A/C upgrades killed master bedroom cooling
A freind of mine got about 20 years out of his central air unit. He got a couple of quotes and then replaced everything. (I’ll get the details about the size and placement tomorrow.) Anyway, ever since he replaced it the master bedroom at the end of a long run of duct work doesn’t cool. He has about 4 tons for the upstairs and 5 ton for the downstairs. The compressors are about 65′ from his master which is on the lower level. The company that replaced them also replaced all of the hard duct with bigger flexible ducting.
In the winter the central heat roasts the master on the lower level. In the summer the room cannot be cooled when the temp gets around 90 degrees. What would cause this if both A/C units are significanly bigger than the units he replaced? I’m wondering if the return air near his master is not big enough. Or, is there such a thing as A/C duct work that is too big in diameter?
The rest of the house cools fine. All did before the new units. Any thoughts? Thanks.
Replies
Most likely the ductwork isn't "balanced". Air is flowing too freely through the new ductwork to other areas and too much air spills out into other areas before it gets to the MBR. Adding manual dampers to the ductwork for other areas might fix this, or it may require a zoned system.
Also, he needs to run the air handlers continuously, to even out the temps.
The size of the AC has little to do with how well it can cool. So long as the AC has enough capacity, more capacity is not "better", and is, in fact, often worse, since it dehumidifies less well.
I agree that an unbalanced system is the likely problem, and that balancing dampers will help, and I also would guess that the return air from the MBR and other rooms may be inadequate.
9 tons of cooling is probably WAY too much, and besides the issue of inadequate de-humidification, the compressors will be cycling far too often and wearing out prematurely.
Sounds to me like another case where the HVAC contractor slapped in equipment without doing a load calc and duct design, and then failed to balance the system afterward.
I think that's exactly what happened: Too MUCH cooling was installed.
Basicly what happenes when the thremostat trips the AC, as sudden flood of cold air flows - much larger than before. Wherever the thermostat is, that room gets cooled off pretty quick. HVAC system says "I'm Done here!" and turns off again, as it has done it's only job: regulating the temprature at the THERMOSTAT.
Before, the smaller system had to run longer to cool that thermostat down, so it got to cool down all the other rooms too.
Here's something else I'll throw out there: I'll bet the house we are talking about has the bedrooms upstairs and the thermostat downstairs: Colonial style. This means all the cold air pools in the downstair, and all the lighter hot air pools upstairs, with nothing to churn, mix, and balance them between.
Best way to fix it?
Thank you all for the information. It sounds like my friend may have a air balancing question and perhaps an overall design issue with location of returns, thermostats, etc. I guess now the first step would be to bring in a different HVAC company to redesign his system starting with the equipment he has already. What is the best way to get this done right?
Making the best of what he's already got should start with a room-by-room load calculation--NOT simply what's called a "block load", which is an estimate of the house's total heating/cooling needs.
This is done using software that crunches the numbers based on inputs, some of which will have to be assumptions on an older house. Things like R-values of wall and ceiling insulation, U-values of windows, etc. Which is not to imply that reasonable accuracy is not possible, it's just that if you have to guess whether or not the walls are R-11 or maybe R-15, you would most likely go for R-11, "just to be safe." But the actual difference that would make in the total heating/cooling loads will be quite small.
Orientation, windows and infiltration rate are the biggies that affect the load. Orientation, of course, is easy to know. Windows are visible and can be calculated quite accurately--choices offered by the software will be things like: vinyl, wood or aluminum frames?--single or double glass?--storm windows?--shading on south facing windows?--etc.
Air-tightness of construction, which affects infiltration rates, will offer 5 basic choices--again, the designer will probably guess on the side of lesser tightness, "just to be safe."
I have very little doubt that this load calculation will show that the equipment is far too large for the acual loads, but it will also show the proportional airflows needed by each room. (Example: the master bedroom, even if it comprises only 10% of the total floor area, may contribute 15% of the load. Thus, it should receive 15% of whatever air the blower delivers.) The load calc will show this for every room or register.
The next step would be for the hvac contractor to intall balancing dampers within the boot of each register, leaving them fully open for his initial readings, using a flow hood to measure and record the airflow at each supply register for each of the 2 systems so that he knows how much total airflow he has to work with for each system. Thus, if the load calc for the area served by 4-ton unit showed needed total airflow to be 1000 CFM, but his actual measured total airflow is 1500 CFM, the target airflow at each register would need to be increased by 50%. Then, he would go back and start closing balancing dampers to registers with excessive airflow, which will force the air to flow to more distant registers, like the MBR.
Also, using a manometer, he must take static pressure readings on the supply and return plenums in order to know if there is excessive resistance in the duct systems. Very often, the return side will show excessive resistance, but there are relatively simple fixes to that. Each bedroom needs an unrestricted return air path (duct or transfer grille or ceiling jumper duct.)
Run, don't walk, away from any hvac contractor who does not readily understand all this, and who does not have the software or the the flow hood to accomplish it.
Great advice
I passed your advice (both the diagnosis and the solution above) to my buddy. He is now ready to move forward with the steps to get it right. He also said that the hvac company that did the work did none of the things rdesigns described above before throwing equipment and ductwork at the project. He's kicking himself for not doing more homework.
Thanks to all!
I'm in SoCal, and I don't have AC... but maybe I will in the future.
Whaen I looked into replacing my 50y.o. furnace, I asked around alot about what i should have done. My answer back was consistently: It's not the equipment that's important, it's the installation.
I went and got three bids. $4500, $7000, and $10,000. I added a stipultion though on the bid: before I accepted, I had to see one installation of their choosing to confirm the quality of installation.
I first went to see the $4500 install. The persons home was the salesman's sister. I looked into her attic, and saw her furnace. At least one duct had popped off the plenum. There was debris from the old asbestos encased furnace all over the place. I thanked her and left a $20 bottle of wine on her doorstep, and did not do business with that company.
I went to see the $7000 company's work (The owners got a $20 bottle of Champaign). I was very pleased with what i saw. All the details were done correctly, and the workmanship was very tidy. I didn't bother with the $10,000 bid, the $7000 bidder was EXACTLY what i was looking for. They did just as good a job in my home as the one I looked at.
As an UNeducated buyer - what was the difference between the two companies? Price. Bother were completely happy with the work done... even if they didn't know the work was done or not.
As an educated buyer, I knew what i was looking for, and I got exactly what I wanted.
I think it's completely short sighted to say that an hvac guy doesn't have his act together because he doesn't mathematically determine a theoretical flow for each area. It doesn't mean they know what they're doing, it just shouldn't be used to discount someone. I use the sharpest most tallented guy in the valley and he laughed when I asked him about this for an existing residential application. It's the theory that someone working on a large complicated new house or commercial space would use, but for an existing house it's peeing into the wind. He said, "...so what would a calculation tell us about a hot room? We already know it's too hot and it's not rocket science to see where the shortfalls are." Of course this requires good judgement which he has, and he is the go-to guy to fix other guy's crappy work, so there are a lot of hvac guys who aren't the brightest bulbs.
Excess calculations are like a painter looking at a room and measuring every little space to the inch, comparing that to coverage on the side of the can of paint, adding a waste factor to a fraction of a percent, calculating any paint additives to the nearest fluid ounce, adding a few ounces for paint left in the roller that never makes it to the wall, and adding a factor for the depth of texture based on a measurement of the roughness of the surface, and adding another factor for the change in color to ensure good coverage. Again, this isn't wrong, it's just a waste of time in the real world to get top results from top painters. The same could be said for all any of the trades - there is a time and place for detailed calculations and the more good judgement and experience someone has the less the calculations make sense.
Oh please....
Room by room calcs on an existing system? That's crazy. You aren't wrong - it's just crazy and not needed.
An experienced residential hvac guy with good judgement can look at what's there and fix that situation easily.
A house with new flex supplies and a hot room screams out for a dedicated return in the hot room, especially since there's good access for flex duct - less than a hour and $150 in materials. I'd balance it after the additional return is in place. If it were my project I'd also blow r60 into the whole works.
You will never go wrong to do a room by room load calc., especially on a house like the OP described.
The load calc gives you a target to aim for in each room or space. Otherwise, it's like buying a new rifle and scope, but never being able to sight it in because you haven't set up a known target at a known distance.
An experienced hvac contractor that routinely does load calcs can do one in less than an hour, and it will give him the targets he needs.
And, no, it's not rocket science, but it IS science. You need to know how much heat each room gains or loses along with the proper duct sizes that will deliver the air efficiently. The OP's friend almost certainly has a grossly over-sized system (2 of them, in fact), and it's likely that duct sizes were guessed.
This is why all nationally-recognized codes now REQUIRE load calcs to be done in accord with ACCA Manual J, and for equipment to be sized by ACCA Manual S, and ducts to be installed by Manual D. Also, the installation instructions of all furnace and A/C manufacturers also direct the installer to size equipment and ducts according to the ACCA manuals.
ACCA is not a code organization, it is a contractors' organization that represents contractors, not a governmental organization. The manuals are all about comfort. Efficiency and energy savings are simply colateral benefits.
Blowing R-60 in the attic would be good, and the load calc should reflect the resulting decrease in loads with a corresponding decrease in equipment sizes.
At this stage, improving insulation, windows and air-tightness will cause the over-sized equipment to short-cycle even more often, resulting in premature equipment failure.
And, of course your hvac friend doesn't see the need for doing something he's probably not proficient in, and he will naturally defend doing things the way he's always done them. And he might even be a guy I would hire to do the install, but first, I would hire somebody to do a proper design that he would follow.
No, the reason I really like my hvac subcontractor is he is very very good at making systems work. Period. There's nothing better than systems that work correctly and last a long time and that's what he's good at. You'll never convice me that doing load calcs on an existing system in an old house is worth the time and money when it just takes common sense and a feel for normally works for a given situation. You're not wrong by saying someone should take out all the commonse sense and experience, but that's a book-smart approach and when the rubber meets the road I'd rather have a guy with 20 yrs experience and a track record of making every system he looks at work and work well. The end results are very similar, but what you're suggesting in many cases is just wasting $100 of my clients' money and an hour of lost productivity.
It's nice that you're satisfied with him, and I hope you were lucky enough to find him before you had to go thru several others who were not as good as he is. You, as a general contractor, are in a very different position from the OP and his friend who got beat up by his hvac contractor.
What I recommended is not just a personal preference--it is the industry's standard, and in answer to the OP's request for help, it is the most reliable, and ultimately, most cost-effective approach for people in his situation.
In my area I can give you the numbers of dozens of hacks that will do calcs all day long if someone wants them to. Doing calcs doesn't mean you have a good hvac guy. There's no substitute for good judgement, knowledge and experience!
An architect friend used a different hvac outfit (going with one of the biggest in the area) and after the house was built now uses my guy to fix what the kids did the first time around.
Any kid coming out of hvac training can run calcs - that's not a very high bar to shoot for! lol
Reading a lot inot a little information.
Since we have no idea where this is, or the age of the house, how does anyone say that the systems are oversized.
It could be a 5200-sf, built in Phoenix in the late sixties, with great big south and west facing windows.
All we do know is the system that did work was replaced with a new, larger capacity system, that included replacement of all the duct work. This tells us that the problem is most probably something to do with the ducting. Either there isn't enough air to the room, or the ducting is passing through a high heat zone and the cooled air is gaining heat.
My first approach, (I'm not an AC guy, just a dumb ol engineer trained in energy auditing), would be to stick a thermometer in the register to see how cold of air is coming out.
Then I'd put the little pocket anenmometer up at the register and check the velocity in feet/sec, and multiply the area of the register in feet, then by 0.8, because the perimeter has less flow than the center, and have a rough idea of the cfs. You don't really need exact numbers here, because hot and cold are relative terms that change by person, time of day, etc. (I know this isn't the "right" way to balance a system, but it costs way less, is far faster to do, and usually will pinpoint issues, that you can start working on. )
Then, I'd check the other registers on that handler, and see what they are putting out. If there is a great difference, in air flow, I'd start looking for a blockage of some kind. It doesn't need to be an old sweatshirt stuffed in the duct, or a collapsed duct, it could be that they ran an 18-inch duct from the handler to the first three rooms with short six-inch stubs to the register, and then decided to run 30-feet of 6-inch to the master suite because it only has one register.
If there is a great difference in temperature, I'd start checking the routing of the duct to see if there is a heat source.
We know the system is oversized because the old system was apparently working fairly well, and the new system is "significantly bigger".
All the methods you mention are useful indicators to help identify problem areas, and there are more that even a "dumb ol' engineer" could use.
You can rig up a 35-gal lightweight trash bag to make a "volumetrc capture hood" that will be quite an accurate "instrument" for measuring and balancing airflows at the registers.
You can also close doors to bedrooms with the furnace blower running and then stand outside the door and feel along the bottom to see if air is rushing out--if so, this indicates inadequate return air from that room.
You can use a thermometer to measure the temperature rise when the furnace is firing and then divide that number into the furnace's rated BTU output, and then multiply that result by 1.08 X ACF (altitude correction factor.), which will give a fairly accurate total CFM value for the duct system.
Or, like Yogi Berra said, "you can observe a lot just by looking."
However, there are American National Standards (ANSI) that have been incorporated in all the nationally-recognized mechanical codes (IRC, UMC, IFGC, IMC) that outliine the steps needed to design residential HVAC systems.Those ANSI standards are ACCA Manuals J, S, and D. The first step is Manual J for load calcs; next is Manual S for sizing of equipment; the next step is proper sizing of the ducts according to Manual D, and the "frosting on the cake" is to balance the system so that each room gets its proportionate share of the air.
All of the above, of course, is useless if the installation of the system is shoddy. Conversely, excellent workmanship and top-of-the-line, high-efficiency equipment can be defeated by a poor design. Neither design nor crafstmanship are substitutes for each other. Both are needed.
I agree....and disagree
Both you and IdahoDon are right and wrong.
Can you go wrong running room by room calcs on a house (or any other buidling)? Absolutely. If you are only Google smart, or inexperienced, or both, you can look up the various values and formulas and/or find the SMACNA idiot sheets (that would be Manual J, et al) crunch the numbers and calculate the wrong loads. IF you do a room by room for heating and cooling, how do you resolve the discrepancies? How much air infiltration do you calculate? Is it same for the whole house? Corner rooms, north and west facing exterior spaces? How do you trea heat loss through the perimiter and/or foundation? What if the big shade tree comes down in a storm? How much will the low-e windws change in the next 5 years? What about the curtains? Color of the roof? The load will never match the capacity of the equipment available, then what do you do? Garbage in, garbage out.
The experienced HVAC engineer and the experienced HVAC installer have learned from their mistakes and from seeing what works and what doesn't.
I ran the loads on all of my houses. I still do this on every house or apartment I buy and/or renovate. Partly because the local code official requires, at a minimum heating loads, for permits and partly because I never cease to be learn how to adjust my early "esitmates". I made my living for over 20 years as an HVAC design engineer and I learned more from mistakes, both mine and those of others, than from books.
I do a room by room on heating loads to determine the minumum size of the furnace/boiler and relative balancing. I do block loads to determine the maximum size of the cooling equipment and air handler sizing. I use a combination of the two to size ducts, after a system layout is roughed out.
Sometimes the calcs will show you something you didn't count on or that doesn't seem to make sense. Engineering Checks, Gut Checks and Rules of Thumb are as important as the pencil and paper (most single family homes, BTW, can be worked up on three sheets of paper with a pencil, calculator and a ductulator in an hour or two). Does it make sense? 400 sf/ton in a SFR? 10 BTUH/SF in the upper Midwest?
The experienced installer/designer CAN wing-it or square-foot it and do a good job because they have the experience to to know what has and what will work. I couldn't count the number of young, degreed engineers that I had to "counsel" when I worked for a major equipment manufacturer.
Calculating without thinking is just another form of hackery, in my professional opinion, as is installing and selling with out thinking.
My only thought after reading about the meticulous room by room flow calc and balancing is that after the tech spends all that time and gets the system perfect and walks out the door.......the wife or kids will come along and mess with the registers and screw the whole thing up.
Red flags
Big red flag #1 - "replaced all of the hard duct with bigger flexible ducting" - never a good idea. It worked before, why change it?
Big red flag #2 - "4 tons for the upstairs and 5 ton for the downstairs" - is this house 10,000 to 12,000 SF?
Big red flag #3 - "both A/C units are significanly bigger than the units he replaced" - Why?
" All did before the new units. Any thoughts?"
Your friend took the lowest bid, trusted a company that was not looking out for his best interest, and is now worse off than before. Unfortunately, this is not uncommon.
Within reason, comfort cooling ang heating ductwork cannot be too big to compromise the function.
Has anything changed around the house near the time this work was done? Big shade tree cut down, renovations, addition, new windows, etc?