For recent systems there are two basic defrost schemes: demand defrost and time/temp defrost.
Time/temp is cheaper and most common. The time can usually be set by jumpers or DIP switches. If time is set to 30 min (for example) the coil temp sensor is checked every 30 minutes and a defrost cycle starts if that temp is below some threshold, like 30 degrees, and stops when the coil temp sensor reaches (for example) cutoff at 60 degrees. There is usually a maximum defrost cycle time limit. In the defrost mode the heat pump acts like an AC unit except the aux/emergency heat strips are on and the outside fan is off. This scheme will cause unnecessary (but short) defrost cycles when it's near or below freezing outside. Ice buildup is actually worst when the outside temp is near and just above 32 degrees and it's humid, not when it's very cold Demand defrost uses one of several methods to estimate air flow restriction through the coils. It may measure the pressure drop or try to compute it from refrigerant pressures or temperatures. In theory this avoids unnecessary defrost cycles and insures the coils really are open. I don't have experience with demand defrost systems. -----Original Message----- From: Mercedes On Behalf Of Kaleb Striplin via Mercedes Sent: Monday, February 15, 2021 10:31 AM To: mercedes@okiebenz.com Cc: Kaleb Striplin <ka...@striplin.net> Subject: Re: [MBZ] Heat pump question Well I went out and checked it. When I looked at in there was no frost on the coils that I could see in the center part. I pulled the cover and sure enough it was blinking a low pressure lockout. With the side cover off I could see some frost on the outside of the coils, not a whole lot though. I shorted the test pins to reset it and it kicked back on. I shorted them again to force a defrost. Defrost ran for about 5-10 minutes and once frost was clear it kicked back into heat mode. Since replacing the defrost board with attached sensor last year, I have not had a problem sense. Why the heck did it happen again? I know it has a temp sensor for the coils. Do these work by going into a defrost cycle on a timed basis if the temp sensor is below a certain temp? Or do they sense the coils are freezing up and enter defrost when needed (not on a timed scheduled). From what I recall, some old systems did not have a temp sensor and just went into defrost every so often. I think some systems sense when it needs to be done. Others I think are still timed but only do it when temp is below a certain point. This is a 2008 Rheem running on 410 so I am not sure. If mine runs on a timer only when temp gets low maybe it is so cold that it simply did not reach its run time to defrost soon enough before it started frosting up, or perhaps it is sucking in blowing snow which caused it to frost up faster? On 2/15/2021 9:03 AM, Kaleb Striplin via Mercedes wrote: > It is now -3 outside, not sure how low it got last night. Anyway, heat > pump has been running pretty much non stop for the last few days with > the aux heat kicking on occasionally. Yesterday afternoon aux heat was > on quite a bit as it never got over 4 yesterday. This morning when I > woke up outside unit was running as normal. A little while later I > happen to look out and it is shut down. Normally when it goes into > defrost it will shut down for a moment then fire back up pretty quick. > It did not appear to be in a defrost cycle. I had issues before where > the temp sensor on the coils was evidentially bad because when it got > below freezing it would seemingly randomly never enter defrost cycle > and would frost over, which would then put it in a low pressure > lockout. I would have to jump the pins on the board to reset it and > force a defrost then it would be fine again for a while. Last year I > replaced the defrost board which has the temp sensor hard wired on it. > Never had a problem since. > > Anyway I was about to throw on a coat and go outside and pull the > cover and see if the board was flashing an error code and it started > back up. It is now about an hour later and I notice the outside unit > is not running again. I am not sure how long it has not been running > but I know its probably be at least 10-15 minutes. Is there some sort > of reason at these low temps that the outside unit would shut itself > down for a period of time then fire back up? Does the system > determine that it is so cold outside that the outside unit is not > producing any heat anyway and shuts itself down for a while? I will > wait a little bit longer and see what happens then go out in the > blowing snow to pull the cover and check the board to see if its > blinking any codes. > > > _______________________________________ > http://www.okiebenz.com > > To search list archives http://www.okiebenz.com/archive/ > > To Unsubscribe or change delivery options go to: > http://mail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com > _______________________________________ http://www.okiebenz.com To search list archives http://www.okiebenz.com/archive/ To Unsubscribe or change delivery options go to: http://mail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com _______________________________________ http://www.okiebenz.com To search list archives http://www.okiebenz.com/archive/ To Unsubscribe or change delivery options go to: http://mail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com
