Geothermal Heat Pump part 1: The new system

Just in time for winter 2014-2015 I had my HVAC system replaced.

Previous system 

I had a dual-fuel heat pump, which is a regular heat pump with an outdoor thermostat that turns off the heat pump and switches to gas heat when the outside air temperature gets below 40 degrees. The system was a Bryant 5 ton, 10 SEER,  7.0 HSPF system.  However I discovered during maintenance that the the outdoor thermostat had never worked correctly, so for all practical purposes I had been heating almost exclusively with propane.  This resulted in some monster propane bills because propane pricing is high, and volatile.  

I also discovered during maintenance that my system was on the verge of failure. It was 15 years old and with most of the equipment in a vented crawlspace, 15 years of condensation and rust had taken its toll.  After several months of deliberation and getting quotes, I decided to replace it with a dual-fuel geothermal system.

What about backup heat?

However unlike the previous dual-fuel system, the new one would switch manually to backup heat, meaning that the backup propane heater would only come on when I manually switched over to it, not automatically at a set temperature.  This way I could control the system and know exactly how effective the geothermal heat pump is at generating heat on its own.

My contractor did not think I needed the backup propane heat at all, and neither did most people with experience in this area that I talked to.  They told me that since a geothermal heat pump draws its heat from the ground, which maintains a steady 60 degrees year round, it would always be able to produce heat year round no matter how cold the air gets, without needing a backup heat source.  This is in contrast to standard heat pumps, which can't draw heat from subfreezing air so they need a backup heat source for when it gets too cold outside. Standard heat pumps also require a defrost cycle, during which backup heat runs, and geothermal systems do not.

But I had another motivation for wanting backup heat: power outages.  Out in the country where I live power outages can last a long time.  I figured if I lost power during an ice or snow storm, the backup propane furnace could be easily run from a consumer grade generator, as you really only have to power the blower.  A gas furnace runs off a standard 110v 15 amp circuit.  A heat pump really can't be run off a normal, relatively inexpensive generator, not only because of the level of current it needs, but because they are sophisticated machines that aren't very tolerant of the kind of "dirty" power produced by a consumer grade generator.  

In 13 years at our place, our power had surprisingly  proven to be pretty reliable with Progress Energy.  But then Duke Power bought Progress Energy in 2012 and almost immediately service got worse.   Shortly after Duke's acquisition of Progress closed, a routine summer NC thunderstorm that might have resulted in a 2-3 hour outage with Progress caused a 2-day outage with Duke.  It seemed clear that with the new power company, I would need to be more prepared for power outages than before.

So I had the propane backup heat installed even though the experts said I didn't "need" it.

The installation: ground field

The installation of the new system was a lot harder than anyone expected, because of the ground field.  The ground field is the network of underground pipes that draw heat from the ground (or in the summer, discharge heat to the ground).

Caved-in geo trench

The original plan was to install these pipes in trenches in my pasture.  This would require seven trenches, each about 150 feet long and six feet deep.   However the digging of the first trench revealed a problem:  Because of the composition and moisture content of my soil, the trenches that were dug were unsafe to work in... several cave-ins occurred almost immediately as the digging was underway, fortunately with no one inside.  The trenches were not going to be a safe method of installation. This required a plan B.

Horizontal drill

Plan B was to use horizontal drilling to install the geo lines.  This is actually a superior, though more expensive method.  Fortunately for me the contractor and subcontractor kept to the originally agreed price, so I got a better method for the same price.  However the horizontal drilling equipment was finicky and subject to multiple breakdowns, and the availability of the right expert personnel to run it.

The bottom line is that the geo field that was supposed to take about 10 days to two weeks to install, took over two months.  But the end result was superior.  The lines were deeper down (8 feet instead of 6) and there was less surface disturbance in my pasture (though it was still significant).

One other advantage of the damp, unstable soil, is that it conducts heat better than dry soil.    So with deeper lines than planned, and wetter soil, I got a better performing geo field than expected.

The installation: equipment

ClimateMaster Tranquility
Geothermal Heat Pump

Trane Clean Air Effects
whole house filter

I definitely went whole hog with this system.  In for a penny in for a pound, I guess. It's a Climatemaster Tranquility 5 ton system, with a Trane Clean Air Effects central filter, backup propane furnace, and desuperator.

Condenser and backup furnace

The desuperator is a feature of geothermal systems that uses waste heat to heat hot water.  The desuperator doesn't provide all my hot water needs, but it preheats water before it goes into the hot water tank, so that the actual hot water heater (propane powered in my case) has much less work to do.

The total system cost was, well, high.  However because it's a geothermal systems it qualifies for green energy tax credits that, in the end, make it cost about the same as standard heat pump system.

The installation: encapsulation

As part of this installation I also had my crawlspace encapsulated and conditioned.  This means that my crawlspace was sealed, insulated, and is heated and cooled like the rest of the house.  This is becoming the new standard in the humid southeast as we are starting to realize that vented crawlspaces in humid climates are a really bad idea.

This is your condenser in a vented crawlspace

The problem with the previous foundation standard is that during the hot summers, the foundation vents draw in warm, humid air which then condenses on anything in the crawlspace that is cooler and drier.  This includes all the HVAC ductwork and all the HVAC equipment that's under there, which in my case includes the condenser and the backup furnace. The amount of mold and mildew this caused was terrible, not to mention shortening the life of the equipment because of rust.  You can see to the left the condition my condenser was in when it was removed.

Showing a crawlspace pier,
this shows how thoroughly
the plastic is applied to seal
the crawlspace

The vented crawlspace had another problem:  critters.  Raccoons had figured out how to remove the foundation vents from my crawlspace and had moved in along with possums and who knows what else, with extremely unpleasant results.  The crawlspace was NOT someplace you wanted to go, and the odors from the crawlspace were getting into the house because the critters had damaged the flexible HVAC ducts under there, resulting in them drawing foul crawlspace air directly into the house.

Showing a side wall of the
encapsulated crawlspace
with R-10 insulation board
the plastic runs up the wall behind
the insulation board to ensure
sealing

In the encapsulated crawlspace, all openings are sealed just like they are in the living areas of the house, which keeps critters and humid air out. The entire floor is covered with 20 mil plastic that goes up the walls and piers to ensure a good seal. Also all the exterior crawlspace walls are covered with R-10 insulation boards. The existing fiberglass insulation under the floor is also removed because it's no longer needed and was in pretty bad shape because of mildew, condensation, and critter damage.

 The space is also conditioned just like the rest of the house, by opening a couple of HVAC vents into the space.  This keeps the humidity down in the space.  The crawlspace essentially becomes an extension of the living space.





The credits

HVAC Contractor: Anthony Maynor
Geo field subcontractor: Lothridge Geo
Crawlspace encapsuation:  Triangle Reconstruction

They did excellent work, handled everything, kept to the original estimate despite unexpectedly bad soil conditions and I would recommend them.