Effect of HP Defrost Cycles on Economic Balance Point

[Calum]

I am a home owner with a dual fuel (natural gas and heat pump) system that was installed this fall. Thanks to guidance from this forum I was able to obtain COP data for my heat pump/indoor coil/furnace installation from American Standard and to prepare a spreadsheet that clearly shows me my economic balance point as my electric and gas prices change. With a better electric rate this month, my economic balance point has dropped to 30 degrees from its previous 40 degrees. I have recently set the thermostat to switch over to the furnace at 30 degrees. I find that the heat pump is able to maintain the 68 degree indoor temperature when the outdoor temperature drops into the low 30's and we are still comfortable but I also notice an increase in the number of heat pump defrost cycles. I understand that this is normal functioning for my heat pump which has demand control of defrost cycles. My understanding is that the increased defrost cycles do, however, decrease efficiency to some degree. My question then relates to where I should actually set my thermostat switch over point. Should I disregard the effect of the defrost cycles and set it at the 30 degrees that I have calculated for the economic balance point or should I make some allowance for the increased defrost cycles. My thermostat allows me to control the balance point in only 5 degree increments so in the current scenario I would have to go up to 35 degrees. My home is in the Pittsburgh, PA area and this is my equipment:

Heat pump: Heritage 14, 2 1/2 ton, 4A6H4030B1000AA
Indoor Coil: 4TXCB031BC3HCAA
Furnace: Freedom 95, Comfort-R, AUH2B080A9V3VA
Thermostat: ACONT803AS32DAA

I would very much appreciate any guidance that you can give.

[amd]

Defrost cycles do lower the balance point to some degree (It's like turning on the air conditioning and using the furnace/strips to counteract what's lost :eek: ), so would would set it a couple of degrees above the calculated economic balance point. (Just my opinion -wait for more replies)

[junkhound]

Very dependent on the local relative humidity of outdoor air.

On own system (dual HP) the resistance heat is disconnected, so defrost uses indoor air as heat source and less impact to balance point. You do need to be able to tolerate a few minutes of cold air from your ducts. At under 45F, own system switches air-air HP to WSHP.

Son's house has dual heat like yours, also uses indoor air for defrost. He is in PNW area with PSE as supplier, break point is below 10F even accounting for defrost. Currently weather is in a 5 year low temp weather (20F) and do activate the gas as the grandkids do like to sit over a hot air register when they come in from sledding in the snow. First time the gas has been activated in 4 years.

To get good numbers for your spreadsheet, you will need to track humidity and the number of defrosts per day and then extrapolate that vs. weather data. For getting some base values for my own home, I put a mirror on a tree so I could see the HP from computer desk and kept track that way for a few different typical days.
Easiest way after all that then for defrost losses (with no dual heat coming on) is to use an empirical COP factor of (1-(1.3X time spent in defrost/ total time HP runs))

[beenthere]

You can use the HSPF instead of COP.
HSPF takes into consideration cycling losses, and defrost times.
Generally you would need the bin data for your area, to use HSPF.

An easy way to use HSPF, without having to look at total BTUs required for the bin time period. Is to convert HSPF to percentage, and multiply COP by it.

EG: With a HSPF of 9.5
9.5 X .1 = .95
COP of 3
3 X .95 = 2.85
Adjusted COP would be 2.85

[tigerdunes]

Calum

I have a very similar dual fuel system albeit Trane including the XL14i HP that is the same as the AmStd Heritage 14. I am going to offer a different perspective. I have what I believe Trane calls a fossil fuel kit with a mech thermostat mounted at the HP condenser controls. This stat is adjustable-I have my changeover also set at 30 degrees.It has a range of 20 degrees Fah to 40 degrees Fah in 2 degree increments. It's not entirely accurate as it probably has a plus 2 minus 2 degree tolerance. As you know, these HPs have "electronic demand defrost".

Have you actually seen your HP in defrost mode as well as the condition of the condenser indicating a defrost condition exists? The reason I ask is that I have never in three yrs seen a defrost condition or what I thought was a defrost cycle. I realize that climate/weather can affect the defrost requirement. I live in SE US. Just a thought but my experience is that defrost cycles with electronic demand should be very infrequent in the low 30s. If you are getting a high number of defrost cycle calls, maybe your installing dealer should take a look.

perhaps the experts on this forum can offer some opinions.

:)

[Airmechanical]

On own system (dual HP) the resistance heat is disconnected, so defrost uses indoor air as heat source and less impact to balance point. You do need to be able to tolerate a few minutes of cold air from your ducts.


i used to do that (disconnect heat strips)

but imo, that cold supply air during defrost is causing a lot of extra run time on the heat pump

which is probably almost equal to running the strips for the defrost period



.

[Calum]

Calum

I have a very similar dual fuel system albeit Trane including the XL14i HP that is the same as the AmStd Heritage 14. I am going to offer a different perspective. I have what I believe Trane calls a fossil fuel kit with a mech thermostat mounted at the HP condenser controls. This stat is adjustable-I have my changeover also set at 30 degrees.It has a range of 20 degrees Fah to 40 degrees Fah in 2 degree increments. It's not entirely accurate as it probably has a plus 2 minus 2 degree tolerance. As you know, these HPs have "electronic demand defrost".

Have you actually seen your HP in defrost mode as well as the condition of the condenser indicating a defrost condition exists? The reason I ask is that I have never in three yrs seen a defrost condition or what I thought was a defrost cycle. I realize that climate/weather can affect the defrost requirement. I live in SE US. Just a thought but my experience is that defrost cycles with electronic demand should be very infrequent in the low 30s. If you are getting a high number of defrost cycle calls, maybe your installing dealer should take a look.

perhaps the experts on this forum can offer some opinions.

:)
I very much appreciate the replys to my inquiry. In response to Tigerdune's question. Yes I have seen the HP go through defrost cycles. Today with a relative humidity of 72% and temperature of 32 degrees the HP has been running continuously (recovering from a slight setback) for the past hour and 10 minutes without defrosting so I don't think that I am experiencing anything abnormal. Over the past few days with similar temperatures but rain and snow and I presume higher humidity there have been defrost cycles but I will admit that I have not yet attempted to track their frequency. When I had the switch-over set to 40 degrees I was not seeing any defrost cycles. I wil attempt to track the defrost cycle time for various weather conditions and with the formula provided by Junkhound I should be able to add the effect of the defrost cycles into my calculations. Again thank you very much for the help.

[tigerdunes]

calum

you have actually seen the need at your HP condenser for the defrost cycle?

td
:)

[Calum]

TD,

Yes, sorry I forgot to say that I had actually watched the frost steadily build up on the coils prior to initiation of a defrost cycle.

Calum

[HPGui]

You can use the HSPF instead of COP.
HSPF takes into consideration cycling losses, and defrost times.
Generally you would need the bin data for your area, to use HSPF.

An easy way to use HSPF, without having to look at total BTUs required for the bin time period. Is to convert HSPF to percentage, and multiply COP by it.

EG: With a HSPF of 9.5
9.5 X .1 = .95
COP of 3
3 X .95 = 2.85
Adjusted COP would be 2.85
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
:)
Thanks, beenthere. This was a VERY useful post. I've been wondering how to factor the defrost cycles in.

[cpetku]

Another way is to monitor/record the duct temperature to figure out exactly how often a defrost is occuring then derate the COP based upon actual conditions. The attached image was produced using data from an IAQ.

[beenthere]

That will work to check how close the calc is.

The OP wants to be able to predict what the economic balance point will be based on his monthly electric rates as they goes up and down.
A 4 cent change in rate from one month to the next, will make a big difference.

With a time and temp defrost, you could use that chart.
With an on demand, the outdoor humidity will effect defrost, so under the same outdoor temp conditions. But with lower humidity, it may run an hour longer before going into defrost. And with higher humidity, it may go into defrost a half hour sooner.

[Calum]

Again I am very appreciative of the information that has been so freely given. I think that I am beginning to get the picture. I could multiply my COPs by the converted HSPF factor as suggested by beenthere to get an average effect of the defrost cycles on the balance point. The alternative is to monitor the time and frequency of defrost cycles under differing weather conditions and derate the COP by the formula suggested by tigerdune to see the immediate effect on the COP. This is a tentative statement since I have not yet made an effort to track these cycles but it seems that with my current economic balance point at 30 degrees there will be enough of an effect on efficiency if defrost cycles (demand triggered) are needed with any kind of regularity during outdoor temperatures in the low 30's to make running the gas furnace more economical. My practical solution for now would be to leave the switch-over point at 30 degrees and kick on the emergency heat when conditions occur that are triggering defrost cycles. If this seems to become too regular of an occurrence and I tire of the game, I will move the switch-over setting up to 35 degrees. This may avoid the defrost cycles entirely but should at least lessen their frequency and will limit their effect since there will be a higher COP at this temperature. My electric rates should be pretty steady for a while but I do expect the natural gas rates to vary so this strategy will have to change accordingly. If my economic balance point goes down considerably, it seems that the economic impact of defrost cycles will diminish and that there will come a point where I can definitely ignore the defrost cycles entirely. On the other hand if my economic balance point goes up there will come a point where defrost cycles are never triggered. This has been a bit of a mind bender for me so I would appreciate anyone pointing out any fallacies that they see in my thinking!