Does anybody here have experience with the design and balancing of zoned residential HVAC forced-air systems?
Specifically, I’m interested in systems that have been designed starting with an accurate load calculation, which will assign CFM’s to each register. I assume that the design would apportion the airflow among all the registers, doing so based on the times when all zones would be open.
Balancing would need to be done with all zones open and the dampers would be set to deliver the designed CFM to each register.
In daily use, one or more of the zones would likely be closed, so the open zones would tend to get more airflow, depending on how the bypass damper is set up. The resulting increase in static pressure would deliver more air to each register in the open zone(s), but the airflows would still be in the same proportion as set in the original balancing procedure. (Register “A” might now get 110 CFM instead of 80 CFM, but it would still be the same % of the total airflow available to that zone.)
The result would simply be that the zone would reach its temperature setting quicker, not only due to increased airflow, but also because the heat exchanger gets extra hot due to the bypassed air that recirculates without giving up its heat.
Is this the way you would do it? Or is there more to it?
Replies
Block loads
Most of my experience is in commercial HVAC systems. The few single family residences I have designed and commissioned did include "zoned" systems.
First, the loads need to be run room-by-room to size the air supply and return for that room under design load conditions.But, you simply don't add all the rooms together to get the size of the system, you have to run "block loads", too. The east side of the house is never going to load up (for cooling) at the same time the west side will. So you run the loads for various time of the day to determine the most representative block load and size the system. When I did this, calcs were done manually for 8, 10, 12, 2 and 4. Heating loads are one block load at night, design conditions. With modern software, this is automatically figured for every month of the year and for the design day.
Each zone will see a design load at different times. A CV system fitted with dampers is not easy to balance. The proportion of the each space design load to the selected system capacity will determine the "all open" balancing target, somewhat less than space design max. So in effect, you balance below design max and let the zone system force more through to meet th design max.
Then, when one zone is loading and another is not, you'll get close to the design demand. Its not perfect, but if there is good diversity in the system, it will work well and be more comfortable than a non zoned system.
In this climate, heating loads will almost always require more airflow than cooling, so if the system is balanced to the heating load, it should perform adequately for the cooling load in each zone, because, as you noted, the zones for cooling won't peak at the same times.
So, I'm thinking that balancing the system with all zones open would provide proportional balance.
Sound right to you?
Sounds right to me
If you ran room by room loads and added them up for both heating and cooling, you will find that the proportions are never the same. When assigning a fixed air supply number to a space, you as the designer/engineer have to make the judgement for each space, which is the more important of the two and set the air flow accordingly.
If heating dominates the air flow demands in your environment, that will dictate the duct size determinations and the balancing requirements. Any compromises required to the cooling supplies will be made up with the zoning system.
Balancing the system with all zone dampers open should provide the best proportional balance.
In this climate, heating loads will almost always require more airflow than cooling
I disagree. Assuming you do have cooling loads, cooling loads should always have more air flow as it is dictated more by supply air temp than loads. Cooling supply air temperature differentials are typically around 20 Fdeg while heating may be more like 30++ Fdeg.
If you have a bypass damper, your cfm to the zone shouldn't change much when the system sees load diversity. That's what the bypass is for.
But, you might consider a variable speed fan system to better serve you (although in most residential applications this may be considered overkill).
I understand your climate ... used to work in Spokane. What I was trying to say was that it is the delta T across the coils that tend to dictate cfm ... with cooling it is like 20 Fdeg ... with heating it is like 30-50 Fdeg. Doesn't matter what the load is, the delta T will be similar. My experience is that if you have too small ducts for cooling, you'll have some serious cooling system problems.
While your heating load is higher, your system's ability to deliver 2-3 times the Btus is more easily done, because you may have supply air temps say 110+ (40 Fdeg delta T). On cooling ... your supply air will/should be no less than 55 degF, so you need a lot more cfm to accomplish the same Btu load.
I think that given the same house, your cfm will be dictated generally by house size. If you assume you need cooling, cooling should dominate the cfm equation ... generally regarless of disparity between the heating/cooling loads. You still deliver the same supply temps and it will be that that affects the cfm. If you have a bad solar exposure in the summer (e.g. west windows, unprotected), your CDD will mean little and your cooling load could easily be triple that of the conductive thermal load (which your load calcs should account for).
You're right, though about 'wanting' AC and 'needing' AC in your climate. But again, it depends on the orientation of the house and the amount of windows unprotected from the extreme sun (e.g. late afternoon sun on the west side). A poorly oriented house could need AC in your neck of the woods even with 'only' 300 CDD. The solar load can easily dwarf the conductive loads on a house ... particularly if oriented poorly.
Your comments highlight the fact that a couple of considerations need attention:
1) The need to deliver the correct amount and temperature of cooling air to each space
2) The need to unload the cooling coil with sufficient airflow
#2 is often a problem with zoned systems, especially with an over-sized condenser in the system--it's true that the bypass damper will open to relieve excess static pressure when only one zone is open, but the bypassed air short-circuits back to the coil with very little capacity to unload the coil, and the possibilty of freezing the coil and/or slugging the condenser.
The heating side can be a problem too, if the already-heated bypass air pushes the temperature of the heat exchanger too high.
This is straying a little from my original Q about balancing, but it all works together, and I appreciate your thoughtful and accurate responses.
Sounds like you have a reasonable handle on this. Comments: On the cooling side, what will happen in light load conditions is that the compressor will cycle frequently, unless you have e.g. a multiple compressor system or maybe a system that is e.g. variable speed (I think these are being made available. Generally, you shouldn't have a freezing coil problem, I don't think as the returned air RH will be relatively low. You shouldn't be able to slug a compressor that is already running; I think that occurs e.g. w/ a cold startup.
On the heating side, the furnace would also have a tendency to cycle on low load (i.e. high return air temp) as it is controlled by the air temp off the heat exchanger.
It seems the primary benefit of a zoned system is to provide better comfort and maybe some efficiency (although that will depend on other factors). The bypass damper is really not the best way to provide zone control. It is the poor man's way to variable flow to zones.
Better methods would be inlet dampers on the supply fan (not typical in residential) or a variable speed fan (more typical). But a variable flow fan system must be accompanied by a system capable of variable Btus ... e.g. a hydronic hot water and cold water coil(s). A hydronic system doesn't have the issues of cycling the furnace/compressor.
Back to the need for zoning. Zoning is great if you have a disparity of loads .... which residences CAN have either due to poor orientation or wide variations of use of your rooms (e.g. living room vs. bedroom vs. family room), etc. We could discuss the true variations of room use at length and get nowhere ... but you have to judge your need for yourself.
If the house is very large, consider separate systems. Also consider rather than closing zone dampers to allow them to go only to a minimum flow. This reduces the girations the system has to deal with.
I may have missed it ... but what is the primary reason you want to zone your system? Also what are some of your parameters? (e.g. house size and system type(s).
... just more food for thought.
Let me clarify that my Q's about balancing zoned systems are not for my house. I am in the process of writing a manual or guide that will be used statewide to help residential HVAC contractors to learn the process of air balancing, which more and more jurisdictions are requiring as the finishing touch to the code-required designs. (Load calcs in accord with ACCA Manual J, equipment selection by Manual S, and duct design by Manual D) Manual D includes the requirement for for balancing, but gives no specific procedures for accomplishing it. ACCA has recently published a balancing manual, but it is too comprehensive for a residential-only application--and it's expensive.
Proportional balancing of simple residential duct systems is relatively straightforward if you have a good design to work from--room-by-room load calcs; CFM requirements; equipment performance; proper duct sizing and, of course, balancing dampers.
But before writing the the methods for balancing zoned systems, I am gathering information from as many sources as possible, and I have found it useful to hear from you and others who have a different perspective than I do--I am a mechanical plans reviewer and inspector and I realize that this gives me different and--in some ways--limited view of the problems in the field.
Much better, thanks. Your approach is correct ... all zone dampers full open. If there is a bypass damper, full closed. And you are right, residential applications are relatively simple. A possible issue would be measuring total air flow of the fan. My balancing is a bit rusty, but I think balancing always checks fan speed and total air flow. Maybe this is largely a non issue, but after you design, you choose a furnace and then you potentially have a much different total cfm. Maybe you just assume supply is e.g. 20% greater than the total design and the intent is to balance to design. Biggest problem there is to ensure it is not substantially oversized (the fan).
You imply that a design is fully laid out on the plan which tends to be a bit unusual for residential ... although I think it would be good to have. Too often a contractor holds his thumb up and says ... eh ... let's put in a 6 inch duct here and an 8 inch there. Even if 'designed', the layout is often done in the field which ends up w/ many elbows that maybe weren't 'in the plan'.
IMO there should be a balance damper, not just adjustable registers. The damper must be accessible ... which is a possible problem depending on the layout. People don't want access doors in drywall if they are in a prominent location.
Quite frankly I think ALL ducted systems should be balanced, not just zoned systems. They should all be designed, too.
We might be a little ahead of the curve here--all new house plans must include a reasonably accurate design that includes the loads, equipment selection and duct design. A balance report must be submitted when the house is finaled.
Although the IRC now (since the 2000 edition) requires residential HVAC systems to be designed according to ACCA standards, I believe there are few juristdictions that actually follow thru with requiring the designs to be submitted.
Many (most?) contractors in the country size equipment and ducts as you said.
I believe that the HVAC system is the most complex system in any house, but, in our area, at least, the HVAC tradesmen are the least-trained of all the licensed trades. It's changing gradually since the State started licensing them 6 years ago, and also started an apprenticeship program that includes schooling. Our local jurisdiction was the first to require designs, and we sponsored training to help contractors learn the process. That was 7 years ago, and most are now competent and comfortable with the process. Since then, other regions of the state have followed suit, and are in various stages of progress in implementing the design requirement.