to spread out larger charts. I think I would end up having it up most of the time. I'll be going offshore some and I want it strong enough to fall against.

 

[Ross]

Roger,
I just did a quick search and came up with approximately 2 btu's per square foot per degree f per hour for heat loss from uninsulated tanks. If that is any help.

 

[Roger Long]

I don't see any flaws in the design.

There is a big one. The hot water heater relief valves are set at a pressure way above what you would want to subject your fresh water hoses and system to, about 120 psi. They are also only intended for emergency use. If they go off routinely, they may start weeping. The arrangement I drew would over pressurize the FW system when the tank heated up.

The solution is an accumulator which is now drawn on the schematic. It doesn't need to be very large, just 5% of the hot water tank volume.

 

[Ross]

The over pressure would only occur if your supply pump is positive displacement or if there is a check valve in the supply line. If you over heat the water it will back flow to the supply tank.

 

[Roger Long]

The over pressure would only occur if your supply pump is positive displacement or if there is a check valve in the supply line.

There does have to be a check valve according to Dickenson. BTW lots of communities are now requiring check valves or backflow preventers at the meter. A plumber comes in and installs it. Someone takes a long shower and draws the water tank down cold then shuts it off and leaves. Usually, it just trips the relief valve on the hot water tank but defective valves or homeowners who have replaced them several times without curing the leaking problem and put a plug in them (Darwin lives) have blown up their water tanks or flooded their houses.

Another nice scenario is older gas hot water heaters which have flues never intended to take these pressures. The flue collapses and the house fills up with CO. Nice.

 

[Ross]

We have been having hot water back flow into the cold water side of the kitchen sink faucets at church. The cure is a check in the supply to the water heater.

 

[Roger Long]

We have been having hot water back flow into the cold water side of the kitchen sink faucets at church.

That's not the issue. The problem is starting with a stone cold tank full of water, something that will be much more frequent on a boat than in a home, and then heating it up in a closed system with no place for the 4-5% expansion to go.

In your church and your house, the expansion just backs out against the street supply pressure. If there is a check valve in the supply, the tank will be pressurized.

 

[Ross]

Quite right. Just like a thermometer.

 

[sailingdog]

So, is it installed yet???