Perpetuating "rules of thumb"

Too often I've seen a set of tender documents put out to bid with an area blocked off on the site plan that says "drilling contractor shall provide GHX that has a capacity of 720 MBH and will be responsible for performance of the GHX." That's kind of like going to your doctor with chest pains and he says "go see the fellow in the nice suit...he's from the drug company. He'll sell you the medication you need"

This engineer is telling his client the he should go see the drilling contractor to get the specifications for the GHX field. Go to the fellow who will benefit the most by selling you the most drilling he can install on the site. No different than the doctor telling you to see the guy who benefits the most by selling you the most expensive medication he can. 

He's not providing the information needed to design the GHX...he's simply saying the "total field heating capacity to be a minimum of 720 MBH". To design a GHX field properly a designer needs a lot more detail, including:

• At least monthly energy loads for the building, but would be more accurate with hourly energy loads. This engineer is only providing the peak heating load for the year. Does the peak load last an hour, a day, a week, a month???
• Thermal conductivity and diffusivity of the ground the vertical boreholes will be drilled into. Is it glacial till, clay, limestone, granite???
• What is the flow rate needed for the project?

This project was built about 2 years ago. It's actually working. The design was based on typical rules of thumb prevalent in the area. The peak heating load is estimated at 720,000 Btu/hr (211 kW). The heated space is 29,000 square feet...about 25 Btu/hr per square foot. Divide 720,000 by 12,000 - that's 60 tons. The engineer is suggesting in the note for the contractor that the boreholes should be 61 m (200') deep...and there should be 60 of them...200' of drilling per ton. 

And the engineer got paid for this? 

The sad thing is that in this case the system actually sort of works. And that's how rules of thumb are perpetuated!