Selecting 30, 60, 90 defrost timer.

[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.