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Old 12-12-2015, 12:14   #1
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AC temperature sensor - how far is too far?

I have a cat with two AC units that I want to use in reverse cycle mode for heat. The temperature sensors were attached to the AC units near the evaporators. The problem is that the units are co-located in one compartment and cannot realistically be moved which means the air temp they are sensing is not necessarily related to the hull they are intended to heat or cool.

The sensors can only be moved a four or five feet unless more wire is spliced into the harness. I need to add about seven feet of wire to one sensor to place in a more appropriate spot; less for the second sensor. Question: will adding this much wire to sensor harness effect its functioning?
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Old 12-12-2015, 13:09   #2
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AC temperature sensor - how far is too far?

The sensors are intentionally placeed where the return air enters the evaporator. This is the correct place so they can sense the cabin air temp. Do not move them IMO.
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Old 12-12-2015, 13:44   #3
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Re: AC temperature sensor - how far is too far?

I don't think this the right answer for this application. As I stated the two units are co-located so they are not sensing the air from each hull as they should and would if installed in the hulls they are servicing. The air from both hulls is just being mixed together.

In addition, unlike other units I've seen where the blower is on continuously, the blower cycles with the compressor so the air surrounding the evaporator doesn't circulate when the compressor is off and therefore doesn't get a chance to sense what is happening temperature wise to the air in the hulls. This means that when the compressor cuts off, it gives off heat which is not dissipated by air circulating from the blower. This creates an even greater temperature discrepancy from the air temperature in the hulls.
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Old 12-12-2015, 13:55   #4
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Re: AC temperature sensor - how far is too far?

If you are correct that the return air sensor is connected by wiring, of course you can move them.

Control wiring can run 4-20 milliamps, 12V, line voltage (120V), etc. Determine the voltage used, should be in the Owner's Manual.

Select an appropriate location: no corners, no exterior surfaces, not in the sunshine, etc. For a boat, 60" above finished floor might be a good choice.

Observe proper industry standards and procedures, and extend appropriate wire in a workmanlike manner. If the sensing bulb will be exposed, I would suggest using in a vented enclosure such as are used for thermostats in public spaces.

Note that this may not be as accurate as you would wish, depending on actual installation, though it should be an improvement on the existing situation you have described.

Consult factory prior to proceeding.

You may also have a choice for fan operation, typically Fan-Auto-Off. Check that manual.
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Old 12-12-2015, 15:57   #5
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Re: AC temperature sensor - how far is too far?

In all the AC units I have seen the fan runs whenever the system is on. This is necessary to circulate air into the evaporator so the temp monitoring will work. I think something may be set up wrong on your system if the fans do not run whenever the system is set to either heat or cool mode. Check with the system documentation to see why the fans don't run. Maybe the previous owner messed with the system thinking they should work like home units. They don't work like home units.
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Old 13-12-2015, 00:09   #6
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Re: AC temperature sensor - how far is too far?

I believe you are correct in that routine way of rigging an AC blower is to leave in on always. I do not why the most manufacturers do it this way (except perhaps because of where they are placing the temperature sensor on self contained units). However if you have ever been on a cold boat using the reverse cycle for heat, then you will quickly wish that the blower stopped when compressor stopped. The cold air being blown out is not welcomed. Using the cooling cycle of course, is a very different story and a constant circulation of the air is normally welcomed.
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Old 13-12-2015, 06:13   #7
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Re: AC temperature sensor - how far is too far?

I agree that when in heat mode it would be nice if the blower stopped. Many units have an automatic fan speed control. This helps because it slows down the air flow when the unit is not running the compressor.

The reason they are designed this way is because any other design would likely result in poor temperature control. In your home the thermostat location is not chosen at random either. It is placed near the return air duct in most instances for the same reason. But even in home units the blower runs for a while after the thermostat stops the compressor. This is necessary to prevent icing.
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Old 13-12-2015, 06:31   #8
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Re: AC temperature sensor - how far is too far?

My Dometic a/c unit is programmable to run fan when heat or cool cycle kicks on, or run continuously. It's about a seven year old unit, 8k btu, so nothing special. Would be surprised if others did not have same option.
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Old 13-12-2015, 13:47   #9
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Re: AC temperature sensor - how far is too far?

The a/c units are functioning on internal pressures. The airflows are effectively there to transfer heat and thus achieve the pressures.
You say you use them for heating, does that mean NOT for cooling? Are they reversed so that they only work one way, or are they true Reverse Cycle units?
To get precise, the Evaporator transfers temperature into the refrigerant, so it does the cooling - the Condenser emits heat, to cool the refrigerant. On a reverse cycle unit, we have a shuttle valve to reverse the functions of the two fixed components (evap. and cond.)

So when you describe a sensor and its relationship to the evaporator, I need to check the terminology - please.
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Old 13-12-2015, 13:58   #10
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Re: AC temperature sensor - how far is too far?

"To get precise, the Evaporator transfers temperature into the refrigerant,"

Ouch! That hurt my brain.

Temperature <> Heat.

Try: the refrigerant in the evaporator absorbs heat.
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Old 13-12-2015, 14:19   #11
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Re: AC temperature sensor - how far is too far?

Quote:
Originally Posted by StuM View Post
"To get precise, the Evaporator transfers temperature into the refrigerant,"

Ouch! That hurt my brain.

Temperature <> Heat.

Try: the refrigerant in the evaporator absorbs heat.
Apologies for the brain pain, yes, whilst the refrigerant absorbs heat, the heat transfer device it is in, the evaporator actually forms the ...........
What the heck - getting way too nerdish here. You get the meaning anyway!

The end result is to ensure we do not relocate something for 1 function and create an issue in another mode.

To consider if relocating the sensor is going to be tricky, the type of sensor and the detail of the control circuit gets important. If it is 0-10V, or similar, then a cable length adjustment would probably be OK, but if 4-20mA it will be more sensitive to the resistance, however this may just be a set point adjustment.
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Old 15-12-2015, 06:12   #12
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Re: AC temperature sensor - how far is too far?

Who manufactured the A/C units? I would contact them. I think Dometic offered an extension to move the temp sensor to another location.
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Old 15-12-2015, 06:29   #13
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Re: AC temperature sensor - how far is too far?

To get back to the question
Some units do in fact shut down the fan when the temp set point is reached, just like the one in your house does, in fact some units even have the temp sensor in the thermostat, just like your house does.
Most units though, usually the less expensive ones run just like a window unit AC, that is fan on all the time, temp sensor right in front of the evaporator sensing return air temp.
You can relocate the temp sense bulb, yes it may in fact change the resistance of the circuit, but if your careful it will still be within the limits of what the thermostat can handle. In other words if it's a digital unit, your temp number displayed my be off a couple of degrees, or if it's just a knob, the knob will be in a slightly different position.

But as much as possible try to position the sensor in the return airflow, if it's not in moving air, you can get some serious temperature swings before the sensor can detect temp fluctuations.

I was going to pop for a King AC unit as it can do all the above, as well as turn the fan down to slower speeds as you neared the set point and I believe even allow different fan operations like fan on for AC, but off when set point is reached in heat etc., but decided to go cheap as my plan is to not use the AC very often.
But depending on how bad you want advanced functions, you may consider a new more advanced unit.
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Old 15-12-2015, 07:24   #14
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Re: AC temperature sensor - how far is too far?

The units are Webasto FCF 16K BTU. I have moved both sensors so they read inside each hull. One could be moved without extending it. The second one needed to be extended about it original length (6ft). Temperature readings are about the same that an infrared temp meter shows. My main problem now is that the port hull AC has a larger temp spread (about 6 degrees) than feels comfortable between cycles. In other words, the temp drops to 68 or 67 before in turns on and the cuts off at 74.

I've called Webasto to see if there is an adjustment but so far have not heard back from them.
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Old 15-12-2015, 07:53   #15
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Re: AC temperature sensor - how far is too far?

You and I have the exact same unit. I located my temp sensor in both my Webasto's in the return air grill, that seemed to give a more accurate room temp reading, the temp sense wire is 6 or 8 ft long?
My other unit is 5K, but same exact thermostat etc.

If possible see if you can duct return air to the unit from the room it is cooling / heating, even if you can only get some of the return air ducted from that room, then you could put the sensor in that duct, even a four inch hose like the supply hose ought to work.
BTW, the inrush current on the Webasto 16K is pretty high, but can be easily cut in half with an inexpensive, simple to add hard start kit.

Really good thing about these units though is they put out an enormous amount of heat, far more heat than my old 16K unit did, and the air output temp is HOT, which is sort of unusual for a heat pump, it appears to be a very good heater.


You know even putting a small fan to blow on the temp sensor might work, I believe your temp swings are not due to the inaccuracy of the sensor, but it's a position problem.
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