How much does the blower off delay at the end of a cooling cycle add to the humidity? It's been discussed that leaving the fan on all the time while in cooling mode increases humidity dramatically, but what about the 90 second blower off delay typical of many furnaces?

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I've tested 60 sec., it's not a problem. Only Nest thermo. has ability to extend blower delay while monitoring humidity.

Blower delay controlled by humidity can also be done with a $15 humidistat. Just wanted to know if it's worth the trouble to hook it up. Then there is also the question of how much efficiency is gained/lost by the blower delay.

Some manufacturers have a ramp down and in some cases a ramp up fan behavior (variable speed blowers) that run at lower speeds for a few minutes at the end and beginning of the cooling cycle to extract more moisture from the air stream.  This uses the indoor coil as a heat sink after the compressor has shut off.  I don't know if there are any studies that prove this works.

I have this capability on my cooling system and there is a good deal of humidity taken out of the system when using a variable speed blower that runs at a slower speed for the first few minutes of the cooling cycle. I have measured about a 10% decrease in humidity compared to a fixed speed blower. There is also a noticable difference in the comfort in the house overall.


My home also has variable speed blower and a special blower setting to maximize system performance and moisture extraction.  I have attached an extraction from the air handler documentation showing how the blower operation can be setup with the dip switches. My system is set to operate using the ramp up and ramp down.  During the last three minutes the compressor is off and the blower operates at 50%.  I don't have any comparison measures other than the system dehumidifies the house well, and can bring the humidity level down to the low 50% RH range on humid, 95 degree days.

This unit is a TRANE UY120R9V5V gas furnace installed with a 5 ton Trane coil and 4 ton Trane compressor.

I've seen this before, it's pretty cool for humid climates. Does the logarithm change if a humidistat is hooked to the "dehumid" terminal? The issue is the above setup blower speed is reduced regardless of humidity conditions.

The real test is not on 95 degree humid days, but on 75 degree humid days when there is little call for cooling...

Good question. According to the installer documents the humidistat can be wired in and call for cooling operation with the blower operation profile that is setup on the dip switches.

 Newer systems with variable speed compressors or multiple stage compressors are set up with sensors outside to check humidity levels, and will adjust the amount of cooling along with the fan speed to alot for more dehumidification. I recentley installed and set up a new 18 seer bryant system with this application, and the home owner has noticed a difference throughout thier home as being more comfortable,even on low temp cooling days.

On systems without this added feature it does not do such a great job, on the system at my house I tend to turn the temp down to adjust for the difference. I have an older system, and I am going to install a thermostat that senses humididty in the home and allow it to adjust the need for cooling and see how that does.

I did the humidistat trick to control the high/low relay. Works like a charm.



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