does it take less power to run heat as opposed to AC?

[MikeP]

I'm on the boat for a while doing some work. It's 33F degrees outside, water temp is 41F and I have all 4 heat/ac systems running. Here's my question... Does it take less current to run the oem cruiser units on heat as opposed to AC? I would have thought it would be the same since, as I understand it, the only difference in operation is which way the reversing valve opens.

The reason for my question is that in the summer, I cannot run all 4 ac systems on 50A shore power at the same time without tripping the 50A breaker at the shore power box. But right now, with all 4 running heat, they are pulling less than 30A.

So what's up with that?

I'm not complaining! In fact I'm tickled to death since the boat is a comfy 74F but I don't understand the difference in AC/Heat power draw.

 

[SeaEric]

It was always my understanding that it took more amps to run the reverse cycle heat as opposed to the cooling. This is supported by the digital meters that I have on my boat that tell me exactly whats happening. AFAIK the numbers are higher on heat.

 

[saltshaker]

It was always my understanding that it took more amps to run the reverse cycle heat as opposed to the cooling. This is supported by the digital meters that I have on my boat that tell me exactly whats happening. AFAIK the numbers are higher on heat.

Heating should draw more than cooling. Check to see if all units are actually running in heat med. If you have a stuck reversing valve that unit will only be running the fan.

 

[Canuck Dennis]

They usually shut off at 40-45 water temp...

 

[SKYCHENEY]

Maybe the whole dock has higher voltage since less people are there in the winter. This might explain the tripping of the dock breaker in the summer since lower voltage will mean more amps drawn.

 

[MikeP]

Well, I have checked very carefully to be sure all 4 units are all actually engaged and supplying heat (as they are this very minute) and when they are all on, they pull approx 25-28A in steady-state operation. In the summer, with cooling, the 4 units will pop the breaker after a few minutes of operation with the (OEM) ammeter on the boat indicating around 46-48A.

Soooo...if cooling actually takes less power than heating, something else is going on and the only thing it could be that I can see is Sky's explanation. I have never checked the marina voltage winter/summer so I can't say for sure that that there is higher voltage now than then but it would make sense.

When I got to the boat yesterday and the interior temp was 41, I figured I would be lucky to just be able to get the master SR to a temp I could deal with - but the Cruisairs had no trouble getting the temp up to 74F - though it took a while to get there initially. Now they are cycling off/on automatically as needed to maintain it.

This added a few minutes later: I just checked the voltage on the OEM meter and it's showing 244VAC. As I said, I never really checked it seriously before but in looking at where I generally remember seeing the needle on the gauge in the summer, I'd say it was at around 230VAC but that's just a guess based on a memory of relative needle position.

 

[MikeP]

Well...can't be the voltage difference.

The load at the moment - 28A at 244VAC is about 7k watts.

If you enter the data into Ohms law the difference between 7000W at 230VAC and 7000W at 244VAC is only about 2.5A. So the current would increase from 28A to about 30.5A with that voltage drop. But the difference in cooling vs heating operation seems to be close to double the current.

Each of the 4 units cool/and heat equally so it doesn't seem to me like there is any problem within the units themselves that could explain the difference...

Something else is afoot!

I'm resisting the death march into this issue because I have REAL work to do here.

 

[MikeP]

OK, I'll admit to being totally mystified.

I tried each unit on heat and then on cool while observing the ammeter. There was no difference at all in the current drawn whether the units were heating or cooling. And since I can't find enough voltage lost between summer and winter at the dock power to account for the amperage difference, I have no clue why all units will run fine now but will pop the shore breaker in the summer.

It will just have to remain a mystery.

 

[SKYCHENEY]

Are there usually other additional loads besides the A/C units? Maybe the water heater kicked on before.

I usually run 2 cords with one dedicated to Ships Service 2 which has all 4 of the A/C units. I have never blow a breaker running all of them on heat or on cool.

 

[carolinacoast]

Could the lack of humidity improve the efficiency and therefore lower the amp draw?
Could there be more heat in the dock wiring from demand and temp during the summer that creates resistance?

I run one cord, and am generally alone in the winter, and only blow a breaker occasionally with the family in the summer.

 

[mdshore]

i use a couple of those 70.00 oil filled heaters and they heat the boat nicely at a use of about 1500w each.. no frozen pumps/intakes to worry with in winter..
at 40 or below water temp you wont get much heat from running the reverse cycle units..

 

[MikeP]

Well, water temp is 40 per the Garmin and the oem heaters will still run you out of here if you want to set them at 80!

 

[mdshore]

... its a dry heat.... lol

 

[rsmith]

Less head pressure with the colder water

 

[sunset grill]

I believe Mr. Smith wins the prize. The compressors are doing the same thing, pumping Freon both summer and winter just in the opposite direction. Since Freon changes pressure based on the temperature, the cooler both the air and the water is, drastically reduces the pressure, so the pump is drawing less amps. Steve

 

[MikeP]

Well, thanks to a SERIOUS "DOH" moment...I think Sky nailed it.

I totally overlooked the water heater because it is not on now but always is normally when I'm here. The dual element, 240VAC heater is rated as 3500W per element with a max of 4k per element. There's the missing 7000 + Watts!

Obviously, during the season, it cycles whenever it needs to and, if 3 of the 4 AC units are running, that easily goes beyond the 50A breaker. The fact that 3 can run for a while and then pop the breaker indicates to me that the popping was occurring whenever the WH cycled. The fact that we normally don't need/operate more than 2 AC units means that the 50A limitation doesn't usually matter.

 

[sandspur1966]

Less head pressure with the colder water

exactly what my guess would be.

 

[Luckyducky]

have you looked at your power cord plugs. Just wondering what if they show any signs of heat or corrosion.