fan time delays

[cg2]

Am I risking any major damage by eliminating the fan time delay on walk-in freezers? I've done it several times over the years and never noticed a problem, but then I wonder why did they put them on if they aren't needed? Then yesterday, while watching a box pull down from ambient, I was kind of annoyed by the switch turning off the fan several times while Im waiting for the box to pull down. Just wondering.

[t527ed]

biggest problem would be blowing warm moist air into box after defrost. could get messy

[NormChris]

It is not a fan time delay. It is a fan thermostat which will not allow the fan to operate until the coil is down to the cut-in temperature of the thermostat.

It serves two functions. It prevents the fan from moving the air over the coil when the coil is still warm from the defrost cycle. And, it allows any moisture left on the coil to freeze so it does not get blown off the coil and accumulate on the box ceiling, walls, floor or product.

Many of the fan delay thermostats are of the three-wire type. The three-wire thermostats have a common wire and two control wires. One wire is the fan delay and the other is the defrost termination circuit. If you disable a three-wire thermostat you are also removing the temperature defrost termination and changing the system to a straight time defrost system. That is not a good idea as it creates two new potential problems.

Norm

[cg2]

OK. That makes sense, I've also eliminated the defrost termination, figuring the other thermostat will shut off the elements if needed and the box will just be off a little longer than is needed. The same thing if the x terrminal on the time clock doesn't work. I have some customers that just don't have a few extra hundred dollars right then and I figure I'll take care of it some other time. Of course that time sometimes never comes.

[NormChris]

Eliminating the temperature termination increases operating cost and decreases product life. The extra cost to fix the temperature termination operation is well worth the savings from having it operate correctly.

When you eliminate the temperature termination, the system remains in defrost longer than necessary. That causes several concerns.

1. The added cost of operating the defrost heaters when they are not needed.

2. The extra heat added to the inside of the box that must be removed by the compressor increases the operating cost of the system. Longer compressor running time at a higher load when recovering from a defrost cycle.

3. The product in the box suffers from the increase in temperature because the box remains at a higher temp for a longer period of time. And, it was not necessary to get that hot to defrost the evaporator properly. Therefore, the product quality suffers plus the shelf life of the product is reduced.

Norm

[cg2]

The only problem is there's a difference between todays dollars and dollars saved a little bit at a time down the road. I guess the bottom line is I ought to carry one on my truck, by the time I diagnose the problem, I could change it in five more minutes. But if I have to go get one, it adds an hour and half to the job. That's the problem with doing too many different things, Hvac/R etc. Is there one temperature range that is most common?

[NormChris]

Originally posted by cg2
The only problem is there's a difference between todays dollars and dollars saved a little bit at a time down the road. I guess the bottom line is I ought to carry one on my truck, by the time I diagnose the problem, I could change it in five more minutes. But if I have to go get one, it adds an hour and half to the job. That's the problem with doing too many different things, Hvac/R etc. Is there one temperature range that is most common?

The money saved more than offsets the cost of doing it the right way. One or two months of operation with the defrost termination and fan delay will save enough energy to easily pay for the replacement of the thermostat.

I believe there is a standard replacement fan delay, termination thermostat replacement. A universal replacement. All the thermostat does is terminate the defrost cycle when the coil gets above 35 degrees or so and later it turn the fan on after the coil temp achieves a low enough temperature to allow the fan to come on without blowing water off the coil or blowing warm air over the product.

Your parts wholesaler will have one or find one in the catalog. Carry one in your truck stock if you service walk in boxes on a regular basis. They are not expensive.

[cg2]

OK. I got one today. That pretty much guarantees that I wont see a bad one again, because the only parts I keep on my truck are ones I never use.