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Old 12-04-2018, 06:24 PM   #1
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Question Webasto FCF Reverse Cycle Freezing?

I'm cross posting this to get as many replies and advice as I can....

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 12-04-2018, 06:51 PM   #2
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You are in Texas, right? Sea water temps can't be down to 35 deg F at this time of year which would freeze up the coil.


OTOH the 170 deg temp that the condenser runs is way, way too hot. Should be 100 or so. Is there plenty of air flow? That could run the temps up, but I can't see why the coil would freeze.


Something is wrong with the pressures that the system is running at. Find another GOOD marine A/C guy to check it out.



David
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Old 12-04-2018, 06:55 PM   #3
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My next experiment is to put a fan just inside of the return grille to help increase the air flow.
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Old 12-04-2018, 09:04 PM   #4
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There should be an automatic defrost cycle kicking in when it frosts up. Check for defrost sensor or defrost relay operation. Once you have thick ice on the coils, the defrost cycle obviously failed and you need to turn the heat pump off. Turn auxiliary heat mode on if your system has one.
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Old 12-04-2018, 11:05 PM   #5
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You don't say what your water temps are. If your incoming water temps are in the low 40's, it's likely the seawater HX coil will freeze. As far as the defrost cycle another poster mentioned, I don't know the specifics of the unit you're working on, but it would be unusual to find a defrost cycle on a marine unit. Residential heat pumps yes, but not marine "reverse cycle". Different animal. The residential heat pump has an air exchange coil, this is water exchange.



If the coil is freezing, it's because there's not enough sensible heat content in the HX coil to boil off the liquid refrigerant as it flashes off into gas. Either the temp of the entering water is too low, or there's not enough volume of water flowing through the coil, so it frosts on the outside, freezes on the inside and eventually chokes off the flow. Blowing additional air across the indoor coil isn't really a solution, although it may help some. Moving more water through the HX would be a better alternative. Switching to cooling switches the HX coil from absorbing heat to rejecting heat, so the ice will melt pretty quickly- minutes.


"Excellent water flow" is subjective. Compare the spec on the unit for required flow against the pump curve. How many units does the pump serve? Is there some plumbing that is restricting the flow of water? Can you measure the actual flow? Absent that, what's the difference in temp of water entering the coil vs. leaving? An IR thermometer is inadequate for checking those temps, you need a thermocouple to attach to the tubing to get a meaningful reading. IR will average in the surrounding temps. Cumulative shortcomings in the seawater flow can be enough to mean the difference between freezing and not when dealing with low (low 40's) entering temps. Even a perfectly tuned system can freeze up if there's not enough heat content in the seawater flowing through the coil. Physics.


The larger systems may be equipped with a TXV, or thermal expansion valve. It uses a thermostatic sensing bulb mounted on the tubing at the end of the coil to control the metering valve that regulates the amount of refrigerant that passes through the expansion device. It's possible that the sensing bulb is not making good thermal contact, or it's not insulated. If the sensing bulb is seeing erroneous higher temps, it will open to allow more refrigerant into the coil, flooding it and causing it to frost/freeze.


As far as the tech experience- "see anything wrong" seems to indicate he looked at it. If he didn't take any readings, how could he possibly know that there wasn't a problem? A good troubleshooter has a routine that he will step through every time. A mental checklist. It will include taking measurements so the system can tell him if there's anything amiss. No information = no diagnosis.
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Old 12-04-2018, 11:27 PM   #6
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Incoming water temps are about 64 right now.
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Old 12-05-2018, 03:33 AM   #7
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I assume you have the manual. Here's the troubleshooting on iced coil:
You can download here: https://www.jamestowndistributors.co...all_Manual.pdf
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Old 12-05-2018, 03:57 AM   #8
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"Switch air conditioner to heat until ice melts or use hair dryer to melt."

LOL! LOL! LOL!

They're funny. Especially since it's freezing up in heat mode.
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Old 12-05-2018, 07:07 AM   #9
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Quote:
Originally Posted by toocoys View Post
"Switch air conditioner to heat until ice melts or use hair dryer to melt."

LOL! LOL! LOL!

They're funny. Especially since it's freezing up in heat mode.

That flowchart addresses cooling, if the evap coil is freezing. Not the case here, so not applicable.



Mr. TooCoys, if your HX coil is freezing with entering water temps of 64, then there's not enough water flow, or the coil is very dirty, preventing heat transfer. Check for obstructions in the seawater circuit. Here's a link to educate yourself and run through some steps to troubleshoot. Without taking some measurements and determining what's actually going on with the system, it's impossible to solve the problem. In order to solve a problem, you must first define the problem; that's not been accomplished.


If your tech didn't find anything unusual about a coil freezing in those circumstances, he failed to define the problem, and you did not receive good value for your investment in his services. You can't fix stuff by looking at it. Measure, document.
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Old 12-05-2018, 08:59 AM   #10
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I am embarrassed that I didn't catch the low water flow theory that Maerin is correctly promoting. That is the obvious cause of freezing. It is the reverse of an evaporator coil freezing up in cooling mode.

Still doesn't explain the reported 170 deg condenser temp which influenced my comments. Is that figure correct? Way, way too high if so and hard to reconcile with the evaporator freezing simultaneously.

One simple test is to measure the temp of the raw water outlet. Probably near freezing with a frozen up coil and low flow is the obvious, probably the only possible cause. Should be about 5-10 degrees lower than sea water temp in heating mode.


Also I have never seen a defrost cycle on marine, sea water cooled/heated system. Many do have a cut out though.



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Old 12-05-2018, 10:27 AM   #11
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This is the current water flow. Doesn’t appear to be obstructed in any way and appears normal. The same as it does in cooking mode.

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Old 12-05-2018, 11:12 AM   #12
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This is the current water flow. Doesnít appear to be obstructed in any way and appears normal. The same as it does in cooking mode.


I'll reiterate my previous post,
Without taking some measurements and determining what's actually going on with the system, it's impossible to solve the problem. In order to solve a problem, you must first define the problem; that's not been accomplished.
How much flow? Put a container under it, measure the output, time how long it takes, convert that to gal/min or a flow rate. Then you have information you can use.
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Old 12-05-2018, 11:14 AM   #13
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I'm voting for not enough water flow. That is a mighty small through hull discharge fitting for a 16k. Looks like 1/2". My 12k uses I think 3/4" and flows maybe double that amount of water.

Also, where are you measuring that 170F? If on the inlet refrigerant line to the condenser, you may be getting some superheat from the compressor. What is air temp coming out of fan or closest grille?

My unit does not have a freeze problem til sea water gets below 40F. And yes, you can put it on AC mode to thaw water coils. Not sure it is good for it, but it worked for me.
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Old 12-05-2018, 04:14 PM   #14
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You need 5 gallons per minute of water flow for it to be working right on that 16k unit.
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Old 12-05-2018, 04:43 PM   #15
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Ok well tomorrow I’ll get a 5g bucket and some hose and some barnacle buster and run it for a cycle. I’ll also measure the output.
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Old 12-05-2018, 04:44 PM   #16
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I'm voting for not enough water flow. That is a mighty small through hull discharge fitting for a 16k. Looks like 1/2". My 12k uses I think 3/4" and flows maybe double that amount of water.

Also, where are you measuring that 170F? If on the inlet refrigerant line to the condenser, you may be getting some superheat from the compressor. What is air temp coming out of fan or closest grille?

My unit does not have a freeze problem til sea water gets below 40F. And yes, you can put it on AC mode to thaw water coils. Not sure it is good for it, but it worked for me.

FWIW... the 16k unit under the aft bed is the same size and has the same thru hull. It works fine, but itís an older dometic unit.
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Old 12-05-2018, 07:42 PM   #17
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Toocoys:


Could you please explain the 170 deg temp noted in your first post. That makes the whole thread suspect IMO.


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Old 12-05-2018, 08:25 PM   #18
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You need 5 gallons per minute of water flow for it to be working right on that 16k unit.

I agree. Rule of thumb is 250 gph per ton. So for 16K, it comes to 5.5 gpm. So if you can get a 5 gallon bucket under the thru hull, it should be overflowing in less than one minute. (If I calculated correctly, the bucket should overflow in 54.5 seconds). I think you'll be close enough for your purposes if the bucket's full in a minute! Do that, then you have a good starting point; you know something for fact. Nothing drags out solving a problem like guesstimated diagnostics!
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Old 12-10-2018, 06:49 PM   #19
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Even though one unit is working well using the same thru-hull, have you had a diver check that thru-hull for marine growth?
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Old 12-12-2018, 11:56 PM   #20
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Even though one unit is working well using the same thru-hull, have you had a diver check that thru-hull for marine growth?
Yes, it was cleaned in September by a diver.
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