Tag: ground-source heat pump

Post #2036: Replacing my heat pumps III: The tax angles.

 

Winter approaches. 

But no pressure, as I slowly work through the tax angles on this HVAC equipment replacement decision.  And bring somebody in for another quote for new equipment. And maybe, eventually, get everything working again.

If nothing else, this whole episode shows me that it’s good to have multiple heating systems in your home.

Even with one heat pump dead, we have some heat.

And that is way better than no heat. Continue reading Post #2036: Replacing my heat pumps III: The tax angles.

Post 2032: Replacing my heat pumps, part II: How efficient are my ground-source and mini-split heat pump options?

 

The key question for this post is about as simple as it gets: If I have two choices for heat pumps, which one will use less electricity?

In my case, one option is the replacement ground-source heat pump that has been recommended, at a base installed price of about $25K per heat pump.  The other option is to replace my dead ground-source heat pump with a modern air-source mini-split heat pump, at somewhere around half that cost (call it 60% after adjusting for likely difference in equipment life, in my particular case).

This is a stupidly hard question to answer well.  As I explain at length below.

But, after doing all the homework that I care to do, for my house and my climate (with mild winters and an efficient gas-fired secondary heating system), the answer is that either style of heat pump (air-source or ground-source) will use roughly the same amount of electricity.  Or near as I can tell, based on published data.

That’s not due to the underlying physics of the situation.  If it were only about the physics, ground-source would win hands-down.  Instead, that appears mainly due to faster technological improvement in air-source units over the past decade or so, compared to ground-source units.  This seems to have fully offset the “natural” advantage of ground-source.  In effect, my real-world choice is between air-source using the current generation of technology, and ground source using older technology.  (The model of ground-source heat pump I have been offered was first introduced in 2016.)  Or, at least, using a less-efficient design for the heat pump itself, disregarding which heat sink (air, ground) is used.  That’s what makes it a tie ballgame, as of now.

This leads me to conclude that replacing one of my dead heat pumps with (e.g.) a name-brand air-source mini-split system:

  • Is substantially cheaper, even accounting for likely shorter equipment life.
  • Incurs no significant loss of efficiency compared to my ground-source option.
  • As a bonus, bypasses my house’s barely-functional 1959-era ductwork.

Ground source systems still have some clear advantages.  All the equipment is indoors, and so likely lasts longer.  They work well even extremes of cold or hot weather.

But the fact is, there just ain’t that many of them, particularly in a relatively mild climate like Virginia.  Of the roughly 4 million annual residential heat pump installations per year (in 2022), maybe 50,000 (call it 2.5%) were ground-source units.  That has big implications for how rapidly the units reflect improved technology, and how much choice you have for who installs and services your unit.

Unless some unforeseen problem arises, I will replace one three-ton dead ground-source heat pump with a pair of 1.5-ton mini-split air-source heat pumps.

And I will not feel the least bit guilty about doing so.

I was going to give full and excruciating details but the overall accuracy of the conclusion does not warrant that.  Below, I sketch out enough to summarize how I arrived at the numbers above.

SEER, EER, HSPF, COP, and all that jazz.

The efficiency of a heat pump varies, based on the how big a temperature difference it is trying to pump against, and how close you are to the maximum capacity of the system.  The bigger the temperature difference, and the closer to maxed out, the less efficiently the heat pump runs.

This means that, despite what you read from many internet sources, you cannot simply convert one heat-pump efficiency measure to another with a simple conversion-of-units number.  Yes, you must do that first, because some of these measures mix BTU/Hs and watts, and others don’t.  But in addition, you also have to make some sort of adjustment for how stringent the test is.

It’s very much like EPA mileage.  The MPG the EPA gets depends on how the car is driven.  Typically, EPA city mileage is much worse than EPA highway mileage.  If you compare the city MPG of one car to the highway MPG of another, you’re making a mistake.  So it is, in spades, with SEER, EER, COP, and HSPF.

Now we get to the hard part:  Things are hazy.

If you Google SEER, say, you’ll see the same zero-details definition everywhere:  It’s the ratio of the cooling power produced (in BTU/H), to the electrical power supplied (in watts).  But as to, how, exactly, that’s measured, it’s hard to find any information at all.  E.g., is the energy used to run the water pumps included, what indoor and outdoor temperatures were used for the test, how were ducts, water pumps, etc. factored in, and so on.

  • The details of the tests are proprietary and reside behind an expensive paywall.
  • For the same measure, ground-source and air-source heat pumps use different methods.
  • Certain aspects of overall energy use — duct system back pressure, water pump electricity use, and resistance electrical heating for backup heat — are either ill-specified, or not stated as to impact.

Among the things that I’ve seen hints for, but no definitive answer, is how these tests treat the waste heat of the electric motors themselves.  I saw at least one credible-looking website showing that ground-source heat pumps add the value of this waste heat to their heating output, as if that heat would make it into your ductwork.  But air source heat pumps do not.  That’s consistent with where the compressor is located (inside for one, outside for the other).  But it boils down to an assumption that the waste heat of the compressor motor somehow warms the air in your ductwork, which clearly isn’t the case for the units in my basement now.  I have yet to find a clear answer on that, and it matters materially to the comparison.

So you need to take the table above with a grain of salt.  My interpretation is that if there is a difference in efficiency across the three units I looked at, it’s small.

Definitions

Each of these measures compares output heating or cooling power, to input electrical power used.

EER (energy efficiency ratio).  Cooling.  Measured at a steady 35C outdoor air temperature, 26C indoor air temperature, and 50% relative humidity (for the outdoor air?).  Heat/cool is measured in BTU/H, electricity is in watts.  I think the test calls for the unit to run full-blast when this is measured.

SEER (seasonal energy efficiency ratio).  Cooling.  Near as I can tell, this is set up to simulate the range of temperatures you would see in a “standard summer”, so to speak.  Heat/cooling power output is measured in BTU/H, electricity input is measured in watts.

COP (coefficient of performance):  Heating:  Generically, COP is simply watts of heat out, divided by watts of electricity used.  Heat pumps have different COP values depending on the temperature tested, and how hard they were running.  But the EPA-reported COP appears to be for one temperature, and I think its with the unit running full blast.  Heat/cooling power is measured in watts, electrical input power is measured in watts.

HSPF (heating seasonal performance factor).  Heating.  Like SEER, this tests the units over a range of temperatures designed to be a sort of “standard winter”.  I believe that, where the unit has a resistance-heating secondary heater, if that clicks on during the testing, the electricity used in secondary heating is counted toward the total.  Heating power is measured in BTU/H, electrical use in watts.

The -2 suffixed versions of these appear to include a more realistic measure of the back-pressure of typical home ducts.  Best I can tell, in the typical situation, you’d expect the (e.g.) SEER2 rating of an appliance to be 5% to 10% lower than the SEER rating.

Accounting for test stringency:  SEER to EER conversion, units-adjusted HSPF to COP conversion.  Here, I found some sketchy internet sources suggesting that where you have SEER and EER for the same unit, SEER is typically 85% of the EER value, due to the more stringent testing cycle.  So I used that to adjust these all to a common EER-style basis.

Conclusion so far

Again, take this table with a grain of salt. There’s a whole lot I don’t know about the details of how each test is applied to each type of machine.  And probably never will know, particularly for the details of testing ground source machines, where tests specifying outdoor air temperature are irrelevant.

That said, if you adjust for the difference in units-of-measurement (BTU/H versus watt), and assume that the tests that use a broad range of conditions (SEER, HSPF) tend to run about 85% of the equivalent tests that use a single set of conditions (EER, and COP as EPA reports it), then you get the comparison above.

Which, honestly, is just about what I came up with, back-of-the-envelope, when I first looked into this some years ago.  The super-high-SEER Japanese-made heat pumps that emerged a decade ago seemed to eclipse (my estimate of) my existing ground-source heat pump’s efficiency.  SEER 25? Maybe I mis-recall.  But I do recall being startled with how high the available SEER ratings got, for air-source units.

Bottom line, efficiency-wise it’s a tossup.  If I weight each units two numbers by local degree-day (3x heating a cooling), I get my estimated all-year efficiency values of 3.6, 3.5, and 4.0 for the three heat pumps examined, respectively.)

If your location experiences lot of time at extremely cold or hot temperatures, ground-source heat pumps still seem to offer some significant efficiency advantages over air-source.  And, for sure, because the equipment is all inside, ground-source is likely to last longer.

But in my case — with a relatively mild climate, efficient (gas-fired) backup heat, and so on — it’s six of one, half a dozen of the other.

Finally, this pretty strongly suggests that the current tax law is out-of-date.  The huge advantage given to ground-source heat pumps might have made sense in 2004.  It appears to make no sense in 2024.

Once upon a time, ground-source heat pumps were king.  But not any more.  And the law has yet to catch up with that.

Post 2031: Both of my heat pumps have died? This should be interesting.

 

 

My house is heated and cooled by two ground-source heat pumps, installed by the previous owner almost exactly 20 years ago.

Well, “was heated and cooled”.  One died last spring.  The other has one foot in the grave, with its most recent repair involving some burned wiring (never a good sign).  Both heat pumps need to be replaced. 

No-brainer, right? Just replace them.

Well …

The only firm in my area that specializes in ground-source heat pumps quoted me a price of $50,000 to replace my two three-ton (ground-source) heat pumps.  That’s for the basic model.  Bells, whistles, and line sets extra.  I’m guessing the final cost would end up around $60K.

 

At this point, the only thing I know for sure is that no matter what, this home repair is going to be about like buying a new car.  Or two.

Minus the fun.

Follow along for the next several posts, as I get a handle on what to do next.

Am I a heat-pump heretic?

I drive an EV.  Cripes, it’s a made-in-USA Chevy EV, for that matter.

I re-calculate my family’s carbon footprint every couple of years.

And I bought my house specifically because it had efficient ground-source heat pumps.

But the world continues to change.  And I’m not sure I’m going to be replacing those with new ground-source heat pumps.

And the fact that I would consider not doing that makes me something of a heretic.  But I’m still in the process of gathering my facts.

  1. Twenty years ago, ground-source was the undisputed king of heat pumps.
  2. In part, that’s because air source heat pumps of the time weren’t very good.
    1. They worked inefficiently when it was cold out.  To the point of essentially not working.  That caused use of “secondary heating”, meaning, typically expensive and inefficient resistance electric heating.  In winter, your fancy heat pump spent too much time operating as more-or-less a big dumb electric space heater.
    2. And they weren’t any great shakes, efficiency-wise, the rest of the time.
    3. Plus, they just kind of generally sucked.  Comfort-wise.  In the winter, they always seemed to blow air that was, upon careful measurement, slightly warmer than the existing room air.  Or, at least, that’s how I recall my Maryland apartment of the mid-1980s.
    4. Basically, they were air conditioners that, in this climate (Virginia), could also put out some heat, for some of the winter.
  3. My impression is that this changed about ten years ago.  At some point, cutting-edge air source heat pumps appeared to be — by my calculation — at least as efficient as my 2004-vintage ground source heat pumps.
  4. That’s because air-source technology improved rapidly, while the technology of ground-source units … lagged?
    1. Part of the improvement was in finding a way for air source heat pumps to function well even at low outdoor temperatures.
    2. That went hand-in-hand with greater efficiency of operation.  E.g., modern air-source units might now have variable-speed compressors, fans, and so on.
    3. But not much seems to have happened to ground source heat pumps.
    4. The slower rate of improvement in ground source heat pumps is a side-effect of the vastly lower volume of ground-source (about 0.5% of the home market) compared to air-source (the other 99.5%).
  5. As a result, ground-source heat pumps are no longer a slam-dunk winner, compared to traditional air-source heat pumps.

    1. As a matter of basic physics, they should be.
    2. But because they seem to be behind the curve in efficiency improvements, they aren’t.
    3. The upshot is kind of a temporary tie:  The rapid adoption of more efficient technology in the air-source sector has offset (or nearly offset) the inherent physics-based advantages of ground-source heat pumps.  For now.
  6. There is no point number 6.
  7. But the tax laws still grossly favor ground-source heat pumps over air-source.  And the subsidies are large.
    1. For ground source, the Feds pick up 30% of the installed cost, no limits.
    2. For air source, if it meets certain efficiency standards, the Feds pick up a maximum of $2000 (or 30% of the installed cost, whichever is less).
    3. And Virginia offers an incentive system for ground-source that is beyond weird, and must be described in a separate posting.  At first blush it appears ludicrously generous toward ground-source units.
    4. Separately, I’m not sure they were thinking about replacements of worn-out old systems when they wrote the law.  Effectively, what I’m doing is repairing my existing system, by replacing the worn-out heat pumps. But, legally, that’s treated identically to putting in a brand-new ground source heat pump system.
  8. So, something is not right here.
    1. Is the law outdated, and out-of-step with the current state of technology?
    2. Or is the law a closet buy-American plan, as these ground-source units seem to be U.S.-made?
    3. Or am I dead wrong about the near-equivalence of air-source versus ground-source efficiency in the modern world?
    4. Or, some thing even weirder — geothermal versus ground source discussion to be added at some point.
  9. Curveball:  My first floor would be ideal for a couple of “ductless mini-split” systems.  These are little air-source heat pumps, but instead of being designed to hook up to your ductwork, they simply blow air around like a room air conditioner.  You pass the refrigerant pipe and condensate drain through an exterior wall, between the inside air-distribution cabinet, to the outside compressor.
  10. So, why not replace one of the dead ground source heat pumps with two mini-split air source heat pumps, half the size.
    1. Near as I can tell, I’d pay only a modest or no efficiency penalty for doing that.
    2. And it looks like it would be quite a bit less expensive, even accounting for likely shorter equipment life of an air-source system.
    3. Plus, we’d possibly have a warm kitchen for the first time since we moved here, because we could bypass our near-useless 1959 first-floor ductwork.
    4. Plus, it’s lower risk — more like an appliance, and less like a fixture in the house.  If one of those dies, I can just toss it and more-or-less just plug in a new one.  Not quite as convenient as a fridge, but not hugely different.
  11. But … but … but … the very thought of replacing a ground-source heat pump with an air-source heat pump is … heresy.  Particularly given that the actual “ground” portion of the ground-source system — the mile of plastic “slinky” pipe buried in my back yard — still functions perfectly.

Conclusion

That’s as far as I can take it in this first post.  I need to pin down some facts to go any further.

I bought this house in large part because it had an efficient ground-source heat pump.

But the world has changed since I bought it.

The next post takes the two real-world heat pumps — one a ductless mini-split air source heat pump, one the ground-source heat pump for which I have been quoted an installed price — and tries to get an apples-to-apples comparison between them, in terms of efficiency.

That turns out to be stupidly hard to do.

That’ll be the next post:  SEER, SEER2, EER, EER2, COP, HSDF and all the rest of that alphabet soup.  And how on earth they measure that, for ground-source heat pumps.