Help with HVAC Contractor

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Larry393

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Help! Should I be hearing air flow at my supply ducts? I purchased the most expensive system to help control air noise. It is my understanding that you should not hear the air flow with low pressure systems. I’m getting the feeling that the system ducting was not designed, just thrown together. The system is the following:


System: Dave Lennox Signature Collection 25 with Hydronic Coil

Size: 4 Ton

Condenser: XC25-048-230

AHU: CBA38MV-048-230 + TDR

AHRI: 201924711

Lennox HCHRV

iComfort WiFi S30 control system

2 Zone


I called initially when it was like a hurricane blowing through the house. The reason told was that the AHU fan motor could not be set lower than 1205 CFM. Their solution was to install a duct directly from the return directly to the supply that has a dampener in-line. I am and was concerned that this was probably not standard practice. Am I correct?


The “fix” did not work completely so I reached out to the installer. They initially wanted to come out and put in larger returns while not addressing the supply velocities. I asked them to tell me what the design flow was that they initially used. No response. This is telling me it was not designed.


I’m an engineer with a lot of construction background. Nothing in HVAC. This is why I am here looking for advice. I am not sure what to ask this company to do at this point. Should I ask to see a complete design before moving forward? Should the duct from return to supply be removed? Is it common to hear the supply air coming out of the duct?


Thank you

Larry
 

SShaw

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Air conditioners require about 300-400 CFM of airflow per ton of capacity to operate properly.

According to the Lennox docs, your air handler allows four choices for the maximum heating and cooling airflow, but it will only operate at two speeds during operation. The operating airflows are either 70% or 100% the set maximum airflow. 1230 CFM is the lowest airflow setting for your model, so your air handler will operate at either 861 CFM or 1230 CFM, depending upon the cooling load.

Is this a replacement or new construction? If you are replacing an existing smaller system, or a system that was not designed for zoning, the existing ducts are likely to be too small to handle the airflow.

Zoning aggravates the problem, as you will be forcing all that air down fewer ducts. The "bypass" the installer added is common practice, but is not a good solution, as you are not delivering that air into the living space. Ideally, each zone of the ductwork would be sized to handle the full 1230 CFM in case either zone operates at the high cooling speed.

What size was the system the Lennox replaced? Was the old system zoned?

There might be a few things that can be done to improve the noise, such as adding more returns, or re-working the supply and return plenums.
 

Dana

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4 tons of AC is a lot for most houses in MA, unless it's a mid-century modern type house with an expansive "sunset view" west facing wall of glass or 5000+ square feet. (FWIW: I have 5 tons of AC for a 2400' antique bungalow type house in Worcester that has only ~2 tons of load at the 1% outside design temp. I've been putting up with it, but will happily swap it out for a 2 ton modulating system when either it's lifecycle is up or I get sufficiently fed up with it.)

Who ran the load numbers to come up with the XC25-048-230/CBA38MV-048-230? HVAC contractors are usually pretty bad at getting this right, compared to professional engineers.

The might not be solved by simple duct tweaks, but lower duct velocity makes a huge difference in total noise, as does duct tightness. If the ducts aren't mastic-sealed on all seams & joints it's a noise factor. Sometimes short sections of flex to mechanically isolate branches from trunks or plenums will help, but it really depends.
 

Larry393

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Air conditioners require about 300-400 CFM of airflow per ton of capacity to operate properly.

According to the Lennox docs, your air handler allows four choices for the maximum heating and cooling airflow, but it will only operate at two speeds during operation. The operating airflows are either 70% or 100% the set maximum airflow. 1230 CFM is the lowest airflow setting for your model, so your air handler will operate at either 861 CFM or 1230 CFM, depending upon the cooling load.

Is this a replacement or new construction? If you are replacing an existing smaller system, or a system that was not designed for zoning, the existing ducts are likely to be too small to handle the airflow.

Zoning aggravates the problem, as you will be forcing all that air down fewer ducts. The "bypass" the installer added is common practice, but is not a good solution, as you are not delivering that air into the living space. Ideally, each zone of the ductwork would be sized to handle the full 1230 CFM in case either zone operates at the high cooling speed.

What size was the system the Lennox replaced? Was the old system zoned?

There might be a few things that can be done to improve the noise, such as adding more returns, or re-working the supply and return plenums.

This is a brand new system on a renovated house that did not have any air prior. Ultimately, I need to know what to ask of the installer at this point. They are responsive and willing to provide a fix. The problem is I think they are knee jerking the fixes rather than doing the math. It looks like I should ask them to resize the ductwork to handle the 1230CFM in each zone.
 

Larry393

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4 tons of AC is a lot for most houses in MA, unless it's a mid-century modern type house with an expansive "sunset view" west facing wall of glass or 5000+ square feet. (FWIW: I have 5 tons of AC for a 2400' antique bungalow type house in Worcester that has only ~2 tons of load at the 1% outside design temp. I've been putting up with it, but will happily swap it out for a 2 ton modulating system when either it's lifecycle is up or I get sufficiently fed up with it.)

Who ran the load numbers to come up with the XC25-048-230/CBA38MV-048-230? HVAC contractors are usually pretty bad at getting this right, compared to professional engineers.

The might not be solved by simple duct tweaks, but lower duct velocity makes a huge difference in total noise, as does duct tightness. If the ducts aren't mastic-sealed on all seams & joints it's a noise factor. Sometimes short sections of flex to mechanically isolate branches from trunks or plenums will help, but it really depends.

This is a 2000 sf renovated house from the 50's with a newer addition attached. All walls spray foamed and roof rafters spray foamed. the house is tight. all new andersen widows and thermatru doors. The system is all new and looks like they did a good job sealing up all joints, connections and seams. Im, assuming they ran the load calcs properly. What are the return velocities and supply velocities that they should be designing for?
 

SShaw

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This is a brand new system on a renovated house that did not have any air prior. Ultimately, I need to know what to ask of the installer at this point. They are responsive and willing to provide a fix. The problem is I think they are knee jerking the fixes rather than doing the math. It looks like I should ask them to resize the ductwork to handle the 1230CFM in each zone.

Best to have a load calculation done and go from there. Your AC is probably 1.5T to 2T oversized. It might be better to rip out the four ton unit and put in a smaller one that's the appropriate size. I suppose you plan to heat with the air handler though, so you would need to consider the airflow necessary for the hydronic coil for heating.
 

Larry393

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Best to have a load calculation done and go from there. Your AC is probably 1.5T to 2T oversized. It might be better to rip out the four ton unit and put in a smaller one that's the appropriate size. I suppose you plan to heat with the air handler though, so you would need to consider the airflow necessary for the hydronic coil for heating.

Where would I go to get a proper load calculation done? At this point I'm not confident that the installer either has the capability or will provide an accurate calculation.
 

SShaw

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Some HVAC companies will do it, but you might have better luck finding a home energy assessment company or an engineer (as Dana suggested.) You could take a look at the list of contractors at this link and see if one of them offers load calculations. You can also search online for energy companies and see if they offer "Manual J" "load calculations."

https://www.masssave.com/en/learn/find-a-contractor

There are also paid software packages and free online tools such as https://loadcalc.net/ where you can do your own calculation. As an engineer, you might find it easy to do.

You said you have a hydronic coil. I don't see that as a factory option in the air handler. Is the coil a third-party add-on? Having an extra coil could contribute to the issue. A hydronic coil could add significant airflow resistance, contributing to the noise. I doubt you'll get the 21 SEER rating for the system with an add-on coil either.

You could also have a contractor measure the static pressure of the ducts as things are now, with only one and with both zones opened, to determine whether there is an airflow issue.

The zoning control system also plays a role. For example, if your system is setup such that stage 2 is never called when only one zone is calling, then each zone would only need to handle the 861 CFM.
 
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rest_in_pasta

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Help! Should I be hearing air flow at my supply ducts? I purchased the most expensive system to help control air noise. It is my understanding that you should not hear the air flow with low pressure systems. I’m getting the feeling that the system ducting was not designed, just thrown together. The system is the following:


System: Dave Lennox Signature Collection 25 with Hydronic Coil

Size: 4 Ton

Condenser: XC25-048-230

AHU: CBA38MV-048-230 + TDR

AHRI: 201924711

Lennox HCHRV

iComfort WiFi S30 control system

2 Zone
............................................
All walls spray foamed and roof rafters spray foamed. the house is tight

Since you have air sealed sealed walls and attic (assuming closed cell spray foam was used), the heat load was significantly reduced.....4-ton AC means you probably have short operating cycles during the warm season and humidity must be unbearable.

Sounds like a case of an oversized HVAC system and undersized/poorly designed duct system. Did the installers commission the system (should have a copy of commission report) and/or check the static pressure of the ducts while the new system ran?

There is a poster on hvac-talk.com that can help you with load calculations by going to his site: mysimplifiedhvac.com
 

JerryR

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I went through this about 5 years ago when we had a new ac unit installed. It is a “5 ton” Daiken inverter system with 3 zones. It actually maxes at 55,000 btu’s. The master bedroom was very noisy when only that zone was open. After complaining about the wind noise they completely redid the duct system and added bigger and more registers to that zone.

They also installed an EWC motorized pressure regulated bypass damper that restricted duct pressure if needed.

I worked with EWC, the zone board manufacture, to develop custom programmable and adjustable parameters since my system was the first 5-ton Daiken heat pump system in Florida.. Depending on which zones called for cooling the communicating zone board limited the AC % output. I have 2 zones set at 30% max each and one zone at 40% max. So when my bedroom is only calling for cool air the unit will not exceed 30% capacity or 16,500 btus and 700 CFM MAX. Normally it runs only at 480 CFM max for that zone.

They also installed 4 filtered return ducts (2-20”, 1-24” and 1-14” in master bedroom)

I bought a portable Anemometer to quantify air speed out of each duct register. https://www.amazon.com/gp/product/B00F3SCSZY/. We used this to balance each duct.

The inverter system modulates CFM anywhere from 450-1850 CFM. The compressor is also fully variable output.

Now it is completely silent, balanced and efficient. The unit rarely runs at 1800 CFM and 100% output but summertime it runs constantly all day long on low output.

Here’s a picture where the 5 ton unit is running at 39% and only 670 CFM

5-ton-readout.jpg
 
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SShaw

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I have a similar system. The twelve-stage compressor modulates from 20% to 100% capacity. The four-ton air handler modulates airflow from 475 CFM up to 1925 CFM to match the compressor stage or aux heat. The zone controller matches the airflow and compressor stage to the current demand in each zone. It literally runs 24 hours a day as long as there's a heating or cooling load. Because the system can lower the capacity and airflow so much, there's no need for a bypass damper and it operates very quietly.

Variable systems like this are much better suited to zoning than a single or dual stage system, but they're not that common yet.
 

Fitter30

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This damper between the supply and return how is it controlled and the whole system? This is duct size.
Calculate the square root of the total. (Example: 4 (ton unit) x 144 square inches = 576 squared. The square root of 576 is 24. Therefore, your return air duct and grill size will be 24 by 24 inches.)
 
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Larry393

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Thank you all for the great advice. Ive contacted Simplified HVAC to do an accurate Manual J. I have also reached out to the contractor requesting a balanced duct design based on the 1230 CFM of the existing AHU. Waiting on their response as well. I will keep you informed as this unfolds.

In response to fitter30: The damper is a weighted damper that is manually adjusted. If the damper needs to be part of the final design i will require a controlled damper as JerryR has installed.

Thank you
 

Larry393

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Update. I reached out to Simplified HVAC to have an accurate Manual J calc. done. I supplied the same information supplied to the contractor. As most of you suspected, Dans calculation resulted in the need for a 2T system, half of the 4T installed. The contractor supplied me the initial design (they used wrightsoft) and I had Dan review it. There were a lot of miscalculated ares and input assumptions. Even with their bloated numbers their output called for a 3T system, not sure why they installed a 4T. I reduced everything in an email and sent to contractor. Waiting for response.
 

Dana

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There were a lot of miscalculated ares and input assumptions. Even with their bloated numbers their output called for a 3T system, not sure why they installed a 4T. I reduced everything in an email and sent to contractor. Waiting for response.

Sadly, this is TYPICAL for the industry, not an outlier. They start out putting their thumb on the scale with invalid inputs because they're overly concerned about getting the call from an irate customer about the system not keeping up, then up-size the equipment a step from the (mis-) calculated result "...just to be sure...".

The comfort cost of a 2x oversize factor is far greater than the comfort cost of undersizing it by 25%. The oversized system can cool off a stagnated overheated house much quicker (ideal only for the folks who leave the system off all day then expect it to reach the setpoint while they jump in the shower after coming home for work) , but usually does a crummier job of latent cooling, and delivers bigger temperature over/under shoots or short cycles when maintaining temp. A right sized system may run hour+ long cycles on afternoons when temps are crossing the 1% outside design temp but doesn't lose (much, if any) ground on the indoor setpoint temperature. A 25% undersized system might lose some ground on the setpoint temperature on those days, but without much of a comfort hit due to the improved latent cooling (drying) of running constantly.
 
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