ductwork can’t support efficient units

chrisc1964

I have a 1956 rambler, 4 bedrooms with ducts routed through a crawlspace.  I need a new furnace and AC and am wanting a dual fuel setup as i’ve got a good solar array. I live in MD and winters aren't too tough, and we will have stretches of days in the low 30’s.  My HVAC guy says that i shouldn’t go with anything other than a single stage setup because my ductwork can’t support anything efficient and the blower motors will go bad quickly.  Does this make sense ? 

Hot_water_fan

No that doesn’t make sense - the airflow rate is low almost always so static pressure is too. I’m in Maryland and a heat pump covers 100% of my heating with no issue. 

Mustangman

I don't know what duct layout you have but I don't agree, across the board, with your Hvac guys opinion. In the day and age of ECM motors, you can get away with some duct issues. When you think about it, lets say the duct is small. A non ECM system will have the blower and furnace come on at 100%. First you will hear air flow noise and you will experience uneven temperatures. The ECM senses the high static pressure from the small duct, and will ramp the system blower motor down. So common sense tells us the longer slower air flow gives us a longer run time. Time to get more air across the coil and create a more even temperature throughout the house. An ECM motor actually converts single phase to 3 phase. 3 Phase is very easy to manipulate the fan speed.
I guess my point is, I would need more information to see how bad the duct is. However, as I outlined above, systems are more forgiving with ECM than with the old single speed motors.
Good Luck

pecmsg

The equipment is sized by Manual “J”
the duct by Manual “D”

now old heating ducts were designed for very low velocity and high temperatures. Those systems are difficult to adapt to todays systems. 

pecmsg

ECM motors Do NOT like high static pressures. One of the major reasons for early failures. 

SuperTech

I disagree with the information above.  In my experience constant torque and variable speed ECM motors don't hold up well to high static pressure and poor ductwork.  They will try to move the amount of airflow they are set for regardless of how restrictive the ductwork is, this always leads to motor failure.  An old fashioned PSC fan motor will only move as much air as its capable of moving in however many RPMs its designed for.  In my experience they can deal with higher static pressure better. 

You probably won't be able to find much new equipment that still has a PSC fan motor.  Any new equipment should be properly sized for the heating and cooling load and then the ductwork can be modified to accommodate it. If it's not possible to modify or correct the ductwork than you would be wise to not have anything installed that the ductwork can't handle. 

Mustangman

First, I said I would have to see the duct work. I have not had any issues, other than cost, with ECM motors. We have a ton of them installed and have had great success. The goal of the ECM motor is to run a .50 static. That is what it is designed to do via the 3 phase converter. I'm sure you are aware of SEER2. For those who do not know, SEER2 represents that the testing on the equipment is now done with a .50 Static. Don't get me wrong, if the duct system consisted of a plenum and a few runs of bologna hose, That is screwed up duct.

ChrisJ

All I have heard on this forum, as well as from others outside of the forum is that ECM blower motors do not tolerate undersized ductwork and will fail.

I've had zero personal experience with such, but it's certainly what the word on the street is.

mattmia2

pecmsg said:

The equipment is sized by Manual “J”
the duct by Manual “D”

now old heating ducts were designed for very low velocity and high temperatures. Those systems are difficult to adapt to todays systems. 

This is where to start. It is very likely the old equipment was very oversized and your ductwork will be ok with smaller equipment that matches the heat loss and gain of the structure or will only require minor modification. If there is a crawlspace it could be modified extensively or replaced although that is likely to be expensive.

chrisc1964

Wow, thank you for all the input…!  albeit i’m still a bit confused.  The ductwork in place is sheet metal rectangular shape running in a crawl space with very old insulation stuffed around them.  Is it that their too rigid, accept a lot of air volume?  Is there anyway to mitigate this?   When i watch the endless hours of video on which heat pump to pick, they almost all recommend at least two stage units.  But my tech is adamant, he sees a lot of failures in variable speed units in older homes, and runs his own business and this hesitant to install them because people are very upset with him.  He installs only Carrier and Trane.