Heat Pump Frosting - Defrosting

[DADoES]

Thought it might be interesting to post some pics of my heat pump getting frosted-up and then defrosting for those who haven't seen one in operation. A heat pump is nothing more than an air conditioner that operates "backward" to move heat from the outside air to inside the house. Even "cold" air contains heat, down to absolute zero (-459.67°F or -273.15°C or 0°K or 0°R). There is less cost in terms of energy consumption to move heat from one place to another (via the refrigeration cycle) than to create it (by "burning" electricity to heat a wire).

Like the evaporator coil in a refrigerator or freezer, a heat pump compressor unit gets frosty in wet/cold ambient conditions and needs to be defrosted. Defrosting is done by switching the system back into an air conditioner for a few minutes to heat up the outside coils and melt off the frost. Standard electric strip heating elements turn on in the air handler during the defrost cycle to counteract the cold air that would otherwise be blowing into the house (the heat strips have nothing directly to do with heating the outdoor coil).

Getting frosty.

 

[DADoES]

A few mins before defrost triggered.

 

[DADoES]

Melting.

 

[DADoES]

The condenser really does heat up a bit during defrost, and it's not unusual for steam/vapor to billow from the unit.

 

[DADoES]

A vapor cloud wafting directly toward me.

 

[DADoES]

The frost is gone but the coils haven't quite reached the target temperature to end the defrost cycle so it continues to run for a few more minutes.

 

[DADoES]

One last shot of a little puff of steam. Note that the condenser fan shuts off during defrost. Steam/vapor billows away by convection and wind, except right at the end when the fan turns back on there may be a larger cloud blown upward for a few moments.

 

[Cybrvanr]

I have a friend that works as an HVAC tech. He says that during the first really cold snap that they get numerous calls that their outdoor compressor units are "smoking" and that the fan has stopped. They just explain to the user that the unit is defrosting and that there's no need to worry! Oddly enough, many still don't believe them!

 

[jamiel]

Yeah, I had a heat pump in an apartment in Atlanta and it was very weird to see the frosty units!

 

[toggleswitch]

Thanks Glenn! I saw my first one not long ago. Fascinating! I find the two-stage thermostst that triggers electric reisistance heat when the heat-pump can't keep up interesting too.

As far as I can tell the unit in question does not have a minimum cut-out temp; (it was 9*F this mornign in its location! It appears to run at any outdoor temp!

Honeywell's chronotherm IV set-back thermostat with adaptive recovery actually starts the temperature recovery and figues out how long it will take to get the space to target temp. by the time on th eclock the user has selected!

It also raises the temp setting on the thermostat incrementally by 2* F at a time to avoid triggering the resistince back-up heaters!

When the thermnostat is satisfied, it waits five minutes then raises the target temp by two degrees until the requested/desired temperature seeting is met.

WAY COOL!

 

[exploder3211]

You can't watch our new heat pump, but from the others i have seen or had are quite shocking to see them run... Although one i had was a Heil and boy did it make a racket when doing this..

 

[toggleswitch]

My favorite sound is when the compressor is laboring for a second or two as the refrigerant reversing vale kicks in for the defrost cycle.

from heating to defrosting=>
This makes the outside coil flip from being the evaporator (cooling)coil to the condenser(heating coil), and the inside coil go from being the condensor to the evaporator.

The KEWLEST feature of a heat-pump is that it takes oudoor heat in winter, concentrates it, adds the wattage consumed to the mix as heat and pushes it it "uphill" to inside the house.

Resistance heat is 4.3+/- BTU/watt IIRC.
a heat pump with a (S)EER of 12.0 and up moves 12.0 BTUs/Watt.
THREE TIMES THE OUTPUT per unit of energy consumed.

In theory this should make it somewhat competitive to fossil fuels in terms of heat /$ where IIRC a good 2/3 of the energy/heat used to make electicity is lost.

Of course the need for back-up emergency heat (usually resistance electric) eats away at this efficiency. Teh unit takes more energy to extract heat as there is leess of it as temperatures plummet outside.

Of course a ground-source heat-pump that uses a loop in the earth to extract its heat is another story. I beleive the earth is a constant 50*f just a few feet down.

(....and count-down till some sharp-shooter corrects me. )*LOL*

What say you techies...and Bobbins...?

 

[appnut]

Steve, mine has a SEER of 18.

 

[appnut]

Steve, the adaptive recovery also adjusts it's "pattern" of recovery from day to day based on previous time of recovery, changes in intensity of temperature of the house or outside.

 

[foraloysius]

Interesting pictures, I don't think I have ever seen one overhere. Natural Gas Central hot water heating is the most popular way of heating here. And central airconditioning is very rare due to the mild climate. Heating and cooling appliances are always indoor anyway.

 

[toggleswitch]

Thanks Bob-load Bob!

Anyone know how resideitial units with multiple compressors work?

Here in this relatively mild summer clamate, we are often discouraged from the high-efficiency central A/C units. They say the complicated mechanics and intital cost are not worth the engery savings gained for the number of hours we would use it.

 

[DADoES]

I've heard of panicked homeowners calling the fire department, thinking their compressor is on fire.

My thermostat has the described adaptive recovery. I didn't see it do that until this season (bought the house in January 2005) and thought the feature wasn't working. Apparently most of the time (and with the 'mild' winters the last couple years) the rate of temperature drop in the house isn't fast enough for it to calculate a need to start early.

I have the auxiliary/backup resistance heat locked out until the ambient reaches some low temp. I don't recall what temp but it's lower than 30°F being as that was the low last night and the auxiliary didn't run. Except during defrost.

IIRC, my target recovery time from 60°F to 70°F is 3 AM. I got home early last night at 1 AM and caught a defrost cycle so recovery may have started a little earlier than that, along with whatever normal running had been happening to hold 60°F with an ambient of 30°F.

By 3 AM the indoor temp was 67°F-68°F. Normally the auxiliary would have triggered at that point to quickly finish recovery to the target 70°F. But I don't care if it takes longer, I just don't want those $$$ heat strips to run. 5KW for the compressor vs. at least 15KW (likely more) for the resistance. Far as I know the system ran continuously until I "heard" it cycle off at 5:22 AM (yes, I looked at the clock).

Under normal conditions the compressor continues running when the auxiliary triggers. Auxiliary doesn't take over completely, it just boosts the compressor's output.

Emergency heat uses the same heat strips as auxiliary (of course, that would be the only heat strips in the system), but in that case the compressor does not run. Emergency is a manual setting that the homeowner activates and normally is not used unless the compressor fails.

Ground source heat pumps (water or earth) are the ultimate (other than solar!). They're also highly efficient for cooling.

I believe Trane (among possibly others) has residential models with two compressors (as opposed to a single two-speed compressor). Same concept as two-speed. Two-stage thermostat, one compressor runs for mild/moderate conditions, both for extremes.

I'm only 12 SEER, dammnit. Previous house was 16.

 

[appnut]

My Trane does have the two compressor style. Teh small one runs for maintenance and cycling on & off after set temp is reached. My air handler runs at low speed then too. The 2nd cmopressor kicks in when there's moe than 3 to 4 degrees diffrence than the set temp and thus boosts output of either heat or cooling. I think before the 2nd compressor kicks in, the air handler increases speed. It's all pretty nifty. I just am not able to run in and out of the house and seee how many of the compressors are running at various stages.

 

[appnut]

My Trane does have the two compressor style. Teh small one runs for maintenance and cycling on & off after set temp is reached. My air handler runs at low speed then too. The 2nd cmopressor kicks in when there's moe than 3 to 4 degrees diffrence than the set temp and thus boosts output of either heat or cooling. I think before the 2nd compressor kicks in, the air handler increases speed. It's all pretty nifty. I just am not able to run in and out of the house and seee how many of the compressors are running at various stages.

 

[sudsmaster]

Chocolate or Vanilla?

Does Heat Pump Frosting go with Heat Pump Cake?

 

[toggleswitch]

oh yes baby, give me that sweet stuff! I'll take a slice.

Now back to business~ I have the auxiliary/backup resistance heat locked out until the ambient reaches some low temp.

How does one do this? What type of control, and where is it located?

 

[exploder3211]

Mine is a 12 SEER (or whatever the Gov. Minumum requierment for 2006 was) and it sucks energy... The heat strips run alot as well, so, i am not sure if it's too small or what.. Great to know the techincal details and such, but they confuse me..

 

[appnut]

Chad, has the unit been inspected and sericed in all those years, had the freon level checked? If it's not working up to snuff, then the heat strips will kick in a lot. My old system had a ciol leak and had to have freon put in about once a year or 18 months.

 

[Cybrvanr]

A 12 SEER should be a reasonably efficient unit. I have a 10 SEER 2 ton unit in my 1000 sf house not too far from you, and I find it to be fairly efficient...giving me uner $100 electric bills as long as it's not too terribly chilly out.

How old is your home? If it's got a bit of age on it, chances are you are fighting a lot of heat loss through leaky windows, cracks, etc. You may want to look into tightening up your house a bit. It worked for me. I installed insulation in the attic, floors, and had it blown into the walls. What really made the diff though was installing new windows! When it's above 55 degrees, the house sort of "heats itself". In other words, I don't need to run any HVAC at all because heat generated through activity inside (running appliances, human bodies, electronics, etc) generates enough heat to maintain a comfortable temp. During the summertime, I can open the windows and it stays pretty cool too. The trick there was planting some maple trees on the south side of my house to break the sunlight!

 

[nasadowsk]

Heat pumps are somewhat rare in the northeast. You see a LOT of forced air, hot water, and even quite a bit of steam heat. I've read that hot water systems can be very efficient, too.

And insulation helps! When I get a house, one of the first things is gonna be expandofoam insulation. This is a pretty new thing where they use whats basically in those cans of expando foam to fill up the entire wall.

I never liked air systems (the one in our office blows...). Hot water's quiet, and really nice, IMHO. Oh yeah, and radiant's even better. Cats love it, too.

 

[DADoES]

Steven, my house two years old. The builder went "deluxe" on several points, but not so much on the HVAC system. I had a two-speed 16+ SEER heat pump with humidity control option at my other house.

Toggles, I added an optional outdoor temperature sensor on the thermostat. In addition to displaying the ambient on the 'stat at the push of a button, it enables a function on the setup menu that allows programming a temp for the auxiliary lock-out. Unless the ambient is at or below the programmed lock-out, the auxiliary cannot run during normal operation, no matter how long the system is taking to reach the setpoint or how much is the differential between setpoint and room temp. Auxiliary still runs during defrost, and if the 'stat is explicitly set to Emergency.

 

[gadgetgary]

I added an optional outdoor temperature sensor on the thermo

Glenn,

Can we see a pic of your thermostat?

Is the outdoor temp sensor something that I can add to mine?
I would love to have that feature of an auxiliary lock-out.

Question: Even though the temp outside is in the teens this morning, will the heat pump produce enough to raise the temp. inside without having to use auxiliary heat(assuming that auxiliary is locked-out)?

 

[appnut]

Gary, I think Glenn has it set so that if the outside temp gets somewhere near the low 20s, then it will unlock the aux strips. Glenn will have to give you the specific temp point at which he's got it programmed to unlock those strips.

 

[DADoES]

GadgetGary, check if your system/thermostat manufacturer offers the sensor/lockout option. Carrier and Lennox both have it. I had Lennox at my previous house, Carrier now. The outdoor temp is displayed on the Carrier 'stat by pressing the temp-up and temp-down buttons simultaneously. The lockout temp is programmed on a setup menu accessed by holding the Fan button for five seconds.

The Lennox thermostat was nice. It continually alternated the display between room temp and outdoor temp, and recorded the daily high/low (auto reset at midnight). A submenu could report the accumulated run-time of low-speed compressor, high-speed compressor, and auxiliary usage.

The lockout setting depends on system performance (the "balance point") and personal preference. The only way to know for sure is experiment and observe the results. The indoor temperature will rise, although it may be very slowly, as long as there's more heat moving in than what's escaping. The ideal system would continually vary its output to exactly balance during normal operation.

My previous house had a slightly oversized system (3.5 tons for 1278 sq ft) and could easily recover a 10°F setback in less than two hours at 30°F ambient, and hold 72°F indoors with normal cycling at 25°F ambient. The Carrier now is closer to "proper" capacity for the house (5 tons for 2550 sq ft, but I have 9' and 10' ceilings so it's a proportionately larger volume of airspace). At 30°F ambient it takes about 4 hours, maybe 4.5 to fully recover the 10°F setback (the setback period is from 10:30 AM to 3:00 AM, 17.5 hours, but it ends up being about 14 to 15 hours when the adaptive-recovery start-time is factored in). Weather conditions have been drizzling/misty so it gets frosty fairly quick. Outdoor temps in the teens I don't think it could handle for a recovery, or hold a normal indoor temp without assistance.

My usage and daily schedule is far from the norm, so YMMV.

 

[toggleswitch]

YMMV?

Your ___ may vary?