Letters This month, Larry Spielvogel discusses HVAC during warm-up or cool-down, and CIBSE Journal technical editor Tim Dwyer responds Overcoming setbacks The article Oversizing: Escaping the trap in the July CIBSE Journal, omits a very important consideration: warm-up and cooldown. Better thermal quality and todays airtight buildings mean sufficient additional HVAC capacity is required to raise or lower the temperature of buildings after being set back, especially after long weekends or holidays, and in severe weather. Safety factors as a percentage of design load are no longer adequate, because the loads are much lower than before. Too many intermittently occupied buildings using the methods described by Richard Green are unable to recover temperatures in a reasonable time, so the buildings maintain comfort temperatures for longer or all of the time, thus wasting energy. There is little support in literature for claims that oversizing HVAC for large commercial buildings wastes energy when properly designed and operated. The ASHRAE Handbook says: Engineering judgment must be applied for the particular project. Armstrong et al1,2 provide a design method to deal with warm-up and cool-down load. Some computer programs and HVAC equipment manufacturers advocate diversity during operation when sizing equipment to reduce its capacity and first cost. But there is rarely any diversity available when warm-up or cool-down from setback is needed. Another important judgment relates to future loads that may be imposed over the HVAC systems life, such as higher internal heat gains or ventilation. Engineers have a responsibility to evaluate all heating and cooling loads, and to use their judgment when selecting HVAC sizes. Larry Spielvogel FCIBSE, ASHRAE past president Tim Dwyer replies In most buildings, there will be a need to size the heating/ cooling plant and distribution system so that they may operate at setback temperatures during unoccupied periods and then be conditioned ready for occupancy. Running continuously at conditions suitable for occupancy is likely to be severely detrimental to the energy use, environmental impact and operating cost. Although this needs to be properly considered and appropriately designed (and controlled), I do not necessarily see it is a deficiency in this article, as it was focusing on controlling oversized VRV systems to operate more effectively during times of normal occupation. There is a good applied discussion of this in section A5.10.3.3 of Guide A Environmental Design (2015) and in section 3.2 of CIBSE TM41 Degree days: Theory and application, (available on the Knowledge Portal See part three of Dwyers overview of Guide A here. References 1 Armstrong, PR, Hancock III, CE, and Seem, JE, 1992a/2b. Commercial building temperature recovery Part I: Design procedure based on a step response model. ASHRAE Transactions 98(1):381-396. 2 Part II: Experiments to verify the step response model. ASHRAE Transactions 98(1):397-410. CIBSE Journal welcomes readers letters, opinions, news stories, events listings, and proposals for articles. Please send all material for possible publication to: editor@cibsejournal.com, or write to Alex Smith, editor, CIBSE Journal, CPL, 275 Newmarket Road, Cambridge, CB5 8JE, UK. We reserve the right to edit all letters.