HVAC

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HVAC Duct Sizing Method Debate

Philip Wemhoff: I enjoyed your recent article in Home Energy. However, I have some comments.

The discussion about slide rules and assumed pressure drops ("FSEC investigators recommend that a friction-loss coefficient of 0.05 inches water column (IWC) be used with slide rules, rather than the almost universally used 0.1 IWC.") is very disappointing. Duct sizing using recognized engineering methods is the correct approach, not these nonsensical pressure drops. In Florida, the energy code specifies how this must be done, but the code is broken by nearly every contractor, every single day. And the result is enormous problems in terms of less comfort, reduced air flow, and energy loss. The Florida Energy Code, in section 610.1.ABC.1, says that "All air distribution systems shall be sized and designed in accordance with recognized engineering standards, such as the ACCA Manual D, ... and based on the following: (a) Duct size shall be determined by supply air requirements of each room (the greater of the heating load or sensible cooling load for that room); (b) the available static pressure, and; (c) the total equivalent length of the various duct runs. Friction loss data shall correspond to the type of material used in duct construction."

Danny Parker: The report on our study (Field Evaluation of Efficient Building Technology with Photovoltaic Power Production in New Florida Residential Houses; FSEC-CR-1044-98. Cocoa, Florida: Florida Solar Energy Center, November 1998) does describe the proper way to do business (Manual D), but prescribing that only and not addressing what is going on in the field, I believe, is the greater travesty. Right now, 99% of residential duct systems in Florida are being sized using duct slide rules with 0.1 inch water column (IWC) pressure drop at a given air flow (the "duct slide rule method," sometimes referred to as the "ductalator method"). The problem is that the 0.1 IWC value assumes smooth, round, metal duct (not flex duct), so even that crude assumption is violated (this would not happen if you use a flex duct multiplier to compensate for duct roughness). My simple observation is this: AC contractors use duct slide rules and undersize ducts (always). Making the simple (and achievable) change to using 0.05 IWC with a duct slide rule will not be in compliance with the proper way to do things (Manual D), but it will result in ducts that can achieve an air flow that is a much closer to the desired air flow.

Saying that everyone must use Manual D is a worthy ideal. However, it is more than a bit quixotic. Remember that only about 40% of residential ACs are sized using any quantitative method. Changing to a much lower friction coefficient, however, is something that can be implemented and will result in better sized ducts. I am also aware of the error of the duct slide rule method in dealing with branches.

So, I wish the article had mentioned Manual D as you suggest, but I still believe that, if taken, our advice would result in a dramatic improvement of the situation relative to what is going on now. Sorry to disappoint. We'll emphasize Manual D next time out.

PW: When sizing ducts, the use of one value throughout will guarantee incorrect duct size for most segments. If 0.05 is used, almost half of the runouts of the system will be oversized, creating zones that are too cold in summer, for example; the remaining runouts, usually the furthest from the air handler, will be undersized, creating zones that are too warm in summer. The result is a design and outcome failure. As you know, the modified equal friction method of Manual D requires that the available static pressure from the fan be "consumed" by the duct through its run from fan to outlet/inlet, with no shortage or excess at the end. Also, pressure drop per 100 ft is not an input--it is a calculated intermediate value. A contractor who knows how to do the calculations to determine the available static pressure, and correctly allocate it to supply and return, is a very rare person.

Further, a recommendation to use 0.05 IWC with duct slide rules will not help. It will still be ignored, because you have not presented overwhelming evidence as to why it should be used. You see, contractors really don't think duct sizing is important, and many don't even use ductalators anymore (the wrong answers are already etched on their memories). Education is needed. Duct size, duct leakage, and AC charge are invisible to them. It is a problem of attribution--to what do we attribute the comfort problem? Well, the answer seems clear to contractors, as demonstrated by their actions: They think the problem is AC tonnage. Duct size, leakage, and charge don't exist for them, so in their view, Manual J must produce incorrect results, and should be ignored. Contractor's conclusion: Let's make the AC larger.

What I'd love to see is an article with side-by-side comparisons of the duct sizes and attendant air flows resulting from 0.1/0.05 IWC duct slide rule method versus the proper method (Manual D). All I'm saying is that we need to promote the correct methods and not reinforce the incorrect ones. Rather than assume a faulty industry standard (and code compliance in Florida), we assume typical practice, which is substandard and a code violation. Where would we be if we assumed that all ducts must continue to leak, because leak sites are the typical practice, and that contractors can't change?

When it comes to duct sizing in Florida, almost every contractor without exception breaks the law on every job he does. Who does that hurt? The public, who in many instances, pays my salary.

DP: You raised some important exceptions to the simple advice we provided on better duct sizing. You are right, of course; one should use Manual D for sizing duct systems. Contractors should also use Manual J to size HVAC systems! However, I still maintain that all other things equal (that is, done wrong), using 0.05 IWC with a duct slide rule will result in more appropriate air flows than the 0.1 IWC that is currently the nearly universal convention among contractors. Regardless of the method used (and this is important), the friction coefficient for flex duct--which is the rage in new construction--is not the same as that for round duct on those duct slide rules or in the tables in Manual D. This makes a big difference in the resulting air flows. Manual D says the pressure drop for flex duct is twice that of round smooth duct. [Editor's note: The book A Builder's Guide to Residential HVAC Systems (see "Resources," p. 46) points out that few data are available for flex duct pressure drops other than for straight runs.]

PW: I do agree that 0.05 IWC will be beneficial. I should have said that. The main benefit is that the air flow across the coil will improve, so the user will benefit from appropriate efficiency and capacity. The downside, as I mentioned, is that the sub­air flows to the zones of the house will likely be wrong, causing spaces that are too warm or too cold. So, the design will be a failure from a comfort standpoint. And energy use may suffer somewhat as well, if the user adjusts the thermostat to achieve comfort conditions in an underflow space.

In any case, technology doesn't always change for the right reasons. Till you came along, Danny, India--a nation with good engineers and little air conditioning--made the best ceiling fans (see "Cutting-Edge Blades Slash Fan Energy Use," HE July/Aug '99, p. 7). But Americans didn't like the looks of them when they were marketed in the 1970s and '80s. They preferred the less efficient paddle fans (now called Casablanca fans), having seen them in the Bogart movie.

Oversized Boiler in Earthen Home?

Even with no heat input, a "normal" house takes several hours to drift a noticeable amount in temperature (even a low-mass house takes at least an hour to lose a couple of degrees of temperature). So if the water heater needs to warm up at the same time as the house, a house that's already warm couldn't drift in temperature enough for even the most paranoid homeowner to notice during the 15 or 20 minutes it takes to heat the water.

Some installers try defending their usual oversizing habits by saying, "What if the house is coming out of a night setback and hot water is needed in the morning? You need that extra capacity then, right?" Wrong. Here's why: Most people use a programmable thermostat for night setbacks anyway. Simply set it to come on 30 minutes sooner, so that the house and the tank of water can warm up in a reasonable amount of time. "But what if the owner doesn't want to warm the house up 30 minutes sooner?" That's probably fine, too, because if the boiler is sized exactly to the heating load alone, then except for 2.5% of a typical winter, the boiler has a fair margin of excess capacity anyway, so there will be no noticeable temperature drift in the house either.

Now if you still worry about those few instances when winter is particularly harsh, the water in the tank will still get hot. It might take a little longer or the house temperature might drift a degree or so, but in my opinion this is a very minor penalty when compared to the cost of the fuel saved during the other 97.5% of the winter and the equipment money saved from not buying the bigger boiler.

Michael Lamb
Energy Efficiency and
Renewable Energy
Clearinghouse
Merrifield, Virginia

Editor's note: The article listed the Voyager unit as a boiler, but it is actually an 80-gallon high-efficiency water heater. (We apologize for the lack of clarity.) However, we are interested in the question of sizing and approached Scott Robbins, co-owner of Madcon Incorporated, the company that designed and installed the system, for a response. He told us:

I always try to size the system to the heating load of the building. The size of the water heater, however, is not relevant to the energy load of the house, because heat is transferred from the water heater to the radiant floor system through a heat exchanger. The water heater loses only 1/2°F per hour. It is not fired when there is a heat demand, but only when the water temperature falls enough below the setpoint differential of the water heater, which is 20°F. The large differential keeps the water heater from cycling during the typically mild weather in Sonoma, California. The Tekmar control determines the amount of energy that is needed and puts in only what is needed through an injection pump.

In any case, we had originally suggested a 130,000 Btu boiler, but the clients wanted a larger water heater to meet the needs of future home expansion.
 
 

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