This is part of the reply I got from my dealer after they worked on the tractor yesterday:
Bobs tractor. Air temp 25. barn cold, within 5 minutes had 80F with heater valve and fan on high, with brake on, load match on, using forward pedal to simulate load intermittently, ran for 17 minutes, WOT 110 at full vent and fan, after 3 to 4 minutes single fan, ½ vent went 102F, idle, full vent, high setting with to 98F.
Blocked off air cleaner with tape, tape did not suck in which told us pressure in roof was compromised. Lifted roof, found both rear corners sealant had broke outside the air cavity box 1” to 1 ½ “ apart. Obviously the sealant applied in factory shrunk, retracted or just plain applied to tight. With the openings outside the air cavity box, you are sucking in cold air and mixing with your heater air and blowing into cab. We went to Gilroys, purchased 30’ of 1/2x1/2 weather strip. Pulled broken ends back together, weatherstripped on top of existing strip, also installed pieces behind broken sections. Installed the roof. By this time tractor had cooled for about 45 minutes. Ran for 15 minutes up and down drive, we got 115F then put load match and under load as best as we could, temp immediately jumped to 120F.
Bob, I talked to Deere, they are now recommending that sealant is replaced each time cab is installed. I concur as long as not stretched to tight, I will want to reseal yours this summer, I want the cab warm when we do it. Also, you have to get the tractor under load to make temp, suggestion run the tractor at ¾ throttle with heater valve off so there is no circulation thru the heater core for 10 to 15 minutes. Then go do your thing.
The water goes from block up left side of cab to the roof to the shutter, blocking the flow ( knob to thin red ) allows engine block to warm the water quicker, once you open the shutter, flow occurs goes thru heater core down right side of cab and back into block. This explains why you have some heat so quickly.
Bob, you definitely had a compromised roof, it is good for now, but I reseal when temp is better. Deere may come out with an improvement. I am ordering sealant for my 3520 in the shop, this will allow me to see how it comes from factory. Temp is wonderful at 120F. When I ran the 3720 of mine and had temp at 100 to 110, I was more than comfortable and would not want anymore, 120F was cooking me.
I plan to get up to my place sometime in the next couple of weeks to see how much of an improvement this is. It looks to me that the design of the system is such that unless some kind of load is put to the engine the best I can hope for is 80 -90 degree output air temp so if I'm doing work where the engine will idle down for long periods the engine temp will drop also.
Although I would like the tractor to be as responsive as my truck I have to keep in mind that the truck has the advantage of a temperature controlled cooling fan thus it would be able to maintain engine temp at idle by simply shutting the fan down.