By John Turner, P.E. | November 15, 2023

One of my favorite seminars of my professional life was by a Boston based Mechanical Engineer.  The topic was how to size a replacement heating/cooling system for an existing building.  The presenter suggested that no amount of theoretical analysis of building envelope R-values and window U-values and building envelope volume could make up for a boots-on-the-ground measurement of “Duty Cycle”.

If it’s 40 Degrees outside, and the 100,000 net BTU boiler is operating at 90% duty cycle, then it’s clearly significantly undersized.  Far more often, however, the reverse is true.  It’s a 2,000 SF Colonial with a 200,000 BTU Steam boiler.  It runs at 10% duty cycle most of the winter and thus is horribly inefficient.  Fortunately, when oversized heating systems are installed, it’s largely just an energy problem.

Cooling systems are another story.  Traditional split system air conditioning systems are all or nothing.  5 tons of cooling sounds way better than three – especially when Houston humidity and heat are outside.  However, if that cooling system drops the temperature too quickly, then the humidity remains – and that’s a perfect recipe for mold.

The good news is that many modern heating and cooling systems can actually modulate heating and cooling capacity based on conditions.  It’s why so many new HVAC systems are incredibly efficient.

The theme here is that in the world of HVAC – at least for New England – the best HVAC design should be versatile – and possibly even utilize multiple technologies.  I recently visited the “Green Team” at Bedford Presbyterian Church.  The main sanctuary is on the Historical Register.  This portion has old, leaky windows, no wall insulation and is only utilized one day a week.  Converting this space – in its current condition – to an electric heat pump – is not the right move.  Keeping the dualing oil fired furnaces – is probably the best move – lots of BTUs to warm the place up quickly on Sunday morning.

Conversely, their Church Offices and meeting rooms – utilized 7 days a week – and constructed in the 1990s with good insulation and windows – is a fantastic candidate for conversion of the current oil fired heating system to high efficiency electric heat pumps.  This is a super example of “hybrid heating systems” – using the right technology based on the building envelope, the use of the space and the best energy choice.

Let your favorite engineers help guide your next generation of HVAC technology choices!

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