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Old 07-06-2022, 06:50 PM   #1
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AC needs a time delay relay on fan circutry

The Coachman 187rb equipt. with an oversized Dometic 13.5k ac is WAY OVERSIZED for the square footage. I want to install a time delay relay for the fan circuit to delay the fan to shut off for appx 45 seconds after the T-sat is satisfied.

As the control is wired now the fan runs continuously when the unit is switched to AC mode. Not only is there a wide fluctuation in temperature but also humidity when outdoor dew point is above 63 degrees f. That plus 73 decibels of fan noise has got to go during the compressor off cycle. When I shut the fan off the decibels are in the 30db range. That's music to my ears.

Specifically what I am in need of is a part manufacturer name & part number. My former career was an HVAC service tech, don't need technical advice as to how to wire it in, that is way to easy, the biggest challenge is getting on & off the roof.

TIA,
Richard in Milwaukee
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Old 07-06-2022, 07:02 PM   #2
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what t-stat do you have? there is no fan setting for On or Auto mode?
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Old 07-06-2022, 07:19 PM   #3
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fan voltage and thermostat wire voltage

you should be able to get a timer from a electrical supply store like grainger
talk to their sales peoples.



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Old 07-06-2022, 09:26 PM   #4
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Built in controls

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Originally Posted by jbflag21ds View Post
what t-stat do you have? there is no fan setting for On or Auto mode?
Similar to what is used on window ACs. Built in T-stat knob & and mode selection knob. Fan operates continuously whether set on low, medium, or high speed.... of course the compressor short cycles due to oversized capacity.....

I'm sure that if all the windows were fully opened & the outer door open it would still short cycle.... I'll I to conduct an experiment next time I have the AC running....

Richard in Milwaukee
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Old 07-06-2022, 09:28 PM   #5
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Looks like this

https://www.amazon.com/Dometic-Air-Conditioners-3314853-000-Control/dp/B07KL4BQVD/ref=asc_df_B07KL4BQVD?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80676783691789&hvnetw=s&hvq mt=e&hvbmt=be&hvdev=m&hvlocint=&hvlocphy=&hvtargid =pla-4584276308956753&psc=1
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Old 07-06-2022, 09:59 PM   #6
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This has been reported to work..
https://www.grainger.com/product/ICM...rchQuery=4E233
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Old 07-07-2022, 06:27 AM   #7
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You may be misunderstanding what is going on. Unlike a home, not all RV thermostats have a temperature sensor in the thermostat for the air conditioner. Our Atwood's, made by Dometic, do not. The temperature sensor in the thermostat is solely for use by the propane furnace.

The room air temperature sensor is stuck to the front of the evaporator coil in the roof unit. That's why our fan runs continuously as well; to draw room air across the room air temperature sensor. If that air flow is not present then the roof unit heats up from the sun and the A/C would come on more often and not in response to the room air temp changes.

If you put a time delay in to shut off the continuously-running fan, what will turn the fan back on when cooling is needed? That sounds like a solution that will work exactly once unless I'm misunderstanding what you're doing.

You did write that the fan runs continuously on Cool so I'm guessing you're flipping the breaker off to kill the roof unit as a test or switching the roof unit completely off via the thermostat?

Ray
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Old 07-07-2022, 06:44 AM   #8
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Quote:
Originally Posted by Richy R View Post
Similar to what is used on window ACs. Built in T-stat knob & and mode selection knob. Fan operates continuously whether set on low, medium, or high speed.... of course the compressor short cycles due to oversized capacity.....

I'm sure that if all the windows were fully opened & the outer door open it would still short cycle.... I'll I to conduct an experiment next time I have the AC running....

Richard in Milwaukee

What Richard is saying is this AC has the switches built into it, you need to reach up to the roof to control the AC, it probably has 2 knobs, one is on-off and fan speed and the other is Temp. I had this problem in a truck camper. As soon as the compressor would turn off the fan would blow all that moisture into the camper and the humidity would climb as fast as the temp.

I recall I put a thermostat somewhere in the coach where I could run the wires t near the AC breaker. I used a relay with 12V coil. The thermostat would turn the relay on and off by 12V and the relay would turn the 120V to the AC on and off, than you still had the fan speeds on the roof controls. Just turn the Thermostat on the roof all the way down and control it with the remote thermostat.

This also got rid if the 5 deg temp swing between on and off.
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Old 07-07-2022, 06:51 AM   #9
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Ahh, thank you, TowPro. I did misunderstand.

When he wrote "Similar to..." I thought that was what he wanted to change to.

Ray
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Old 07-07-2022, 07:23 AM   #10
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That's another method

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I recall I put a thermostat somewhere in the coach where I could run the wires t near the AC breaker. I used a relay with 12V coil. The thermostat would turn the relay on and off by 12V and the relay would turn the 120V to the AC on and off, than you still had the fan speeds on the roof controls. Just turn the Thermostat on the roof all the way down and control it with the remote thermostat like you did.

This also got rid if the 5 deg temp swing between on and off.
I thought about putting in a remote T-stat, however because of the large volume of air being moved in such small cubic footage (appx 800 cubic feet, 126 square feet, typically for a home the rule of thumb in this area is 1 ton per 600 to 800 square feet) the T-stat would false sense no matter where it is mounted, though I just might go with a remote stat.


By the numbers a 1 ton evaporator fan moves 400 CFM per ton. The 13.5 k AC is WAY OVERSIZED for the application, turning the volume of air over every two minutes. With the fan running my DW does not need to use her hair drier :-), the stoves burner flame gets blown out when set to a simmer setting, and yes the temperature swings are aggravating. I'm sure some would love to have a predicament like the one I'm dealing with.

I can't locate a time delay relay with a 120 volt control voltage for a reasonable price. A typical heat sequencer is 24 volt control selling for appx $12 to $18. The prices I'm getting for a 120 volt sequencer are $60 to $85.

Richard in Milwaukee
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Old 07-07-2022, 07:45 AM   #11
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my truck camper was 10' long, Plus the bunk space over the cab. it had a 9200 BTU: it was enough for 90% of the time, but you get out in the middle of a field in a hot summer day and you realize that if you did a Manual J calculation on an RV its no where near as efficient as a home


Plus the 9200 (Colman mac 8 polar cub) was louder than the 13.5 K on the outside because it used a fan for the condenser that was like a small electric radiator fan, and it made lots of noise.
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Old 07-07-2022, 09:10 AM   #12
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Quote:
Originally Posted by Richy R View Post
I thought about putting in a remote T-stat, however because of the large volume of air being moved in such small cubic footage (appx 800 cubic feet, 126 square feet, typically for a home the rule of thumb in this area is 1 ton per 600 to 800 square feet) the T-stat would false sense no matter where it is mounted, though I just might go with a remote stat.


By the numbers a 1 ton evaporator fan moves 400 CFM per ton. The 13.5 k AC is WAY OVERSIZED for the application, turning the volume of air over every two minutes. With the fan running my DW does not need to use her hair drier :-), the stoves burner flame gets blown out when set to a simmer setting, and yes the temperature swings are aggravating. I'm sure some would love to have a predicament like the one I'm dealing with.

I can't locate a time delay relay with a 120 volt control voltage for a reasonable price. A typical heat sequencer is 24 volt control selling for appx $12 to $18. The prices I'm getting for a 120 volt sequencer are $60 to $85.

Richard in Milwaukee
Of course you realize the fan is also the outside fan for the condenser. One motor two fan blades.
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Old 07-07-2022, 09:36 AM   #13
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As you live in Milwaukee Wisconsin that big Clock Tower Company down the road makes timer relays....

I found this reasonably priced one on Amazon. I am not an AC expert so trusting you understand the cooling/AC application. This is an Off Delay Relay like you were requesting in your post.

https://www.amazon.com/dp/B07CG6Z6L7...xpY2s9dHJ1ZQ==
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Old 07-07-2022, 10:28 AM   #14
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https://www.amazon.com/Delay-Timer-R...41649728&psc=1
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Old 07-07-2022, 10:37 AM   #15
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@Larry-NC:
I saw that relay you posted also, but the OP said he needs an off delay function. I don't believe your posted relay provices that.
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Old 07-07-2022, 05:44 PM   #16
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Quote:
Originally Posted by aircommuter View Post
Of course you realize the fan is also the outside fan for the condenser. One motor two fan blades.

this is not true for all RV air conditioners. The Colman polar cub series have 2 separate fans.
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Old 07-07-2022, 05:45 PM   #17
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I "think" one of the problems with disconnecting power to turn AC on and off is the startup capacitor discharges so it draws more amps to startup?
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Old 07-07-2022, 06:10 PM   #18
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this is not true for all RV air conditioners. The Colman polar cub series have 2 separate fans.
I thought he had a Dometic?
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Old 07-07-2022, 07:14 PM   #19
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Huh?

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I "think" one of the problems with disconnecting power to turn AC on and off is the startup capacitor discharges so it draws more amps to startup?
Huh? First of all the capacitor is charged and discharged 60 times a second--it's AC you know--so it's going to charge every time it starts no matter how it's regulated. About the only trick you can play with startup current is to add some (short) delay between the compressor motor and fan motor so the fan doesn't start until the compressor surge has passed. We're talking a couple of seconds here. Companies can charge an outrageous fee for a time delay relay for this application simply because they also provide a custom harness or wiring diagram for each individual model.
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Old 07-07-2022, 07:19 PM   #20
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I disagree

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@Larry-NC:
I saw that relay you posted also, but the OP said he needs an off delay function. I don't believe your posted relay provices that.
@RV Bicyclist.

Go back to the link I sent and hover your mouse over each of the images. Pay attention to where the captions say "Delay Off."
https://www.amazon.com/Delay-Timer-R...41649728&psc=1 ,
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