Heat Pumps

[Jeff Wisnia]

I'm still trying to get my brain to accept that heat pumps can put out 2 to 3 times as much heat as the kilowatt hours they use. I keep trying to find a description of what the limitations are and why that performance number is what it is and not something different.

This page is the best I could find, but it still doesn't explain what the limiting factors are:

I'm gonna try a post to sci.physics.

Speaking of heat pumps... Yesterday I had to dig out, throw away, and replace a wheelbarrow full of topsoil from one of the Pachysandra beds alongside our AC pad. The "ignoranus" who carted off the old units must have let the refrigeration oil spill out right there. Shortly after they did the job a patch about three square feet in size started turning yellow and dying, and it just kept getting worse.

From my experience with fuel oil delivery stuff (With it's attendant spills around houses.) I knew that I wouldn't live long enought to see that soil come back to where it could support vegetation by itself again, so replacement was the right solution.

Arrrrgh!!!

CU Sunday, but que hora?

Love,

Dad

[Stormin Mormon]

Air conditioners also put out several times more heat than the wattage would suggest. Same principle.

Just stand outdoors in the hot blast, and you'll see what I mean. Much more heat than the wattage indicates.

Sorry to hear that your pachyderm rolled over dead.

[Jeff Wisnia]

Diditagain dammit!

I forgot that my Netscape 7.2 was sitting on the newsgroup and went to send that message by e-mail to my son!

Sorry guys, but a little love to you all can't hurt, can it?

Jeff

[nicksanspam]

That is sort of magical...

The basic limit comes from Carnot, who said the maximum possible heating COP (energyout/energyin) with absolute heat source and sink temps Tcold and Thot is Thot/(Thot-Tcold.) For example, if Thot = 70 F (= 460+70 Rankine) and Tcold = 50, COPmax = (460+70)/(70-50) = 26.5 :-)

Friction and heat loss make it closer to 3, and electric motors with 80% vs 100% efficiencies, and finite-size heat exchangers with less than 100% efficiency, which have the effect of raising Thot and lowering Tcold. Fans and pumps make heat exchangers more efficient, but they also use energy. If house air is 70 F and the hot fluid is 80 vs 70 F and it's 50 outdoors but the cold fluid is 40, COPmax = (460+80)/(80-40) = 13.5.

And the fluid must be economical and safe, and real heat pumps waste energy in throttling vs perfect fluid expansion.

Nick

[Joseph Meehan]

Heat is like water in that he can be pumped and if it has an opportunity it will roll downhill. A heat pump does not make heat, it just moves it. As long as it does not need to move it too far uphill, it can move it using less energy than making it. In fact the energy that is uses to move it, will also turn to heat and much of that can end up in your home.

[Jeff Wisnia]

That Carnot part is eggsackly what I was seeking, THANKS!

Even though I didn't mean to post here, (It was an intended e-mail to my eldest son) I very glad I did. We're both EEs (MIT '57 and U Vermont '87) so between us we don't remember much beyond PV=nRT about thermodynamics.

But our curiousities were piqued when I had to finally have my home's pair of 20 year old Rheem "compressor units" replaced last month.

The COP was dramatically demonstrated this last winter when a fan motor bracket on the main floor "compressor unit" cracked and let the motor tilt down so its lower edge wore its way through a refrigerant line and dumped the gas. We had to heat with just the resistance heaters for a couple of months until the snows let up enough to enable an easy replacement of both outside units.

Even though I played scrooge with the thermostat, and was very glad I wasn't a brass monkey, the electric meter "spun off the wall" during that period with a resulting bill far heigher than we'd ever had before.

Hopefully, the two new Rheem "compressor units" with get our power consumption back down to where it was 20 years ago. Now if I could only get the *rates* back down there too....sigh...

Jeff

[jumpnut30]

I live in the Dallas Ft. Worth area and I'm considering installing a heat pump rather than a standard A/C unit. Any recommendations as to reliability and efficiency?

[John Gilmer]

The technology is "mature."

The relatively "new stuff" includes dual speed compressors and variable speed fans which can also be "programmed" to reduce humidity.

A heat pump is essentialy as reliable as the A/C it replaces. There are a few extra parts (changeover valve, an extra Pressure/Temperature Valve, and

2 check valves) and, of course, you will use it year round rather than just the cooling season.

You have to "work the numbers" to see whether it's cost effective compared to how you heat your home during cold spells. If you use resistance (electrical) heating, it's a no brainer. If you use oil or gas then it gets a lot more complicated.

Regadless of your final decision, it's always a good idea to have a back up source of heat that will function without electric power.

[SteveBell]

I prefer gas heat. Right now, gas is cheaper that electricity. The process of converting gas to electricity at the station, transmitting the electricity a house, then converting it to heat in a heat pump is necessarily less efficient than just getting the heat directly from the gas.

I have a heat pump, and it works just fine. Gas is not available in my neighborhood. Oil furnaces aren't available _anywhere_ around here that I know about. Our electric bill was a little over $200 the last two months for a 2300 square foot home, and we keep the thermostat at 67ºF.

The units themselves are just an air conditioner with a few extra parts. The thermostats are a little more expensive, since heat pumps require a 3-5 minute delay between off and on.

Your HVAC contractor can work the numbers with you for cost of operation. Compare the costs of producing BTUs of heat for gas and heat pump.

[Sanity]

You appear knowledgable concerning heat pumps. I live in Western North Carolina. I have a combo unit. Heat pump and gas furnace in the attic. There is a point where the heat pump shuts down and the gas furnace comes on. At first it was set to switch over at 30 degrees outside temp. Since it doesn't get too cold here, I found the heat pump running most of the time and running excessively. They changed the setting to 40 degrees and the house heats up almost instantly however , even though my electric bill lowered slightly, my gas bill jumped through the ceiling. Is there a temp setting for the switchover that is more or less standard and economical?

[dpb]

Around 30F or so is pretty typical.

What the most cost-effective switchover point is is dependent on relative costs of electric vs backup heat source (gas in your case) and the actual efficiencies of the two systems. I'd guess you'd "win" somewhere lower than 40F; that seems awfully high to cut out the heat pump entirely if that's the way your system is set up.

Not had heat pump in location that had gas so never actually had that combination, but one advantage of the resistance "emergency" heat was it could be staged easily as temperatures dropped rather than the "all or nothing" that it sounds as though your system is w/ the gas furnace. Perhaps you could look into whether that could also be done w/ your system so it had the ability to use less gas when only needs a small boost as well.

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[SteveBell]

First, I'm not an expert on heat pumps or heating calculations. I only have a general knowledge of the way heat pumps work and a tenuous grasp of physics. :-) Maybe more knowledgeable people who participate in this group can help.

Your case is complicated. To _calculate_ the best setting, you'll need advice from someone who knows the physics of the system, and you'll also need the costs of gas and electricity. The size and insulation values of the house come into play too, as well as the outside temperature.

If I were in your shoes, I would use experimentation. Change the cutover to 35o and see what happens to the bills. Based on that result, I'd then change the cutover to either 33o or 37o, then based on those results....

Was the total of your electric and gas bills higher or lower after the tech made the change?

[hibb]

My heat pump is an air transfer type. It is a very efficient air conditioner. It doesn't put out enough heat to keep the house warm in the winter if the temps drop below 35. I have a pellet stove that I use a lot in the winter but when I have trouble with it, the heat pump just doesn't help much.

But I live in Michigan. I don't know how bas your winters are in Dallas. If your area doesn't have freezing temps for weeks at a time, you might be happy with an Air Transfer heat pump instead of the more expensive (but better) Geothermal type.

David

[Sanity]

The electric lowered slightly and the gas bill went out of sight. The heating company is coming down to do their semi-annual servicing in a week or two. I'll discuss it with the tech and most likely will have him change the settings to 35. I do know that it can only be changed at the thermostat in increments of 5 degrees. Thanks

[LouB]

While they are there ask them to check if gas heater is working properly.

Lou

[dpb]

Sanity wrote: ...

...

You might want to check on what the efficiency of the gas aux heating units is. I could see if it was a) a somewhat older unit, and b) to cut initial cost (particularly since it's "emergency" heat in a moderately benign climate) they may well only be 50% efficient units as was fairly typical of older natural draft systems. If it's not forced draft, it will have to be under about 80% or so.

The lower that efficiency rating is, the lower the break-even temperature is going to be where you'll be better off running the heat pump longer.

I'm confident that 40F is too high to cut out the heat pump entirely for virtually any system.

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[Bob F]

I would like to see documentation of this last claim, because it isn't obviously so.

Plus, most utilities don't produce most of their electricity with gas.

[Bob F]

If the heat pump is running "most of the time", the setting is probably reasonable as it was. When it gets colder than "running most of the time", meaning it can't keep up, then you need the auxillary heat. Obviously, in your case, the 30F setting was way better than the 40F setting.

[dpb]

...

... Well, assuming one doesn't account for the energy inputs into producing and transporting the gas which I'm quite sure aren't in the thought process here.

Of course, thermal efficiency of central generation runs in the 30-40% range whereas (modern) gas furnaces are 80% or higher so superficially one knows where it's coming from.

But, of course, what's significant to the homeowner is what is the cheaper alternative which can be the heat pump in many areas; particularly if were geothermal instead of air-exchange.

But, as you say, only comparatively small amounts of the overall electric grid are gas. I was looking at some data the other day and iirc w/o looking it up again, it's about 9% nation-wide, about half of nuclear. Much of that is peaking capacity although there are baseload plants in areas that do have local large supplies or in some areas where it was all that could be permitted when utilities had no choice but needed more capacity.

It is, of course, a really silly way to use up natural gas supplies...

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[BobR]

If you have the option of GAS...DON"T DO IT! THEY SUCK and I would never again have a house with one.