Posted By jonr on 07 Jun 2012 09:38 AM
I would put some care into balancing the supplies and returns such that you aren't pressurizing or depressurizing rooms. That causes infiltration and exfiltration which leads to heat loss and possibly wall moisture issues. Anything short of an open door will not self balance.

Not an open door, but pretty self-balancing nonetheless.

I was actually a bit surprised at how well they fared under third party field testing.

Stuart, if you do end up proceeding with a third zone, a couple of hopefully helpful comments:

First, since I think you have a Web Energy Logger (WEL), it would be a good idea to set up a monitor (and an alarm) on the air temp coming right off of the evaporator.

Here's an example for me, at http://www.welserver.com/perl/plot/...rTemps.png 



The dashed brown line is the air temp right off the evap coil for my 3 ton unit for the past 4 weeks - it stays at 49 - 50°.

You may have to do some damper adjustments for each zone to ensure enough air is always flowing through the evap coil to help it avoid a freezing scenario.    I.e., all dampers, in the closed position, may actually have to be set partially open.

To avoid freezing the coil, you'll need to ensure a Leaving Air Temp (LAT) of at least 10° higher than freezing - i.e. 42°.

In my case I have 3 zones on my 3 ton unit.  I had to do damper adjusments (resulting in none of them completely closed in the 'closed' position) to keep the LAT high enough.  I've got my WEL alarm set at 45°.

The second suggestion is to look at using a zoning controller that has connections for temp sensors.  These controllers will open up all zones if LAT gets too cold.

Overall, I think doing zoning properly is particularly challenging.  It's critically important to ensure LAT is above 45° no matter what's being called for by an individual tstat.

Best regards,

Bill

I am still wavering on the zoning. I've put out a few requests to companies that do external solar shades for the skylights. If the cost is right, I might try that route first.

But, Bill, I u detest and what you're saying. I could put a sensor right behind the huge exchanger in the air handler to measure the temp immediately as it comes across the coil, then make sure it never goes below 42, which it could if the zoning was restricting the airflow too much...right?

I kinda like the idea that when the system is on, a little air goes to every zone anyway, but most air goes to the zone(s) calling for it. Would probably help with balancing the temp anyway. I usually always leave the fan running anyway.

Thanks for the ideas. I will update if/when I proceed.

Posted By decafdrinker on 11 Jun 2012 06:01 AM
... put a sensor right behind the huge exchanger in the air handler to measure the temp immediately as it comes across the coil

... make sure it never goes below 42

... which it could if the zoning was restricting the airflow too much...right?

I kinda like the idea that when the system is on, a little air goes to every zone anyway, but most air goes to the zone(s) calling for it. Would probably help with balancing the temp anyway.
 
I usually always leave the fan running anyway.

My temp sensor for LAT is indeed right "behind" the evap coil.  'Closed' damper positions have been adjusted such that LAT is almost always about 49 - 50° regardless of which single zone may be calling for cooling.  I also have my zoning controller set such that the compressor can't move to 2nd stage if just one tstat is calling for cooling.

If you want to implement a control strategy instead of simply bleeding air to zones that are not calling for cooling, then you'll need one of the fancy zone controllers that has inputs for LAT temp sensors that then cause the controller to open all zones at a certain set point temperature.  (My WaterFurnace supplied zoning controller does not offer this feature.)

Indeed, if you don't have minimum blower fan speed set appropriately and/or you have many zones (more than 2) and/or one of the zones calls for markedly less air than the others then it would be pretty easy to freeze up the evap coil (LAT = 42° or less).

Zoning, done in a manner that does not reduce the long term reliability of the HVAC system, is tricky.  If you have another alternative, such as external shades, I recommend prioritizing the alternative above doing zoning.  Especially if the zoning scenario is 3 or more zones.

BTW, since you leave the blower fan running, you must be residing in a much more arid climate than me.  Doing so here in the Dallas area would increase inside RH considerably.  The cyan line on the chart I illustrated above is my inside RH, where over the past 4 weeks it's ranged from 46 to 42%.

Best regards,

Bill

Thanks Bill (and geodean). I don't think my zoning panel has the option for either LAT inputs or locked Stage 1 unless multi-zone is calling. It's ZP432 Honeywell, but I've read the manual a few times and if it had that option, I probably would have enabled it. It makes sense in many situations.

Thanks Bill (and geodean). I don't think my zoning panel has the option for either LAT inputs or locked Stage 1 unless multi-zone is calling. It's ZP432 Honeywell, but I've read the manual a few times and if it had that option, I probably would have enabled it. It makes sense in many situations.

Hey Bill,

Impressive charting. Just curious..what are the two lines on the chart in the 20° range?

Posted By geodean on 11 Jun 2012 09:27 AM
Impressive charting.
 
... what are the two lines on the chart in the 20° range?

Thanks Dwayne.  After a few years of using the WEL, and with about 20 supported, I've come to appreciate that the WEL's capabilities are extensive, and charting is one of its stongest capabilities.  For my personal implementation (WEL0043), my charting ranges from 36 hours (Power Usage) all the way out to multi-year (i.e. Heating & Cooling Cost).

The two lines on the 4 week Evap Temps chart, that are in the 20° range, are the air DeltaT values across the evap coils (blue = 5 ton unit, dashed violet = 3T).  (I can't make the chart appear any larger here because the forum s/w prevents image illustration > 500 pixels).



I use this 4 week chart to occasionally glance at to affirm all is well.  These values are sampled and held, BTW, to eliminate all of the graph 'noise' that otherwise would occur when the units are not running.

The DeltaT values are further used to compute EER, which is displayed on the real time system diagram (but charted offline because EER needs to be displayed in a statistical histogram manner).

This 4 week chart, showing return air temps (EAT), supply air temps (LAT), and DeltaT temps, for both units, is a pretty fundamental chart indicating overall 'health' of the equipment.  Anything other than generally straight lines across the chart would indicate there's something to go check on.

About a year ago I added a 7th line to the chart - inside RH (cyan line).

Holler if you have any questions or if there's anything I can be helpful with.

Best regards,

Bill

> put some care into balancing the supplies and returns

That applies to ANY duct system and has little to do with zoned supply system.

It has everything to do with it if you zone the supply system and don't zone the return system in an otherwise sealed room. It will depressurize the room and cause infiltration.

Early in the inspection biz an old pro commented on duct work "it doesn't hafta hold water".
He is exactly right. While all the wisdom leans toward duct sealing et al, there are some benefits to duct leakage. For instance temporing a crawl space to protect plumbing in the winter. Even worse I had a commercial retrofit where the ME had me seal all the duct work at the cost of many man hours and then cut basement registers in the same trunk.

By the same token zone dampers are not meant to hold water. You want the path of least resistance to be the area you are trying to condition. What you don't want to do is reduce airflow to a level unsuitable for the appliance- particularly on the return side which is why all the pros will tell you they never damper the returns.

Path of least resistance = greatest air flow to that area. If you can't get it cooled or heated then; you have a bigger problem.

Point is hyper tight duct work means you probably have to design in bleed for both supply and return. Returns are intended to be a little more passive or less active and capitolize on free communication of air between zones. No they don't have to be, but they generally are. You wanna dampen returns, you are going to have to go back to manual J and look at the whole system.

I've never dampered return air. If a customer absolutely had to have a zoned system able to maintain radically different teperatures in different zones, then dampering return air would become necessary and particular caution taken to ensure minimum airflow under all single zone call scenarios.

In general, I tell customers that the object of zoning is to maintain all areas of the house at nearly the same temperature even as solar and internal loads change throughout the day. A corrallary benefit, ductwork permitting, is to allow the system to concentrate all its capacity into the central "public" zone on special occasions. If they are hosting a major get together with lots of guests, cooking and door opening, raising setpoint in unused parts of the home such as an upper floor, bedroom wing or master suite during the party allows the system to focus all its power on the central zone for the duration of the event.

Zoning allows for energy savings when part of the home is unused during the day. A recent customer likes to chill bedroom zone at night but let it ride up during the day. Near bedtime they raise the main area stat and lower the bedroom area stat. That allows the system to operate in low stage nearly all the time - quiet and efficient.

Can they have 75 in the bedroom zone and 85 in the main zone at the same time? Of course not, owing to common return, but they understand that.

For zoning to actually save energy requires tight ducts and a tight house. At typical existing home duct and building leakage rates found in utility survey testing in CA, the increase infiltration and duct leakage losses to the house from unbalancing the system by closing registers or zone valves exceeded the savings of letting the zone rise in temp, increasing rather than decreasing total energy use. This subsequently which led to duct tightness & remediation requirements for new construction & remodels under CA Title 24, and a requirement that duct designs comply with Manual-D on all new construction.

Even with tight ducts and a tight house the savings (when they exist) are quite modest- almost a wash- zoning ducted systems is more about comfort than true energy savings, despite what our intuition tells us should happen.

I completely agree - zoning is about comfort, quiet, and better dehumidification. The energy savings alone won't pay for it. It does typically allow me to round down the tonnage with greater confidence, and that is valuable.

I won't risk a downsize w/o before and after ductblasts, and any project involving substantial ductwork gets blasted.

Not only would duct zoning never pay for itself, energy savings of duct zoning are NEGATIVE, a energy COST in many (or even most) instances. Even if the ducts are tight, it doesn't mean the house is. But the tighter both are, the less any of it matters.

Besides increased energy costs, I'd also consider the increased potential for moisture in the wall problems. Pushing/pulling moist warm air into an exterior wall is risky.

That's not as risky on the left half of the US as it is on the right half, but yes, infiltration/exfiltration drives can increase moisture condensation on the leakage paths.

Cooling season infilrtation condensing moisture inside building assemblies is primarily an issue in the southeastern US, where dew points can linger in the high-70s, and rooms might be cooled to the low 70s. In New England or the upper midwest the outdoor dew points are only rarely above the conditioned space temperatures for days on end. But during the heating season exfiltration paths matter in those regions, since outdoor temps can stay well below indoor dew points for months.

Posted By Dana1 on 14 Jun 2012 04:11 PM
Not only would duct zoning never pay for itself, energy savings of duct zoning are NEGATIVE, a energy COST in many (or even most) instances. Even if the ducts are tight, it doesn't mean the house is. But the tighter both are, the less any of it matters.

Are there studies or data that support the contention that zoning costs energy?

In 2003 the Lawrence Berkeley National Labs' Energy Performance of Buildings Group (http://epb.lbl.gov/residential/res_index.html published the seminal study that demonstrated the problem. It used to be directly downloadable from the LBL publications site, but it doesn't seem to be working today- maybe the server is down for something- it may come up later:

http://epb.lbl.gov/Publications/lbnl-54005.pdf

It's widely cited & quoted, and subsequent studies by utilities (IIRC PG & E was one), have replicated the results. It's often referred to in advertizing for duct-sealing services, but also referenced by state energy conservation departments, etc. eg:

http://www.consumerenergycenter.org/myths/vents.html

Somewhere near the bottom line, tight ducts, tight house= no problem. Typical house, typical ducts- increased energy use is measurable.

Fair points.

Several counterpoints:

1) As noted above, we make a reasonable effort to seal any ducts we work on

2) We do not zone single stage systems, nor systems without ECM blowers

3) We control duct velocities and pressures in cases of small zones by either or both of providing extra duct capacity to small zones and / or cracking (leaving somewhat open at all times) the damper to a larger zone; never do we use bypass dampers.

4) Most of our zoned systems are in homes with sttic sprayfoam, rendering duct leaks nearly moot.

Attics & homes with spray foam shouldn't be considered air-tight unless VERIFIED air-tight with blower doors. While they're usually at least somewhat tighter than "typical" construction, it's far from assured.

With very low leakage tested ducts the increased leakage due to the additional pressure of zoning puts the increased energy use "in the noise". Duct commissioning with a tested leakage limit of 6% max is now required for new construction under CA Title 24 (incorporated largely in response to the LBNL-EPB and PG & E field testing.) There is a revision in the code (to take effect in 2013) under consideration to convert that commissioning spec to a max of 24 CFM & 25 pascals number rather than a percentage, along with inserting flow requirements and other specifications not in the currently active version of Title 24. They're also considering adopting the IRC2012 3ACH/50 whole-house limit See:

http://www.energy.ca.gov/2012publications/CEC-500-2012-062/CEC-500-2012-062.pdf

http://www.energy.ca.gov/title24/2013standards/prerulemaking/documents/2011-05-31_workshop/presentations/Res_Infiltration_and_Testing-053111.pdf

If you're doing compensation for velocity & pressure and taking measures to seal ducts you're probably at the head of the class nationally, and would likely hit CA Title 24 2013 spec by adding a verification testing/duct-commisioning step.