Oversizing Equipment

At Thoughtful Balance we are currently designing a space for Hilltop Community Health, a non-profit dedicated to providing health care to under-served populations. The space is a small commercial space about 8,000 sf. It is the goal of the project to minimize utility bills. We designed to meet Passive House standards, and the building is challenging as it occupies a space in an existing building, and of course there are thermal bridges galore! When we received the drawings back from the mechanical engineer we were surprised to learn that our equipment was sized at 22.5 tons! How could that be? ... a Passive House project using 22.5 tons? We reviewed the plans with him in great detail and prepared to challenge his assumptions. We discovered a clerical error that brought our loads down to 18 tons, an honest mistake that happens to us all, but underscores the need for the architect to circle back and check assumptions. We quickly realized that the model our engineer was using wouldn't allow for a wall as well constructed as our wall due to air infiltration assumptions. So, after fiddling with the model, they were able to get a wall that looked closer to ours.

What we were not prepared for was the majority of the problems were in the assumptions that AHSRAE makes. For peak temperatures ASHRAE models for 2 degrees beyond the hottest and coldest days in your climate data. We think this is nuts. Yes, it does protect the engineer in terms of calculating the absolutely largest possible load, but should buildings really be sized for the exception rather than the rule? We also found that assumptions for the equipment and lighting loads were running around 3 watts per sf and our estimates were closer to 2 watts per sf. In addition, ASHRAE stipulates that ventilation rates should be three times what Passive House ventilation rates are. Now, Passive House is known for having aggressive ventilation rates; is ASHRAE's ventilation rate because they are concerned about health? If so, couldn't that be handled better by passing the air past a UV light? This pushed our ventilation rate to one air change per hour: an astounding rate! We are not bashing our engineers - we love them! What can they be asked to do when the industry standard is making wildly conservative assumptions without regard to energy use? They really must follow ASHRAE. So, that is the dilemma: one approach is to try and accurately assess all of the loads, the climate data and the need for ventilation and to hit it on target; the other, ASHRAE's, is to create a large cushion by over-sizing equipment and minimizing risk. It is clear we need to challenge conventional wisdom.