Another Roof Top AC/heat pump question.

[WVPete]

Hi,
We left our 2005 Dutch Star plugged into 50 amp with the two heat pumps running in 45 degree weather. We were gone about three hours and when we returned, the front unit was humming but not making heat. The rear unit was working properly.

I shut off the front unit and then restarted it after about two minutes. It started up and was making heat again. It sounded normal. A while later, it started humming with no heat. I went to the roof and some of the cooling fins are iced over.

I have shut the unit down so the ice will melt. But, why would it ice over at this temperature? I am pretty sure there is a heating strip function but I thought it only operated at lower temperatures.

Does anyone have any idea what could be going on?
Thanks,
Pete

[USMCRET]

Hey Pete, I can’t allow a fellow mountaineer’s question to go unanswered. Native Charlestonian here, long gone.

What you are experiencing is perfectly normal. When the unit is humming like that, it is deicing itself. If you let it continue to run, you’ll find that the unit will resume heating in a few minutes.

Ed

[wildtoad]

Agree. I only have one but it goes into defrost mode for a while then resumes normal operation.

[Dutch Star Don]

"WVPete"......when you use your unit as a heat pump, it just reverses the function of air conditioning. When you have it running as an AC unit, it's producing cool air inside and bowing the hot air outside. When you use it as a heat pump, there is a reversing valve that causes the unit to run just like an AC unit, but backwards. Now the hot air is blowing inside and the cold air is blowing outside. I imagine certain humidity levels make it ice faster. There is no heat strip in the unit.

Lastly, if you aren't aware, once the unit becomes ineffective, somewhere in the low 40's, it will cause your coach to switch over to your furnace.

[bruceisla]

As Don said ... it's probably normal. However, icing can be caused by other factors. Make sure your airflow is sufficient (clean the filters). Here's the documentaion from Dometic:


"Defrost Cycle (Heat Pump Models Only)
To avoid frost formation
on the outside
coil and to obtain
maximum
performance
when the outside
temperature
is less than 42 °F (6 °C) and greater
than 30 °F
(-1 °C), a defrost
cycle will be initiated.
While operating
in this temperature
range the
compressor
continuous
run time is limited
to 25 minutes.
When this time is accumu
-
lated, the fan will shut OFF, the refrigerant
flow will be reversed,
and the compres
-
sor will continue
to run for 4.5 minutes.
During
this 4.5 minute
period,
the LCD will
toggle
between
HP and Defrost.
The refrigerant
flow will then be reversed
and after
a 30 second
delay the fan will resume
operation.
This cycle will remove
any frost
formation
on the outside
coil. This cycle will repeat
itself until the outside
tempera
-
ture is greater
than 42 °F (6 °C). If the outside
temperature
becomes
less than 30 °F
(-1 °C) the heat pump will shut OFF and the auxiliary
heat (if provided)
will turn on"

[WVPete]

Thanks for all these replies. I have had this DS for a little over two years and I am still learning things. I think the icing may have happened because my wife was not feeling well and kept turning up the thermostat so the unit had to run longer. When I brought the set temp back to 65, it doesn't ice up, at least not when it is 45 degrees out.
Thanks again for all the help. I'm really happy to hear it is functioning properly. I had cleaned the fins this past summer and the filters too but I think the filters can stand to be cleaned again.
Pete

[harleyjt]

Originally Posted by bruceisla

As Don said ... it's probably normal. However, icing can be caused by other factors. Make sure your airflow is sufficient (clean the filters). Here's the documentaion from Dometic:


"Defrost Cycle (Heat Pump Models Only)
To avoid frost formation
on the outside
coil and to obtain
maximum
performance
when the outside
temperature
is less than 42 °F (6 °C) and greater
than 30 °F
(-1 °C), a defrost
cycle will be initiated.
While operating
in this temperature
range the
compressor
continuous
run time is limited
to 25 minutes.
When this time is accumu
-
lated, the fan will shut OFF, the refrigerant
flow will be reversed,
and the compres
-
sor will continue
to run for 4.5 minutes.
During
this 4.5 minute
period,
the LCD will
toggle
between
HP and Defrost.
The refrigerant
flow will then be reversed
and after
a 30 second
delay the fan will resume
operation.
This cycle will remove
any frost
formation
on the outside
coil. This cycle will repeat
itself until the outside
tempera
-
ture is greater
than 42 °F (6 °C). If the outside
temperature
becomes
less than 30 °F
(-1 °C) the heat pump will shut OFF and the auxiliary
heat (if provided)
will turn on"

Good to know. Thanks for posting.
jt

[bruceisla]

Originally Posted by WVPete

I had cleaned the fins this past summer and the filters too but I think the filters can stand to be cleaned again.
Pete

FWIW, I clean filters once per month ... sometimes more frequently.

[WVPete]

Originally Posted by bruceisla

FWIW, I clean filters once per month ... sometimes more frequently.

I cleaned the filters again today. I was surprised at the amount of dirt that was left in the water.

Also, I discovered that the one of the vents that put out heat was aiming air at the main thermostat control. So when the back unit comes on, warm air is blowing on the front thermostat preventing it from running. This is sort of un-related to my original post, but changing the aim has made the front unit cycle more regularly and keep the front warmer at a lower setting.

Like I said earlier, I am still learning stuff about RVing!!

[bruceisla]

Originally Posted by WVPete

I cleaned the filters again today. I was surprised at the amount of dirt that was left in the water.

Also, I discovered that the one of the vents that put out heat was aiming air at the main thermostat control. So when the back unit comes on, warm air is blowing on the front thermostat preventing it from running. This is sort of un-related to my original post, but changing the aim has made the front unit cycle more regularly and keep the front warmer at a lower setting.

Like I said earlier, I am still learning stuff about RVing!!