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Originally Posted by VTP99

. I've also seen coils with a tee at the u that can have a mounting spot there.

Exhibit A

On a hot pull down, it's not uncommon to see the fans cycle on and off a few times until the box cools down some.

Originally Posted by VTP99

It was a considerable amount of time but then it was from a 80° box starting point.
Typically I'm servicing a pulled down box so that makes a huge difference.
This was a Heatcraft coil with the switches mounted on the end sheet. I believe the return bend type would respond much quicker. I've also seen coils with a tee at the u that can have a mounting spot there.

Originally Posted by ammoniadog

Sometimes new walk in freezers do take forever before the fans turn on, even when the DFTD starts to get surrounded by frost. If I get tired of waiting I will sometimes grab an upside down CO2 cylinder and blast it with dry ice particles. That usually does the trick and gets it to start working.

We have a tech that does that on self contained.... “fans not coming on had to blast fan delay with CO2”
And when we run the Callback we find an undercharge or evap starved for refrigerant.

Originally Posted by shellkamp

I have had to bang on brand new DTFDs with the handle-end of a screwdriver for it to start the fans before. I knew for a fact they were seeing the correct temperature.

I never had a problem with it after that so I'm thinking that just by sitting in storage for a while maybe caused it to get lazy.



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Sometimes new walk in freezers do take forever before the fans turn on, even when the DFTD starts to get surrounded by frost. If I get tired of waiting I will sometimes grab an upside down CO2 cylinder and blast it with dry ice particles. That usually does the trick and gets it to start working.

I think it was an old Larkin, from when they were Larkin, not a part of Heatcraft, that had a part soldered on to one of the 'u-turns' that was copper and about the same size as the flat mounting flange of the common DTFD. Those worked great!

Originally Posted by VTP99

Thanks for the clarification.
Just this past week I had to start up a walk-in freezer.
The high limit had failed open and was not permitting defrost.
I picked up a new one as well as the DTFD switch.
After starting it up a good half hour had lapsed with the coil well frosted over but the switch not triggering the fans. I had to get going to my next call so I installed the new DTFD to play it safe. It still would not switch over so I moved it over to the power terminal to get them running knowing I'd be back the next day.
Next day box was down to temperature and I repositioned the fan wire where it belonged. All was good so I came to the conclusion that that box just had to pull down before the surface at the switch was down to temp.
I wonder how much flooding would have happened had I just let it rock as was ?

I have had to bang on brand new DTFDs with the handle-end of a screwdriver for it to start the fans before. I knew for a fact they were seeing the correct temperature.

I never had a problem with it after that so I'm thinking that just by sitting in storage for a while maybe caused it to get lazy.



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Originally Posted by MilfordsMrFixIt

VTP99: Understandable. You need to determine the right amount of time. I usually use a thermometer embedded in the coil close to the suction to determine how long it takes to prechill

It was a considerable amount of time but then it was from a 80° box starting point.
Typically I'm servicing a pulled down box so that makes a huge difference.
This was a Heatcraft coil with the switches mounted on the end sheet. I believe the return bend type would respond much quicker. I've also seen coils with a tee at the u that can have a mounting spot there.

VTP99: Understandable. You need to determine the right amount of time. I usually use a thermometer embedded in the coil close to the suction to determine how long it takes to prechill

Gleng: Right, you still need to terminate defrost. The fans can be connected to the compressor output and a delay timer to delay the fans. 5 minutes is the non-adjustable setting on the Grassling timer, but you might need more or less. In a -20 Warehouse, there's 8, 10 hp units, that we control with an Arduino, that we delay the fans by 6:30 after a 35 minute defrost every 8 hours.. Just one unit defrosts at the top of every hour.

Originally Posted by MilfordsMrFixIt

VTP99: Leaving the fan so it starts as soon as defrost ends just blows foggy air around. That leaves moisture in the air to condense on literally everything. Flooding? Nah. I have seen units that have had the 3 wires simply spliced together and the "X" wire disconnected at the timer. Some have been that way for YEARS! Just an annoyance in most cases, but the frost covering everything is often the giveaway.

In my case the box was empty so it didn't matter. But as for the flooding I'd still be concerned. Superheat on most freezers is low to begin with like 4-6 so it wouldn't take long for liquid to pass right through that coil. Especially with the condensing unit right above it.

VTP99: Leaving the fan so it starts as soon as defrost ends just blows foggy air around. That leaves moisture in the air to condense on literally everything. Flooding? Nah. I have seen units that have had the 3 wires simply spliced together and the "X" wire disconnected at the timer. Some have been that way for YEARS! Just an annoyance in most cases, but the frost covering everything is often the giveaway.

good alternative to the fan delay mounted to the coil to bring the fan on.i assume you still use a sensor going to X to take defrost out, once the coil has melted the frost.so no fan on thermostat, only a defrost termination stat?Does 5 minutes seem to be a proper time for the coil to frost up and bring the fans back on?

Originally Posted by MilfordsMrFixIt

Well, there are a number of timers that control the fan delay. Usually formatted in 'time initiate and time terminate' mode. There is a terminal "F" for this purpose,.

Thanks for the clarification.
Just this past week I had to start up a walk-in freezer.
The high limit had failed open and was not permitting defrost.
I picked up a new one as well as the DTFD switch.
After starting it up a good half hour had lapsed with the coil well frosted over but the switch not triggering the fans. I had to get going to my next call so I installed the new DTFD to play it safe. It still would not switch over so I moved it over to the power terminal to get them running knowing I'd be back the next day.
Next day box was down to temperature and I repositioned the fan wire where it belonged. All was good so I came to the conclusion that that box just had to pull down before the surface at the switch was down to temp.
I wonder how much flooding would have happened had I just let it rock as was ?

True!!!Frost allover the products.i like the defrost sensors in between the fins,They work much better,If they are mounted wrong, they don't work well.

Originally Posted by MilfordsMrFixIt

You know how it is; the timer itself is actually made by Paragon, and used by a half dozen companies. They just have different boards, as you can see in the pics. They stink. I have had a number of them that just don't click into defrost, the dial needs a little push to the left to get it to go into defrost. Most times it's a random thing where it works ok for a period of time, then messes up... freezes solid...service call. I usually drop a good old time tested 8145 in place of it. A 9145 if I can get the customer to spend a few extra bucks.

I agree. The mechanical timers would last many years. It was quite common the relays on the solid state timers, at one time, would weld shut. A Grasslin timer was good for a year if you were lucky

Not to mention frost all over the products!! Most ARE mounted the the side of the coil. The delay is generally desirable. KolPak units for example, often have it mounted on a bracket in contact with the coil. If not touching just right, the defrost ends up too long and the fan takes forever to come on.

you don't want the fans to come on until the temp of the coil is below 32.The coil should be frosted up when the fans turn on.this prevents moisture from being blown of the coil, and putting frost on the freezer walls, fan blades,and fan screen.Where is the sensor mounted.If mounted on the side of the coil, rather than a bulb into the fins, it takes longer for the fans to come on.

You know how it is; the timer itself is actually made by Paragon, and used by a half dozen companies. They just have different boards, as you can see in the pics. They stink. I have had a number of them that just don't click into defrost, the dial needs a little push to the left to get it to go into defrost. Most times it's a random thing where it works ok for a period of time, then messes up... freezes solid...service call. I usually drop a good old time tested 8145 in place of it. A 9145 if I can get the customer to spend a few extra bucks.

ICM using a Grasslin timer?

What is this world coming to?

To clarify the Timer with fan delay question;

Well, there are a number of timers that control the fan delay. Usually formatted in 'time initiate and time terminate' mode. There is a terminal "F" for this purpose, however most clocks just use it to isolate the fan output to prevent feedback issues.
• This Grasselin control delays the "F" contact by a fixed 5 minutes. We found that out when the fans just would not come on. Of note, there are 5 relays on the board while most have just 2 or 3. Impatience on that one...LOL We rewired it to a standard DTFD as the fans were coming on just a little too soon causing fog in the freezer- alarming the users! One called the fire department reporting a 'Freon™ leak'! So we had to come up with a solution. (note the currently empty "F" terminal)
• The ICM 550 on the other hand just turns the fans on as soon as the defrost terminates. Note there are 3 relays, the small one signals the termination cycle.
• The electronic Paragon 9145 clock (love it) seems to be just a changeover as well. Great timer if you need to keep the times for defrost at exact times of day, or if you need to run short defrosts during the day, IE 15 minutes, and you can schedule a 90 minute at 4 am. The DTFD will still stop the defrost if it finished early.
◘There are a few models out there that delay the fans, few, but some.
▬ We have a customer that had a drive-in frozen warehouse that had 8 separate sets of condensing units and evaporators. that were all controlled by a single timer... and a boat load of relays and contactors! What a cluster---- that was. Initially I Converted them to individual timers. Imagine the inrush when 80 HP of condensing units all started at the same time!!! Their power demand went down enough for the accountant to notice!!
--As of now, I installed an Arduino Uno that's programmed to defrost one at a time for a max of 30 minutes, then hard delay the fans for 6 minutes and 30 seconds with a standard DTFD stat backing it up. So, at the top of each hour, a different unit defrosts, giving a defrost every 8 hours to each unit in succession. This keeps energy usage in line and also the temperatur regulation, which is controlled separately. It's been running with the Uno for 10 years now trouble free, there's a spare at the site pre-programmed if it's needed.