Electric Resistance Heating in HVAC units.

[Ace18]

After thinking about it im pretty sure heat pump defrost are pretty much all the way I described in previous post.

The heat pump indoor coil on almost all air handlers that use electric heat strips is ahead of the heat strip in the air stream. So turning on the heat strip would be pointless as far as defrosting is concerned. I guess that it would still be putting heat into the heated space if the heat strip were on but the defrost cycle is short enough that it is still pointless.

The outdoor coil is the one one that needs defrosting. The electric heat comes one during defrost to offset the cold air from the defrost cycle.

 

[Dennis Alwon]

You not only have 20 kw x 125% you also must add in the blower.
I sure hope this is not a house. Ouch what a bill

I have hooked up a few 20 kw units in my years but lately they are unheard because of the insulation and tightness of the houses we do.

A 20 kw furnace generally doesn't have 20 kw heat strips but rather 19. XX kw strips. I think with the load of the motor you would have 20kw.

 

[augie47]

I did some digging and found an installation manual for this unit. It lists FLA for all size electric heaters. 20kW has a FLA of 83.3. So, 83.3 x 125%=104
using the 75? column I would need a #2 wire. Am I doing it right?

Now for page 2 Your 104 is the correct figure to work with FOR THE HEAT, As for wire size, depends on what type wire. THHN with conduit you could use a #2. NM or SE (under the 08 Code) you would need a #1 Cu or a 2/0 AL.
Also don't forget your blower motor (for an Air Handler) and/or compressor (for a package unit-- unless you can assure the heat & compressor can't run simultaneously).
And 424.22(B) still comes into play so somewhere, either factory installed, or electrician installed, the OCP for the individual heat (in this case usually two-10kw ) can't exceed 60 amps

Keep in mind too how you want to feed all this. Added together it's obviously a sizable load and, depending on how your loads are distributed in your service panels, you may not want to being it all from one panel.

 

[Mgraw]

Every 20kw unit I have installed call for two 60 amp circuits.

 

[Dennis Alwon]

Every 20kw unit I have installed call for two 60 amp circuits.

I have done a few, about 15 or more years ago, that called for one circuit but the unit had 2- 60 amp breakers mounted inside the unit to comply with NEC rules. The handles for the breakers were accessible from outside the unit.

 

[One-eyed Jack]

After thinking about it im pretty sure heat pump defrost are pretty much all the way I described in previous post.

The heat pump indoor coil on almost all air handlers that use electric heat strips is ahead of the heat strip in the air stream. So turning on the heat strip would be pointless as far as defrosting is concerned. I guess that it would still be putting heat into the heated space if the heat strip were on but the defrost cycle is short enough that it is still pointless.

The heat strip is turned on in defrost to offset the cooling effect of the evaporator during this cycle. If you did not you would have a loss of heat during the defrost cycle. Normally just enough kw to match btu of AC. Typically in this area 5kw is dedicated to defrost cycle only.

 

[Mgraw]

I looked up a couple of different brands and on one circuit electrical they all call for around 110 amp min 125 amp max and #1 wire.

 

[Ace18]

Now for page 2 Your 104 is the correct figure to work with FOR THE HEAT, As for wire size, depends on what type wire. THHN with conduit you could use a #2. NM or SE (under the 08 Code) you would need a #1 Cu or a 2/0 AL.

Where can I read that about the NM?

 

[augie47]

334.80 (you might want to look at 338.10(B0(4) also)

 

[suemarkp]

A heat pump installer that I frequently work with usually installs a lockout thermostat to shut down the heat pump when the outdoor temperature falls below the point where the efficiency is compromised. This is essentially a SPDT thermostat switch that switches from normal heat thermostat lead to backup heat thermostat lead, and the backup heat comes on immediately on call for heat.

As far as I know most heat pumps simply operate the reversing valve during defrost, which causes the unit to go into cooling mode - Heat is transferred from inside to outside just like during air conditiong season. this will remove the frost on the outdoor coil fairly quickly and then the unit resumes normal operation.

This may be a locality thing, but here in Seattle, heat pumps defrost a lot because of all the moisture in the air and it doesn't get below 30F very often. My defrost cycles typically take 8 to 10 minutes if the heat pump has been on high heat. Maybe you're OK with freezing cold air coming out of your heat vents for 10 minutes, but most people are not. When defrosting, the heat pump is in air conditioner mode -- that can do a lot of cooling when that outdoor coil is 30F.

If you're using strip heat as the aux/supplemental heat, it doesn't make sense to turn the compressor off until about 0F to 5F. Even at 0F, most heat pumps still have a Coefficient of Performance of 1.4 or more. So they are making 40% more heat than the equivalent amount of power in a heat strip. Turning off the compressor at the balance point (generally near 30F) is inefficient unless you have gas or oil backup heat.

 

[Mgraw]

Ace18 If you post the model number of the unit I will try to find you the specs.