Heating Offset

Sunday, June 9, 2013

The prevailing wisdom in peak load calculation is to ignore internal gains (heat energy generated by occupants and equipment) on the heating side. This approach guarantees that the heating system will be able to keep the space at set point temperature in the event that these heat sources are turned off or removed from the space. This is the most conservative design practice and should be followed in all cases if there is a reasonable possibility that the usage profile of the building will change.

That said, there are many LoopLinkers that have requested a means of accounting for static internal gains (energy that will always be there) to minimize the size of the installed loopfield and equipment costs based on a designed usage profile. This posed a difficult problem for our engineers and sparked surprisingly lively debates about whether accommodating these loads was ‘valid’ and in the best interest of the industry.

Ultimately, we decided to add the new Heating Offset field to each zone. The Heating Offset allows the system designer to account for static internal gains in each zone. The energy in the heating offset is pulled out of every heating bin during the bin analysis so it can have a significant impact on the final loopfield design. Additionally, the heating offset will change the calculated outdoor air temperature at which the geothermal system will start supplying heat (Heating Start Temperature). Which is why, we at Geo-Connections all agree that Heating Offset should be used with caution.

Consider Yourself Warned

The Heating Offset has the potential to significantly reduce loop lengths on the heating side. If an arbitrarily high heating offset is defined, the loopfield will not be sufficient to meet demand. In other words, your customer will not be happy. Also important to note is that if the next building owner changes how they use the space, your carefully calculated usage profile is irrelevant and your undersized loopfield could leave them with a low opinion of geothermal.

When is Heating Offset Appropriate?

Heating Offset should only be used in zones with known long term usage profiles. Accounting for a refrigerator in the kitchen is a reasonable application since there is a low probability that the next homeowner will not have a fridge. Accounting for six televisions in the living room is probably not a great idea since the next homeowner isn’t likely to have a similarly awesome vegetative command center.

Heating Offset can also be used with systems that include passive heating strategies that have a known minimum amount of energy that they will store in the building. The key here is that the heating offset should not exceed the worst theoretically possible performance of the passive system in the design year. This will minimize the likelihood of under-sizing the loopfield and becoming overly dependent on supplemental or dual fuel systems on your coldest days. This also helps in the case of a future building owner not understanding the passive heating strategy and planting trees in front of those south facing windows.