Solving the Difficulties of the Low-Load Home
I found out last month that the Air Conditioning Contractors of America (ACCA) is working on a new design manual. You certainly know of some of their other manuals: Manual J, Manual S, Manual D, Manual T…maybe even Manual P. They have quite a few others as well (H, Zr…) but now they’re working on one that will address a part of the market we encounter more and more often. I’m talking about low-load homes, of course.
ACCA put out a call for volunteers to work on the manual in the spring of this year. (Applications were due in April so it’s too late to apply now.) The objectives of the new manual, at least as specified in the call for volunteers, are:
- Defining low load homes characteristics (i.e., low infiltration, sealed combustion appliances, ducts in conditioned space, low CFM exhaust fans, etc.).
- Resolving ventilation requirements (for occupant health and safety) while maintaining moisture control.
- Addressing ancillary dehumidification equipment for humid locations.
- Offering air distribution strategies for occupant comfort; especially problematic when a 2000 square foot home (and larger) may only need one-ton of air conditioning (hence, only 400 CFM of total airflow is available).
At the ASHRAE conference in Houston, I spoke with someone who’s on the task force and was told they’re defining a low-load home as a house that has a house-size-to-load ratio of 1,500 square feet per ton or greater. That sounds about right to me. At the low end, that means a 2,000 square foot house would need only about 16,000 BTU per hour of heating or cooling capacity. That’s an air conditioner or heat pump smaller than 1.5 ton!
And that’s just the starting point for low-load homes. We’ve done load calculations for homes that are in the 2,500 to 3,000 square feet per ton range. (See my 2016 article with data from our results.) Now we’re talking about a 2,000 sf home that needs less than a ton of cooling. As the objectives above point out, heating or cooling 2,000 square feet with 400 cubic feet per minute (cfm) of air flow is a challenge.
This is an important issue because manufacturers have been slow to make low-capacity equipment for low-load homes. Yeah, I know what some of you are thinking: Why should manufacturers make smaller equipment since low-load homes are just a tiny niche in the larger market?
Ah, but that’s not really true. Low-load homes may be a tiny niche among single-family detached homes but not when you get to multifamily homes (apartments and condos). Because of all the adiabatic walls, floors, and ceilings, many multifamily dwelling units hit that low-load threshold of 1,500 sf/ton simply by meeting code. And to make it even worse, they’re smaller.
Back in 2012, Professor John Straube gave a full-day presentation on mechanical systems for low-load homes at Building Science Corporation’s Experts’ Session. He’s a Canadian and gave only a number for heating, but he defined a low-load home as one having a peak heating load of 15,000 to 30,000 BTU/hr. Most code-compliant multifamily dwellings would be there or lower.
Passive House is the ultimate low-load home program. They’re all about increasing the insulation and airtightness while decreasing the heat transfer through thermal bridges. Their requirements result in homes with heating and cooling loads in the range of 2,500 to 3,000 sf/ton.
In our HVAC design work at Energy Vanguard, our clients often end up going with something other than conventional equipment for the homes with really low loads. Ducted and ductless mini-split heat pumps are the standard choice in those cases.
I’m interested to see what the new ACCA committee comes up with.
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