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Old 30-07-2010, 16:32   #1
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Airtronic vs Hydronic - Cabin Heat

Hello All,

We're looking to install a diesel cabin heater before this winter. Ignoring the Espar vs. Webasto consideration, I'm trying to get a good sense of exactly what the differences are between a comparable sized forced air unit vs. a hyronic or coolant type unit.

From what I gather so far (and feel free to correct me):

Pros of an airtronic type:
- Better job combating humidity if installed to draw outside air.
- No radiators required.

Pros of a hydronic type:
- Much smaller hoses to run through bulkheads etc.
- Can also heat domestic water.
- Less heat loss over distance.
- Simpler technology?

Can anyone tell me if either type is easier/cheaper to service on your own? Which generates less noise? Heat output and electrical draw seem pretty comparable. Anything else I should be considering?

We live aboard full time in Vancouver BC. Thanks!
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Old 30-07-2010, 16:46   #2
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I've owned both systems. Your initial analysis is spot on. One "pro" you missed about hydronic systems is that if you run the piping under the floorboards where you commonly walk, they provide a good source of radiant heat. Nothing like walking barefoot in the winter on warm floorboards.

One of the pros you missed of forced-air systems is that they only take a fraction of the time to heat up the boat. In five minutes they'll accomplish what will take a hyrodronic system at least a half hour to do. This is because the hydronic system has to heat all the fluid in the system before the fans will turn on. It can seem really slow on a cold winter morning, and sometimes the heat wouldn't come on until I was just about to leave for work.

We ultimately switched from hydronic to forced air (in successive boats) because of the humidity issue. If I were not a liveaboard I would go with hydronic because it will heat water for the shower while at anchor. However, the hydronic systems do absolutely nothing to control humidity, and in a situation where we were showering and cooking on the boat during the winter rainy season we were having to supplement the system with a dehumidifier working 24/7.

Both our systems have been Webasto and both have been maintenance free. We had the hydronic system for eight years, and have had the forced air system for four. The forced-air system is slightly more quiet than the hydronic was, but I don't see this as a significant enough factor to make a difference. The most annoying element of both systems was the fuel pump, and it's the same pump for both.

Both systems consumed more DC power than I'd hoped, especially when the furnaces are in start-up mode.
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Old 30-07-2010, 18:03   #3
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We live in the Puget Sound area as well and just recently worked through the same dilemma on our new boat. We decided on hydronic for the ability to heat water without running the engine in addition to the primary function of heating the interior. In order to overcome the humidity issue, we have one of our heat exchangers (servicing the head) pulling in fresh air from an intake outside the boat which effectively pressurizes the cabin and moves humid air out through the dorades or a cracked hatch.

Another pro of hydronic is the ability to have different heat zones, we very much like being able to turn up the heat in the head prior to showering without making the rest of the boat a sauna!

As Bash said, it does take a bit longer for the heat to kick in after turning it on, but we don't find our system taking more than about 10 minutes to kick in.

Hope this helps...
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Old 31-07-2010, 12:10   #4
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For me, the biggest issue is the humidity. Even though our system pulled in outside air, the temp differential between the hull/hatch outside vs. inside was always great enough to create a high amount of humidity.

Having said that, there is NO comparison between the heat produced on a hydronic system and a forced air system. We had both systems during our first winter aboard. We quickly determined that the forced air system would only get used if the hydronic system broke down.

IOW, hydronic = HOT air. Forced air = air that feels cold even if it is actually warm.

Fair Winds,
Mike
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Old 19-09-2010, 10:45   #5
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Within the same topic I'm curious if any of you have insulated or cored hulls. How cold would you go in either of these situations in a production fiberglass boat? How does it feel when you hit -10 in Vancouver with either system?
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Old 19-09-2010, 10:53   #6
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Quote:
Originally Posted by YOGAO View Post
... IOW, hydronic = HOT air. Forced air = air that feels cold even if it is actually warm...
Nonsence.
A Hydronic system uses water, instead of air, as the heat-transfer (distribution) medium.
Air or water can be heated to any temperature, by a number of means.
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Old 19-09-2010, 11:00   #7
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Quote:
Originally Posted by GordMay View Post
Nonsence.
A Hydronic system uses water, instead of air, as the heat-transfer (distribution) medium.
Air or water can be heated to any temperature, by a number of means.
either system can heat a cabin as hot as you like. The difference here is that a hydronic system heats air already in the cabin, which means it doesn't take the moisture out. the air merely passes through a radiator, much like the radiator in your car.
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Old 19-09-2010, 12:55   #8
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I will confine myself to Espar heaters I have owned both types

Firstly the hydronic system in my experience is very unreliable with many boiler problems. It's also very sensitive to installation errors.

As to heat the airtronic system is far superior it's heats the boat faster and hotter. I used lagged ducts now and these improve the end temperature.

If a hydronic system leaks then it's messy whereas the airtronic is clean. The other issues is if you use a fan matrix system it's very noisy even louder then the latest airtronic.

Both airtronic and hydronic recirculate cabin air neither take in outside air.

The biggest gain is the instant heat of the air system. Boats arnt heated like houses ie with heat potentially running 24/7 you tend to turn on and off the heat depending on whats happening. Air system deliver unrivalled temp rise.

Find a way of heating your hot water ( I do it from an investor) airtronicd the way to go IMHO

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Old 19-09-2010, 13:46   #9
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Quote:
Originally Posted by goboatingnow View Post
I will confine myself to Espar heaters I have owned both types

Firstly the hydronic system in my experience is very unreliable with many boiler problems. It's also very sensitive to installation errors.

As to heat the airtronic system is far superior it's heats the boat faster and hotter. I used lagged ducts now and these improve the end temperature.
The HX's in these boilers are very low mass therefore it is 100% imperative that all air be purged out of the system and that the water circulate across the HX fast enough to not "flash" off. Flashing can cause the water to bubble or boil as it crosses the HX and results in a awful racket. The low mass commercial and residential boiler industry also has problems with this when a contractor uses the wrong circulation pumps. The hydronic systems are more complex but also offer the ability to heat hot water. They also tend to use more DC current with both the need for h20 circulation and multiple fans for each air handler / buss heater.

With a properly installed FHA system it should still be very comfortable and the boat should be nice and dry but you do lose accurate zone control and the ability to heat hot water. We owned a boat with hydronic and a good friend owns one with FHA. Will likely be installing a FHA system on this boat, possibly this winter, if time allows. The air heated boat is considerably drier and heated up faster but was not as "controllable" as the hydronic system. Both are good, but both have pros and cons.
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Old 19-09-2010, 15:57   #10
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Thanks for all the replies. We decided on the forced air version, primarily to combat humidity.

Interesting to note, we saved over $1000 by purchasing the unit and accessories from a truck supplier as opposed to the local marine distributor. Exact same unit and parts.

Quote:
Originally Posted by Jude2010 View Post
How does it feel when you hit -10 in Vancouver with either system?
It doesn't hit -10 in Vancouver We have a solid fiberglass hull, the coldest I remember in the past 3 winters is maybe -5 for a few days. 2 electric space heaters managed to keep us warm. Humidity is always a big issue though, I'm looking forward to getting the airtronic installed.
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Old 19-09-2010, 16:37   #11
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I do not understand the humidity control problem mentioned above with the hydronic systems. Our boat has such a system and it stays totally dry in the Pacific NW all winter. In fact it is dry enough that water in the heads evaporates if they are not flushed periodically. We do have muffin fans that ventilate the bilges overboard which draws some outside air into the hull, and we maintain 60 degrees F during the winter to eliminate any chance of mildew formation. Our system uses a larger that normal number of fan heaters, and separate thermostat zones for each cabin.

Our favorite benefit of the hydronic system is having heated towel bars in the heads. We always have nice dray towels. Further details are here: Traditional

When we shut the system down, the time to delivering heat is 12 minutes. Based on our experience over the past 22 years on two different boats, I will always use take the hydronic option.
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Old 19-09-2010, 21:07   #12
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I've lived in Vancouver as-well. Two winters ago through record snow and only 2 snow plows for the city, with 90% of the cars on summer tires..good times.. heated towel bars, thats redunkuless :P
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Old 20-09-2010, 07:35   #13
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I am researching diesel heating products.
Do not understand why there is such an issue with humidity. And why one or the other methods of heating the boat is allegedly better.
Most homes which are heated have problems with air being too dry - excessively so. there is a huge market for humidifiers to compensate.
Humidity is actually relative humidity. When the air is heated and moisture content remains the same, the relative humidity drops - typical situation on a heated space.
I suspect the high humidity is caused by people(sweating) and cooking, and inadequate heat capacity. Seems would be easy to allow a modest, variable amount of ventilation in on a cruising boat. Too much and you pay too much to heat the fresh air, and reduce capacity.
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Old 20-09-2010, 08:19   #14
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Have not owned either but have used airtronic alot-a comment I got from an experienced liveaboard was that for fulltime liveaboard the hyrdronic is the "only way to go" - in short, the constant heat (especially below the floorboards see above) is the difference, when the airtronic is cycled off there is no heat produced, the hydronic works like a typical home hot water system and, in his experienced opinion (40+ footer in NE) makes the boat comfortable at all times. Condensation is always as issue I guess in cold climes
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Old 20-09-2010, 08:52   #15
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I am not sure why there are so many concerns regarding humidity using hydronic systems either. We did try to proactively address humidity concerns by ducting outside air directly to two of the three heat exchangers and it seems to have done the trick. We cook and shower in our 37' boat when the temp is in the low 40's outside and have no humidity problems at all. Our friend has a 42' Catalina with an Airtronic and he is regretting going with it because they have to run the engine for hot water... with our hydronic system we can be at anchor for days and have hot water whenever we want / need it.

And while making comparisons, our hydronic system is definitely quieter than the Airtronic system on our friend's Catalina. In case it helps, we have an Espar Hydronic M12 boiler with four zones (two staterooms, salon and head) and an aquastat on the water heater.
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