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.