Heat Pump and electric supplemental heat non-coincident loads?

[DimlyLit]

Are a heat pump and a supplemental electric resistance heat source non-coincedent loads for an HVAC installation? Does the heat pump remain on at low temperatures where it is not efficient when it calls for heat from the supplemental resistance coil source, or does it shut off and allow the supplemental heat to be the full supply until the temperature rises for the heat pump to kick back in? I am wondering as I am sizing a feeder to the two units, controlled off the same stat and wonder if I can just calculate the larger load at 125% rather than both loads.

 

[ptonsparky]

My heart pump is water source and coincidental with the back up strips. An air to air may be different. Ask your HVAC provider.

 

[LarryFine]

They come both ways, so yes, you must ask. I believe emergency locks out the compressor.

 

[GoldDigger]

In addition to circumstances where the heat pump becomes ineffective, many heat pump systems will also kick in the electric heat when the differential between the set point and the inside temperature exceeds a built in value. The pump continues to run.
This is one of the reasons that steep night time setbacks are discouraged with heat pumps. Nighttime energy savings are lost in the electric heat when the temperature set point suddenly rises in the morning.

 

[DimlyLit]

Thank you. That's very helpful.

 

[ptonsparky]

My heat pump ran 274 minutes continuous some time since the 25th. In the week prior when it was 0 and below I'm sure it ran closer to 18 hours a day with the aux cycling as needed.

 

[MD Automation]

In addition to circumstances where the heat pump becomes ineffective, many heat pump systems will also kick in the electric heat when the differential between the set point and the inside temperature exceeds a built in value.

Yes, this value is typically 3 degrees.

In addition to situations where the heat pump output is "falling behind", the other common situation for aux heating to engage is when the heat pump goes into defrost mode. When outdoor temps are near freezing and the outdoor coil sensors indicate there is icing, the heat pump goes into defrost (reversing valve engages and the system runs "backwards", like it was in AC mode) to melt any ice buildup. If the aux heat did not engage during defrost, people inside would complain about the (sudden) cold drop in air temps from the registers. The heat pump is running (briefly) as an air conditioner like it was summer-time.

Defrost mode won't last long, a few minutes typically, but thought to point it out.

 

[jaggedben]

Are a heat pump and a supplemental electric resistance heat source non-coincedent loads for an HVAC installation?

No, I've seen them both on at the same time. Just set the thermostat different enough from the room temp.

Perhaps there are exceptions but I'd say for NEC purposes you'd need some pretty solid documentation from the manufacturer.

 

[Little Bill]

My heart pump is water source and coincidental with the back up strips. An air to air may be different. Ask your HVAC provider.

I would hope it worked on blood source!
Man, those HVAC guys work on hearts to?

 

[ptonsparky]

I would hope it worked on blood source!
Man, those HVAC guys work on hearts to?

ahhh jeez...

 

[ptonsparky]

Yes, this value is typically 3 degrees.

In addition to situations where the heat pump output is "falling behind", the other common situation for aux heating to engage is when the heat pump goes into defrost mode. When outdoor temps are near freezing and the outdoor coil sensors indicate there is icing, the heat pump goes into defrost (reversing valve engages and the system runs "backwards", like it was in AC mode) to melt any ice buildup. If the aux heat did not engage during defrost, people inside would complain about the (sudden) cold drop in air temps from the registers. The heat pump is running (briefly) as an air conditioner like it was summer-time.

Defrost mode won't last long, a few minutes typically, but thought to point it out.

MDSW wants cold at nights. My set back has been to 64 degrees from 69. I changed it yesterday.
I happen to graph my water pressures, cycle time, and reason for well operation. Pressure, Water Source Heat Pump, or Lawn. That has been updated recently.
This morning I notice that apparently only the first of two stages of water for HP came on throughout the night. First time in 12 years I've seen or had evidence of that happening. (I didn't need to bury my head under the covers to keep warm either.)

I don't expect significant changes to cost of operation, but Thank you.

 

[MD Automation]

This morning I notice that apparently only the first of two stages of water for HP came on throughout the night.

Interesting. So I assume you changed your set back to be a little narrower than 69 -> 64. And that kept your water for HP operation out of "full beans" mode during the night?

The 3 degree number I posted is what I recalled from memory (from way before digital thermostats), then I saw your post and looked up more info to see if I was right. I think that my number is close, I saw places that mentioned 4 degrees. And some new digital devices can, naturally, make that delta anything they want.

In the old days, it was done with a second mercury bulb that sat on the same bi-metallic coil as the primary bulb switch - just set at a slightly different tilt angle. The setback delta was not crazy accurate, nor was the temp itself. It depended on how level you could mount the base. Maybe not accurate, but it was consistent. And that was good enough back then.

 

[ptonsparky]

In all but the coldest nights I don't think the house ever got below 64. It may have cycled on only the first stage but I was never aware of it, that I can remember The updated graphing readily shows first and second stage for water flow.

 

[Dennis Alwon]

When heat pumps were fairly new the aux. heat would come on when the temp got below 30° or so but as the system improved the heat pumps got more efficient so they now wire them that both the strip heat and the heat pump can run together. It can easily be wired, thru the low voltage, so that they run independently.

 

[mbednarik]

I would not size it that way, its a configurable setting in the thermostat. You can have them interlocked, but you don't have to. They can be run simultaneously. Another thing to keep in mind is the resistance heat will the cycled on when the HP defrosts, so both would run for a short period in that instance.