Supply side plenum temperature

[kwilcox]

Currently mine is 150 degrees. Can I contact my contractor and get this lowered by 10 degrees? I'm assuming that this is not advisable but wanted to check with the pros here first.

The reason is that I want to upgrade my inline duct fans to a more effective model that is rated for 140 degrees. The current SunCourt fans that I'm using have no issues operating at this temp but they are axial design. The Fantech fans I'm looking at are centrifugal design and run efficiently at .5SP. My supply plenum is split to accommodate higher headroom in the basement living areas. To eliminate a big soffit running down the middle of the library ceiling, the west half of the supply plenum was moved to the middle of the house (it runs next to the central beam now). Two 8 inch ducts connect it to the original plenum and duct fans are used inside these to balance everything out.

The design works well and has run pretty much trouble-free over the last 5 years. However, the axial design fans can make a ringing sound at the low equalizing speed that they are set to. This is especially noticeable during the summer when the HVAC system is cooling the house. Also, the motor design in the Suncourt fans doesn't lend itself will to fine speed adjustments.

Here's some pics to help visualize this:

Furnace side of plenum


Split plenum running down center of house.


Shot showing the library with improved headroom. Circular stairs will be added in the future.

[hvac-learning]

why do you have fan boosters at the end of supply duct?

[kwilcox]

Good point. The original idea was to preserve full 8' headroom in a hallway that hasn't been built yet. Also was to place all equipment in a furnace room. I suppose I could have the fans moved closer to the other end where plenum temps could be lower. It sure wouldn't hurt to check the temp at the other end but I'd be surprised if it dropped 10 degrees over the 15 foot length.

Edit: 2 degree drop only as expected

Another angle: can Fantech fans tolerate a 6-7% spec variance I wonder... Am I being AR just worrying about this small of a variance???

Edit2: maybe I missed your question HVAC-Learning. Those fans are actually very close to the middle of the supply plenum. The split part shown in the second picture is about as long as the original supply plenum on the other side of the centrally located furnace. There are 4 six inch takeoffs in each side so the duct fans were speed matched until the SP in both sections was the same. It didn't take much, only enough to overcome the added friction introduced by the 4 additional right angles.

Here's a pic showing the furnace from the area of the library. Don't worry about the blocked return air vent as it was extended into the library after it was finished. It's purpose is to draw air from the living areas above through the circular stair opening.

[beenthere]

What kind of door, are you going to use to isolate the furnace room from the other areas.

Don't for get, it needs combustion air.

[kwilcox]

The furnace room will actually be open to the hallway and the rest of the basement at another location about 15 feet away. No door was planned for that one for the reason you just cited. We were going to finish putting in 5/8 fireboard (library wall against the plenum has it now) then just leave the opening on the other end for maximum noise abatement near the library. There's really not much sound now due to 5/8 inch fireboard - R12 fiberglass filled 2X4 studs - 1/2 inch drywall on the library wall side so this job may never be completed...

[kwilcox]

you know.... a delta T of 80-85 seems high for my Heil furnace. It's certainly not tripping the limit switch though and the plenum sp is about .5. Maybe its my temp gauge....

Just for kicks, I'm going to try a reading with the filter pulled too. Temp rise on this furnace (NTG3100GFG1) is supposed to be 40-70. To tell the truth, I was surprised to see a 150 degree reading. I had assumed it would've been closer to 120-130 with a 70 degree return air temp.

[beenthere]

If you have a sp of .5" in the supply plenum alone, you have a very restricted supply system.

[kwilcox]

ok, time to have a chat with my HVAC guy then. Thx.

[beenthere]

When a unit say ESP .5", it means total.

The supply and return combined.

If your system is under .8", your doing ok.

But, since your .5" in the supply already, I doubt your under 1".

PS: if you have an A/C coil on a furnace, then the supply static is measured between the coil and furnace.

Some coils will have a PD of .3" or greater.

[kwilcox]

So you're looking for an optimal total PD lower than the .8 range. Something to ponder I guess.

I would've thought a restricted return would raise return side negative sp, lower the supply side positive sp and increase delta t. Conversely, a restricted supply would raise supply side sp, lower return side negative sp and also raise delta t. Restrictions in both sides would show up as low sp readings all around and also high delta T.

A normally operating system therefore, would have low total sp (PD), higher flow, and lower delta t. I guess I understand your point.

[beenthere]

Its easy to have both, a restricted supply and return.

You can also, have a supply that measures high, and a return that measures low.
And when you fix the supply, the return now has a high static.

[kwilcox]

Ok, so my HVAC guy was out today, took a look, issued dope slaps and re-balanced the supply plenum. I ummm had lowered the booster fan speeds in an attempt to play with the ringing sound and forgot to set them back last summer.... He was anxious to have me keep doing that so he could come out and replace the furnace itnf too... That got us into a chat about air density diffs when the system was cooling vs. heating (which is why the duct fans ring in summer but not in winter) and the possibility of having my HA system automatically balance the plenums by adjusting fan speeds itself once fans with effective variable speed control were installed. This stuff is quite the rocket science IMHO. That makes you guys rocket scientists I guess....

He also measured the supply plenum temp afterward and his read 140. I thought that was high still and he pointed to my bypass humidifier and issued a second dope slap. Two dope-slaps in one day is generally my limit. Thank God I went to High School with the guy or a major brawl would've ensued in the furnace room...

Soooo I ordered a set of Fantech FKD8XL blowers to replace the SunCourt fans and am considering a true-steam upgrade. The blowers should be installed in a couple of days. Should take a week or two after that to tweak out the HA programming. Thankfully, I have an understanding family/HVAC contractor...

[kwilcox]

Well, the fans were installed 2 weeks ago and the results are just short of amazing. Here's some pics:






What's amazing is this: The HA system runs these fans at 40% whenever the furnace blower is active except during AM recovery. When this is occurring, the HA system runs the fans at 80%. At this speed, the fans are adding almost 1000 cfm @ .5sp to the east side of the supply plenum. This is where the living (as apposed to sleeping) areas of my house are. What's happening is that recovery time is being reduced by 25-30%.

It has to be due to the increased mixing effect since you can now feel a breeze from the combined supply registers when the system is recovering.

[tedkidd]

Put in an infinity with zoning, lower your energy bill, be able to closet off the furnace, and solve all these distribution problems.

[captube]

That looks like something Myth Busters would rig up.

[kwilcox]

Well, my home is actually an experiment in alternate ways to conserve energy through active Home Automation control. Remember how car engines were before they were computer controlled? The same thing can apply to residential HVAC. My house is an easy test bed for this concept because as its ranch layout suggests an obvious zoning strategy to segregate sleeping and living areas. We're just trying to see if there's a more efficient solution to traditional damper based forced air zoning here.

[kwilcox]

It's also important to understand what moving the West side supply plenum did to the living spaces downstairs:

This shot shows how bookshelves/alcoves were used during construction in the spaces directly under the moved plenum:


Finished. You can see an unfinished area of the ceiling where the circular stairs will connect the library to the living room above.

[captube]

Well, my home is actually an experiment in alternate ways to conserve energy through active Home Automation control. Remember how car engines were before they were computer controlled? The same thing can apply to residential HVAC. My house is an easy test bed for this concept because as its ranch layout suggests an obvious zoning strategy to segregate sleeping and living areas. We're just trying to see if there's a more efficient solution to traditional damper based forced air zoning here.

That was sure a good time, things change

Try tightening the structure so energy consumption drops.

[kwilcox]

That's coming. It's a bit scary to me though. In our older house, I don't have to worry about how often the air changes etc because it was insulated in the 1960's. Sure, I've done the obvious improvements to attic insulation, replaced windows etc, but modern houses are a whole different animal. Air changes, humidity etc have to be carefully considered in a modern envelope. Otherwise mold, moisture buildup and worse can happen. This happens quite often in newer homes. I'm not there yet. Still learning. One step at a time. There's a lot of older homes out there and the goal is to cost-effectively reduce their energy consumption too.

[captube]

Just saw the last photos looks good