Marine Aire (Dometic) 18,000 BTU HVAC freezing

[RandB Davis]

Our 2009 Hunter 38 (Mariner Package) has a Marine Aire (Dometic) 18,000 BTU HVAC unit in the main salon and had a similar 6,000 BTU unit in the aft cabin. Both use separate Dometic Passport I/O Digital Controls. The aft unit was not working at all when we purchased the boat in October 2020, so I have pulled it out, hoping to replace it sometime in the not-to-distant future.

The main unit is model SVCD18K/2 – HV (115VAC/60Hz)

Meanwhile, the main cabin unit regularly freezes up. Two key programmable settings come into play: fan speed and de-icing cycle.

For fan speed, I have the Passport control set to A (Automatic) and I believe it is running at its highest speed; however, I will double-check that P-2 (High Fan Limit) is set to 95 as it can be set from 65-95. Likewise, I will make sure that P-3 (Low Fan Limit) is set to its highest value (75).

For the de-icing cycle programmable setting, it can be Off, 1, or 2, where 1 looks for a 5-degree temperature differential between the thermostat setting and the thermostat temperature sensor, and 2 looks for a 7-degree differential. It was set to 1 and the coil froze, so following the troubleshooting guidance in the Passport manual, I changed it to 2 and it still froze. When I say froze, I mean solid ice coating the pump and coil.

I have had the refrigerant checked by an experienced, competent marine HVAC technician—twice. Both times, he suggested that what is needed is more air flow, that the output vents, collectively, do not allow enough air flow through the system. He suggested adding a new 4x8-inch output vent in the main cabin forward bulkhead. I am not opposed to this type of solution, but I do not want to cut a hole in that bulkhead. And my co-owner is strongly opposed to that solution!

Rather, I am inclined to provide more air flow on the opposite side of that bulkhead in the v-berth; either (1) larger vent at the top where a 4x-4-inch vent exists, or (2) add a second vent down low, which is directly in-line with the output of the HVAC unit under the starboard forward settee.

Has anyone experienced this issue with this model Hunter, and if so, did you come up with other solutions, or have you cut additional output vents? Are there other Passport control settings I should change? If so, which and why?

I would prefer a 4x8-inch outlet vent (cherry), but am only finding 4x4 and 4x12 at VHMK12X4SP Supply Air Grille, African Cherry (Makore), 4-Way 12" x 4", SATIN VARNISH by MSI). Are you aware of any other sources of these premade vents that match the 2009 h38 decore?

Finally, has anyone replaced the 6,000 BTU aft HVAC unit, and if so, what did you chose and how do you like it?

Thanks in advance!
Regards,
Roger

 

[Rich Stidger]

I have a different Marine Aire unit and in conditions of high humidity and long running it will ice up. I don't have an automatic de-icing setting, but switching to heat for a minute or two melts the ice and you can switch back to cooling for another couple of hours.

 

[RandB Davis]

Thanks for the very quick response Rich.

The tech suggested the same thing, however, it invariably freezes up when I am away from the boat. There is another potential controller setting according to the manual. It is called Moisture Mode. I have not yet tried that, but it looks like an option when I am away from the boat during hot & humid periods: "While in the On Mode, simultaneously press the Power and Down buttons. The first cycle starts in one minute. Every four hours, the fan circulates air for 30 minutes. During this time, the air temperature is sampled and entered into memory. If necessary, the cooling cycle starts and continues until the temperature is lowered 2°F (1.1°C) or until the compressor runs a maximum of one hour. Four hours after the temperature is satisfied or the compressor times out, the cycle repeats. The “HU1” code displays while in Moisture Mode. Press the Power button once to end Moisture Mode."

Regards,
Roger

 

[Sailgunner]

I have a similar unit and I leave it in Moisture Mode all summer. The boat gets hot but the humidiy is lower and no mold grows in the boat and I am in SW Florida.
Your unit should shut down and correct itself if the coil starts to freeze up. I had that issue and just turned up the fan speed. You can set the speed whereever you want. Don't leave it in Auto speed mode.

 

[Rich Stidger]

Thanks for the very quick response Rich.

The tech suggested the same thing, however, it invariably freezes up when I am away from the boat. There is another potential controller setting according to the manual. It is called Moisture Mode. I have not yet tried that, but it looks like an option when I am away from the boat during hot & humid periods: "While in the On Mode, simultaneously press the Power and Down buttons. The first cycle starts in one minute. Every four hours, the fan circulates air for 30 minutes. During this time, the air temperature is sampled and entered into memory. If necessary, the cooling cycle starts and continues until the temperature is lowered 2°F (1.1°C) or until the compressor runs a maximum of one hour. Four hours after the temperature is satisfied or the compressor times out, the cycle repeats. The “HU1” code displays while in Moisture Mode. Press the Power button once to end Moisture Mode."

Regards,
Roger

I have the equivalent to your Moisture Mode. My system calls it Humidity Control. It runs the A/C as you describe and because it is not running continueously it does not freeze. Upon arrival I switch to normal mode to cool down the boat.

 

[SEMPERAVANTI41]

I had the same problem in my Hunter 41, which I sold. Try setting the fan to a constant speed higher than the lowest speed. When I set the fan to automatic, the controller would lower the fan speed to the lowest setting, and the coil would freeze solid. If that does not work you may want to install a ECL Evaporator Control Limit Switch from Retrozone.
I have replaced the 16,000 BTU unit in my 41 with a Dometic replacement unit, which required fitting as it is NOT a direct replacement. In my 50 I have replaced with a Websato. I prefer the Webasto as it has a better warranty, is less expensive and works just as well.

 

[Don S/V ILLusion]

Another possible cause is external- if bilge air comprises some of the external air source through the fins, the moisture content exacerbates the condition which causes freezing and it’s often simply analteration of air flow pattern through the coil.

 

[quadrille38]

Our 2009 Hunter 38 (Mariner Package) has a Marine Aire (Dometic) 18,000 BTU HVAC unit in the main salon and had a similar 6,000 BTU unit in the aft cabin. Both use separate Dometic Passport I/O Digital Controls. The aft unit was not working at all when we purchased the boat in October 2020, so I have pulled it out, hoping to replace it sometime in the not-to-distant future.

The main unit is model SVCD18K/2 – HV (115VAC/60Hz)

Meanwhile, the main cabin unit regularly freezes up. Two key programmable settings come into play: fan speed and de-icing cycle.

For fan speed, I have the Passport control set to A (Automatic) and I believe it is running at its highest speed; however, I will double-check that P-2 (High Fan Limit) is set to 95 as it can be set from 65-95. Likewise, I will make sure that P-3 (Low Fan Limit) is set to its highest value (75).

For the de-icing cycle programmable setting, it can be Off, 1, or 2, where 1 looks for a 5-degree temperature differential between the thermostat setting and the thermostat temperature sensor, and 2 looks for a 7-degree differential. It was set to 1 and the coil froze, so following the troubleshooting guidance in the Passport manual, I changed it to 2 and it still froze. When I say froze, I mean solid ice coating the pump and coil.

I have had the refrigerant checked by an experienced, competent marine HVAC technician—twice. Both times, he suggested that what is needed is more air flow, that the output vents, collectively, do not allow enough air flow through the system. He suggested adding a new 4x8-inch output vent in the main cabin forward bulkhead. I am not opposed to this type of solution, but I do not want to cut a hole in that bulkhead. And my co-owner is strongly opposed to that solution!

Rather, I am inclined to provide more air flow on the opposite side of that bulkhead in the v-berth; either (1) larger vent at the top where a 4x-4-inch vent exists, or (2) add a second vent down low, which is directly in-line with the output of the HVAC unit under the starboard forward settee.

Has anyone experienced this issue with this model Hunter, and if so, did you come up with other solutions, or have you cut additional output vents? Are there other Passport control settings I should change? If so, which and why?

I would prefer a 4x8-inch outlet vent (cherry), but am only finding 4x4 and 4x12 at VHMK12X4SP Supply Air Grille, African Cherry (Makore), 4-Way 12" x 4", SATIN VARNISH by MSI). Are you aware of any other sources of these premade vents that match the 2009 h38 decore?

Finally, has anyone replaced the 6,000 BTU aft HVAC unit, and if so, what did you chose and how do you like it?

Thanks in advance!
Regards,
Roger

I have a 2008 version of the same boat.
I did experience icing in the aft cabin unit at one time. It was because I had slowed the fan speed down in an effort to make it quieter at night when we were sleeping. Primarily happened in really humid weather. I had to return the program settings for the unit to the default settings specified in the manual.

So some suggestions for you.
1. Do you have the manual that gives you the program settings ? It so return them to the default values. If you don’t have the manual let me know and I’ll take a picture of the settings pages in mine and send them to you.
2. When air conditioning make sure the fan is set to continuous and not cycle. I find that with air conditioning we want the constant air movement as it makes the cabin feel cooler and keeps air moving past the condenser. When heating I set the fan to cycle because the constant moving of the air makes things feel chillier.
3. Make sure the screen behind the lovers is clean and dust free. It’s a metal mesh material. If it’s clogged it restricts air flow.
4. There should be a thing plastic type filter material that slides in front of the condenser radiator on the air intake side. If it dirty is seriously restricts air flow And will cause icing. If it’s missing you have a related problem And need to get a replacement for it. See #5 for the related problem.
5. clean the radiator. If it’s really dirty the air flow will be restricted. If there was no filter it will be really restricted. Be very careful and gently spray a cleaning solution in the radiator and then rinse thoroughly to get the dirt out.

I just had a marine hvac guy come and look at mine yesterday. The rear unit was acting strange. Wouldn’t heat or cool. He found the control circuit board on the unit is failing. When he wiggled it the compressor kicked on like it was supposed to. He ordered a new one and should be installing it next week for me.

 

[RandB Davis]

I have a similar unit and I leave it in Moisture Mode all summer. The boat gets hot but the humidiy is lower and no mold grows in the boat and I am in SW Florida.
Your unit should shut down and correct itself if the coil starts to freeze up. I had that issue and just turned up the fan speed. You can set the speed whereever you want. Don't leave it in Auto speed mode.

Thank you Dennis.
I would think it would shut down, but it does not and that is the one more irritating thing about the situation.

I tried setting it in Moisture Mode yesterday evening, but was not sure it went into that mode. I will try to get to the boat today/tomorrow to see how it is performing.
Regards,
Roger

 

[RandB Davis]

I have a 2008 version of the same boat.
I did experience icing in the aft cabin unit at one time. It was because I had slowed the fan speed down in an effort to make it quieter at night when we were sleeping. Primarily happened in really humid weather. I had to return the program settings for the unit to the default settings specified in the manual.

So some suggestions for you.
1. Do you have the manual that gives you the program settings ? It so return them to the default values. If you don’t have the manual let me know and I’ll take a picture of the settings pages in mine and send them to you.
2. When air conditioning make sure the fan is set to continuous and not cycle. I find that with air conditioning we want the constant air movement as it makes the cabin feel cooler and keeps air moving past the condenser. When heating I set the fan to cycle because the constant moving of the air makes things feel chillier.
3. Make sure the screen behind the lovers is clean and dust free. It’s a metal mesh material. If it’s clogged it restricts air flow.
4. There should be a thing plastic type filter material that slides in front of the condenser radiator on the air intake side. If it dirty is seriously restricts air flow And will cause icing. If it’s missing you have a related problem And need to get a replacement for it. See #5 for the related problem.
5. clean the radiator. If it’s really dirty the air flow will be restricted. If there was no filter it will be really restricted. Be very careful and gently spray a cleaning solution in the radiator and then rinse thoroughly to get the dirt out.

I just had a marine hvac guy come and look at mine yesterday. The rear unit was acting strange. Wouldn’t heat or cool. He found the control circuit board on the unit is failing. When he wiggled it the compressor kicked on like it was supposed to. He ordered a new one and should be installing it next week for me.

Thank you for your detailed response, especially given you have the same model Hunter.

I have the manual, and have been through the settings so many times that I practically know them by heart. The one difference between the aft unit and the forward unit is the P-18 setting as one has the Split Capacitor (SC/aft) and the other is Shaded Pole (SP/fwd).

Though I seem to have good air flow, having checked and cleaned all the intake screens as you have suggested. I had been using the Automatic fan speed setting (P-2=A), but have switched that to 6, the highest. Time will tell if there is any improvement.

I plan to clean the radiator (coil) as you have suggested as soon as I can locate the proper cleaner for that task. I will blow compressed air through it first, but like I said (and the HVAC tech confirmed) i seem to be having good air flow; however, he emphasized the need for an additional output vent needed to be cut into the bulkhead as he opined the small round ceiling vents and v-berth 4x4 vent simply were too small.

As to your aft unit and the bad circuit board--interesting. Before calling the HVAC tech, I swapped the fwd/aft Passport controllers (thermostats) and nothing changed, however, I did not think to swap the controller boards mounted in the white box on the units. when he checked the aft unit, he said it was dead, old, and beyond fixing as it was very expensive to get replacement parts. The quote for a replacement unit was a bit steep, so I am shopping around. I may see if I can find the circuit board and give that a try.

Regards,
Roger

 

[RandB Davis]

Another possible cause is external- if bilge air comprises some of the external air source through the fins, the moisture content exacerbates the condition which causes freezing and it’s often simply analteration of air flow pattern through the coil.

Thanks Don.

Fortunately, no direct bilge air comes into the cabin air flow, and we are also fortunate to have a relatively dry bilge. I would be dryer still if I could get the bilge switch/bilge pump to extract the last inch of water!
Regards,
Roger

 

[RandB Davis]

I had the same problem in my Hunter 41, which I sold. Try setting the fan to a constant speed higher than the lowest speed. When I set the fan to automatic, the controller would lower the fan speed to the lowest setting, and the coil would freeze solid. If that does not work you may want to install a ECL Evaporator Control Limit Switch from Retrozone.
I have replaced the 16,000 BTU unit in my 41 with a Dometic replacement unit, which required fitting as it is NOT a direct replacement. In my 50 I have replaced with a Websato. I prefer the Webasto as it has a better warranty, is less expensive and works just as well.

Thanks for the response.

I have recently changed the setting to 6, rather than A/Automatic. Time will tell. Also, thanks for the advice on the ECL Evaporator Control Limit Switch from Retrozone. I will look into that.
Regards,
Roger

 

[quadrille38]

Thank you for your detailed response, especially given you have the same model Hunter.

I have the manual, and have been through the settings so many times that I practically know them by heart. The one difference between the aft unit and the forward unit is the P-18 setting as one has the Split Capacitor (SC/aft) and the other is Shaded Pole (SP/fwd).

Though I seem to have good air flow, having checked and cleaned all the intake screens as you have suggested. I had been using the Automatic fan speed setting (P-2=A), but have switched that to 6, the highest. Time will tell if there is any improvement.

I plan to clean the radiator (coil) as you have suggested as soon as I can locate the proper cleaner for that task. I will blow compressed air through it first, but like I said (and the HVAC tech confirmed) i seem to be having good air flow; however, he emphasized the need for an additional output vent needed to be cut into the bulkhead as he opined the small round ceiling vents and v-berth 4x4 vent simply were too small.

As to your aft unit and the bad circuit board--interesting. Before calling the HVAC tech, I swapped the fwd/aft Passport controllers (thermostats) and nothing changed, however, I did not think to swap the controller boards mounted in the white box on the units. when he checked the aft unit, he said it was dead, old, and beyond fixing as it was very expensive to get replacement parts. The quote for a replacement unit was a bit steep, so I am shopping around. I may see if I can find the circuit board and give that a try.

Regards,
Roger

Interesting - sounds like you’ve tried a lot. I have just the standard output vents built into the ceiling of the main cabin port and starboard and the one large vent in the bulkhead on the starboard side of the v berth. Thr service guy who is helping me called the factory while at my boat. Found the circuit board was in stock. This was on Tuesday and he said he’d probably have it by the end of this week.

The aft cabin unit has vents in the sides of the hanging lockers on both sides of the cabin and there is a vent in the cabinet right under the head sink.

i did find this schematic on line a while back. Maybe it will offer you some insights. It shows the installation and duct runs on our boats

 

[RandB Davis]

Interesting - sounds like you’ve tried a lot. I have just the standard output vents built into the ceiling of the main cabin port and starboard and the one large vent in the bulkhead on the starboard side of the v berth. Thr service guy who is helping me called the factory while at my boat. Found the circuit board was in stock. This was on Tuesday and he said he’d probably have it by the end of this week.

The aft cabin unit has vents in the sides of the hanging lockers on both sides of the cabin and there is a vent in the cabinet right under the head sink.

i did find this schematic on line a while back. Maybe it will offer you some insights. It shows the installation and duct runs on our boats

Thank you VERY much for the documentation! Where the heck did you get that level of detail? If I could get that level of detail for the actual wiring harness lay out (e.g., the NEMA 2000 routing that seems to be incomplete on our boat--NOTHING is feeding into my Garmin n GPSmap 740 GPS Chart Plotter), I would be grateful.

Question: when you say, "... and the one large vent in the bulkhead on the starboard side of the v berth"; how large is large? We have a vent in the same location that is 4" x 4". As mentioned above, if I cannot find any other satisfactory solution, I will follow my HVAC tech's advice and either put a larger vent there, or add a vent directly below that, but near the bottom of that bulkhead, opposite the output of the HVAC unit.

Thanks again,
Roger

 

[ice breaker]

I never tried this, but a tech from one of the AC manufactures, told me that if you reside in colder water, and are getting iced up that by closing the through hull exit point valve a bit, it will reduce the water flowing over the condensing coil keeping the refrigerant a bit warmer so that when it expands in the evaporator the coils will be just above the freezing point.

 

[quadrille38]

Thank you VERY much for the documentation! Where the heck did you get that level of detail? If I could get that level of detail for the actual wiring harness lay out (e.g., the NEMA 2000 routing that seems to be incomplete on our boat--NOTHING is feeding into my Garmin n GPSmap 740 GPS Chart Plotter), I would be grateful.

Question: when you say, "... and the one large vent in the bulkhead on the starboard side of the v berth"; how large is large? We have a vent in the same location that is 4" x 4". As mentioned above, if I cannot find any other satisfactory solution, I will follow my HVAC tech's advice and either put a larger vent there, or add a vent directly below that, but near the bottom of that bulkhead, opposite the output of the HVAC unit.

Thanks again,
Roger

I think I found it in the links on here that take you boat specific information from Hunter / Marlow Hunter

Here is a link to schematics that you might find useful.

https://www.marlow-hunter.com/wp-content/uploads/h38ServiceManual120105.pdf

Also look here. You will see information on various models and there is a listing for wiring schematics.

Index of /wp-content/export

in the one labeled electrical schematics you should see a link to the 38. Keep drilling down and when you get to the DC electrical system schematic the 38 starts on page 15.

Got to the boat a few hours ago. The vent in the forward cabin is 4x4.