Zoning a Geothermal System
Zoning splits one system into separate areas, each with its own thermostat, using motorized dampers and a control panel so conditioned air goes only where it is called for. Geothermal takes to it unusually well: a variable capacity machine can throttle down to serve one small zone, and the ground loop is a shared resource every zone draws from.
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I am Dave Hartzell. 47 years in this trade, 15+ years running Hartzell’s Heat & Air out of Kingfisher, IGSHPA accredited for geothermal.
What zoning solves
One thermostat serves one spot well and every other room by accident. A west facing great room takes the afternoon sun, a north bedroom never sees any, a walkout basement stays cold in July, a second story collects what the first sheds. Those rooms peak at different hours, so one thermostat can only average them.
That house is short on control, not equipment. Generic dampers and panels are on my HVAC zoning systems page. This page is what changes when the machine is geothermal.
Zoning versus separate systems
One machine, zoned
Dampers in the duct, a control panel, a thermostat per area. Less money than a second system, one machine to maintain. The catch is that everything hangs off it, so when it is down the house is down.
Separate systems
Each area gets its own equipment and costs more up front, always. That buys redundancy, one unit down leaves the rest conditioned, plus an exact size per area.
The part people miss is the loop. Separate geothermal units can share one loop with the flow split between them, or run separate loops, and sharing is usually cheaper, because the loop is the largest line item on the job. A shared loop is sized for what they do together, not two nameplates added up, which is why what a ton means matters here.
I pick one zoned machine when areas share peak hours and the duct carries it, separate systems when they behave like different buildings: a guest wing, a shop, a pool room.
Geothermal is unusually well suited to zoning
The loop is a shared resource. The ground is the heat sink and the heat source for every zone at once, so when one zone calls and the rest are satisfied, the loop is simply doing less. That makes a shared loop across zones a clean design, not a compromise.
A variable capacity machine can throttle to a small zone. A single stage air source unit has one output, full. Close most of the zones and it makes more capacity than the open zone can absorb, satisfies it in minutes, and shuts off. That short cycling is why a house sits at temperature and feels clammy, the mechanism behind geothermal and humidity. A modulating unit steps down toward the load calling, and long and low is what dries the air. See how a geothermal heat pump works.
I do not publish a turndown percentage, it belongs to the specific model. I check the machine’s minimum capacity against the smallest zone before I design.
The load calculation is the whole ballgame
Zones come out of a room by room load calculation, not a floor plan and not square footage. It gives me a heating and a cooling load for every room, and groups the ones whose loads behave alike.
Get zone loads wrong and you manufacture the problem zoning was bought to solve: a zone that overshoots, one that never satisfies, a damper hunting all afternoon. By then it is in the duct, which is why I will not quote zoning by phone. It comes out of a geothermal site analysis with the Manual J load calculation in it, free on a new system.
Bypass and airflow
When zones close, the duct the blower pushes into gets smaller, but the machine still needs air across its coil. The old answer is a bypass duct, supply back to return with a damper. Done badly it wrecks efficiency and comfort, because air routed around the house comes back already conditioned: in cooling it ices the coil, in heating it trips the limit.
So I would rather not lean on bypass. I match capacity to the smallest zone, use an ECM blower that cuts airflow as dampers close, and give what is left a dump zone in a large open area. I fix the return path too, because most houses here are short on return air.
I do not publish a static pressure target or a bypass size, they belong to the equipment and the duct in your house. I put a manometer on the air handler and design to what I measure.
Special load rooms deserve their own zone
Standard zoning assumes every zone is a room with people in it. Some are not: an enclosed pool or spa room, a sunroom that is mostly glass, a shop with doors that open, a wine room holding a narrow band year round, a server closet making heat in January. Their difference is latent load, the moisture, not sensible load. The load calculation decides which.
An enclosed pool room is the clearest example
I have run exactly this: a lake house retreat with an enclosed pool area that had to be its own zone, worked through with my distributor’s engineering group.
- Water evaporates constantly
- The pool surface never stops evaporating, a latent load running around the clock, unlike a bedroom whose moisture comes and goes with the people.
- There are two setpoints
- The water has a target and the room has one, linked: the further apart they sit, the harder the water evaporates. Heating pool water suits a ground loop.
- Chlorine and salt differ
- Both put a corrosive atmosphere in the room, changing what the coil, cabinet, grilles and duct are made of, and it condenses in wall cavities unseen. Salt water is not chlorine with the chemistry swapped.
Thermostats and controls
What a homeowner lives with is the control system, because the dampers sit above the ceiling. Placement beats features: a thermostat in direct sun or above a supply register reports a room that does not exist, and the zone chases it.
Retrofit zoning, and where it stops
Retrofit works, and the honest limits are in the duct. Zoning redirects airflow, it does not create any. An undersized branch improves slightly when dampers close elsewhere but is not fixed, and undersized returns get worse.
So I measure first: static pressure, duct sizing, the return path, the loads. Sometimes the duct needs work before zoning is worth buying, and I would rather say so in your hallway than after you paid for dampers. On a new system it is designed with the duct, part of how I install a geothermal system.
I go statewide for geothermal
My seven daily service counties are the everyday heat and air territory. Geothermal is not limited to them. I have worked in and around Kingfisher, Okarche, Hennessey, Watonga, Piedmont, El Reno, Yukon, Edmond, Enid and Oklahoma City, and I go to Lawton, Broken Bow and Durant for geothermal work. One shop, in Kingfisher, and I drive.
Questions people ask about zoning a geothermal system
How much does it cost to zone a geothermal system in Kingfisher Oklahoma?
Any figure quoted before the load calculation is a guess. What sets it: how many zones the loads justify, whether the duct can carry them, and whether a special load room needs its own equipment. Estimates are free on a new system.
Is zoning better than installing two separate geothermal systems?
Neither is better in general. One zoned machine costs less and fits a house whose areas peak at similar hours. Separate systems buy redundancy and an exact size per area, and either way they share one loop.
Does an indoor pool room need its own zone?
Yes, in nearly every case. It runs a constant moisture load off the water surface, it has two setpoints pulling against each other, and the air is corrosive, which changes the equipment.
Do you design geothermal zoning outside your daily service area?
Yes. Geothermal is statewide Oklahoma for me, from Lawton to Broken Bow to Durant, not just the seven counties I run heat and air in daily. Call 405-375-4822.
Let me run the loads before anybody draws a zone
Architects and builders: send the plans. I will run the loads, name the zones honestly, and design the loop and duct around them.
Estimates on a new system are free. A Geothermal Diagnostic on a down system is $348 flat, any hour. IGSHPA accredited, Master HVAC licensed. Hartzell’s Heat & Air, 602 S Main St, Kingfisher OK 73750.