There's no such thing as a "40-ton" GHX (140 kW)

One of my pet peeves is people referring to a GHX as if you can pick it out of a supplier's catalog...much like selecting a cooling tower or boiler. Let me give you an example. Let's say you were asked to design two systems across the street from each other. One building is a church, the other a store. You calculated the peak heating and cooling loads for both buildings, and by coincidence, both buildings had identical peak heating and cooling loads. Both the store and the church had a peak cooling load of 40 tons (140 kW) and a peak heating load of 300 kBtu/hr (87 kW).

Great! This will be simple! The geology is pretty much the same...the buildings are across the street from each other. Both have about the same land area for a vertical GHX. Two 40-ton (140 kW) systems. This will be simple! 40 tons, 180' per ton (52 W/m) in this area is what everyone else is putting in...so 7,200' (2,195 m) of drilling should do it. 

OK...back up a bit. Do these buildings have average loads? Think about it...the church may have a 40-ton (140 kW) cooling load, but when will that load be there? Sunday morning for a few hours and that's about it. The rest of the week there's no one in the church, there's no lighting on every day. The store on the other hand has a lot of customers every day, the lights are on, and, oh yeah...the front of the store is facing west with a lot of glass, and the glass is fairly clear so people can see in, and the door is open a lot. So the store has the same 40-ton (140 kW) cooling load, but it's running all the time. Maybe we should take a look at the annual energy loads. These charts show the peak heating and cooling loads of the church and store, with the peak cooling load of 140 kW highlighted in the right hand blue column and the peak heating load of 87 kW highlighted in the right hand red column. The peak loads in both buildings are identical. The graphs to the right of the charts illustrate the energy profile of the two buildings graphically. You can see how much more cooling the store requires than the church over the year, and how much less heating it needs on an annual basis.

The annual energy loads of the buildings are shown in the left hand blue and red columns. Compare the total annual kWh at the bottom of each of the kWh columns in cooling (the blue column) . The total cooling kWh of the store is just under 280,000, about 4 times the total cooling kWh of the church (just under 70,000). Also note that the total heating kWh of the church (just under 180,000) is about 3 times the total heating kWh (just under 60,000) of the store. The amount of heating and cooling each of the buildings needs is much different, even though the peak heating and cooling loads are identical. Does it make any sense that the size of the GHX is the same? So when we use the complete energy profile to calculate the size of the GHX field, what do we get?

We can calculate the amount of borehole required for the GHX to operate at a minimum temperature of 35°F (2°C) and a maximum temperature of 90°F (32°C) using commercially available GHX design software that takes into account both the peak cooling and heating loads as well as the annual energy loads. The results are shown on the graph.

The borehole length required for the church is close to the estimate for based on the local "rule of thumb", but the GHX for the store is 22% longer and the temperature can be expected to continue climbing after the 5 year prediction shown on the graph. So which one of these is a 40-ton (140 kW) GHX?