In Touch With Underground Physics

Blog #78 CaliforniaGeo 12-10-20 

One of those twice-a-year events-     

I live in the northern Sierra Nevada at a latitude of 40°North.  Like everyone else we are captives of (or influenced by) the Earth’s seasons.  I’m very much aware that this week we are only about 10 days away from Winter Solstice.  That means there will be two minutes less in day length until we hit December 21st, and then the days will get 2 minutes longer for the next six months until Summer Solstice.  As a solar PV-equipped homeowner, I’m always concerned with my “catch,” and long days are better—especially if they’re cloudless.

A related astrological relationship-

Sure, my solar effectiveness is important as a ratepayer to my electrical utility.  But I am even more fascinated with what happens underground because my home was built with heat exchange in mind from underneath the lot our home was built on.  There are natural temperature cycles at work there that are highly correlated to above-ground  astrophysics.  Unless you’re talking about temperatures below a depth of 25 feet, there are seasonal thermal cycles underground.  The closer to the surface, the greater the variations.  These are the kinds of things that you take into consideration when specifying a ground source (geo) heat pump for heating, cooling, and (usually) hot water production.

The longest (and hottest) days of summer occur well before the temperature under the ground does.  The shortest (and coldest) days of winter occur before the coldest underground temperatures.  This is a “lag effect.”  

The deeper you are underground, the later in the year that the extreme highs or lows occur.  The deeper you are, the cooler the dirt is that you’re rejecting unwanted summer heat into.  The deeper you are, the warmer the dirt is as you pass through the worst of the heating season.  This means that your ground source heat pump doesn’t have to work very hard, particularly when the outside air temperatures threaten indoor comfort the most.

Taking advantage-

The temperature relationships underground allow me to leverage the thermodynamics for my benefit while the above ground conditions display hot, medium, or cold days and nights.  I’m  going to steal copious heat from my dirt in my dominant heating season here in the mountains and push excess summer heat into cool ground.  My heat pump lives in my garage, but it is thermally served by a ground loop that maintains that connection to the earth’s thermal battery without anything but pumped water through a polyethylene pipe.

While air-based heat pump equipment is forced to deal with dramatic changes in air temperatures across days and seasons, I’m working with a stable thermal medium that stores heat energy underground.  Outside air cannot store thermal energy.  My climate is more extreme than most, and we have day/night air temperature that ranges up to 50°F, common in higher altitude arid climates. 

Early humans inhabited caves not just to avoid predators or stay out of storms, but to occupy a moderated thermal environment.  In the case of my geo heat pump equipped home, I get to live above ground, but my heat pump gets to concentrate those thermal cave benefits into my house for me.  My underground deployment of pipe is horizontal, using what’s called a Slinky® configuration.  There are also parallel straight pipe runs, and the most common method is for drilled and grouted vertical bore holes that carry these pipes.  For them, the temperature below 25 feet never changes.  That’s very helpful if it’s sub-zero outside in northern Minnesota.


I’ve lived in my current house since 2013, but built and occupied one for eight years in 1977 that was served by an air-sourced heat pump in this same valley.  I can testify to the difference in comfort and efficiency.  During winter, the outdoor portion of my split system air source unit had to undergo many electrically powered, defrost cycles to keep its fanned coils from building up ice.  (In heating, this unit had to cool outside air in order to export its heat into my heat pump’s refrigerant and then into my living space.  Moist air condenses when cooled, and when the starting temperature gets under 39°, iced coils were often not far behind.

I no longer have this problem because my geo loops are in soil that will not freeze.  They gather heat by conduction (direct contact) while their air-source cousins require transfer heat by convection.  You can simulate the difference by holding your hand 3” behind your idling car’s radiator instead of placing your hand directly on it.  The other advantage of my current system is that its conduction is handled with a methanol/water mix that’s 3,000 times as dense as the air that air-sourced equipment uses.  This means the small pumps I use consume far less electricity than the fans found in air-sourced equipment. 

Toward winter, I experience a sliding reduction in underground temperature where my loops are located, but nothing like the daily grind I’d face if I was using an air-sourced heat pump.  My advantage is shown at right with the black line where my loops live as a much higher temperature than the daily variation shown in the extreme ups and downs of the magenta air temperature.  You can also see the advantage of being seven feet deep instead of three feet deep.  A closer look at the orange and black about six weeks earlier features a twice-a-year crossover, where the shallower depth which has been at a higher temperature in summer now responds to the cold air at the ground’s surface.  The black line exhibits a steadier thermal profile.  Again, in the more common vertical loops below 25 feet, you’re in “the promised land.” 

These same advantages are present in hot climates.  When your insulation can’t protect inside temperatures any longer, a heat pump will start to correct the deficit.  No matter the kind of heat pump, buildings will respond identically.  The difference is that in that hot afternoon or cold middle of the night when your building needs mechanical help—the geo heat pump works with a medium of unchanged temperature.  The air-source unit is working against the most difficult conditions (hottest part of the day, or coldest part of the night).  They consume more power over longer run cycles.

At left, you can see an example.  This morning there was a 27.5 degree difference between what an air-sourced heat pump would be working with (A) compared to what I was working with underground (B).  In in this screen shot, my small geo heat pump is working in Stage 1, and that’s 67% of total capacity.  Paste a copy of the blue URL (from the upper right of the graphic) and you can see a live display of this data anytime, unless my Internet is out.  I’ll confess a prejudice for ground source (geothermal) heat pumps over every other HVAC technology, but that’s a prejudice borne of experience.

Carbon and Zero Net Energy-

When a geo heat pump takes the place of a fossil-fueled heating unit, it’s obvious that no on-site carbon is burned, contributing to emissions known as greenhouse gases (GHG’s).  There are off-site emissions to be considered, and as efficient as geo heat pumps might be, they will consume more than the electricity used to power a fossil-fired heating unit.  But they will consume less than an air-sourced heat pump or an air conditioner that is linked to a fossil furnace.

Off-site electricity generation still uses a variety of fossil sources.  Most states have policy or regulatory targets for increasing the proportion of off-site electricity generated on their grids by carbonless renewable sources.  California’s renewable targets are 33% in 2020 (achieved in 2018) 50% in 2030, and 100% in 2045.  The warmer the climate the less would be this off-site difference in carbon generation by a geo heat pump because the air-sourced unit or standard air conditioner would consume greater electricity for equal cooling duty.  All three types of cooling equipment would consume less off-site non-renewable electrons if they made some of their own using solar PV electricity.

Zero Net Energy is most often defined as consuming no more energy in a year’s time than is produced on-site by renewables.  Wind turbines could provide this but the only practical residential method is solar photovoltaic generation.  And unless an occupant is in love with maintaining batteries and inverters—the cheapest and simplest course is for an NEM (net energy metered) connection with the local electrical utility.  In such a case, the utility serves as your electrical battery.  Therefore, when you obtain or reject heat from/to the earth’s thermal battery using the most efficient equipment available (recognized by the EPA over 30 years ago) you’ll install fewer solar panels to hit that ZNE target. 


HVAC equipment that relies on ambient air often requires design and orientation considerations.  Placement of the outdoor unit on the south or west exposures?  Great for winter but what about summer?  Can that outdoor unit avoid being compromised by snow build-up?  Is noise a factor for homeowners or close-in neighbors?

A ground source (geo) heat pump is independent of all the above worries.  It’s also isolated from airborne debris build-up and vandalism.  And because it works with that thermally steady medium of underground formations by the tie to its heat exchanger, it can operate for up to twice as long as the air-sourced unit.  

The thermodynamics of an underground heat exchanger allows achievement of the lowest LCC (life cycle cost) of any HVAC equipment.  Having used both air and ground source in homes I’ve occupied, my preference for the latter helped me hit zero cost electricity four times out of six operating years with two of those actually achieving Zero Net Energy.

—Bill Martin