Carrier gas furnace horizontal installation

jesmed1

I'm not a heating pro, but as a mechanical engineer, my church asked me to troubleshoot a heating problem that our HVAC company couldn't figure out. The short story is the HVAC company installed a new Carrier 110MBH furnace in a low-ceiling mechanical room. Because of the height limitation, with the AC coil on top of the furnace, there was only about 6" of height left under the furnace for the bottom return plenum. The result was that the furnace constantly overheated due to insufficient airflow.

After taking static pressure measurements at various places, I found the static pressure across the return plenum was 0.6 inches water column, which by itself exceeded the entire total external static pressure allowed by the mfr (0.5 inches W.C.) This became obvious when consulting the install manual, which requires a much larger return plenum.

When I pointed this out to the HVAC company, they shrugged and said there was nothing they could do because of the limited space. I said it was their job to size the furnace correctly, and if they didn't have sufficient space to install the furnace so it could run without overheating, they should have either installed it horizontally or selected a different furnace that would have worked within the space available.

Now we have to run the furnace locked in low fire mode (40% capacity) so it doesn't overheat, but that means in very cold weather it can barely keep up with demand. In looking for alternatives, I've considered various reconfigurations of the ductwork, but the only workable solution I can see is removing the furnace and re-installing it horizontally. That would allow a properly sized return plenum, and should bring down the total external static pressure from the current 1.4 inches W.C. to a still excessive but probably workable 0.8 inches W.C.

I wouldn't ask these HVAC guys to do the work because they've proven themselves incompetent. I'd find another HVAC company to do the work, but before suggesting this horizontal re-installation to church leadership, I'd like to make sure it's doable, and I'd welcome any advice from experts. Is a horizontal install pretty straightforward? Any potential pitfalls?

gpjazz

You say that TESP is 1.4" and static in return is 0.6".. So that means static in supply is 0.8". Which is still higher than return. So even if things were modified to get return static to 0.2" - 0.3" for example, TESP would still be higher than desired.

So it sounds like there are more ductwork issues with the supply side rather than the return side.

Unless your measurements are not accurate

jesmed1

https://forum.heatinghelp.com/discussion/comment/1885590#Comment_1885590

Sorry, I was quoting that number from memory and wasn't quite accurate. Here are the actual measurements, which I took numerous times with a digital manometer and am confident are correct:

Total supply static: +0.45" W.C., measured between furnace output and AC coil

Total return static: -0.70" W.C., measured at bottom of furnace just above return plenum

TESP: 1.15" W.C. (don't know why I misremembered as 1.4", sorry)

But get this: when the entire return duct system is disconnected from the return plenum, and with the intake end of the return plenum totally exposed to atmosphere, the return static under the furnace is -0.60" W.C. So the badly undersized return plenum itself is responsible for 0.60" of the overall 0.70" static in the return system. Which means the return ductwork before the plenum is contributing only 0.10" W.C. This makes sense because the furnace is quite close to the large return intake grille in the room, with a short run of duct to the return plenum under the furnace. So if you saw the install, you'd see why only 0.10" W.C. is reasonable for the return ductwork before the plenum. Also, if you saw the return plenum under the furnace, you'd see why 0.60" W.C. is believable for that alone. It's a 6" tall rectanglular "pancake" that extends out behind the furnace, and the return air comes from the top vertically down into the pancake, makes a 90 degree turn under the furnace through a narrow chokepoint, then makes another 90 degree turn up vertically into the furnace. You couldn't design a worse return plenum if you tried.

So if the furnace was laid sideways and a full-sized return plenum was installed, with the return air making a straight horizontal shot into the plenum, the plenum itself would add almost no static pressure, leaving the supply at +0.45" and the return at -0.10" for a total of 0.55".

You could argue that now, with a better return plenum, the CFM's will increase, and so will the TESP. OK, so maybe we end up at 0.7" or 0.8" TESP. It's still better than 1.15", and although still above the spec of 0.5", some HVAC people I've talked to said it's not uncommon to see furnaces with 0.8" TESP running normally. It wouldn't be ideal, but it would be better than what we have now.

pecmsg

You need someone on site that knows how to measure static pressure and what the numbers mean. Bad supply, bad return, both.

As far as the furnace a complete model 3 is needed. If the ducts cannot handle the required CFM then a) Down size the furnace or b) increase the ducts.

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EBEBRATT-Ed

@jesmed1

If you use the same furnace you need to make sure it is compatible. Some can be installed in almost any position. In addition some are horizontal left discharge or horizontal right discharge. Just need to read the manual to figure it out

jesmed1

@EBEBRATT-Ed Thanks, I didn't know there was a right vs left discharge thing. This would only work with a left discharge, so I'll see what the manual says.

EBEBRATT-Ed

Some can be reversed from left to right

HVACNUT

So the furnace is sitting on a return duct pedestal, but the return duct sizes at the furnace is too small? Is there any room to add a return at the side of the furnace?

jesmed1

https://forum.heatinghelp.com/discussion/comment/1885785#Comment_1885785

Correct. The return "pedestal" is only about 6" high, with insulation inside that makes it more like 4" high inside. So the return air has to go through a rectangle the width of the furnace and just 4" high, then makes a 90-degree turn vertically up into the furnace.

The return can't be reconfigured to come through the side because the furnace room is so narrow that a side return would block access to the back of the room.