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Old 04-12-2018, 16:21   #1
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Question Webasto FCF Reverse Cycle Freezing?

We have a Webasto FCF unit in our salon. It is a 110V, 16K btu unit and it was installed in January of 2015. It was installed under the settee and it has a recommended 120sq inch return air grill. It also has a 500gph pump, and has excellent water flow.

It ran pretty much non-stop this summer and cooled just fine. Never had an issue.

However, on reverse cycle heating, the coils freeze over. I first noticed it when it was running and there wasn't any water going through. I defrosted it and it worked again, however the condenser runs around 170 degrees.

I had an A/C tech out the other day and paid him $152 to tell me that he couldn't see anything wrong with it, and my two options were to have him pull it and run it at the yacht service's shop, or replace it. Since I wasted that money, its been heating and cooling fine, however I noticed a thick layer of frost on the coils again today.

Most people have told me that its low on refrigerant, however these units are not serviceable without tapping the lines and adding a valve. Additionally, it cools just fine.

Has anyone had this problem, and if so, what was the solution?

The photos are of the first time it froze over.
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Old 04-12-2018, 17:57   #2
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Re: Webasto FCF Reverse Cycle Freezing?

I cannot answer you question..
I just installed this unit in our boat.. the same.. no issues but haven't used it for heat much yet.. only installed it over Thanksgiving..
I seem to remember that they want 140-160 sq inches of intake area..we don't have this much..to time will tell.


what is the water temperature?


-dkenny64
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Old 04-12-2018, 18:08   #3
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Re: Webasto FCF Reverse Cycle Freezing?

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what is the water temperature?


-dkenny64


Currently 64 degrees F.
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Old 04-12-2018, 20:09   #4
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Re: Webasto FCF Reverse Cycle Freezing?

It could be low on refrigerant or your water flow is too low. Or your heat exchanger is dirty (scaled inside?). If it worked in cooling without freezing your evap coil, it's probably a water flow issue. It's not related to your return air.
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Old 04-12-2018, 20:16   #5
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Re: Webasto FCF Reverse Cycle Freezing?

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It could be low on refrigerant or your water flow is too low. Or your heat exchanger is dirty (scaled inside?). If it worked in cooling without freezing your evap coil, it's probably a water flow issue. It's not related to your return air.
dl

Nope. Nope. And Nope.

A 500gph pump is supplying ample water flow without obstruction.
The internals were descaled when the tech was out last week.
And there's enough refrigerant for it to cool.

You haven't said anything that hasn't been said by 100 other people, so we checked all that. Unfortunately we're still coming up with nothing.

I've emailed Webasto about an hour ago, hopefully I'll hear back from them in the next few days.
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Old 05-12-2018, 01:49   #6
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Re: Webasto FCF Reverse Cycle Freezing?

If the water is really 64F it seems impossible to freeze the condenser coil like that unless one of two things (or both). Refrigerant is low or water flow is restricted. If you can measure the water outlet temp what to you get? If the difference between sea and outlet temp is more than single digit degrees then water flow is an issue. My guess would be low refrigerant pressure.
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Old 05-12-2018, 02:02   #7
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Re: Webasto FCF Reverse Cycle Freezing?

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My guess would be low refrigerant pressure.
The unit is only 3 years old and is a sealed system - no ports to recharge. And if refrigerant was low, wouldn't it cause the same issues in cooling mode? It doesn't make sense that it works perfectly in one mode, and freezes up in the other.
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Old 05-12-2018, 02:14   #8
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Webasto FCF Reverse Cycle Freezing?

How about a video of your water flow?

Also, as previously suggested, a measurement of the outlet water temperature.

Does this ice build up to the point that it effects anything?
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Old 05-12-2018, 02:25   #9
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Re: Webasto FCF Reverse Cycle Freezing?

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How about a video of your water flow?

Also, as previously suggested, a measurement of the outlet water temperature.

Does this ice build up to the point that it effects anything?


I'll try to take water temps, and shoot a video this morning.

If I allow it to, it will ice to the point that it prevents water flow.
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Old 05-12-2018, 02:49   #10
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Re: Webasto FCF Reverse Cycle Freezing?

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The unit is only 3 years old and is a sealed system - no ports to recharge. And if refrigerant was low, wouldn't it cause the same issues in cooling mode? It doesn't make sense that it works perfectly in one mode, and freezes up in the other.

To freeze the sea water the water exit temp has to be below 32F. Thatís more than a 32 degree delta. So now my money is on a water/heat exchanger issue. There should be no possible way that 62F water can freeze in such a short path. The flow is either too little or the scale inside was not removed. Possibly there is a foreign object stuck inside the heat exchanger but the ice seems too uniform. You need more water flow or do another descale operation. Water flow increase seems the best option.
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Old 05-12-2018, 02:59   #11
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Re: Webasto FCF Reverse Cycle Freezing?

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To freeze the sea water the water exit temp has to be below 32F. Thatís more than a 32 degree delta. So now my money is on a water/heat exchanger issue. There should be no possible way that 62F water can freeze in such a short path. The flow is either too little or the scale inside was not removed. Possibly there is a foreign object stuck inside the heat exchanger but the ice seems too uniform. You need more water flow or do another descale operation. Water flow increase seems the best option.
Everyone keeps saying its a water flow issue, but then I call the yacht service out and they say that its adequate water flow. I would think a 500gph pump is nearing overkill.
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Old 05-12-2018, 03:11   #12
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Re: Webasto FCF Reverse Cycle Freezing?

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Everyone keeps saying its a water flow issue, but then I call the yacht service out and they say that its adequate water flow. I would think a 500gph pump is nearing overkill.


The pump is adequate, but any restriction in the line will greatly reduce the flow. These pumps are not positive displacement, they generate very little pressure.
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Old 05-12-2018, 03:13   #13
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Re: Webasto FCF Reverse Cycle Freezing?

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The pump is adequate, but any restriction in the line will greatly reduce the flow. These pumps are not positive displacement, they generate very little pressure.

I'll take a video of the output but I promise you it's not restricted.
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Old 05-12-2018, 04:07   #14
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Re: Webasto FCF Reverse Cycle Freezing?

This thing is trying to absorb 16K BTUs /Hr of heat from what appears to be a relatively small heat exchanger. It would require full water flow and more importantly a very clean heat exchange area, internally within the area where heat exchange is intended. For example, if the interior of the exchanger tubing is coated so as to restrict heat transfer, you may have a good water flow but it is 'insulated' from collecting the correct / engineered amount of heat. This would cause the condenser to perform poorly, gradually allowing its external surface to lower below 32F and build ice as a consequence.

Suggest you have the exchanger properly cleaned, water pump flow rates tested and enquire if the system has an operating de-ice device.

It is not short of gas.

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Old 05-12-2018, 05:31   #15
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Re: Webasto FCF Reverse Cycle Freezing?

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Everyone keeps saying its a water flow issue, but then I call the yacht service out and they say that its adequate water flow. I would think a 500gph pump is nearing overkill.
We are just looking at the physics. I ran the math and to freeze 62F water (you said the water inside the loop freezes) your flow rate has to be < 100 GPH. It's a simple formula for pure water.

Power = 265 * GPH / 60 * Delta-C

Power for your unit is about 6,000
Delta-C is about 17-18

A little algebra solves for GPH = 6000 / 265 * 60 / 18 = 75GPH

If the flow rate is higher than that it cannot freeze. ergo....

In case you are wondering, if the water is not pure the water constant 265 is less which means the flow rate has to be less than the above formula predicts.

Maybe the flow rate gradually slows. Or maybe the pump doesn't start flowing right away giving time for the coils to get really cold. But the math says it has to be something related to water flow.
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