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3-Tiered Cake Return


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Lots of HVAC guys here tell me the return doesn't need to be as big as the supply. Not that I believe it.

Maybe the 3 layer tier gives the installer enough room to stick his hand in-between supply and return to install the tape and isolator.

Marc

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So, what about it?

Why don't we like it? What problems can it bring? What should be in it's place?

It's a bottleneck, a restriction, to the flow of air. If it's in the D calc, it's fine, otherwise I write them up.

Marc

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So, what about it?

Why don't we like it? What problems can it bring? What should be in it's place?

It's a bottleneck, a restriction, to the flow of air. If it's in the D calc, it's fine, otherwise I write them up.

Marc

Bill says it is the return, so it is an expansion, not restriction.

I would put a picture of it in my report so my client knows it is there. I would write nothing about it because nothing is likely to ever be done about it.

It is like they bought a car with an ugly grill. Why waste breath or words on it?

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So, what about it?

Why don't we like it? What problems can it bring? What should be in it's place?

It's a bottleneck, a restriction, to the flow of air. If it's in the D calc, it's fine, otherwise I write them up.

Marc

Bill says it is the return, so it is an expansion, not restriction.

I would put a picture of it in my report so my client knows it is there. I would write nothing about it because nothing is likely to ever be done about it.

It is like they bought a car with an ugly grill. Why waste breath or words on it?

So, this is one of those goofy retro fits that doesn't necessarily compromise the system?

It's another of the many head shakers we see and walk away from?

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If it wasn't comprising function in any way, I'd might put it in a report as an FYI, i.e., "dipsquat but functional".

It's garden variety stupid. It wouldn't make my "top 10 stupidest things I've ever seen" list.

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It looks like the supply plenum is about 20x20. The bottleneck at the return is about 9x12. It's about 1/4 the size that it should be. There's no way that this system is working well.

I'd tell them that their return is too small and to have an HVAC tech make up a properly sized one, per the furnace installation instructions.

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I've found a lot of HVAC stuff over the years that "can't be working", but it does.

But not working well. By far the biggest complaint I've found most people have is the poor heating & cooling about the house, with uneven heating/cooling about the house leading the pack.

My pre-drywall inspections find extremely poor insulation installs 99% of the time. Add that to the lack of *science* in the design and installs of the HVAC systems, and you've got my own (tract) house, one with an electric space heater running this very moment by my desk.

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Here's a very good explanation from someone who knows (and can explain) HVAC issues better than anyone I know:

"This is pretty basic. You can't take a 2" water line, reduce it to 1/4" and then reconnect it to a 2" line and expect great water flow.

Supply air and return air need to be properly sized. If the return air is undersized you're starving the unit for air. Numerous nasty side effects occur when you start to restrict air flow. In the heating cycle the lack of air flow could be causing the high limit to start turning the burners on and off as there is not enough air flow to carry the heat away from the heat exchanger. Obviously this isn't a desirable condition as one of the side effects is adding a lot of stress to the heat exchange (overheating and cooling down repeating itself numerous times during a heating cycle) which it wasn't designed for. Efficiencies of the furnace tank, outlying rooms, farthest from the furnace, do not warm properly etc.

What you show in your picture is cause for concern. You can see how much the R/A has been reduced from the size of the R/A opening on the furnace itself.

If the manufacturer wanted a 12x12 duct attached to the furnace they

wouldn't provide a 24x24 opening.

If you knew the CFM of the unit it would be easy to calculate proper return air using a simple Trane Ductulator however, for the purpose of home inspector discussion, flag it as a possible defect and recommend (insert boilerplate here)".

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I've found a lot of HVAC stuff over the years that "can't be working", but it does.

But not working well. By far the biggest complaint I've found most people have is the poor heating & cooling about the house, with uneven heating/cooling about the house leading the pack.

My pre-drywall inspections find extremely poor insulation installs 99% of the time. Add that to the lack of *science* in the design and installs of the HVAC systems, and you've got my own (tract) house, one with an electric space heater running this very moment by my desk.

I know what you're saying, but sometimes stuff does work OK even when it's stupid. I'm not saying how to report a condition; everyone can do whatever they want. But, sometimes systems are installed stupidly and they work fine. I see lots of small houses that work fine, even though supplies and returns are all "wrong".

That said, stats say the #1 home complaint on the north shore is inadequately performing HVAC. Put a stupid system in a giant house, and they usually don't work.

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It comes down to the D calc but I'd be surprised if the system in the OP, with that 3 tier cake, would mesh with any scheme you could fit into such a calculation and still yield an adequate system CFM.

I'd be writing it up but what I'd say depends on what else the duct system looks like. I often either 'throw the book at em' or let it go. I stay away from marginal write-ups.

My apologies, John K.

Marc

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Here's a very good explanation from someone who knows (and can explain) HVAC issues better than anyone I know:

"This is pretty basic. You can't take a 2" water line, reduce it to 1/4" and then reconnect it to a 2" line and expect great water flow.

Supply air and return air need to be properly sized. If the return air is undersized you're starving the unit for air. Numerous nasty side effects occur when you start to restrict air flow. In the heating cycle the lack of air flow could be causing the high limit to start turning the burners on and off as there is not enough air flow to carry the heat away from the heat exchanger. Obviously this isn't a desirable condition as one of the side effects is adding a lot of stress to the heat exchange (overheating and cooling down repeating itself numerous times during a heating cycle) which it wasn't designed for. Efficiencies of the furnace tank, outlying rooms, farthest from the furnace, do not warm properly etc.

What you show in your picture is cause for concern. You can see how much the R/A has been reduced from the size of the R/A opening on the furnace itself.

If the manufacturer wanted a 12x12 duct attached to the furnace they

wouldn't provide a 24x24 opening.

If you knew the CFM of the unit it would be easy to calculate proper return air using a simple Trane Ductulator however, for the purpose of home inspector discussion, flag it as a possible defect and recommend (insert boilerplate here)".

You cannot directly compare the two. A liquid versus a gas and typically much different pressures. Length of the restriction is an important factor.

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