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Old 03-16-2013, 11:19 PM   #1
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Using reverse cycle air ducts for Webasto heat

I'm considering the purchase of a 2 cabin 45' trawler outfitted with reverse cycle heat and A/C. It has ducting throughout. Problem is...at anchor on cold days we'd have to run the genset to power this heat pump system. So, my question is: Has anyone installed a diesel heater (Webasto air?) that uses the same ducting already in place for the reverse cycle system? My other option is to install a separate diesel furnace hydronic system (Kabola, Hurricane) but at considerable cost. Any suggestions will be greatly appreciated -- Thanks!
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Old 03-17-2013, 06:50 AM   #2
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considered it but the RC ducting is much larger and I found an easy run for my Wallas furnace ducting. I also was concerned the heat would go in both directions in the RC ducting and half the heat would wind up not where I wanted it.

could be done though...just was easier another way for me.

what actually changed everything is the Wallas was going to be harder to mount in the engine room where I first wated it due to fuel line install requirements...so it wound up in a cabin locker making the ducting runs easier.
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Old 03-17-2013, 08:34 AM   #3
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Anything is possible. You first have to find a spot for the forced air heater that has access to return air, fuel supply and exhaust. Then you have to route its hot air duct to the existing duct. And you will have to install a damper that will shut off the duct going back to the reverse cycle A/C system.

It won't be easy.

Another way would be to run the duct from the forced air system to the suction side of the reverse cycle A/C and use the A/C fan only to circulate the heat. But that of course requires AC power. It is only a few amps and could be supplied from an inverter.

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Old 03-17-2013, 09:40 AM   #4
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........my question is: Has anyone installed a diesel heater (Webasto air?) that uses the same ducting already in place for the reverse cycle system? .......!
It doesn't really matter if anyone else has done this, someone has to be the first so why not you? It seems like a great idea to me.

It would be wise to consult with an HVAC contractor or technician. Someone with training and experience in the field.
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Old 03-24-2013, 09:37 AM   #5
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The only real difference would be the temperature of the heating air.
units output.


Reverse cycle is warm not HOT , compared to a diesel burning furnace.

I would get the duct materials from the furnace folks and simply replace the existing feed ducts with the higher temp accepting stuff.
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Old 03-24-2013, 10:02 AM   #6
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Hi All,

Thanks for the advice, as i am thinking of doing the exact same thing with a Wallas 40 heater on a Krogen 42. Won't do it right now, but it will be on my fall project list.
Richard
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Old 03-25-2013, 04:59 PM   #7
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Make sure what ever the brand that there is parts and service in your area. 15 year ago in the ONW Webasto was 90% of the installed, a few Hurricane and no Kaboto/Wallas. Many Diesel mechanics can also serve Deisel heaters. Many service Webasto. Once you have decided the brand than call an installer to come out to your boat. The reason is the installer can buy and sell you the components the same price as you can buy direct. The installer will help size and design the system for you.

For the Eagle the only choice was the hot water as the air would have cooled before reaching the front stateroom, which was the primary reason to heat as I want the Webasto I the engine room so the run as 40+ ft through the bilge.
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Old 03-27-2013, 02:25 PM   #8
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I have the reverse question. I have Webasto hydronic heat with radiators that blow air through ducts around the boat. Thinking about installing A/C. Has anyone got experience with A/C that cools the water in the hydronic system so the radiators blow cold air?
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Old 03-27-2013, 02:56 PM   #9
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I have the reverse question. I have Webasto hydronic heat with radiators that blow air through ducts around the boat. Thinking about installing A/C. Has anyone got experience with A/C that cools the water in the hydronic system so the radiators blow cold air?
Yes I have! I do not like/want raw water cooled refrigeration as I donít like through hulls and raw water running into the boat. I was hoping when Webasto developed their AC system that they would incorporate the heating system as the exchangers, fluid, manifold, water pump expansion tank would be used. So instead of heating the coolant it would run though a refrigeration/freezer unit. Several years ago where was a discussion about doing this.

The biggest concern was the condensation cause had how to collect and get rid of the water. My ideas as to have pans under the change units that drain to the bilge, and since we already have a refrigeration unit in the bilge that we do no use the coolant could be run though a large ice box.

However, I decide to install regular domestic AC window units, that I make wood enclosure and butted up against the port windows and salon window. two, 6000 btu in the staterooms and one 8000 btu in the salon, total cost about 1,000 bucks. Since the temps only goes above 80 degrees a couple of days per year not worth a big investment but we do use them during the winter to clean and circulate the air.

Anyway its possible!
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Old 03-27-2013, 03:00 PM   #10
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That type of system is used all of the time in commercial buildings, some homes and some boats, usually larger ones. It should be straightforward to set up, but you need someone with marine experience to do it. Probably someone who does this for large boats.

Dometic/Marine Air makes a modular water chiller with a 16-24,000 btu capacity. See products - Dometic

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Old 03-28-2013, 06:53 AM   #11
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One advantage is the entire chilled water package will allow 1,2, or 3 compressors to operate as needed.

But the BIG advantage is a failed compressor package can be yanked for repair with only simple electric and system water disconnects.

The Freon stays in the package till on the bench at the repair folks.
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Old 03-28-2013, 07:11 AM   #12
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What is the fuel savings running a diesel burning heater over a diesel burning genset? I would add up all the mods and stuff your going to have to buy and try to figure out how often your going to be using the heater to make sure it is worth it.
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Old 03-28-2013, 08:14 AM   #13
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My Wallas burns a maximum of 2 gallons of diesel a day....many gensets needed to heat a boat electrically would maybe be around 1/2 gallon per hour so 12 gallons per day

Plus the noise and vibration would be a factor for many.

I installed my Wallas in late September in New Jersey. Even leaving in Dec and traveling south to Georgia....and back in April...I have put around 850 hours on the heater. Had I anchored out more, I could have easily doubled that.

I would think the heater easily pays for itself in a couple years.
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Old 03-28-2013, 08:37 AM   #14
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What is the fuel savings running a diesel burning heater over a diesel burning genset? I would add up all the mods and stuff your going to have to buy and try to figure out how often your going to be using the heater to make sure it is worth it.
A diesel generator converts about 25% of the energy in a gallon of diesel into electric energy. A reverse cycle, electric driven heater "moves" 2-3 times the electric energy as heat. So overall it is about 50-75% efficient. An electric resistance heater converts 100% of its electric energy to heat,so that system is about 25% efficient overall.

A diesel heater has a flue, so some of its energy is lost up the stack, so overall it is 70-80% efficient.

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Old 03-28-2013, 09:36 AM   #15
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I have a Espar D12L diesel heater and use it when the water temp drops below 50deg and the rev/cycle heat no longer works. There are ducts in to each cabin and it keeps the boat warm all winter.
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Old 03-28-2013, 10:17 AM   #16
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Its not as question of cost is a question of heating the boat 24/7 - 9 months of the year, at 70 degree, set and forget, with little noise and electrical amp demand, 20 amps 12 volt DC. When the temps are below freezing we use about 20 gallons per week, 30 to 40 Ė 15 gallons, 40 to 50 Ė 10 gallons, above 50 we change over to electrical as is a little cheaper than diesel. The Webasto is sized down to 0 F degrees, but the boiler is on 50% of the time. During the 9 months we will use 400 to 500 gallons.

When the dock AC goes out for days at a time below freezing weather, we are warm and dry. Its not cheap but its better than little no heat. I installed about 75% of the Webasto, most of the grunt work, so the total cost was about 12,000 bucks, but well worth the money. That was 14 years ago. However I do have the Webasto serviced every year, had to replace the burn tube, water pump and have the head rebuilt. The reason I choice Webasto is itís the #1 install and parts/service is readily available in the PNW.

Oh in the winter it also reheats the hot water, keep the engine room at 60+ and can preheat the 671.
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Old 03-28-2013, 12:59 PM   #17
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A diesel generator converts about 25% of the energy in a gallon of diesel into electric energy. A reverse cycle, electric driven heater "moves" 2-3 times the electric energy as heat. So overall it is about 50-75% efficient. An electric resistance heater converts 100% of its electric energy to heat,so that system is about 25% efficient overall.

A diesel heater has a flue, so some of its energy is lost up the stack, so overall it is 70-80% efficient.

David
A reverse cycle heater and an electrical resistance heater are both free to operate if you're at a marina so as far as we are concerned, they are 100% or more efficient.

So what's the efficiency of an electrical resistance heater powered by an inverter connected to batteries that are charged by a diesel engine that's also moving the boat?

That's what I've had to resort to a couple of times. I'm a warm weather boater.
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Old 03-28-2013, 05:01 PM   #18
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So what's the efficiency of an electrical resistance heater powered by an inverter connected to batteries that are charged by a diesel engine that's also moving the boat?
The efficiency is pretty low. An engine driven alternator is only about 50% efficient. Multiply that by the diesel's efficiency at turning diesel energy into work at the shaft and it is less than 20%. Multiply that by the inverter's efficiency of about 90% and it is 18% or so.

But if you need it to stay warm underway, who cares about efficiency.

David
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Old 03-28-2013, 05:54 PM   #19
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The efficiency is pretty low. An engine driven alternator is only about 50% efficient. Multiply that by the diesel's efficiency at turning diesel energy into work at the shaft and it is less than 20%. Multiply that by the inverter's efficiency of about 90% and it is 18% or so.

But if you need it to stay warm underway, who cares about efficiency.

David
I should have placed a "wink" smiley after my question. I know it's pretty low. I probably had half as much heat coming out of the inverter as the heater.
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Old 03-28-2013, 10:32 PM   #20
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If you are thinking of using your AC ducts for heat, the heat will likely be blowing into the spaces near the ceilings and the lower areas will stay cold. We bit the bullet and installed the Huricanne and have been extremely happy. The pex tubing below the floors in the bilge keep the floors warm, the heat from the blowers is dry and warm. I asked if it made sense to circulate cold water for cooling in the summer, but was advised against for the same reason...wrong location for the airflow.
Good luck
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