Sunday, 4 January 2015

Dometic HB2500 Air Conditioner

When I converted Jim I installed a floor mounted air conditioner in the form of a Dometic HB2500. The air conditioner runs from a 240 volt AC mains supply, and so we did not get a huge number of opportunities to run it on our trip around North and Central America last year, despite temperatures getting up to 44 degrees centigrade. I originally thought that I could run the air conditioner from a 120v mains supply, by powering the air conditioner from the batteries through our 24v to 240v inverter, and putting the charge back into the batteries from our universal input battery charger. The reason that we ended up not running the air conditioner in this way often, was that the combined heat output from running the inverter and battery charger, pretty much negated any cooling effect from the small 8,500 BTU air conditioner. Both the battery charger and inverter are located inside the living accommodation; in hindsight this was a mistake and it would have been more sensible to locate them in a locker thermally isolated form the inside of the truck body.

The only occasions that we ran the air conditioner, were either when we were desperate enough to use the generator, or when we had a 50a power supply available. A lot of small commercial electrical installs in North America have single split-phase 240v supply, with 120v from either live to the neutral, but 240v between the lives. The big 50a sockets are the only commonly found supplies that offer both lives, and these allowed us to plug the truck into a 240v 60hz supply through a cable which swapped one of the live legs to the neutral leg of the 32a ceeform input socket on Jim. The air conditioner ran fine from these supplies and on a handful of occasions we had a chance to escape the heat in this way.

This sporadic and scant use of the air conditioner changed when we returned to the UK. The HB2500 has the capacity to generate a hot air supply by reversing the refrigerant cycle, and so we used the electrical supply we ran from the house to save some diesel and give the Eberspacher air heater a rest. The noise from the blower is too obtrusive to run all night, but we have found the unit useful for keeping us warm during the day. It is not a particularly effective heater, partly because the air outlets are all mounted at ceiling level, but it is powerful enough to keep the truck comfortably warm when the temperatures outside are around freezing. Being mounted at floor level, and now being run for around 14 hours a day, the air inlet regularly sucks in enough dog hair and dust to block the filter, and so every 2 weeks or so I have to take the filter out and clean it.

Dometic HB2500 floor mounted air conditioner, fitted in Jim the overland motorhome truck

Unfortunately over the recent cold snap the unit has started cutting out periodically. The display shows a flashing orange light, and the unit will stop blowing for a few minutes before restarting again. To see if I could diagnose the problem, I decided to take the unit apart. Access to the air conditioner is fairly simple, I just had to remove the bottom drawer and undo the 10 screws holding the cover panels on, and on taking the polystyrene cover off I could immediately see the problem. The condensating coil was frozen solid. This happens occasionally on air conditioners working hard in extremely humid conditions, but it has never been an issue on our HB2500 before.

The insides of a Dometic HB2500 air conditioner

The frozen condensating coil of a Dometic HB2500 air conditioner

Unfortunately I can see now why air conditioners do not make effective heaters. Any ice generated during in operation in hot conditions is likely to thaw and melt off fairly quickly, but when operating as a heater in below freezing conditions, the incoming air blown over the evaporating and condensating coils is too cold to allow the ice to melt. Being mounted inside the truck, I can turn the unit off and allow it to slowly thaw out, but a roof mounted air conditioner sitting in the cold air would never have the chance to thaw until the outside temperature rises. A heater that stops working in below freezing conditions is a bit of a liability, and so I would not recommend that anyone relies on the HB2500 for heating their vehicle.

One good thing to come out of taking the unit apart, was the chance to remove two spider nests before they hatched millions of arachnids into our home. I sincerely hope that the spiders were harmless domestic critters, but there is every likelihood that they hitched a lift from Mexico or America and may have been less friendly than the spiders that I'm used to.

A spiders nest that was found when dismantling our air conditioner


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