Selecting 30, 60, 90 defrost timer.

[georgelass]

Is the recommended setting in the install manual or what?
I'm not understanding why there should be options since
the temp sensor doesn't have an option (and there is no
humidity sensor).

I'm not having any particular issues at work. I'm merely
reading up on heat pumps and I thought maybe an explanation
of the need for these various timer options would provide
insight into an icing problem were I to encounter one.

Thank you. (Expect another heat pump question in the
next few days. I'm still reading.)

[Rob_in_WV]

Units located in shady areas will require more frequent defrost intervals. By setting the board timer from 90 to 60 it means that after defrost stat closes it won't have to run for an extended amount of time before a defrost.

[SgtBone]

I suppose it would depend on where you live. Units in Fl may only need 90 while units in the north need 30 min intervals.

[Snapperhead]

On my Own unit , I can set it 60 minutes during mild winters , but if it snows alot , or freezing rain , I will set to 30 min , otherwise it will frost up pretty good if I dont

[georgelass]

So if you set it for 30 minutes in mild winters, couldn't we assume that the
thing wouldn't go into defrost often, and were it to go into defrost, it would
be brief? What would the downside be of setting it to 30 minutes in mild
winters?

[Kon99lbi]

To best answer this question, I think its better to understand the defrost cycle on heat pumps. In a timed-temperature defrost, both timed (defrost timer motor) and temperature (temperature sensor) conditions must be met in order to initiate defrost. All the temp sensor does is close when the outdoor coil has reached a temperature below freezing. In environments with erratic temperature swings, the coil temp could vary in and out of the sensors threshold. Defrost cycle on heat pumps is inefficient because it initiates cooling and tempers the supply air by energizing the auxiliary heat. This is why another condition must be met in order to ensure the defrost cycle is needed. The timer motor does a brief trial defrost every 30, 60, or 90 minutes. In colder areas of the country, your unit might require more frequent defrost cycles, thus the timed portion of defrost is adjustable to occur more often and visa versa in warmer parts of the country. Hope this helps

[Kevin O'Neill]

It depends on how humid your climate is. In dry climates, you will not need defrost very often. In humid climates, the moisture in the air condenses on the outdoor coil during heating mode. If the outdoor coil temperature is below 32 degrees, it turns to frost. In those humid climates, there is more moisture in the air, so defrost will be needed more often.

[Snapperhead]

So if you set it for 30 minutes in mild winters, couldn't we assume that the
thing wouldn't go into defrost often, and were it to go into defrost, it would
be brief? What would the downside be of setting it to 30 minutes in mild
winters?
if you have it set at 30 , it will defrost every 30 min ( as long as its cold enough )

How long it stays in defrost is determined by the sensor on the coil. When it senses the coil is clear it will turn off defrost.

[air1]

if you have it set at 30 , it will defrost every 30 min ( as long as its cold enough )

How long it stays in defrost is determined by the sensor on the coil. When it senses the coil is clear it will turn off defrost.
It will only defrost if the defrost thermostat is closed.
90 minute setting should be used whenever possible to avoid unnecessary defrost cycles. You should only increase defrost frequency if you are experiencing problems with ice build up on the coil. After a while you should get a sense of what the optimal setting is for your area or for special circumstances like equipment located in cold damp environments.

[Kevin Weaver]

I have always chosen 60, in central Indiana. And that's 60 RUN minutes.

I have always said that if you can get a heat pump to defrost, you won't get called back, even if its not running exactly perfect.

[fliks]

Here in central texas I use 90.

[ADillon]

The correct setting will vary from application to application, area to area, and year to year (or even month to month depending on the outdoor conditions) It is usually set as high as possible and then reduced if defrost issues are encountered. Remember that a time temp board initiates on BOTH time and TEMP and so you wouldn't adjust the 'sensor' which is simply a bi-metal switch. Getting these set right has always been a little bit touchy feely but the longer the better as defrost cycles = electric heat = high bills.

[Kevin Weaver]

Thats the reason manufactures have went with DEMAND DEFROST. now the question is, what temperature do you select for termination?

I choose 80 degrees.

[m singer]

The conditions that cause the coil to frost up the fastest, requiring the shortest defrost interval, are misty rain/very hi humidity with outdoor dry bulb temps in the thirties. Any time that the outdoor temp is below 40 the outdoor coil is going to be operating below freezing.

Cold air holds less moisture at any given relative humidity when compared to warmer air. A coil will frost up faster in 30 degree weather with hi humidity then it will at 10 degree weather with the same RH. In East Tennessee I set the timer to 60. 30 is a waste of energy in my area. I used to set the timer tom 90 and very rarely did it cause a problem. Setting at 60 in my area has never caused a problem for me. YMMV

[54regcab]

Thats the reason manufactures have went with DEMAND DEFROST. now the question is, what temperature do you select for termination?

I choose 80 degrees.

Is demand defrost used as part of the HSPF or SEER calculations?

[garyed]

I'm a 90 minute guy & I'm in humid SC. It doesn't get that cold normally but we have a lot of damp 30 & 40 degree weather here.
On a rare occasion I'll set one at 60 but its usually because the customer complains about frost some times & doesn't wait for it to defrost off. If you set them at 30 you'll probably have more complaints about cold air blowing before the strips kick in during defrost.
I think heat pumps should not have any sequencers on their heat strips so they can come on immediately to offset that initial blast of cold air during defrost.

[m singer]

Is demand defrost used as part of the HSPF or SEER calculations?

The type of defrost control is not a factor at all in SEER. HSPF does not account for strip heat use in defrost and assumes that there will be no defrost below 17 degrees. Demand defrost has has performance advantages over time and temp that will not be fully appreciated in HSPF.

[larnot]

Short (I hope) essay from a (tired)grunt in the field...
There are 2 main types of defrost control. Time and Temp, and Demand.
Time and Temp uses a timer that says 'we've been running for 30/60/90 minutes, is our sensor (that can be anywhere from 28 to 50 degrees) closed?' If the answer is yes, then let's defrost. If we are not defrosting, or cannot force test a defrost, check one of us.
Demand is a combination of things set into an algorithm designed by the manufacturer. It senses the outdoor coil temp and the outdoor air temp and tries to keep defrost cycles to a minimum as these call for electric heat to come on to temper the cold discharge air.
You need to be able to check these sensors/thermocouples for the correct resistance.
Of course, then you have manufacturers (like Tempstar for one), who decide that the original design is not good enough, and tell you to change demand board and sensor to their 'new' time and temp system.
Good luck, hope we can answer your questions in the future.

[54regcab]

I'm a 90 minute guy & I'm in humid SC. It doesn't get that cold normally but we have a lot of damp 30 & 40 degree weather here.
On a rare occasion I'll set one at 60 but its usually because the customer complains about frost some times & doesn't wait for it to defrost off. If you set them at 30 you'll probably have more complaints about cold air blowing before the strips kick in during defrost.
I think heat pumps should not have any sequencers on their heat strips so they can come on immediately to offset that initial blast of cold air during defrost.

+1. I don't see why they just don't use contactors. Sure they may be noiser, but it shouldn't be a problem for most installations. Contactors could use time delays so they don't all kick on at the same time.

[54regcab]

The type of defrost control is not a factor at all in SEER. HSPF does not account for strip heat use in defrost and assumes that there will be no defrost below 17 degrees. Demand defrost has has performance advantages over time and temp that will not be fully appreciated in HSPF.

What is bad is the typical consumer doesn't have any idea about this and demand defrost wont show the real world advantage in the HSPF.

[beenthere]

What is bad is the typical consumer doesn't have any idea about this and demand defrost wont show the real world advantage in the HSPF.

Defrost cycles are taken into consideration in HSPF. And on demand defrost is how some manufacturers get a high HSPF, with low BTU output.

[Kevin O'Neill]

Defrost cycles are taken into consideration in HSPF. And on demand defrost is how some manufacturers get a high HSPF, with low BTU output.

That is what I understood also.