In my last three blogs, I discussed the basics of heat-loss and cooling load calculations. The unfortunate truth about these calculations is that fast methods aren’t particularly accurate, and accurate methods require making measurements, checking specifications, and entering data into a computer program — in other words, a significant investment of time.
So how should builders go about making these calculations? There are several ways:
- You can use a rule of thumb (along with experience) to estimate your equipment needs.
- You can ask your HVAC contractor to make the calculations. (I don’t recommend this method.)
- You can make the calculations yourself using a simple spreadsheet or an online calculator (like this one from the Build It Solar website).
- You can buy Manual J software and learn to use it.
- You can hire a consultant (usually an energy rater or an engineer) to perform the calculations for you.
Why do we need to perform these calculations?
There are at least two reasons why we need to perform load calculations: to size heating and cooling equipment (ideally, using ACCA Manual S), and to design heating and cooling distribution systems (using ACCA Manual D). Neither Manual S nor Manual D can be used unless Manual J calculations are performed first.
These are valid reasons, so a room-by-room Manual J load calculation makes a lot of sense. If you perform such a calculation, you may save money on your heating and cooling equipment (because it is less likely to be oversized), and there will be a lower chance that the homeowners will have comfort complaints arising from a poorly designed heat-distribution system.
In most areas of the country, a room-by-room Manual J load calculation is required by code. If you don’t have the software yourself, you’ll have to hire an energy rater or engineer to perform the calculations. Very few HVAC contractors are capable of performing an accurate load assessment, so I’d be wary of leaving this task to your furnace guy.
You don’t always have to perform a Manual J
As long as there is no code requirement in your jurisdiction for a Manual J calculation for the type of work you are contemplating, you may not need a Manual J calculation. To understand why, we need to examine two myths that have long been promulgated by energy experts. The first myth is that rules of thumb are inappropriate; the second is that oversizing of equipment is disastrous.
In fact, if you are an experienced builder who understands Manual J calculations, and if you have already built a few new homes, you probably already have a good rule-of-thumb understanding of how big a heating or a cooling system is needed in your climate. This rule-of-thumb method may be perfectly adequate for sizing heating and cooling systems for new homes.
There are a few caveats, however:
- A rule of thumb is only useful if the homes fall into the same general category, with similar insulation levels, glazing specifications, and air leakage rates.
- A rule of thumb won’t work for a house with unusual features (especially unusual glazing features).
- My advice only applies to people who have actually performed an accurate Manual J calculation or who have hired a knowledgeable professional to prepare one for them. If you’re just guessing, you will almost certainly oversize your equipment.
Why am I comfortable with these seemingly risky recommendations?
- Even skilled users of Manual J software usually end up with slightly oversized equipment.
- Oversized equipment doesn’t really carry an energy penalty or a comfort penalty anymore. (Newer modulating or two-speed furnaces operate efficiently under part-load conditions, solving any possible problems from furnace oversizing; and oversized air conditioners aren’t really as terrible at dehumidification as many energy experts claim.)
- As houses have become better insulated and tighter, heating and cooling distribution systems become less important, because room-to-room variations in temperature are much smaller than in older homes.
Most comfort problems in existing homes are due to a poor building envelope (not enough insulation, low-performance windows, and a high rate of air leakage), stupid design details (like big unshaded west windows), and leaky ductwork. Once you control these factors, it’s much easier to avoid comfort problems.
In an old, leaky house, it’s not unusual for occupants to complain that one room is chronically cold or hot. A variety of factors are usually responsible, but it’s a good bet that the walls and ceiling are leaking air, the windows have terrible glazing specifications, and the leaky ductwork is located in an unconditioned attic. The problem is almost never due to undersized heating and cooling equipment.
Passivhaus designers are now building homes with a single heat source (for example, a gas space heater with through-the-wall venting) in a central location. As these homes make clear, as building shells become tighter and better insulated, distribution system design is becoming less relevant.
Why is Manual J so complicated?
The better the envelope, the easier it is to perform load calculations. Moreover, if your envelope is good enough, errors in load calculations don’t matter as much as they do when you have a leaky envelope.
It could be argued that current code requirements for Manual J calculations are only necessary because so many new homes have lousy thermal envelopes. If code enforcement officials were willing to insist on high-performance envelopes, full-scale Manual J calculations wouldn’t be necessary.
It’s easy to imagine the development of a simplified version of Manual J for homes that exceed certain minimum airtightness, insulation, and window-performance specifications. After all, if we know that our envelope is well built, we should need fewer inputs when performing our load calculations.
The wild card is occupant behavior
Energy nerds can be fetishistic about their load calculations. The everyday variety of this species is the Manual J Fetishist — usually an engineer who warns homeowners that they will be uncomfortable and will face high energy bills unless they invest in more engineering.
The more exotic variety of this species is the PHPP Fetishist — usually a young architect who did a year of post-graduate study in Germany. This fetishist spends days at his or her computer, trying to reduce the U-factor of a troublesome thermal bridge in hopes of achieving the magical goal of 15 kWh per square meter per year.
Both types of fetishists are easily defeated by the Common American Homeowner, a casual oaf who buys several big TVs at Best Buy, installs an extra refrigerator, leaves the bedroom window open, and never turns out the lights.
Whoever performs your Manual J calculations, remember this motto: “Be aggressive.” This advice comes directly from ACCA Manual J, version 8: “Manual J calculations should be aggressive, which means that the designer should take full advantage of legitimate opportunities to minimize the size of estimated loads. In this regard, the practice of manipulating the outdoor design temperature, not taking full credit for efficient construction features, ignoring internal and external window shading devices and then applying an arbitrary ‘safety factor’ is indefensible.”
Last week’s blog: “Calculating Cooling Loads.”