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Another HVAC Question - Zoned system reduce tonnage?
Posted on 6/4/20 at 8:04 am
Posted on 6/4/20 at 8:04 am
Been racking my brain over this. Current AC was a 1998 3 1/2 ton. Total sq ft is around 1600 with 450 being a poorly insulated upstairs bonus room(not really sealed from attic properly at this time). All companies that have come out automatically jump to a 4 ton variable speed or 2 stage based on my current setup.
What I'm wondering, but haven't spoken to AC company about, does it make sense to drop to a 3 ton variable speed with 2 zones (upstairs/downstairs). I don't see the need to ever require a complete cooldown between both zones. Once I properly insulate upstairs, that should be a big help in keeping that room cooler.
I know I probably truly need a Manual J, but not sure who actually does that in BR,and how accurate it can be in an older home that I don't know what type of insulation is in walls, etc.
What I'm wondering, but haven't spoken to AC company about, does it make sense to drop to a 3 ton variable speed with 2 zones (upstairs/downstairs). I don't see the need to ever require a complete cooldown between both zones. Once I properly insulate upstairs, that should be a big help in keeping that room cooler.
I know I probably truly need a Manual J, but not sure who actually does that in BR,and how accurate it can be in an older home that I don't know what type of insulation is in walls, etc.
3
Posted on 6/4/20 at 8:23 am to mctiger1985
Good news! You don't have to guess assuming your current one still runs.
Stepping down will usually result in more comfort (better humidity control) for more times of the year, reduce up front costs and also save on energy usage. If it can be done it is usually the right option.
If you want to know if 3-tons will be enough, measure the duty cycle of your current system. In order for 3-ton to be enough your current system needs to always stay below an 85% duty cycle (or 85% loaded) because 85% of 3.5 tons is 3 tons. A duty cycle is calculated by measuring the length of an on-cycle and the immediately following off cycle, adding the times together and then dividing the on-cycle length time by the combined times.
ON/(ON+oFF)
To figure this out you need to measure your AC run times and off times during peak heat. If your current system runs for more than an hour continuously during the peak heat, your current system is properly sized and stepping down in size might result in the house temperature raising a couple degrees during the afternoons of the hottest days. That may or may not be a big deal.
If your current system, for example runs for 20 minutes straight, turns off for 5 minutes, then starts up again. That is an on cycle of 20 minutes and off cycle of 5. 20/(20+5)=80%. In this case your current system would have a duty cycle of 80% (loaded to 80%) and is using the equivalent of .8*3.5=2.8tons. In this scenario downsizing to a 3-ton system would be the proper choice.
Good luck.
Stepping down will usually result in more comfort (better humidity control) for more times of the year, reduce up front costs and also save on energy usage. If it can be done it is usually the right option.
If you want to know if 3-tons will be enough, measure the duty cycle of your current system. In order for 3-ton to be enough your current system needs to always stay below an 85% duty cycle (or 85% loaded) because 85% of 3.5 tons is 3 tons. A duty cycle is calculated by measuring the length of an on-cycle and the immediately following off cycle, adding the times together and then dividing the on-cycle length time by the combined times.
ON/(ON+oFF)
To figure this out you need to measure your AC run times and off times during peak heat. If your current system runs for more than an hour continuously during the peak heat, your current system is properly sized and stepping down in size might result in the house temperature raising a couple degrees during the afternoons of the hottest days. That may or may not be a big deal.
If your current system, for example runs for 20 minutes straight, turns off for 5 minutes, then starts up again. That is an on cycle of 20 minutes and off cycle of 5. 20/(20+5)=80%. In this case your current system would have a duty cycle of 80% (loaded to 80%) and is using the equivalent of .8*3.5=2.8tons. In this scenario downsizing to a 3-ton system would be the proper choice.
Good luck.
Posted on 6/4/20 at 9:15 am to mctiger1985
I think the manual J is essential, particularly if you do insulate the upstairs bonus room, b/c that should be considered in sizing a new system. If you know when the house was built the experts that do do manual J s can unusually provide a good estimate on the insulation the wall.
Have you considered the existing tonnage might be correct for the house but issue with cooling the upstairs may be more duct work and air flow issue and not AC tonnage per se? If you increase tonnage to 4 tons, have the HVAC firms you’ve spoken with going to upsize the duct work to accommodate a 4 ton unit? A 4 ton unit with 3 ton sized ducts is only going to provide you with 3 tons of cooling capacity.
I will suggest you think about this, many professional HVAC tech recommended separate units for multiple floors, as opposed to zoning often because many HVAC firms are not good at installing zoned systems properly. You might consider a single stage system for the first floor and a mini-split for the upstairs bonus room. The $3-5K added cost for a 2 stage or multistage system over a single stage system should easily pay for a second mini split for the bonus room.
There are companies in BR that can do manual J s but I don’t have one to recommend. I’m thinking about having this guy (Dan) in FL do one for my house - he provides excellent advice on the AOC HVAC Talk forum. (can’t find the link to his website now, but as soon as I do I’ll come back and add it to this post). ETA: Dan Racine LINK. Pricing for Manual J starts on page 3
In fact, I’d recommend you go to that forum LINK and join and post your question there and get the advice of experienced HVAC techs, Dan in FL who is a manual J guru is sure to respond. It would cost about $200-250 for him to do a manual J, double that if you also had him do the manual D (duct sizing) calculations. Those experienced HVAC techs routinely provide advice/suggestions on their forum to questions such as yours.
Have you considered the existing tonnage might be correct for the house but issue with cooling the upstairs may be more duct work and air flow issue and not AC tonnage per se? If you increase tonnage to 4 tons, have the HVAC firms you’ve spoken with going to upsize the duct work to accommodate a 4 ton unit? A 4 ton unit with 3 ton sized ducts is only going to provide you with 3 tons of cooling capacity.
I will suggest you think about this, many professional HVAC tech recommended separate units for multiple floors, as opposed to zoning often because many HVAC firms are not good at installing zoned systems properly. You might consider a single stage system for the first floor and a mini-split for the upstairs bonus room. The $3-5K added cost for a 2 stage or multistage system over a single stage system should easily pay for a second mini split for the bonus room.
There are companies in BR that can do manual J s but I don’t have one to recommend. I’m thinking about having this guy (Dan) in FL do one for my house - he provides excellent advice on the AOC HVAC Talk forum. (can’t find the link to his website now, but as soon as I do I’ll come back and add it to this post). ETA: Dan Racine LINK. Pricing for Manual J starts on page 3
In fact, I’d recommend you go to that forum LINK and join and post your question there and get the advice of experienced HVAC techs, Dan in FL who is a manual J guru is sure to respond. It would cost about $200-250 for him to do a manual J, double that if you also had him do the manual D (duct sizing) calculations. Those experienced HVAC techs routinely provide advice/suggestions on their forum to questions such as yours.
This post was edited on 6/4/20 at 9:57 am
Posted on 6/4/20 at 9:44 am to CrawDude
quote:
manual J and D
I plan on replacing my A/C units this fall. Is this something that companies should routinely do as part of the quote process? Is is something they normally charge extra to do?
Posted on 6/4/20 at 9:48 am to CrawDude
Thanks, I'll take a look at that site. Unit has bit the dust, so I don't have too long to get this right.
Here's my data from a 95 degree day last day from my Ecobee where it did run for quite a while in the afternoon. I definitely know there is an airflow balance issue, and the bad upstairs room is killing my performance. I do need new ductwork as well, so that will be replaced. I also know I have a Freon leak, so no idea if this unit was performing optimally at the time. So many factors going on, it's blowing my mind. ETA: ALso, that upstairs room does have a 12x12 return, so it's mixing that 94 degree air back into the AC.
Also, I did get quotes on minisplit, but not sure I want to have 3 ac units to maintain.
[url=LINK ]
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Here's my data from a 95 degree day last day from my Ecobee where it did run for quite a while in the afternoon. I definitely know there is an airflow balance issue, and the bad upstairs room is killing my performance. I do need new ductwork as well, so that will be replaced. I also know I have a Freon leak, so no idea if this unit was performing optimally at the time. So many factors going on, it's blowing my mind. ETA: ALso, that upstairs room does have a 12x12 return, so it's mixing that 94 degree air back into the AC.
Also, I did get quotes on minisplit, but not sure I want to have 3 ac units to maintain.
[url=LINK ]
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[/url]This post was edited on 6/4/20 at 9:54 am
Posted on 6/4/20 at 10:17 am to mctiger1985
based on the ecobee data you probably do not want to downsize until you get insulation and cooling load improvements are made.
Posted on 6/4/20 at 10:21 am to Major Dutch Schaefer
quote:
I plan on replacing my A/C units this fall. Is this something that companies should routinely do as part of the quote process? Is is something they normally charge extra to do?
I’m not in the HVAC business so I’m just going to relate what I’ve learned in the past 2 years studying this as I’m going to be the change out mode for my 15 year old unit in the near future. A top notch company should do manual J (heat load) and if needed manual D (duct sizing) but apparently few do, and that seems to be consistent throughout the USA. Now in LA it is required by code that a manual J be conducted, starting in 2015, for new construction, but not replacement units.
Now, a HVAC installer may do a manual J but not give you the results until signing a contract with them - I understand that is normal. Reasoning being is that they don’t want you to take their results from their work at their cost, give it to you and then have have you take their findings and go find someone cheaper to do the job. That’s understandable.
But you can hire independent firms to do this, and some HVAC firms do it with in house employees or sub it out for the calculations.
Here is the guy in FL I mentioned that does this - it’s worth reading, he provides pricing and example reports. LINK Although I know there are people/firms in BR that do this I’ve not found a website that remotely comes close to providing these details.
This post was edited on 6/4/20 at 6:36 pm
Posted on 6/4/20 at 10:39 am to mctiger1985
quote:
mctiger1985
Understood you are in a bind for time. I’ll leave you with this, duct work and sizing is as important as sizing tonnage for home comfort and most good HVAC Techs say the biggest issue the come across with clients with home comfort issues is undersized ducts matched with oversized equipment (tonnage).
Ask any company you are considering if they provide a commissioning report after installation, which includes static pressures, air flow measurements, balancing the air flow to rooms, etc. Here is an example of a commissioning report form LINK
This post was edited on 6/4/20 at 10:59 am
Posted on 6/4/20 at 10:49 am to CrawDude
quote:
I’m going to be the change out mode for my 15 year old unit in the near future.
Sounds like we are in the same situation. Thanks for the information.
Posted on 6/4/20 at 11:18 am to CrawDude
quote:
HVAC Techs say the biggest issue the come across with clients with home comfort issues is undersized ducts matched with oversized equipment (tonnage).
What about the other way around? Our house was built in the late 60s. My HVAC guy was out to service a heat pump the other day. He's told me this in the past, but it appears the main unit was downsized about the time the heat pump for upstairs was installed. All prior to us moving in back in 2007. The units are from the early 2000s. It was probably a 4.5 or 5 ton unit originally, with ductwork running to both floors. Lower wall vents on both floors that come from the unit in the basement. Combined with a gas furnace. The "downstairs" unit Lennox is a 3.5 ton, but the ductwork to the upstairs rooms are still connected.
The heat pump's inside unit is installed in the attic and has flex ducting run to ceiling vents. So technically the upstairs is getting A/C from both. But I noticed that the air coming out of these upstairs vents isn't blowing very hard. Could this be from a smaller unit than was originally installed? Meaning, not enough blowing power for the old ducts that were on the larger unit?
We're going to replace the heat pump soon, it is on its last legs, but his comments and this thread are making me rethink what we want to go with. Should I go back to a larger downstairs unit to get more air to these upstairs vents, or leave it as it is and just go with replacing the upstairs unit?
Downstairs Sq feet is about 1,500 and upstairs is around 800.
Posted on 6/4/20 at 12:34 pm to TU Rob
From the professional HVAC Talk forum I read, nearly daily, these HVAC techs state in usually 30+ years experience they have have not seen ducts over-sized, but see undersized ducts daily.
Again, the only way to determine proper HVAC sizing for your house is through a heat load calculation (manual J) for each floor, usually room by room. What size unit you require depends on the age of your house, insulation, windows, how leaky the house is etc. Some houses might require 1 ton of cooling per 500 sq ft (old rule of thumb), some modern houses 1 ton per 1,000 sq ft or more. Unless your house leaks like a sieve and is poorly insulated, a 3.5 ton heat pump for 800 sq ft 2nd floor is way over-sized. Even the 3.5 ton downstairs unit for 1500 sq ft might be over-sized.
Again, not a HVAC tech but I don’t believe ductwork should be shared between 2 units, sounds like the house at one time had a single unit, and then a second unit was added just tying into existing ductwork if I understand you correctly.
Air flow rule of thumb is 400 cubic feet per ton (range: 350 to 450 cu ft). Duct work must be sized to accommodate that. The most common method HVAC techs use to determine air flow is to measure total external static pressure at the air handler on the return side and supply side, then the static pressure is compared to to the equipment manufactures blower performance tables to obtain air flow. High static pressure means lower air flow and is no bueno and poor system performance; low static pressure, good air flow and good system performance (usually). Under-sized ducts relative to HVAC tonnage equates to high static pressure and poor air flow, and vice-versa. Air handler blower motor speeds can be adjusted manually (or electronically) to increase or decrease air flow to the proper air flow per tonnage and this should be measured, and adjusted and included in the commissioning report when a new system is installed, and apparently too often it is not.
Choose your HVAC company wisely.
Again, the only way to determine proper HVAC sizing for your house is through a heat load calculation (manual J) for each floor, usually room by room. What size unit you require depends on the age of your house, insulation, windows, how leaky the house is etc. Some houses might require 1 ton of cooling per 500 sq ft (old rule of thumb), some modern houses 1 ton per 1,000 sq ft or more. Unless your house leaks like a sieve and is poorly insulated, a 3.5 ton heat pump for 800 sq ft 2nd floor is way over-sized. Even the 3.5 ton downstairs unit for 1500 sq ft might be over-sized.
Again, not a HVAC tech but I don’t believe ductwork should be shared between 2 units, sounds like the house at one time had a single unit, and then a second unit was added just tying into existing ductwork if I understand you correctly.
Air flow rule of thumb is 400 cubic feet per ton (range: 350 to 450 cu ft). Duct work must be sized to accommodate that. The most common method HVAC techs use to determine air flow is to measure total external static pressure at the air handler on the return side and supply side, then the static pressure is compared to to the equipment manufactures blower performance tables to obtain air flow. High static pressure means lower air flow and is no bueno and poor system performance; low static pressure, good air flow and good system performance (usually). Under-sized ducts relative to HVAC tonnage equates to high static pressure and poor air flow, and vice-versa. Air handler blower motor speeds can be adjusted manually (or electronically) to increase or decrease air flow to the proper air flow per tonnage and this should be measured, and adjusted and included in the commissioning report when a new system is installed, and apparently too often it is not.
Choose your HVAC company wisely.
This post was edited on 6/4/20 at 12:38 pm
Posted on 6/4/20 at 12:49 pm to CrawDude
quote:
Unless your house leaks like a sieve and is poorly insulated, a 3.5 ton heat pump for 800 sq ft 2nd floor is way over-sized. Even the 3.5 ton downstairs unit for 1500 sq ft might be over-sized.
I don't think I was clear there. 3.5 ton unit, old ductwork that is wrapped with insulation. These ducts go to all rooms in the house.
quote:
Again, not a HVAC tech but I don’t believe ductwork should be shared between 2 units, sounds like the house at one time had a single unit, and then a second unit was added just tying into existing ductwork if I understand you correctly.
2 ton heat pump upstairs, new flex ducting in the attic and this is a whole separate system than the 3.5 ton downstairs and the two aren't tied together. The room I'm in now (upstairs) has a lower wall vent from the downstairs and a ceiling vent from the heat pump.
Posted on 6/4/20 at 1:49 pm to CrawDude
Nm
This post was edited on 6/4/20 at 2:10 pm
Posted on 6/4/20 at 2:12 pm to TU Rob
I think I understand now. So the upstairs floor receives air from the newer 2 ton unit, but does the upstairs also receive air from the downstairs 3.5 ton unit or are those 2nd story floor vents from downstairs closed off? Maybe when the new 2 ton unit was installed for the upstairs floor the HVAC tech closed dampers in the duct work to prevent conditioned air from the downstairs unit from going to the upstair’s floor. That’s what they should have done.
Anyway 3.5 tons for 1500 sq ft, 430 sq ft per ton, seems like it would possibly oversized and maybe you can size even further, using the old rule of thumb of 500 sq ft per ton, suggest you could downsize to at least a 3 ton unit for the first floor, maybe even more and that is what a manual J load would tell you.
If you want you can do your own manual J estimate for the first floor using this free on-line calculator. LINK. For example, my 2,500 sq ft conditioned area is cooled by a 4 ton unit. The manual J calculator I linked indicates a 3.5 ton unit would be sufficient to cool my house and I believe it. I hope to add some energy savings improvements to the house to be able to down size to a 3 ton unit when I replace my HVAC in the next year or so.
Anyway, based on everything I’ve read, I would not be concerned that your original ducts for the whole house are oversized. It’s much better to have oversized ducts than undersized.
Anyway 3.5 tons for 1500 sq ft, 430 sq ft per ton, seems like it would possibly oversized and maybe you can size even further, using the old rule of thumb of 500 sq ft per ton, suggest you could downsize to at least a 3 ton unit for the first floor, maybe even more and that is what a manual J load would tell you.
If you want you can do your own manual J estimate for the first floor using this free on-line calculator. LINK. For example, my 2,500 sq ft conditioned area is cooled by a 4 ton unit. The manual J calculator I linked indicates a 3.5 ton unit would be sufficient to cool my house and I believe it. I hope to add some energy savings improvements to the house to be able to down size to a 3 ton unit when I replace my HVAC in the next year or so.
Anyway, based on everything I’ve read, I would not be concerned that your original ducts for the whole house are oversized. It’s much better to have oversized ducts than undersized.
Posted on 6/4/20 at 2:38 pm to CrawDude
quote:
I think I understand now. So the upstairs floor receives air from the newer 2 ton unit, but does the upstairs also receive air from the downstairs 3.5 ton unit or are those 2nd story floor vents from downstairs closed off? Maybe when the new 2 ton unit was installed for the upstairs floor the HVAC tech closed dampers in the duct work to prevent conditioned air from the downstairs unit from going to the upstair’s floor. That’s what they should have done.
That's right. It comes from both but they are separate vents on separate systems. They might have closed them some, but I can definitely feel air flow when I get near the vent. It just isn't really strong flow is why I was wondering. Thought it might have had something to do with how long the runs are from the unit 2 floors down or the blower not being strong enough for all vents.
quote:
Anyway, based on everything I’ve read, I would not be concerned that your original ducts for the whole house are oversized. It’s much better to have oversized ducts than undersized.
Posted on 8/31/23 at 6:39 am to mctiger1985
Hey, sorry for bumping the old thread. I just came across it and wanted to share my thoughts. You're on the money thinking about a zoned system, especially considering you have an upstairs space with clear insulation issues. Downgrading to a 3-ton variable speed with 2 zones could turn out to be a great solution once you fix the insulation issue upstairs.
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