HVAC Replacement

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Last summer, our pilot house and master stateroom HVAC units stopped working. Since we’d just got out of the yard in Stornoway Scotland, it was just about 100% certain that the two failures were correlated, and caused by air in the system. We bled the pump and there was air. But when we tested the systems, these two units still were not working. It seemed very unlikely that we would have two independent systems fail at exactly the same time. These systems are very independent where each HVAC unit shares only the central water pump providing cooling water. Other than that, they have no components in common.

Since two failures at the same time seems unlikely, we took the cooling hose off the pilot house unit, pictured below, to ensure it wasn’t air locked. It wasn’t and the outlet hose was free flowing. We checked the master stateroom unit and found it was fine as well.

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Pilothouse HVAC cooling hose removed

The water temperature was 58F (14.4C) and these systems normally operate well at temperatures below 50F (10C), so that wasn’t a problem either. We had changed nothing, put the system back into operation and tested it. What we found was even harder to explain, where the pilot house system started working and the guest stateroom unit stopped working. So we still had two faulty units but a different two. Fortunately the one we use the most, the PH, at least was functioning. Again, it seemed unlikely that three independent systems would fail, and the problem was somewhat baffling.

In the next stage of our investigation, we detached the outflow line from the master stateroom system and installed a short length of hose in order to pull any potential air out of the line. The system is below the waterline, so we had to be careful to keep that hose raised above the water level (where James is pointing to in the left picture below). We vacuumed the HVAC outflow line, at right below, to remove any air and get the system working again. We were absolutely positive no air was in the system. The reason why we’ve chased water flow and potential air in the system so aggressively is that we had multiple units all failing at the same time, with the only thing they have in common being the shared water flow.

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MSR Outflow Removed
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Vacuuming air from MSR outflow

The next step was to try replacing the control board for the master stateroom unit with a spare. Given the symptoms, this was highly unlikely to improve the situation, but we decided to give it a try. This also didn’t fix the problem.

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Replacing MSR control board with spare

We don’t have air conditioning pressure gauges, but as a quick test on whether the system was still able to maintain a charge, we just touched the valve, pictured below, to see if there was pressure. There was ample pressure in the system and, externally, there was no evidence of coolant leaks or oil residue left by coolant leaks. At this point, refrigerant charge didn’t seem a likely cause of the problem.

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Checking for pressure

We concluded that the reversing valve on both units had stuck in the middle, allowing the compressor to pump into the reversing valve and straight back to the compressor. The reversing valve is switched positively from end-to-end by pressure differential in the system (see TheEngineeringMindset.com for a detailed explanation). Since the compressor was producing very little flow and there was almost no temperature differential between the low and high pressure sides, it likely was not producing much pressure differential, making it more likely that the reversing valve wouldn’t switch positively in either direction.

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Reversing valve (from HvacPartsShop.com)
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Reversing valve solenoid energized (from TheEngineeringMindset.com)

To confirm the hypothesis that the reversing valve was stuck in the middle, we ran the compressor for long enough that it was building up heat and tracked the hot coolant flow using an IR temperature gun. We could see that the flow was going from the compressor to the reversing valve, and directly back to the compressor. We also could test this by switching back and forth between heat mode and cool mode, which normally produces an audible click as the reversing valve spool swings to the other end. (When doing this test, you have to ensure that you’re not hearing the solenoid that controls the reversing valve click and that it actually is the reversing valve itself traveling from end-to-end.)

We definitely had a failed reversing valve in both these systems. This is a common failure mode in reverse-cycle HVAC systems and is best avoided by switching between hot and cold every three months. The most effective way of correcting this problem is to switch the unit between hot and cold repeatedly while firmly tapping the reversing valve with a plastic hammer. (Be careful to only tap on the ends of the reversing valve, since a dent along the longitudinal sliding surface will cause the valve to stick rather than correct the issue.) This approach didn’t work on either failed unit. We have a pretty unusual situation where two units have independently failed at almost the same time. However, someone has to win the lottery.

We knew for sure that both units had a reversing valve stuck midway, so they’d both need a new reversing valve. Unfortunately, when the reversing valve sticks in the middle, it short-circuits the system, where the compressor pumps right back into the compressor, just about guaranteeing that the compressor inlet will have liquid refrigerant. This will destroy the compressor as well, so there was a very high probability that we need to replace both reversing valves and both compressors. These are eleven-year-old modular HVAC systems and, at that age, they’re barely worth servicing. So we decided to replace them. We also decided to replace the pilot house unit at the same time because it had been used more heavily than any other HVAC system in the boat, had failed for a few days before starting to work again, and in cooler water often cannot produce heat where the other systems can.

Unfortunately these are 60Hz appliances, so they needed to be sourced from North America. We later discovered that the manufacturer, Dometic, has moved from having two separate product lines, one of 50Hz and one of 60Hz, to a single worldwide SKU supporting both power frequencies. So we actually could have sourced these two HVAC systems locally in Europe. We have a diesel boiler, so we had heat, and we were in Norway, so we really didn’t need cooling. After checking prices around Europe and the US, sourcing from the US was more cost-effective, even with the additional shipping costs, and replacement wasn’t urgent. And we could also include with the shipment a few other important parts and spares. We purchased the needed items in the US and arranged with GAC Logistics Norway to ship the package to Norway. It arrived into Bergen in late January and we took delivery in Farsund a few days later.

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Pallet ready to ship from US
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Pallet delivery in Farsund, Norway
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Unpacking pallet
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New HVAC units and other parts from pallet

We replaced the master stateroom unit first, since it was the most difficult and the one we wanted to have running the most. The pictures below show us detaching the original unit and sliding it out of a locker just inside the stateroom door.

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Original MSR system in locker by staterooom door
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Detaching MSR system
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Original MSR control board
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Removing MSR unit

After living aboard Dirona for eleven years as we’ve cruised the world, we’ve generally been really impressed with the job Nordhavn does of providing access panels to support maintenance and replacement. The two pictures below show access to the rear of the HVAC unit locker through a removable panel in the adjacent head (James’s fingers can be seen as he is removing the panel in the picture above captioned Detaching MSR system). It would have been pretty much impossible to replace the HVAC unit without access to the rear of the locker. Having the access panel made the job much easier as we otherwise would have had to create an opening ourselves, which would have added hours to the job.

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MSR unit removed from locker with access panel at rear
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Looking into locker through access panel from MSR head

The pictures below show the new master stateroom HVAC unit being installed. The job went remarkably quickly. Typically when an eleven-year-old part is replaced, the new part has some slight modification that requires customization to install. In this case we didn’t have to do anything extra beyond removing the hoses and ducts and reinstalling them once the new unit was in place.

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Installing new MSR unit
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New MSR unit in place
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Wiring new MSR board
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New MSR board ready to mount

We tested the system and it worked perfectly. It was also much quieter than the previous unit, a nice bonus. Seeing the system producing heat again was awesome! It was especially impressive that it was working with the water temperature at 41F (5C). All the units struggled, and particularly the pilot house, to run when were in London for the winter, when the water temperature was down in the 45-47F (7.2-8.3C) range.

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And we have heat!

Next we tackled the pilot house unit, mounted under the pilot house settee. This would have been as quick a change as the master stateroom unit, except that we wanted to mount the unit higher to have a more positive condensate drain with a slightly steeper slope. We also changed the mount position to slightly move the system away from the wall to reduce noise from vibration.

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Original PH system under settee
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Original PH system removed

And finally, we replaced the guest stateroom unit, mounted under the guest stateroom berth. This also was slightly more difficult than the master stateroom unit due to a custom-designed fabrication to put a ‘T’ into the outlet flow. Once we understood that this hose was using a custom-designed flange, we just left it in place and attached it directly to the new HVAC unit, replacing the factory flange.

By this point, the water temperature had dropped to 39F (4C) and the unit was not able to heat. However, we could tell by the frost build-up visible in the second picture below that it was working. And when the surrounding water warmed up a few days later, it worked beautifully.

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New GSR unit in place under berth
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New GSR unit heating

All three units are working perfectly now and are much quieter. The job went much better than we expected and we’re very happy to have three new HVAC systems on board. Not having these systems working for the past few months has been no hardship since we have a diesel furnace. But we will need the air conditioning later this year as we return to North America and pass through warmer climates. Underway we don’t like to leave the overhead hatch from the master stateroom to the Portuguese brow open if conditions are even the slightest bit rough, and with that window closed the master stateroom can warm up even in relatively cool climates. We elect to just run the air conditioning underway on most longer passages. It slightly reduces fuel economy and range, but makes for a much nicer living experience.

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12 comments on “HVAC Replacement
  1. Rick Miller says:

    How many HVAC/Heating units do you have throughout the boat and where are they located in the boat? Is 10 years about the average lifespan of these units?

    • We have 5 HVAC units on the boat. We had a control board failure in the Master Stateroom System after about 2 years. The failure mode was the compressor relay and we think it’s caused by the increased inrush current when operating 60 cycle systems in 50 cycle mode. Dometic had at the time approved the use of their 60 cycle systems on 50 cycle power but subsequently stopped supporting that operating mode. Since they cut support and since we had seen a failure, we moved to only using them on 60 cycle power.

      The next two failures we saw were after 10 years where we had two systems fault at almost exactly the same time where both had stuck reversing valves leading to compressor failure. This fault mode is common but not really age related. It’s caused by not switching between heat and cool frequently enough or other factors like contaminants leading to the valve hanging up. We switch between heat and cold operating modes every 6 months at minimum and usually more but frequently making lack of use a less likely issue. And, both units failing at just about exactly the same time is strange as well so I suspect we don’t fully understand the failure.

      My take is the 2 units that haven’t been replaced yet could easily go another 5 to 10 years but we view 10 years as a good lifetime for these integrated heat pumps. So, even though only 2 units had failed, the PH system is important so we elected to replace it as well even though it was still operating without issue.

      Generally, there is no special time when the systems fail and well maintained systems often go 10 to 15 years.

  2. Chris Barber says:

    James, I’ll be replacing my MSR unit in the next couple of months, soon as it is delivered. How/where do you get to the removable access panel to remove it? This is obviously a key piece of the puzzle. And I too really love the new Dometic DTGs. I have already replaced my PH and one of the main salon units, increasing it from 12 to 16.

  3. Peter Smith says:


    We will have Webasto HVAC units aboard our Nordhavn 41 and will take delivery of the boat in Istanbul. Although our HVAC will be 60Hz, we expect to run the air conditioning on 50Hz for several years while cruising in Europe. Did you use 50Hz current on your old HVAC while Dirona was in Europe? Do you think that running 60Hz units on 50Hz current will harm the air conditioners?

    • The newest Dometic units (DTG) are 50hz/60hz and the older Dometic units (DTU) will run on 50hz but it’s not recommended. The Webasto line appears to be rated to run on either 50/60hz so you should be fine but you need to make sure that your system is rated to run on 50hz. Either ask the manufacturer or get your exact model number and check with this: https://www.webasto-comfort.com/fileadmin/webasto__media/webasto-comfort.com/INT/Documents/marine-catalogue.pdf.

      We had a letter from Dometic saying they would warrant ours on 50hz and we ran that way for a year or so before concluding that the additional amperage draw was just too much at 50hz. We had a compressor control board failure and decided to stop running them on 50hz. We went to a model where we ran the entire boat including the HVAC off of inverters and then used the shore power to drive the the inverters through the batter chargers. More details on what we did here: https://mvdirona.com/2014/08/a-more-flexible-power-system-for-dirona/.

      If you can confirm what I looked up above and your Webasto systems are fully supported on 50hz, you’re fine with your plans.

  4. Steven Coleman says:

    Hello James,

    I’m glad you got that sorted out. Heating of course wasn’t an issue but cooling will be at some point.

    I wouldn’t necessarily recommend it now but, if you need to replace your boiler a couple years down the road you might check on the state of development for fuel oil fired condensing boilers.


    • Thanks Steve. We do currently have a 11 year old Becket Boiler. It’s in a horrible to access location, there have been some water leaks in that area so it’s rusty, and generally I’m just not looking forward to needing to replace it. Fortunately, it’s an unbelievably reliable system and just doesn’t cause issues. I like designs with a long, successful history outside of the marine industry.

      Thanks again for the help with the HVAC issues last summer.

      • Steven Coleman says:

        The pressure vessel and burner are really the heart of the system when it comes to boilers. I’ve seen many that looked like you should get a tetanus shot after working on that actually had many years if not decades left.

        Fuel line leaks are the biggest issue with oil fired burners, next would be ignition electrodes followed with over time and a lot of use, nozzle erosion.

        • This is our first experience with a Becket apartment boiler. On previous boats we used a small Webasto primarily used to heat over the road trucks. Within the first couple of years we had learned just about every detail of that Webasto. The Becket boiler runs exactly as you said — it just keeps going without service. We’ve never even given it a new burner. I suspect it’s first failure will be a rusted out boiler since water gets forced down the exhaust when we are in really rough weather. It doesn’t happen often but salt water is tough on it.

  5. Chris C says:

    Hi, Been lurking on here for a while, just out of curiosity in purely economical terms is it more efficient to use the diesel boiler or run the HVAC in heat mode from the generators.

    • Generally, the diesel heat system is more power efficient than the reverse cycle system operating on generator power. I suspect the diesel boiler is a more efficient heating system even when on shore power but that’s less clear. For sure we do know that the efficiency of marine water cooled reverse cycle systems declines with the water temperature and will stop producing heat down in the 40F to 45F range. Because the HVAC systems are big electrical loads then can really only reasonably operated on generator or shore power whereas the boiler can be run 24×7 without unreasonably increasing generator run frequency. Technically our inverters can run several HVAC units at once but it probably just makes more sense to run the generator when you have a big electrical draw like reverse cycle HVAC.

      HVAC has a potential advantage when plugged into shore power in that power is sometimes cheaper than burning diesel and, even when the diesel compares well economically, it can still be more of a hassle to fill the tanks whereas shore power is essentially always available without any additional work. The reverse cycle system also has a potential advantage when only heating a small part of the boat. The diesel boiler will deliver some heat to all places where the hot water lines are run even if you aren’t intending to heat that room and the fan units are not on. This isn’t a huge factor and can be mitigated by insulating the hot water runs. We chose not to insulate them only because we like systems ability to deliver a little heat everywhere in the boat to reduce moisture build. But, when heating just one room, the HVAC system is more precise and might be more efficient.

      The diesel boiler is far quieter and is a more gentle and more comfortable heat. We sometimes use the diesel boiler when we have electrical power available just because it’s a more comfortable heat. And, on the other side, we sometimes use the reverse cycle system when we’re spending a long time on a dock on cold climates to avoiding needed to fill the diesel tanks. And, of course, in really hot weather, the reverse cycle system is wonderful whereas the diesel furnace can’t do much for you. Overall, there are good reasons for both and we like having both.

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