Panned cold air returns

[MTLMIKEscott]

Hello I am moving the location of a cold air return that is in the same joist cavity. When researching online I have come across sites saying how bad panned cold returns are. To be sure, a panned cold air return is when sheet metal is nailed under joists to create the return flow to the furnace? I haven't looked in detail at mine yet, but if they have lined the sides of the joist in sheet metal, is this acceptable or should I be concerned. Thanks

Another future renovation would be removing a load bearing wall that has a cold air return. I really do not like floor mounted cold air returns and if I am unable to find a new spot for the cold air return, can i just eliminate one? I understand that every room should have them but my last house was built in 1969 and it did not have cold air returns in every room and it never noticed any problems.

 

[Dana]

Hello I am moving the location of a cold air return that is in the same joist cavity. When researching online I have come across sites saying how bad panned cold returns are. To be sure, a panned cold air return is when sheet metal is nailed under joists to create the return flow to the furnace? I haven't looked in detail at mine yet, but if they have lined the sides of the joist in sheet metal, is this acceptable or should I be concerned. Thanks

Panned joists always eventually leak due to the seasonal dimensional changes of the wood. Simply lining the joist bay with metal doesn't air seal it, but installing sealed rigid duct fully in the joist bay is fine.

Another future renovation would be removing a load bearing wall that has a cold air return. I really do not like floor mounted cold air returns and if I am unable to find a new spot for the cold air return, can i just eliminate one? I understand that every room should have them but my last house was built in 1969 and it did not have cold air returns in every room and it never noticed any problems.

Rooms with supply-only registers end up a a higher pressure relative to the outside whenever the air handler is running, using "The Great Outdoors" as part of the return path. It may still deliver enough air to heat the room, but it's forcing outside air into the house via some other path. An Energy Star duct system would have no more than 3 pascals (0.012" water column) pressure difference between rooms at all air handler speeds, with the doors open and with doors closed. You'll never get there without a dedicate return path for doored-off rooms. A door cut at the threshold isn't usually anywhere near enough.

But it's possible to use partition wall stud bays as "jump ducts" for the return, with a ventilation grille into the cavity near the floor on one side, and another near the top on the other side to provide some light & sound abatement. Like panned joists, partition wall jump ducts will leak. Be sure to install and air seal some blocking above the top grille and below the bottom grille to limit how much of that leakage can end up in the attic or basement, places where you'd least be wanting your return air to be drawn from.

There are several other ways to provide more legitimate return paths, some of which can be found in this response to an earlier thread.

 

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[MTLMIKEscott]

Panned joists always eventually leak due to the seasonal dimensional changes of the wood. Simply lining the joist bay with metal doesn't air seal it, but installing sealed rigid duct fully in the joist bay is fine.



Rooms with supply-only registers end up a a higher pressure relative to the outside whenever the air handler is running, using "The Great Outdoors" as part of the return path. It may still deliver enough air to heat the room, but it's forcing outside air into the house via some other path. An Energy Star duct system would have no more than 3 pascals (0.012" water column) pressure difference between rooms at all air handler speeds, with the doors open and with doors closed. You'll never get there without a dedicate return path for doored-off rooms. A door cut at the threshold isn't usually anywhere near enough.

But it's possible to use partition wall stud bays as "jump ducts" for the return, with a ventilation grille into the cavity near the floor on one side, and another near the top on the other side to provide some light & sound abatement. Like panned joists, partition wall jump ducts will leak. Be sure to install and air seal some blocking above the top grille and below the bottom grille to limit how much of that leakage can end up in the attic or basement, places where you'd least be wanting your return air to be drawn from.

There are several other ways to provide more legitimate return paths, some of which can be found in this response to an earlier thread.

Thank you for the quick response!

 

[Dana]

Also note, most furnaces installed in 1969 were ~3x or more oversized for the heat load of the house at the 99% outside design temperature, and most houses have had upgrades in windows/insulation/air-sealing since then. While it's unlikely that the original furnace is still in service, most HVAC installers spec replacement equipment with roughly the same (often slightly higher) heat output rather than risk pulling a defeat from the jaws of victory under the theory that "It heated the place just fine for a few decades so it's fine". While an oversized furnace still heats the place, oversizing is the enemy of comfort.

If your furnace is more than 15 years old it's worth running a fuel-use based load calculation, which is a measurement of the heat requirements of the house in it's current condition that includes all of the distribution losses such as leaky ducts and the infiltration losses of the illegitimate return paths created by the unbalanced duct system, etc.. ASHRAE recommends a 1.4x oversize factor, which is enough to cover the load during the extreme cold snaps during Polar Vortex disturbance events, but still runs at a (1/1.4=) 71% duty cycle during the normal peak cold temps. This reduces the hot-flash followed by the extended chill experience that typifies 3x oversizing that people have somehow gotten to think of as "normal". While 3x oversizing allows for more rapid recovery from overnight setbacks, it's really nowhere near as comfortable as right-sizing.

When right-sizing a replacement furnace with a big reduction in size, the cfm of the air handler drops, which reduces the amount of room pressurization from unbalanced duct systems, reduces noise and wind-chill/draft effects and even improves the performance of filters, reducing dust.

The overall, effects on efficiency are negligible (unless the ducts are grossly out of balance, and that balance isn't being fixed)- there is no "payback" in financial terms from retiring a furnace early, but as a rule right sizing does pay back in comfort. So if you can prove to your own satisfaction what the loads really are by tracking gas bills & comparing fuel use to heating degree-days you can avoid the common mistake of replacing like-for-like. The heat loads of homes are MUCH lower than most people (including HVAC contractors) generally believe.

Nate Adams is a consultant/contractor in Cleveland OH who recently published a book on how to achieve and maximize home comfort, in which he complains/explains at length the problems associated with oversizing the equipment. It's worth reading his free download chapters and watching the short videos on Home Comfort and HVAC stuff any time you're hacking on the heating system or replacing the heating equipment. There is merit to his other stuff as well- he has plenty of insights that casual observers often miss, and explains it reasonably well. The comfort equation can't be solved by HVAC or insulation & air sealing upgrades alone- it's multi-dimensional, but it's not rocket science either.

 

[MTLMIKEscott]

Also note, most furnaces installed in 1969 were ~3x or more oversized for the heat load of the house at the 99% outside design temperature, and most houses have had upgrades in windows/insulation/air-sealing since then. While it's unlikely that the original furnace is still in service, most HVAC installers spec replacement equipment with roughly the same (often slightly higher) heat output rather than risk pulling a defeat from the jaws of victory under the theory that "It heated the place just fine for a few decades so it's fine". While an oversized furnace still heats the place, oversizing is the enemy of comfort.

If your furnace is more than 15 years old it's worth running a fuel-use based load calculation, which is a measurement of the heat requirements of the house in it's current condition that includes all of the distribution losses such as leaky ducts and the infiltration losses of the illegitimate return paths created by the unbalanced duct system, etc.. ASHRAE recommends a 1.4x oversize factor, which is enough to cover the load during the extreme cold snaps during Polar Vortex disturbance events, but still runs at a (1/1.4=) 71% duty cycle during the normal peak cold temps. This reduces the hot-flash followed by the extended chill experience that typifies 3x oversizing that people have somehow gotten to think of as "normal". While 3x oversizing allows for more rapid recovery from overnight setbacks, it's really nowhere near as comfortable as right-sizing.

When right-sizing a replacement furnace with a big reduction in size, the cfm of the air handler drops, which reduces the amount of room pressurization from unbalanced duct systems, reduces noise and wind-chill/draft effects and even improves the performance of filters, reducing dust.

The overall, effects on efficiency are negligible (unless the ducts are grossly out of balance, and that balance isn't being fixed)- there is no "payback" in financial terms from retiring a furnace early, but as a rule right sizing does pay back in comfort. So if you can prove to your own satisfaction what the loads really are by tracking gas bills & comparing fuel use to heating degree-days you can avoid the common mistake of replacing like-for-like. The heat loads of homes are MUCH lower than most people (including HVAC contractors) generally believe.

Nate Adams is a consultant/contractor in Cleveland OH who recently published a book on how to achieve and maximize home comfort, in which he complains/explains at length the problems associated with oversizing the equipment. It's worth reading his free download chapters and watching the short videos on Home Comfort and HVAC stuff any time you're hacking on the heating system or replacing the heating equipment. There is merit to his other stuff as well- he has plenty of insights that casual observers often miss, and explains it reasonably well. The comfort equation can't be solved by HVAC or insulation & air sealing upgrades alone- it's multi-dimensional, but it's not rocket science either.

Wow, I had no idea, thanks for all the info. I will look more into this for sure! Thanks