Are Zoned Heating/Cooling Systems a Good Idea or a Bad Idea?

For starters, I live in Atlanta, Ga and I am not an HVAC pro so this is a serious question that I would like answered for one of my clients.  I understand the premise of a zoned system and on the surface, it sounds like a good idea. Then I considered the fact that I have always told my customers that it is a bad idea to close off vents in rooms they are not in because of duct leakage.  Also, the second law of thermodynamics simply states the hot goes to cold, so now the unheated areas are doing their best to rob warm air from the heated areas.  Finally, if your zoning a single system, aren't you creating on oversizing issue because the unit (that was probably oversized in the first place) is now servicing a smaller are than what it was designed for?

 

I look forward to your input.

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If all zones from a single HP system are on the same floor level or above grade walls, it would probably be OK in Atlanta. However, conditioned basements are best left on a separate system that can handle lots of moisture. Attic rooms with insulated roof decks may be better served with a minisplit.
This is a 3700sf 2 story on a slab.

I live in the Dallas/Ft Worth area and am not an HVAC pro myself but I have talked to variety of reputable HVAC professionals on this topic and offer the following.

 

Strictly from an economic viewpoint, in the short term, zoned systems tend to have an installation price advantage over non-zoned systems on larger structures simply because less equipment can be better utilized over a larger area. However, in order to achieve better equipment utilization requires a more complicated control system which tends to create more maintenance issues in the long term.  So, in my view, zoning provides the installer with a tool to reduce the first cost which is the name of the game to get the job.  The long term costs of owning that sysem is pushed off into the future which I consider a bad idea.

 

Technically, I think your last question is addressed by zoning proponents with variable speed compressors.  However, the slower the compressor speed, the higher the air coil temperature which affects the unit's ability to dehumidify the home adequately.  Install HVAC equipment with warmer air coils in a home that has received a good job of air sealing in an area with high humidity in the summer and you have yourself a recipe for mold disaster in the home.  This is a big deal in Dallas/Ft Worth and, I suspect, a bigger deal in Atlanta.

 

When it comes to dehumidifying an air tight home in a high humidity location, the coldest air coil is your best friend.

I am not an HVAC installer but a design professional, but do have solid grounding in field practice.
In my experience, the question goes to, "How closely is my equipment sized to the load?". Secondly, "How well will my system adapt to changing loads?" (ie: modulating or staged fire or modulating AC capacity).

We all know that an over-sized system, be it heating or cooling, is less efficient and tends to cycle more. In cooling, a loss of dehumidification is the first casualty. In heating, we know that every time a burner ignites, it goes through a minute or two of combustion stabilization, before anything resembling "steady-state efficiency" even begins. If a system cycles off within this time, such momentum is lost.

Now, take a home with a heat loss, on a design cold day of 40,000 BTUH (40 MBH) in a single zone. Your system (furnace, boiler, does not matter), has an input of 60 MBH and a net output of 54 MBH (90% combustion efficiency). This was the smallest appliance available, it is what it is. Let us also say, for discussion purposes that this system has on-off control, no modulation, all or nothing.

What this means is that, on the coldest day, your system is 35% over-sized. Every day, every hour it is warmer outside, this margin of over-sizing increases. If your outdoor design temperature is 20 degrees and the temperature outside is 40 degrees, your heat loss is, for discussion purposes, 24 MBH. Now your system is 125% oversized or 2.25 times the needed capacity. (54 / 24 = 2.25).
That is a lot of cycling.

Now, take this 40 MBH system and divide it into two equal 20 MBH zones. (Just works out this way for discussion.) Say that one of the zones calls on the coldest day, the other one is in setback or sleeping, no load.

Now your system demand, 20 MBH, meets with a single-zone 54 MBH output appliance. The over-sizing surplus is now 170% or 2.7 to 1, this being the coldest day. When the outdoor temperature rises to 40 degrees and your zone heat losses are now 12 MBH, your over-sizing is now 54 / 12 = 4.5. A far cry from an ideal 1:1.

Extend this to more zones, even a smaller single family room for a cold night huddle with the remainder of the house at setback. Greater over-sizing.

What this means in an air system: Obviously more cycling, possibly more "cold between cycle" perception.

What this means in a water system: Cycling, sure, but if the system has cast iron radiation, higher mass or a buffer tank, one can bridge over any "cold-between-cycles" and not perceive discomfort. Hot water also allows differential control or firing the burner in a variable range around a setpoint, to reduce cycle periods and losses.

A few thoughts anyway!

Brad
I appreciate the input, but in our area we don't use boilers or radiators. 75-80% of the systems are gas heat and electric. Also, the case that got me started on this question is a 4 ton unit w/ 4 zones (3 upstairs and 1 down)

Hi Jon, 

4 zones? What's your sq footage approx? Very surprising that you have so many zones. Variable speed system?

I'm retrofitting my home as we speak into 2 zones. Top floor, main floor.  I have the option of a third with my controller but the third zone basement would have been 288 sq ft with 8 ft ceiling. So, going to save my third zone on the controller for any future relay fails on the board. It can happen. Enough about me. Lets talk about you.

How satisfied are you with your home's HVAC? Do you know if the home has _ever_ called for all the zones at once?

(Sorry to hear about your clients drama. Felt like I was watching Mike Holmes)

Jon,

Answer to your initial thread question: zones are good. MUCH better than oversizing.

Hi PJ,

Cut and paste:

  • Economics- Over sized equipment has a higher initial cost and costs more to operate.  Higher operating costs include lower system efficiencies due to short cycling and higher power draws.  The short cycling also reduces the life of the compressor. Starting the compressor, compressor fan, and indoor fan motor causes high amp draw while just running longer rather than quicker start stops can decrease costs.
  • Reduced System Efficiency – The highest efficiency rated HVAC equipment is the multi-stage equipment.  These systems are designed to run in the lower output stage most of the time, which allows longer run-times.  When a multi-stage system is oversized, the efficiency that was to be gained, is lost as the oversized system will cycle on and off in lieu of running continuously.
  • Reduced Comfort – Oversized HVAC equipment reduces the system’s ability to remove moisture during the cooling season – a very important function of the AC system.  The relative humidity in the home plays an important role in comfort.  Homes with lower humidity levels (50%RH to 60%RH) feel more comfortable at higher thermostat settings.  This saves energy.
  • Higher Noise Levels – Oversized HVAC equipment produces more noise than properly sized equipment.  They can also create annoying high air speed levels that interfere with paper and book use.

From http://www.energyht.com/ManualJ/ManualJ.htm

Also good is http://optimalbuilding.com/files/elephant_in_the_room.pdf which talks about HVAC for High Performance Home

Oversized equipment sucks from a comfort prospective, especially for gas furnaces. The heat comes on, blows you out of the room, then it's freezing before it cycles again. Furnaces do tend to be sized for CFM in the southern climates. A 45,000 BTU is good for 1200CFM, any more than 3 tons of A/C requires a bigger furnace.

Seems like we're sliding into politics here.

I read the study you are showing. http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-328-97/

Figure 4 shows that 17 hours out of the day the oversized units used more electricity than the undersized. 


Conclusion: There is an increase in annual cooling energy use for homes that have oversized systems.


Reading their conclusions, I don't see it being trivial that oversizing units puts excess demands of electrical infrastructure. This goes beyond costs. Why #1pay more for a larger unit and then #2 waste electricity?

Poorly trained HVAC companies multiplied by number of units installed per year times number of years in business. Every drop in the ocean counts. Everything counts.

From the sidelines, this is a draw. Your both right, if you live in Florida. North of the dehumidifier line, it's all about heat. Here in the mountains  we don't need AC and contractors will tell you that the most cost effective and comfortable system is a HEAT pump with humidity controls. I am saving about $100 a month this winter because my 180% over-sized heat pump has a new thermostat that virtually eliminates the strip heat. About cooling, the new thermostat has a programmable fan only option (which runs in the slow mode) and a humidistat. My cooling cost was less than $20/month last summer. This spring we will add a hot water heat pump that will dehumidify the basement in addition to the hot water at about 1/3 of the resistance heater cost.

However, the cost effective part may soon disappear when the subsidies dry up and taxpayers get wise that electricity is not 100% efficient. How that factor got into energy modeling programs is a testament to the lobbyist in DC. How about somebody blogging on the cost benefits of taking the AC out of DC. 

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