Efficiency in 'heat pump' type of HVAC

Thread Starter

Externet

Joined Nov 29, 2005
1,546
Hi. Seems like dedicated to #12...

In summer heat, the compressor liquifies the refrigerant, it is cooled to outside ambient and evaporated inside the dwelling to absorb heat.

In winter, the refrigerant flow is reversed by an electrovalve and the compressor liquifies the refrigerant and heats it up, with its heat being released inside the dwelling. The warm gas that delivered heat inside goes back to the compressor outside and is compressed/heated again...

About efficiency; how can these systems be claimed as more efficient in winter; if a kilowatt is a kilowatt. The energy of the compressor, fan, remnant heat in coils/pipes is vented outside in the cold warming the ambient; say those items dissipate 1 KW. Instead, a 1 KW electric heating element inside would not waste anything outside.
Is this 'heat pump in winter' a good efficient idea?

Shouldn't the liquid refrigerant pipe running bare by a crawling space be insulated in winter ?
 

tcmtech

Joined Nov 4, 2013
2,867
Factor in the temperature differentials from inside to outside against the relative system efficiency at those differentials.

At some temperature differential the system is not able to do anything but waste energy but above that the heat pumping effect works in its favor to move more heat heat energy in from outside than its equivalent electrical energy consumption is equal to.
 

#12

Joined Nov 30, 2010
18,210
You're missing a couple of concepts but I can answer your questions.
A heat pump has a "fail" point which depends on outside air temperature.
Annnd...it looks like the early bird got here first.:(

and, yes, all exposed pipes should be insulated. That's why I take new machines apart and install proper insulation before I install them as air conditioners.
 

crutschow

Joined Mar 14, 2008
25,690
A kilowatt is indeed a kilowatt, but it doesn't take a kilowatt of energy to move a kilowatt of heat from one location to another, which is what a heat pump does.
Since it take less energy to move heat than to create it (depending upon the temperature difference between the two locations), a heat pump is more efficient than a heater in heating a house.
In the winter it can actually be slightly more efficient than cooling in summer since the heat energy loss of the compressor adds to the heat inside the house.
The efficiency of this transfer depends upon how well the heat pump is designed to move this heat compared to the amount of applied power.
This is given as the SEER or ESEER efficiency.

Thus in the winter, the refrigerant is piped outside (usually to a long tube buried deep in the ground) to evaporate and pick up heat (thus making the ground colder).
This gas is then pumped inside and compressed by the pump to condense back to a liquid, giving up the absorbed heat to warm the house.
 

#12

Joined Nov 30, 2010
18,210
This is given as the SEER or ESEER efficiency.
There is another term used specifically for heat pumps. COP = Coefficient of Performance
Starting with 3.413 BTUs per watt hour, COP is the number you multiply that with to get winter performance ratings.
Last time I looked, the usual COP was slightly above 3.
Mathematically, it looks stupid to make up a new label that just means "multiply" but it also means, "winter performance" while SEER means summer performance.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,546
Thanks.
OK; understood the limitations... At an exterior ambient temperature being too cold, the 'heat pump' mode will not be efficient and then is when the system switches to electric heating element and stops the exterior compressor, right ?

The narrow pipe intended to carry liquified refrigerant -say in a crawling space- towards the living space should be insulated but only in winter, right ?

Which would be a hassle for most homeowners -knowing the laziness or skills level-

If two paralleled 'narrow pipes' were used for liquid refrigerant flow, one insulated and the other not, it would be more efficient, yes ? Inserting valves in each so the hot fluid in winter at 'heat pump' mode only flows in the insulated one avoiding losses warming the crawling space; and in summer, the crawling space helps cooling the liquid refrigerant, right ? And would take care of the intervention hassle.

Exterior compr.==========valve open in 'heatpump' mode=======>>>inside evap. [Insulated for winter]

Exterior compr.----------------valve open in cooling mode ---------------------->>>inside evap. [Bare for summer]

Am I missing something like the liquid refrigerant flows only in one direction for both cooling and 'heat pump' modes in the narrow pipe ?
 

wayneh

Joined Sep 9, 2010
16,402
The narrow pipe intended to carry liquified refrigerant -say in a crawling space- towards the living space should be insulated but only in winter, right ?
In an air conditioner setup, the only reason to insulate the cool freon gas returning to the compressor would be to minimize condensation on that tubing and the water damage that could ensue. That's probably a minimal risk. I can't see that it would be a problem if it absorbed a little more heat on its way out.

The incoming liquid freon is hot, though. It has to be at least as hot as the outdoor temperature and probably will be several degrees above that due to heat exchanger inefficiency. I suppose it makes sense to keep that heat in contained in the freon rather than release it into the room and have to take it back out.

Rats, now I have to go look at how my pipes are insulated.
 

tcmtech

Joined Nov 4, 2013
2,867
In an air conditioner setup, the only reason to insulate the cool freon gas returning to the compressor would be to minimize condensation on that tubing and the water damage that could ensue. That's probably a minimal risk. I can't see that it would be a problem if it absorbed a little more heat on its way out.

The incoming liquid freon is hot, though. It has to be at least as hot as the outdoor temperature and probably will be several degrees above that due to heat exchanger inefficiency. I suppose it makes sense to keep that heat in contained in the freon rather than release it into the room and have to take it back out.

Rats, now I have to go look at how my pipes are insulated.
Yea. Mine aren't insulated but then they run under the house where condensation drips don't bother a thing.

If anything given my ~35 - 40 F return line temps most good summer days I have often pondered on adding a second smaller heat exchanger and fan set down there to act like a dehumidifier being that crawlspace air doesn't get exchanged with the main house are any too fast unless I have the front room floor vent fan running all the time to pull that air up into the house.
 

wayneh

Joined Sep 9, 2010
16,402
I have variable rate electrical pricing, and there are times in the summer that the rate actually goes negative. I've considered putting a chest freezer full of water/ice in the basement and running my A/C's incoming freon line through it. When electricity is cheap or free, I could run the freezer to make ice. This would give extra free cooling to the warm liquid freon entering my house. I may have done it by now if it didn't require cracking open my system. Freezers show up free on craigslist once a week or so. Of course I'd want to vent the freezer's coils to the outdoors also. Just enough work to keep me from doing it.
 

#12

Joined Nov 30, 2010
18,210
Working this out as I go:
In A/C mode, the warm liquid goes to the air handler (small pipe) warmer than ambient air and should radiate unless it's in the attic at 120 F. Small pipe insulation depends on its route. The cool gas returning to the compressor (large pipe) tends to pick up heat from anywhere, so insulate the large pipe.

In heat mode: The compressor makes hot gas which is going to the indoor coil and so the large pipe should be insulated. The warm liquid refrigerant returning to the outdoor coil in the small pipe needs its heat contained, so insulate the small pipe.

What are the results? Insulate both long pipes in the winter. Insulate the large pipe in the summer. Insulate the small pipe in the summer if it travels through a hot attic.

Now here's my method: Put both pipes inside the same insulation tube. (Just be sure to strap them together so they don't vibrate holes in each other.) In both seasons, whatever one pipe loses, the other pipe wants.
 

wayneh

Joined Sep 9, 2010
16,402
Now here's my method: Put both pipes inside the same insulation tube. (Just be sure to strap them together so they don't vibrate holes in each other.) In both seasons, whatever one pipe loses, the other pipe wants.
Counter-current heat exchanger, I like it. I'm hoping that's what I find when I examine mine.

As long as we're here, I have a related question. My daughter just bought a house. It has what appears to be the largest home A/C I've ever seen sitting out back. I speculated it might be a heat pump, but it's not. The previous owners complained about it being hot upstairs, so they installed this $8K beast. It was installed last summer and barely used. I haven't read the label specs yet but I'm expecting it to be oversized. (The fan didn't seem absurdly large, only the heat exchanger 'column'.) Is there a first-pass rule-of-thumb I can use to make this calculation? It's already installed so it's not like I need a precise recommendation. I just want some sense of how bad it might be. They're probably going to need a dehumidifier in the basement, same as me.
 

tcmtech

Joined Nov 4, 2013
2,867
Is there a first-pass rule-of-thumb I can use to make this calculation? It's already installed so it's not like I need a precise recommendation. I just want some sense of how bad it might be. They're probably going to need a dehumidifier in the basement, same as me.
Look up the model number and specs then use the standard HVAC sizing calculations to see how well it matches the house dimensions.

If its a properly insulated and sealed house and it's not in a climate or location where extreme summer heat is an issue and the units rating are no more than 10 - 20% over the houses thermal load estimations it's probably close enough.

Now if it's like my old trailer house where by the HVAC book calculations it should need no more than 30,000 BTU (2.5 tons) worth of cooling system capacity which is what it came with when I got it some ~18 years ago.

Reality is given the design of the house it was half of what it takes to cool this place. (ran constantly, burned a huge amount of power and largely did nothing but piss me off when I got the electric bill) The system I put in two summers ago is a used ~5 ton commercial system (~55 - 60 KBTU) and it just barely keeps up on the hottest most humid summer days here and we rarely ever see over 100F at that. :(

The common sayings I have come to equate this place with is 'In the winter it's like trying to heat a corn crib.' and, 'In the summer it's like trying to cool a greenhouse.' and very much an exception to the standard HVAC calculations and system sizing suggestions.
 

wayneh

Joined Sep 9, 2010
16,402
It's a 4-ton unit (14kW) for a 2,482 ft^2 house. It does say it's a 2-stage unit. Maybe that means big whopping heat exchanger?
 

#12

Joined Nov 30, 2010
18,210
Counter-current heat exchanger, I like it. I'm hoping that's what I find when I examine mine.
You won't. This kind of quality is up to the installer, and nobody (except me) installs the extra $10 worth of insulation with your multi-thousand dollar air conditioner.
It does say it's a 2-stage unit.
Probably means 2 speed compressor. Lots of expensive electronics.:D
Better get some K-Y jelly to go with that.
Look in the air handler and see if it has a variable speed motor. If it does, you can pay most of $2,000 to replace a 1/2 HP motor when it goes bad.

I keep telling people not to buy this high-tech crap. You get screwed on the purchase price and every repair.:mad:
"Oh, but it's Prestigious.":p
Nobody is going to read the label on your air conditioner and drop jaw at your awesome ability to pay the highest possible retail price.:(

PS, Big heat exchangers work better. They need less fan motor and keep the Freon pressures down. That causes, "efficiency".
 

tcmtech

Joined Nov 4, 2013
2,867
I keep telling people not to buy this high-tech crap. You get screwed on the purchase price and every repair.:mad:
"Oh, but it's Prestigious.":p
Yep.

Either you pay $4000 more than necessary to own a unit that will save you less than $50 a year on electricity (but you can stream from the roof tops look how wise and green I am!) and it will likely breakdown beyond affordable repair before it 10 years old or you paid $4000 more than necessary for a system that was probably worth $2000 or less had you bought it yourself from a reputable online source.

In my life I can say I have come to know a of maybe 5 honest HVAC techs besides myself. #12 is one and two others died of old age in the last few years and the last living one I know of largely works on systems like me. By request only just so he doesn't have to deal with the hassle of all the other idiots and runaround in the industry.
Sadly, most everyone else I have met and got to know had largely gave me every reason to believe they were either outright crooks or drugged up half wits working for crooks. I don't mind a guy making a fair profit on a job but when you charge $5000+ to put in a $1800 system that took less than 6 hours start to finish you or the guy you work for is a GD crooked SOB. :( :mad:
 

#12

Joined Nov 30, 2010
18,210
drugged up half wits working for crooks.
I've seen that, too.

Imagine a Coked up doofus that connected 240 VAC to the wrong terminals on the contactor and laughed himself silly when the thermostat launched off the wall in a ball of flames.:confused: Then he takes the customers' check to the boss who drinks whiskey for breakfast and tries to give me driving instructions by telling me which bar to turn left at.:mad:

"South on Starkey to ABC liquors, turn left to Joe's Bar & Grill, then right to The Green Iguana..."o_O
 

Thread Starter

Externet

Joined Nov 29, 2005
1,546
Thanks, guys. I think I recognize my goofing :

In summer cooling mode, the non-insulated narrow pipe carries warm refrigerant towards the inside and by being not insulated actually helps passing trough the crawling space to cool a bit more. Good.

In winter heat pump mode, the hot refrigerant does NOT pass by the narrow non-insulated pipe because the flow is reversed; it flows towards the inside by the wide insulated pipe and that same insulation actually helps conserving heat passing by the cold crawling space and delivers the heat inside with little thermal loss. Good.

So insulating the narrow pipe in winter only is not necessary ! It already flows insulated in heat mode. Good.
The only thermal losses in 'heat pump' mode is the compressor and fan motor and some pipe portion dissipation in the outside unit wasted to the cold exterior ambient.

I will re-think about having both pipes in the same insulation. Something does not fully click well in me yet.
 
Last edited:

tcmtech

Joined Nov 4, 2013
2,867
I've seen that, too.

Imagine a Coked up doofus that connected 240 VAC to the wrong terminals on the contactor and laughed himself silly when the thermostat launched off the wall in a ball of flames.:confused: Then he takes the customers' check to the boss who drinks whiskey for breakfast and tries to give me driving instructions by telling me which bar to turn left at.:mad:

"South on Starkey to ABC liquors, turn left to Joe's Bar & Grill, then right to The Green Iguana..."o_O
Yea one of the old guys who explained the basics to me (friend or distant relative of my grandparents?) had similar thoughts about the generation of people who were taking his work form him in the end. Said something to the effect that if he ever showed up at a customers door looking, smelling and acting like those people did then sent them a outrageous bill for the inexcusably bad work they did while there he would have hung himself in shame.

I didn't know what he was referring to at the time but once I started doing my first self employment work and saw and heard about what most HVAC tech were doing and what they charged for it I had a pretty good understanding what he was talking about. His generation built there names and reputations on doing quality work at fair prices and being as nice and respectable to the customers as if they were in church with them.

Now you get after a contractor in any field for crappy work and bad service and the first thing out of there mouth is 'that's what everyone else does and what they charge so if you don't like try and find someone else." and the few who don't do that get badmouthed and lied about until they give up or go out of business. :mad:

My concrete expert buddy is in that group. When he was bringing in piles of cash for his employers (but not for himself of course ) he was getting job offers from every concrete business in town trying snag him up to pick up work on his reputation but the day he went independant they couldn't make up a story bad enough fast enough to undermine his reputation and work. Two years on his own and he had to fold up and go to work for another company in another town just to get away from it. :(
 

#12

Joined Nov 30, 2010
18,210
I will re-think about having both pipes in the same insulation. Something does not fully click well in me yet.
That's OK. It's a zero sum game with 2 pipes in one sleeve. Energy flowing one way enhances the effectiveness of the refrigerant and the pipe that provided that advantage inflicts the difference squarely on the compressor. Three apparently "free" BTUs at the coil cost an additional watt hour at the compressor and the net effect is zero. Primitive technicians debated this until somebody did the measurements in the design lab. There was no appreciable efficiency change. That's probably why the nanny state hasn't made Siamese pipes a law.

At least you're thinking correctly now. The only thing coupling the pipes together accomplishes is a zero net loss instead of trying to decide between winter and summer insulation. A naked small pipe in the winter radiates heat it shouldn't. A naked small pipe in the summer radiates heat like it should (if it's in a crawl space). Win some/lose some or go for a null equation?

At my house, and most Florida houses, the pipes are in the attic and most people don't have heat pumps. Both pipes should be insulated in both seasons, so I just strap them together and install slightly larger insulation on one large pipe bundle instead of one large insulated pipe and a small one strapped outside the insulation, picking up attic heat all summer.

It is always good to insulate the large pipe in the outside machine for efficiency. It also avoids that constant condensate drip that rusts out the base all summer.
 
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