The Engineered Approach
We often see engineers approach a geothermal system for large residences as though they are designing small commercial buildings.
This is very covenient for them because they simply take an old boiler cooling tower design and modify it for the new building. The boiler and cooling tower are replaced by an earth loop.
Larger commercial buildings often generate excess heat in their cores because of computer centres and high occupancy loads, even in winter. Usually, only buildings over 200,000 sq. ft. generate enough internal gains to make this type of heat recovery cost effective.
Residences on the other hand, generally do not behave like commercial buildings and even if they did, there would likely be insufficient internal energy available to justify the expense of a large internal loop installation.
Although an engineer’s intention is to secure the best price on a well designed job for their customer, if the job is over designed in the first place, even the lowest price tendered will be significantly higher than it should.
E.g., I recently consulted with an architect who was building his own home in Toronto. A four storey residence (including basement), to have a very large amount of architectural glass, the architect wanted separate temperature control over three air conditioned floors along with a mix of radiant floor heating and forced air heating. The house is well shaded, well insulated and very tight having a total heated area of 4,222 sq. ft. (including the basement). The engineer had calculated a heat loss of 122,000 Btuh. for this building.
The engineer designed a geothermal system for the owner that would utilize a number of small heat pumps situated around the house for zone control as follows:
| Equipment | Description | Service |
| HP1 | 1.0 ton water to air heat pump | heating and cooling basement area |
| HP2 | 1.5 ton water to air heat pump | heating and cooling part ground floor |
| HP3 | 1.0 ton water to air heat pump | heating and cooling part second floor |
| HP4 | 1.0 ton water to air heat pump | heating and cooling part second floor |
| HP5 | 2.5 ton water to air heat pump | heating and cooling third floor |
| HP6 | 3.0 ton water to water heat pump | heating and cooling part ground floor and RFH |
| FC1 | 800 cfm fan coil unit | heating and cooling part ground floor |
| FC2 | 420 cfm fan coil unit | heating and cooling part ground floor |
| ERV1 | 320 cfm energy recovery ventilator | ventilation for HP1, HP2 and HP3 service areas |
| ERV2 | 320 cfm energy recovery ventilator | ventilation for FC1 and FC3 service areas |
| ERV3 | 320 cfm energy recovery ventilator | ventilation for HP4 and HP5 service areas |
| Total | 10.0 tons | approximately $190,000 with ductwork and RFH |
Although the design was professionally drawn up and presented very clearly, installing a total of 10 tons of heat pump equipment and drilling 5 x 300′ deep holes for a home of this size would simply be problematic. Not only would the price to install the system above range somewhere in the $190,000 range once all the drilling, ductwork and radiant floor heating, (RFH) was included, there was very little space available to install the equipment inside.
I was suspicious of possible design errors in the engineers heat loss right from the start because experience told me that a house of this size that was to be as well built as this one, would certainly not require 10 tons of equipment to keep it warm or to keep it cool.
Upon reviewing the original heating and cooling load calculations I noticed several differences in the insulation (R) values used for calculating the thermal loads for the walls, the ceiling and the high performance glass and those specified by the architect. The engineer also calculated the infiltration rate at a higher value than the owner was targeting. The house would be sprayed with urethane foam over all the exterior walls as well as the joist pockets, so essentially the owner was planning on building an air tight building, expecting performance similar to an R2000 home.
I recalculated the heating and cooling loads using the correct vales. I used a calculation method based upon CSA Standard F280, as recognized by the Ontario Building Code. The result was a much smaller heat loss calculation of 66,000 Btuh., much closer to my expectation.
Needless to say this saved the architect a lot of money. The re-designed installation sold for close to $80,000.00, a saving of around $110.000.00. In fact, without the re-design the architect would never have installed a geothermal system – “too much money”.
The new design provides all the zoning flexibility the customer desires along with lower projected operating cost, due to running one higher efficiency, two stage compressor rather than six commercial grade compressors as specified previously. The new design also provides better automation for the winter to summer switchover from heating to cooling mode. Please See Table 2.
| Equipment | Description | Service |
| HP1 | 5.0 ton water to air heat pump with demand hot water heating |
whole house forced air, selected areas of RFH |
| AUX 1 | 9.7 kW internal electric heater | supplements all forced air heated areas |
| AUX 2 | 4.5 kW Rheem mass tank | supplements RFH areas |
| ZCRL | InteliZone Digital Zone Control Panel | provides three individual air conditioned zones |
| HRV1 | 300 cfm dual core heat recovery ventilator | ventilation for HP1 service area |
| Total | 5.0 tons | approximately $80,000 with ductwork and RFH |
One could ask why this happens? Why could there be such a difference?
We all seem to go about things the way we learn them. Most designers learned through the fossil fuel side of the industry. If a sloppy load calculation results in a gas furnace sized one size larger it’s not considered a big problem because it’s only a couple of hundred dollars more and that would never kill a project. With geothermal however, go up one size of heat pump and for an urban residence that can cost as much as few thousand dollars, depending upon drilling conditions and expense.
Also, the larger manufacturers really know residential sizing well. Just as they know not to go too small in northern climates, they also know that you don’t need to cover the whole heat loss with the heat pump running at minimum capacity in January or February. Just cover the large majority of the annual load, say 95 to 99% of the annual Btu’s. and give the short amount of peak cold hours to electric supplemental for $50.00 per year.