Will smaller zones damage HVAC system or save money?

[Mike Mc]

Hi,

My house is in the Sacramento area (a Hot-Dry climate) and the power company is forcing everybody to move to a TOU (time of use) plan that charges upto 38 cents per KWh in the summer, so I'm looking for ways to reduce electricity usage and AC is the biggest power consumer.

My house is 3000 sqft 2-story facing south and has a 12 year old 120kBTU natural gas 2 stage furnace with a 5 ton AC coil. It has a 20" round return and three 12" round hard pipe supply (upper west, upper east, downstairs). Each supply has a damper. Currently the upper east and west supplies are controlled as a single zone (two of the 12" dampers are open for upstairs). Only one 12" damper is open when downstairs runs. Each 12" supply has 3-4 floor mounted registers.

Since the zone controller has a unused 3rd zone and the system has 3 dampers, I'm considering adding another thermostat and controlling the upper east and upper west supplies separately. My concern is restricting a 120kBTU / 5 ton system to a single 12" round supply could damage or shorten the service life of the system, cause leaks in the ducts and increase duct noise. My research suggest this system should have 1800-2000 CFM, but a 12 round pipe should only have 700-750 CFM. I have a similar concern in heating mode, but slightly less so since the 1st stage heat is usually sufficient and has lower CFM requirements.

Is there a minimum CFM requirement for this system? Are there other things to consider before separating the control of the upper west and east zones? Google couldn't find any specs for these units.

Could I potentially save money separating the upper zones? I realize it is difficult to answer since there are lots of variables with 4-5 (adults and teenagers) coming and going all day. The potential savings would come from not cooling or heating one of the upper zones as much or at all during the day.

Thanks

-Mike

Equipment:
2 Ecobee4 thermostat (just installed)
American Standard Freedom 90 2-Stage furnace AUX120R960W5 (92 AFLUE)
American Standard AC coil TXC065S3HPC0 (SEER 13)
Honeywell universal mini zone controller (supports 3 zones).

 

[JerryR]

Wow, up to 38 cents/kWh? Outrageous. Our rates are 10 cents/kWh all year all times and days. My 3100 sq ft Florida house averaged $160/mo with $216 highest electric bill in last 2 years. $40 of that $160 is from my pool pump motor running 6 hours a day.

You may want to consider replacing the 12 year old ac unit with a newer 20+ SEER inverter system. Trane, Amana, Daikin all make good systems.

My 5 ton Daikin inverter system will modulate from 500 cfm to 1800 and the compressor also changes output based on what each thermostat calls for.

I have 3 zones. The bedroom zone is the smallest. At night we shut bedroom door and turn up the thermostats in other zones to 82 degrees. The ac then limits output to 600 cfm max and the compressor runs at max 30%. We don’t hear any noisy vents.

Even with this setup we have a powered bypass damper that maintains supply duct static pressure regardless of how many zones are open. http://www.ewccontrols.com/acrobat/090377a0061.pdf
https://ewccontrols.com/

You can see here where it’s 86.8 degrees outside, the combined thermostats are calling for 39% compressor and delivering 39% compressor output. You can also see that zone board is calling for 680 CFM and delivering 670 CFM output.

 

[Sponsor]

 

[WorthFlorida]

You have no chance to save with teenagers, fact of life.

FYI the SEER 20+ units get very expensive and the added cost you'll need to do the math if the payback is in a practical range. These smart SEER high efficiency units really work well until something breaks. To control the fan speeds, the outside condenser fan and the air handler fan, are ECM type motors. That is the 220v AC at the motor is converter to DC. A data lead with voltages of 0-12v dc is what controls the fan speed. Another data lead reads the RPM. It work really well until one of these motors go bad. Right now the dealer cost for one of these is around $600 and AC companies are charging over $1000, usually. Don't forget the control boards for these, also very expensive. Fortunately, CA does not have many electrical storm as we do in Florida so right there you'll have better longevity.

All HE units are heat pumps so changing yours out can help in the heating time of the year. A five ton unit, with today's efficiency AC units may just be too large. As far as the duct size, these smart high SEER units read the static pressure to regulate the airflow, if its too much back pressure or lack of airflow, the unit may error out and shut down and assumes that it iced up. Carrier units will do this and the error is on the thermostat display. You may have to get measurements on your current system.

This is not answering your questions but if you're going to upgrade, questions to ask.

 

[JerryR]

At 38 cents/kWh in a hot climate, would think that the payback would be pretty quick.

My 4 year old, 5 ton Daikin Inverter system came with a factory 12 year 100% parts and labor warranty. Daikin support has been exceptional. We recently had a compressor fault when running heat. Service rep was out in 1 hour, verified compressor fault, called Daikin second level support and they approved replacing the complete outdoor condenser unit per their policy. At 8 am the next morning a brand new 5 ton unit was delivered and installed.

We ran heat in emergency heat mode (heat strips) that one night before new unit was delivered.

 

[JerryR]

When I posted my electric usage I forgot to include that my average $160/mo bill not only includes running the 21 SEER 5 ton Daikin Inverter system to heat or cool our 3100 sq ft home and run the pool pump 6 hours/day but it also includes charging my Nissan Electric car.

 

[Jadnashua]

Restricting the flow over the evaporator coils in an a/c unit can cause them to ice up, especially if it is humid. There's usually a min/max air flow listed in the manual that you should respect.

 

[JerryR]

Restricting the flow over the evaporator coils in an a/c unit can cause them to ice up, especially if it is humid. There's usually a min/max air flow listed in the manual that you should respect.

That’s the beauty of an Inverter system. There’s a DAT (discharged air temp) sensor that modulates compressor output to match air flow. For example my unit will modulate compressor output so not allow the DAT to drop below 54 degrees no matter if air flow is 600 or 1800 CFM.

Compressor, condenser fan and evaporator fan motors are all DC variable speed motors.

 

[WorthFlorida]

My 4 year old, 5 ton Daikin Inverter system came with a factory 12 year 100% parts and labor warranty. Daikin support has been exceptional.

Daikin really seems to be pushing hard to get a good foothold in the residential market and I’ll be sure looking at them the next time I need to replace my unit. My Carrier, after three years the fan motor (variable speed 5 year warranty) failed. It still cost me $345 for labor. Like Hyundai, first to offer 100,000 mile warranty to get market share and it worked.

 

[Jacobsond]

.38 cents wow. I cant help much with reducing your energy use,but getting your friends and neighbors to push for a new power plant to take up the slack with your electricity shortage. Being CA probably a snowballs chance in hell tho.Good luck with your project.

 

[WorthFlorida]

Assuming Mike Mc has PG&E, I looked up Pacific Gas and Electric and sure enough here are their rates for Time of Use. And CA companies design and built electric vehicles. At off peak hours rate it may cost as much as petro fuel to run your car, but if you need a charge while at work at peak hours . (The columns go off after submitting this post.)

https://www.pge.com/tariffs/assets/pdf/tariffbook/ELEC_SCHEDS_E-6.pdf

TOTAL RATES
Total Energy Rates $ per kWh) PEAK PART- PEAK OFF-PEAK

Summer
Baseline Usage $0.37490 (I) $0.25963 (I) $0.18285 (I)
Over 100% of Baseline $0.46220 (I) $0.34693 (I) $0.27015 (I)

Winter
Baseline Usage – $0.20402 (I) $0.18719 (I)
Over 100% of Baseline – $0.29132 (I) $0.27449 (I)

Total Meter Charge Rate ($ per meter per day) $0.25298
Delivery Minimum Bill Amount ($ per meter per day) $0.32854
California Climate Credit (per household, per semi-annua lpayment occurring in the April and October bill cycles) ($39.42)

 

[Jadnashua]

I read one article of a guy somewhere in the southeast that was using stored, chilled water that was made when the sun was out (he had a lot of solar panels) and then used that for his cooling. Essentially, he had large, super insulated storage containers, similar to a vacuum bottle to store the chilled water in. This could help for time shifting, too. Sort of acted like a big battery, well, stored energy, regardless.

Time of use electric rates will likely become more common as time goes on. As batteries become more energy dense and cheaper, it will become more common to store some at your home for use when the load is greater so things even out rather than having to plan for significant peak loads.

 

[Dana]

First off, in a 120K furnace is ludicrously oversized for your house (probably 5x oversized for your 99% load), and a 5 ton split AC system is also oversized (perhaps not as badly), no matter what the hack HVAC rules of thumb say.

Yes, there is a minimum flow requirement, below which the coil temperature drops below the freezing temperature and ices up, or worse, the coil is sending liquid refrigerant back to the compressor. At the low to negative latent cooling loads of Sacramento you can probably get away with cheating that a bit, but you'll chew up most compressors really fast if it's taking in liquid on the intake side.

At the low latent loads of CA extremely high 30+ SEER mini-split heat pumps work very well. An SEER of 20 or less air might still be required in more humid areas to have a sufficient sensible heat ratio to deal with the humidity, but in your part of CA that doesn't matter at all.

Short of replacing the system with something more appropriately sized with an appropriately high SEER, programming the thermostats to pre-cool the house by a few degrees prior to the peak hours is probably your best bet. If you're going to bite the bullet on a complete HVAC replacement, have qualified third party (not an HVAC installer) run an aggressive Manual-J cooling & heating load calculation on it. Use an engineer, a RESNET rater etc to run the numbers- somebody who makes their living & reputation on the accuracy of their numbers, not somebody whose bread & butter is installing & maintaining HVAC equipment. You may end up spending $500-1000 for that service, but it will often save that much in up front equipment costs.

To get a sense of just how oversized 5 tons of AC might be for your house, check out this graphic a consulting company in GA made plotting house size against square feet per ton based on a few dozen Manual-Js performed for clients. Most of these houses are in the humid gulf coast states, with much higher latent loads than yours:

The very WORST performing house in the 3000 square foot range came in at about a ton per 900'. That worst-case house could be cooled b 3.5 ton unit (assuming a reasonable sensible heat ratio). The middle range is about a ton per 1400', throwing out the one outlier up there at about a ton per 3300'.

And that's why it's worth paying a pro to run the real numbers before buying any replacement equipment. Manual-Js performed by HVAC companies have all sorts of bias toward the high side- they're far more concerned about the overheated irate customer screaming on phone when it's 103F outside than the customer who can stay cool no matter what, but is paying more than they should on power bills.

So if your house is a middle-range house you'd be looking at about 2 tons of cooling load and the 5 tonner is probably ~2x oversized, running such a low duty cycle that it won't likely meet it's (fairly low by current standards) SEER numbers.

Sacramento policy makers have been steering people away from natural gas. If you replace the system there are probably incentives to go with a heat pump (and a heat pump water heater), if you're willing to go all-electric, giving up your gas connection altogether.

 

[Jadnashua]

A modulating system, if it has enough range of operation, can work with zoning. The thing that gets messy is if it can't (and many cannot). Short cycling both heating and cooling systems is inefficient and also affects the comfort level. Things work best when the conditioning can run longer cycles.

While I may be on borrowed time, I typically run my circulating fan in my Trane air handler continuously. It's a multispeed (16-speeds, I think) unit, so unless actually heating or cooling, it's only on a very low speed. It was installed about 1988, so pushing 30-years...still working fine. I have a spare motor sitting around, but so far, I've not needed it.