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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.