HVAC Sizing

HVAC Sizing: What is This All About?

Heating, Ventilation, and Air Condition (HVAC) is an extremely complicated process. It is not sexy, but it is a very important part of building a new home. So let’s start with what it is and how it is done: HVAC sizing, or load calculations, determines the size or amount of BTU’s that a furnace needs to produce to work properly. It works the same way as air conditioning with tonnage.

How it is Done

Load calculations need to be done to meet ASHRAE Manual J specifications. ASHRAE, or the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, includes the Manual J: a very detailed and designed manual to figure out how many BTU’s and tonnage is required for a particular building.

Remember – I did say it was a complicated process, but there is good news. There are many third party software companies that make it very easy to complete this process, like Wright Soft and Elite Software.

Why is it Important?

Having a furnace or air conditioner that is too large causes the furnace to “short cycle,” or run for a short time and produce a lot of heat quickly but then shut down. This does not allow the system to operate properly. Most furnaces should operate on a longer cycle. Think about it this way: you have to warm up the inside of the furnace and all the duct work every time it comes on. Energy is thus being wasted every time you do this; this also applies to air conditioning.

If the unit is too small, it will not produce enough heat to keep the building warm. You may notice this on really cold days or even really warm days. The units will run longer or constantly run to meet the setting on the thermostat.

What Needs to be Done?

Many factors are key when sizing a unit, such as: insulation type and levels, window types, sizes and solar orientations, building air tightness, and many other things the software figures behind the scene automatically. This short video gives a good explanation why one size does not fit all:

Your HVAC contractor should be completing these calculations when designing and choosing the equipment for your new home. Unfortunately, a large number of them never do. They go by a rule of thumb estimate that is not very accurate. When taking bids form HVAC contractors, ask them to include the load calculations with their quotes. If the contractor is not asking about insulation types and levels or window types and sizes, it is likely that the load calculation are not very accurate.

According to buildingscience.com, “Our experience has shown that typical air conditioner sizing generally results in cooling system over-sizing by about 40-50%.” This also rings true for heating systems. In fact, by design Manual J will size heating systems by at least 110%. So when you have an HVAC contractor use the “rule of thumb” method, he could be installing a system that is twice as big as required. Who do you think is paying for that?

While Manual J load calculations are important, they are only part of the process needed to have a comfortable home. HVAC contractors also should be conducting a Manual D. Manual D will tell you room by room correct sizing, layout and number of supply, and return duct work needed for proper air flow to each room. I can tell from experience that very few contractors do this.

What Does HVAC Have to do With Spray Foam?

With spray foam (because it is both an air seal and an insulation), it simply works better than other types of insulation. You can downsize HVAC equipment, which will save you money when purchasing and operating the equipment. I have witnessed in cases where cutting the size in half when using spray foam works perfectly well. Unfortunately, Manual J doesn’t figure this in automatically. I have also seen remodels with additions added that do not require upsizing of existing equipment because replacing the insulation in the remodeled part of the home makes up the difference and allows the current oversized furnace to use its extra sizing to heat the addition.

You wouldn’t buy a pair of pants two sizes too large. Make sure your HVAC system is sized correctly and save some money when you hire a qualified spray foam insulation contractor to insulate your next project. We happen to know a good one!